Soil Water Regime

1.1. Definition and Essence of Water Balance

When beginning to study the water regime, it is first necessary to distinguish between two fundamental concepts:

• Water balance is the quantitative expression of water input and output in the soil over a specific period (White, 2006; Mukha et al., 2003). This answers the question "How much water came in and how much went out?".
• Water regime is the totality of processes of water input, movement, retention, and expenditure in the soil (Mukha et al., 2003). This answers the question "How does water behave in the soil?" — at what speed does it move, where is it retained, what forces act upon it.

The water balance is described by an equation based on the law of conservation of mass. In its most general form for the soil profile, it looks as follows (Mukha et al., 2003; Weil & Brady, 2017):

where:

• W₀ — initial water storage in the soil;
• P — atmospheric precipitation;
• I — irrigation water;
• G — water input from groundwater (capillary rise);
• C — condensation of water vapor in the soil;
• R_in — surface and lateral subsurface inflow;
• ET — total evapotranspiration (physical evaporation from the soil surface + plant transpiration);
• R_out — surface runoff;
• D — deep percolation (infiltration beyond the root zone);
• W₁ — final water storage in the soil.

In a simplified form for crop production needs and without considering groundwater contribution, the water balance equation is often written as (Weil & Brady, 2017):

where ΔW is the change in soil water storage over the period under consideration.

1.2. Components of the Input Part of the Balance

Atmospheric precipitation (P) — the main source of water input to the soil in most natural zones. However, not all water that falls with rain or snow reaches the soil surface. A significant part is intercepted by the vegetation cover (drip, stemflow) and returns to the atmosphere through evaporation. In forest ecosystems, interception can range from 10 to 40% or more of the total precipitation (Weil & Brady, 2017).

Irrigation (I) — artificial replenishment of water reserves, playing a critical role in arid regions. When calculating the water balance, it is important to consider not only the amount of water applied but also its distribution and losses due to evaporation during irrigation.

Capillary rise from groundwater (G) — the process by which water rises through capillary pores from the groundwater table to the root zone. This source is particularly significant in soils with a shallow water table. The height of capillary rise depends on the texture: in sands, it does not exceed 0.5–0.8 m; in loams, it can reach 3–6 m (Mukha et al., 2003).

Condensation of water vapor (C) — the process of water vapor transitioning from the soil air into the liquid phase. It is especially important under conditions of sharp daily temperature fluctuations, characteristic of desert and semi-desert landscapes (White, 2006).

1.3. Components of the Output Part of the Balance

Evapotranspiration (ET) — total water loss, consisting of two processes:

• Physical evaporation (E) — the transition of water from liquid to vapor state from the soil surface;
• Transpiration (T) — the evaporation of water by plants through the stomatal apparatus of leaves.

For most agricultural crops, transpiration constitutes the major portion of water expenditure. It is important to understand that the intensity of transpiration is regulated not only by soil moisture availability but also by plant physiological mechanisms (stomatal closure under water stress) (Weil & Brady, 2017).

Surface runoff (R_out) — water that did not have time to infiltrate into the soil and flowed over the surface. Two mechanisms of runoff formation are distinguished (White, 2006):

• Infiltration excess — occurs when the rainfall intensity exceeds the soil's infiltration rate;
• Saturation excess — occurs when the soil is completely saturated with water and cannot accept new portions of moisture.

Deep percolation (D) — water that has moved beyond the root zone, into deep horizons, and replenished groundwater reserves. In some contexts, this process is called percolation or drainage losses (Weil & Brady, 2017; White, 2006).

1.4. Soil Water Storage and Its Quantitative Assessment

For agronomic practice, two indicators characterizing the amount of water in the soil are most important:

Total water storage (TWS) — the total amount of water in the entire soil profile. It is calculated by summing the water storage by genetic horizons (Mukha et al., 2003):

where:

• W_i — horizon moisture content (in % of dry soil mass);
• ρ_bi — horizon bulk density;
• h_i — horizon thickness.

Productive (available) water storage (PWS) — the amount of water available to plants. It is defined as the difference between the total water storage and the storage of unavailable (dead) moisture — water retained by the soil with a force exceeding the root's suction force (permanent wilting point moisture) (Mukha et al., 2003):

where UWS is the unavailable water storage (at wilting point moisture).

In agronomic practice, productive moisture reserves are assessed using the following scale (for the 0–100 cm layer) (Mukha et al., 2003):

1.5. Factors Affecting the Water Balance

The soil water balance is determined by the interaction of three groups of factors:

1. Climatic conditions — amount and distribution of precipitation, air temperature, wind speed, relative humidity. These parameters determine the potential evaporation rate (evaporativity) (White, 2006; Weil & Brady, 2017).

2. Soil properties — texture and mineralogical composition, bulk density, structure, organic matter content. These determine the water-holding capacity, permeability, and hydraulic conductivity of the soil.

3. Vegetation cover and anthropogenic impact — vegetation type, root system depth, irrigation, drainage, agronomic practices.

2.1. Definition and Distinction from Water Balance

If the water balance is a quantitative characteristic describing the ratio of water input and output in the soil over a specific period (how much water came in and how much went out), then the water regime is a qualitative-dynamic characteristic reflecting the totality of processes of water input, movement, retention, and expenditure in the soil, occurring over time (Mukha et al., 2003).

Thus, the water regime answers the question "how does water behave in the soil?" — at what speed does it move, in what forms is it retained, in what direction does it move, how does its state change during the day, season, or year.

The water regime includes four main processes (Mukha et al., 2003; Weil & Brady, 2017):

1. Input — how water enters the soil (infiltration of rainfall and irrigation water, capillary rise from groundwater, condensation of water vapor);

2. Movement — how water moves within the soil profile (downward and upward flows, lateral movement);

3. Retention — how and in what forms water is held in the soil (sorptive, capillary, and gravitational forces);

4. Losses — how water leaves the soil (physical evaporation, plant transpiration, surface runoff, deep percolation).

The water regime is the integral result of the interaction of climatic conditions, soil properties (texture, density, structure, organic matter content), topography, and vegetation cover (White, 2006). Therefore, the water regime of a particular soil is variable over time: it depends on the season, weather conditions, plant development phase, and the nature of anthropogenic impact.

2.2. Forms of Water in Soil: The Basis for Understanding the Water Regime

For proper management of the water regime, it is necessary to know in what forms water can exist in the soil and how these forms differ in mobility and availability to plants. Water in the soil is retained by forces of different natures — sorptive (attraction to the surface of solid particles), capillary (meniscus forces), and gravitational (gravity) (Mukha et al., 2003; Weil & Brady, 2017).

2.3. Soil Hydrological Constants and Their Significance

Soil hydrological constants are soil moisture values corresponding to a specific energy state of the water. They serve as boundaries between different forms of moisture and allow the assessment of water availability to plants (Mukha et al., 2003; Weil & Brady, 2017).

pF — the decimal logarithm of the negative pressure of soil moisture, expressed in centimeters of water column (Mukha et al., 2003). The pF scale allows the comparison of the energy state of water in different soils, regardless of their texture. The higher the pF, the more strongly the water is bound to the soil.

Practical significance of these constants: water located between field capacity (FC) and the permanent wilting point (PWP) constitutes productive moisture — the main reserve for plant water supply. Water content above FC is gravitational (excess) moisture; below PWP is unavailable (dead) moisture.

It is important to emphasize that the wilting point depends not only on soil properties but also on plant species. Different crops have different root suction forces, so the lower limit of available moisture for them varies (Mukha et al., 2003):

2.4. Availability of Moisture to Plants Depending on the Form of Water

The relationship between water forms, their availability to plants, and their modes of movement in the soil can be presented in the following table (Mukha et al., 2003):

The optimal moisture range for most agricultural crops lies between the capillary break moisture and field capacity (Mukha et al., 2003). However, the ecological moisture optimum varies among crops: for example, for rice and moisture-loving grasses, it is shifted towards higher values; for drought-resistant plants, towards lower values.

2.5. Regulation of the Water Regime

Managing the water regime includes a set of measures aimed at creating optimal conditions for plant water supply with minimal unproductive water losses. Depending on the climatic zone, the regulation tasks differ (Mukha et al., 2003; Weil & Brady, 2017).

In zones of insufficient and unstable moisture (steppes, dry steppes, semi-deserts), the main techniques are aimed at:

• Moisture accumulation: snow retention, retention of meltwater (furrowing, slitting, cross-slope tillage);
• Moisture conservation: surface loosening (harrowing) to break capillaries, mulching, creating shelterbelts;
• Additional moistening: irrigation, considering the soil's physical properties and the biological characteristics of the crops.

In zones of excessive moisture (forest, forest-steppe zones with a shallow water table), the main techniques are aimed at:

• Removal of excess moisture: open or closed drainage, ridge cultivation, leveling micro- and meso-depressions;
• Creating a regulated water regime: using drainage systems capable of supplying water through drains during dry periods (dual-purpose drainage).

On irrigated lands, regulating the water regime requires the mandatory combination of irrigation with drainage — otherwise, secondary salinization and waterlogging are inevitable. Modern drip irrigation systems allow moisture to be maintained within the optimal range, using water economically and without creating a risk of over-wetting (Weil & Brady, 2017).

Thus, knowledge of the patterns of the water regime and its quantitative characteristics is a necessary condition for the development of scientifically based farming systems, land reclamation, and soil resource protection.

3.1. Definition and Essence of the Process

Infiltration is the process of water entering the soil through its surface (Weil & Brady, 2017; White, 2006). Infiltration begins when water — rainwater, meltwater, or irrigation water — comes into contact with the soil surface and continues as long as there is a supply of water to the surface and the soil can accept it.

Infiltration is a key process determining how much of the atmospheric precipitation or irrigation water will replenish soil moisture reserves and how much will contribute to surface runoff. Therefore, infiltration is the first link in the chain of plant water supply and the first barrier against possible water erosion (Weil & Brady, 2017; Shukla, 2023).

In the context of our key question — how water comes, moves, is retained, and leaves the soil — infiltration represents the process of water input into the soil.

3.2. Mechanism of Infiltration. Driving Forces

Water penetrates the soil under the influence of two main forces:

1. Gravitational force — gravity, tending to move water downward. The gravitational component becomes dominant as the wetting front advances deeper and the matric potential gradient decreases (White, 2006).

2. Capillary (matric) force — arises from the difference in water potential between dry and wet soil. The matric potential of dry soil is significantly lower (more negative) than that of wet soil, creating a powerful gradient that "pulls" water into the pore space (Weil & Brady, 2017; Marshall et al., 1996).

It is the matric component that explains the high initial infiltration rate into dry soil: water is literally "sucked" into the empty pores by capillary forces. As the upper horizons become saturated, the matric gradient decreases, and gravity begins to play the leading role.

It is important to understand that infiltration is a non-steady-state process. The infiltration rate is not constant but continuously changes over time, decreasing as the soil wets (Marshall et al., 1996; Weil & Brady, 2017).

3.3. Infiltration Rate and Factors Determining It

Infiltration rate (infiltrability) is the volume of water passing through a unit area of soil surface per unit time. Usually expressed in mm/h or cm/day (Weil & Brady, 2017).

A characteristic feature of infiltration is its exponential decrease with time. At the initial moment, when the soil is dry, the rate is maximal. Then, as pores fill with water and the matric gradient decreases, the rate drops sharply, tending towards a constant value — the steady-state infiltration rate. This steady-state rate is close to the saturated hydraulic conductivity of the soil (White, 2006; Weil & Brady, 2017).

Factors determining the infiltration rate:

1. Soil texture. Larger particles create larger pores through which water passes with less resistance. Therefore, sandy soils have high initial and steady-state infiltration. Clay soils, on the other hand, are characterized by low permeability, especially when clay minerals swell. However, in a dry state, clay soils with a developed network of cracks can have a very high initial infiltration rate (Weil & Brady, 2017).

2. Structural condition. Aggregated, well-structured soils have higher permeability due to a system of macropores — inter- and intra-aggregate voids, root and worm channels. Destruction of structure (e.g., by plowing, compaction) sharply reduces infiltration (Weil & Brady, 2017).

3. Initial moisture content. The drier the soil, the greater the matric gradient and the higher the initial infiltration rate. Wet soil absorbs water more slowly (Marshall et al., 1996).

4. Soil crust. The formation of a compacted surface layer under the impact of raindrops, which destroy aggregates and clog pores with fine material, can sharply reduce the infiltration rate. A crust promotes the conversion of water to surface runoff even at moderate rainfall intensities (Weil & Brady, 2017).

5. Vegetation cover and plant residues. Plant roots create additional macropores, improve structure, and protect the surface from the destructive action of raindrops. Forest litter and mulch slow down surface runoff, increasing the contact time of water with the soil and, consequently, infiltration (Weil & Brady, 2017).

6. Presence of restrictive layers in the profile. If a dense horizon (fragipan, illuvial horizon, plow pan) is located at a shallow depth, infiltration may slow down or completely stop after the saturation of the overlying layer (Weil & Brady, 2017).

3.4. Darcy's Law as the Basis for Describing Infiltration

For the quantitative description of water movement in saturated and unsaturated soils, Darcy's law is used, formulated by the French engineer Henry Darcy in 1856 (Weil & Brady, 2017; White, 2006; Radcliffe & Šimůnek, 2012):

where:

• J_w — water flux density (volume of water passing through a unit area per unit time), m³/(m²·s) or m/s;
• K — soil hydraulic conductivity, characterizing the soil's ability to transmit water, m/s;
• dH/dz — hydraulic head gradient (change in head per unit length in the direction of flow).

The minus sign indicates that water moves in the direction of decreasing head — from higher to lower potential (White, 2006).

In unsaturated soil conditions (and infiltration into dry soil is precisely an unsaturated flow), hydraulic conductivity K is not a constant but depends on moisture content (or matric potential). As the soil wets, K increases, approaching the saturated hydraulic conductivity K_sat (Radcliffe & Šimůnek, 2012; Weil & Brady, 2017).

For unsaturated flow, Darcy's law is written as (Radcliffe & Šimůnek, 2012; White, 2006):

where K(θ) is hydraulic conductivity as a function of moisture content θ.

The hydraulic head H in Darcy's law consists of two components (White, 2006; Weil & Brady, 2017):

where:

• h — matric (capillary) head, related to the forces of attraction of water to the solid phase (negative in unsaturated soil);
• z — gravitational head, determined by the elevation of the point relative to a reference level.

Thus, the head gradient can be represented as:

Substituting into Darcy's law, for unsaturated vertical flow we get:

In the initial period of infiltration, when the matric head gradient dh/dz is large, it contributes the main part of the driving force. As the profile wets, dh/dz decreases, and the gravitational component begins to dominate (White, 2006; Marshall et al., 1996).

3.5. The Green-Ampt Model

One of the classic approaches to describing infiltration is the model proposed by Green and Ampt (Green & Ampt, 1911), which is based on the concept of a sharp, piston-type wetting front (Weil & Brady, 2017; Marshall et al., 1996; Radcliffe & Šimůnek, 2012).

The main assumptions of the Green-Ampt model are:

1. The wetting front is a sharp boundary between wet and dry soil;

2. Behind the front, the moisture content is constant and close to saturation;

3. The matric potential at the wetting front is constant.

The Green-Ampt equation for the infiltration rate (Weil & Brady, 2017; Marshall et al., 1996):

where:

• i(t) — infiltration rate at time t;
• K₀ — hydraulic conductivity of the soil behind the wetting front (close to K_{sat});
• h₀ — depth of ponded water on the surface (during irrigation or flooding);
• ψ_f — matric potential at the wetting front (negative value);
• Δθ — increase in moisture content from initial to maximum;
• I(t) — cumulative amount of infiltrated water up to time t (cumulative infiltration).

The equation shows that when I(t) is small (initial moment), the infiltration rate is high due to the large term (h₀ - ψ_f)·Δθ / I(t). As I(t) grows, this term decreases, and the rate approaches K₀ (Weil & Brady, 2017; Marshall et al., 1996).

For cumulative infiltration I(t), the Green-Ampt equation has the form (Radcliffe & Šimůnek, 2012):

The Green-Ampt model gives good results for coarse-textured soils and for ponded infiltration conditions. For fine-textured soils, where the wetting front is diffuse, parameter adjustment is required (Marshall et al., 1996; Radcliffe & Šimůnek, 2012).

3.6. Preferential Flow

In real field conditions, infiltration is rarely homogeneous. Preferential flow is the movement of water through individual macropores (cracks, root and worm channels, inter- and intra-aggregate voids) at a speed significantly exceeding the movement through the soil matrix (Weil & Brady, 2017; Radcliffe & Šimůnek, 2012).

Preferential flow has important consequences:

• Water and dissolved substances can quickly reach deep horizons and groundwater, bypassing the bulk of the soil profile;
• This accelerates the leaching of pollutants (nitrates, pesticides, pathogens) into groundwater (Weil & Brady, 2017);
• During intense rainfall, preferential flow can contribute to the rapid saturation of deep layers and the formation of perched water tables (Radcliffe & Šimůnek, 2012).

The study of preferential flow is an important area of modern soil hydrology, especially in the context of protecting water resources from pollution (Weil & Brady, 2017).

3.7. Soil Permeability

Closely related to infiltration is the concept of soil permeability — the soil's property to transmit water through itself. Permeability is quantitatively characterized by the hydraulic conductivity (K) — the volume of water (in mm) passing through a unit area of soil per unit time at a hydraulic gradient equal to 1 (Mukha et al., 2003).

Permeability depends on the same factors as infiltration: texture, structural condition, density, presence of macropores. Permeability can vary over a very wide range — from fractions of a millimeter to several meters per day.

Gradation of permeability according to V. A. Kovda (Mukha et al., 2003):

Soils with excessively high permeability are unable to create sufficient moisture reserves in the root zone; water quickly moves downward. Soils with unsatisfactory and very poor permeability become waterlogged, promote surface runoff, and the development of erosion (Mukha et al., 2003).

3.8. Practical Significance of Infiltration

Infiltration is a key process determining the efficiency of using precipitation and irrigation water in agriculture. The rate and depth of infiltration affect:

• Replenishment of soil moisture reserves — the higher the infiltration capacity, the greater the portion of precipitation and irrigation water that enters the soil rather than being lost to runoff (Weil & Brady, 2017);
• Irrigation rates and schedules — for soils with low infiltration, frequent irrigations with small amounts are required to avoid runoff and waterlogging (Weil & Brady, 2017);
• Risk of water erosion — surface runoff, which occurs when rainfall intensity exceeds infiltration capacity, is the main cause of soil loss (Weil & Brady, 2017; Mukha et al., 2003);
• Feasibility of different irrigation methods — sprinkler and drip irrigation are preferred on soils with low infiltration capacity, as they do not create a layer of water on the surface and do not provoke runoff (Weil & Brady, 2017).

Thus, managing infiltration is a critical task in agronomy. Techniques that increase infiltration include: preserving and improving soil structure (applying organic fertilizers, minimum tillage, using green manures), mulching, creating shelterbelts, slitting, and other measures to retain water on the surface (Weil & Brady, 2017; Mukha et al., 2003).

4.1. Definition and Essence of the Process

After the supply of water to the soil surface ceases — the rain has stopped, irrigation is complete, snow has melted — infiltration stops; however, the water in the soil does not remain stationary. The process of redistribution begins — the movement of moisture within the soil profile from wetter zones to drier zones under the influence of potential gradients (Marshall et al., 1996; White, 2006).

Redistribution answers the question: how does water move within the soil after it has entered through the surface? Unlike infiltration, where water comes from outside, redistribution is internal movement of moisture within the soil mass.

Redistribution includes two main directions of movement (Marshall et al., 1996; Weil & Brady, 2017):

1. Downward redistribution — the continued movement of water deeper under the influence of gravity and residual matric gradients;

2. Upward redistribution — the movement of water upward under the influence of capillary forces arising from surface evaporation or the pull of moisture by plant roots.

Redistribution is a non-steady-state, decaying process over time. As moisture content in the profile equalizes, the driving forces diminish, and water movement slows, tending towards an equilibrium state (Marshall et al., 1996; Radcliffe & Šimůnek, 2012).

4.2. Dynamics of the Soil Moisture Profile After Wetting

Immediately after heavy wetting, a characteristic moisture distribution forms in the soil profile (Marshall et al., 1996; Weil & Brady, 2017):

1. Saturation zone (or near-saturation) — the upper part of the profile, where pores are maximally filled with water. Here, the matric potential is close to zero, and water is primarily in gravitational and capillary forms.

2. Transmission zone — the region with moisture content close to field capacity. In this zone, moisture is relatively constant, and water moves downward mainly under gravity with a small matric gradient. This zone essentially acts as a "conduit" for water to the wetting front (Marshall et al., 1996; Weil & Brady, 2017).

3. Wetting zone (front) — the transitional region where moisture content drops sharply from values near saturation to the initial (background) values of the dry soil. This is the wetting front — a sharp or diffuse boundary between wet and dry soil. Behind the wetting front, the matric gradient is maximal, ensuring the front's advancement (Marshall et al., 1996).

4. Initial (dry) zone — soil whose moisture content has not yet changed due to this wetting event. The matric potential here is minimal (most negative).

As redistribution proceeds, the wetting front advances deeper, and its shape becomes increasingly diffuse due to the decreasing gradient and the influence of pore space heterogeneity.

An important feature of redistribution: unlike infiltration, where the water movement rate is mainly determined by the input intensity, during redistribution, the rate is determined by the internal properties of the soil — its hydraulic conductivity and water storage capacity (Marshall et al., 1996; Radcliffe & Šimůnek, 2012).

4.3. Rate and Nature of Wetting Front Advancement

For the quantitative description of redistribution, the Richards equation Richards equation is used, which combines Darcy's law and the continuity equation (Marshall et al., 1996; White, 2006; Radcliffe & Šimůnek, 2012):

where:

• θ — volumetric soil water content;
• t — time;
• z — vertical coordinate (positive upward);
• K(θ) — hydraulic conductivity as a function of moisture content;
• h — matric head (negative in unsaturated soil).

The Richards equation describes the change in moisture content over time at each point in the profile and is the basis for numerical modeling of redistribution (Radcliffe & Šimůnek, 2012).

In a simplified form, neglecting the gravitational component (for horizontal movement), the solution to the Richards equation leads to the known dependence (Marshall et al., 1996):

or explicitly for the position of the wetting front (at constant diffusivity):

where x(t) is the depth of front advancement at time t, Δh is the difference in matric head between the wet and dry zones, and Δθ is the increase in moisture content.

This relationship means that over equal successive time intervals, the wetting front advances by decreasing distances: first quickly, then more and more slowly. For example, in the first 10 minutes, the front might advance 5 cm; in the next 10 minutes, another 3 cm; then 2 cm, etc. (Marshall et al., 1996; Weil & Brady, 2017). This is explained by the decrease in the matric potential gradient as the path length from the surface to the front increases.

4.4. Capillary Rise as a Special Case of Upward Redistribution

A special role in redistribution is played by capillary rise — the upward movement of water through capillary pores from the groundwater table or from a wet layer to a drier overlying horizon (Mukha et al., 2003; Weil & Brady, 2017).

Capillary rise occurs due to meniscus forces — the tendency of water to rise in thin capillaries until the weight of the raised water column balances the capillary pressure (Weil & Brady, 2017; White, 2006). The height of rise is determined by the radius of the capillary pores (Jurín's equation):

where:

• h — height of capillary rise, m;
• σ — surface tension of water, N/m;
• α — contact angle (for most mineral soils, close to 0);
• ρ — density of water, kg/m³;
• g — acceleration due to gravity, m/s²;
• r — capillary radius, m.

Height and rate of capillary rise depending on soil texture (Mukha et al., 2003):

Important rule: the thinner the capillaries, the higher, but slower the water rises. In sandy soils, water rises quickly but to a small height; in clay soils, it rises slowly but to a significant height (Mukha et al., 2003; Weil & Brady, 2017).

Capillary rise has important agronomic significance:

• With a shallow water table (less than 2–3 m), capillary rise can provide additional moistening of the root zone during dry periods;
• However, if the groundwater is mineralized, capillary rise causes secondary soil salinization — salts accumulate in the upper horizons upon evaporation (Mukha et al., 2003; Weil & Brady, 2017);
• Breaking capillaries (by harrowing, loosening) is one of the effective agronomic techniques for conserving soil moisture: by disrupting the continuous capillary connection, we prevent water from being drawn to the surface and evaporating.

Capillary-suspended and capillary-supported water are two states of capillary moisture. The first is formed by wetting from the surface and is held above a dry layer; the second rises from the groundwater table. There may be a dry layer between them if the height of capillary rise is insufficient for them to meet. If they do meet, a continuous capillary connection is created, and water begins to rise actively to the surface, which can lead to salinization (Mukha et al., 2003).

4.5. Redistribution in Layered Profiles (Capillary Barrier)

In natural and agrogenic conditions, the soil profile is often layered: horizons differ in texture and density. The behavior of water at layer boundaries follows important patterns (Weil & Brady, 2017; White, 2006).

Situation 1: wet fine-textured layer over dry fine-textured layer — water will move downward relatively freely, as the capillary properties of the layers are similar.

Situation 2: wet fine-textured layer (loam, clay) over dry coarse-textured layer (sand, gravel) — a capillary barrier arises (Weil & Brady, 2017). The issue is that the matric potential in fine pores is more negative (water is held more strongly) than in coarse pores. Therefore, water cannot pass from fine pores to coarse pores until a sufficiently high positive pressure (saturation of the overlying layer) is reached. As a result, water accumulates above the coarse-textured layer, creating a perched water table — a temporary or permanent saturation (Weil & Brady, 2017).

Practical implications of the capillary barrier:

• A layer of gravel or sand in the soil profile can serve not as drainage, but as an impediment to downward water movement. This principle is used in the construction of turf profiles (sand layer over gravel) to maintain moisture in the root zone (Weil & Brady, 2017).
• In natural conditions, layered deposits (e.g., loess over gravel) can create conditions for the formation of a perched water table and gleying in the upper part of the profile.
• When designing drainage systems, it must be considered that backfilling drains with gravel can, conversely, slow down the entry of water from the fine-textured soil into the drain if positive pressure is not created.

Situation 3: wet coarse-textured layer over dry fine-textured layer — water moves downward relatively easily, as large pores do not create capillary resistance, and the fine-textured layer actively draws water in due to its high matric potential.

Thus, when analyzing redistribution in layered soils, it is necessary to consider not only the hydraulic conductivity of the layers but also the relationship of their capillary properties.

4.6. Redistribution During Evaporation (Upward Fluxes)

If evaporation occurs from the soil surface, the upper layer loses moisture, its matric potential becomes more negative, and a gradient directed upward is created. This causes upward redistribution — the pulling of moisture from deeper, wetter horizons to the surface (Marshall et al., 1996; Shukla, 2023).

During evaporation, two zones form in the profile (Shukla, 2023; Weil & Brady, 2017):

1. Drying zone (dry layer) — the upper part of the profile, where moisture content has dropped below the capillary break point. In this layer, capillary connection is disrupted, and water moves primarily in the vapor phase (slowly).

2. Capillary pull zone — below the dry layer, where capillary connection is still maintained, and water reaches the surface through capillaries.

The rate of upward redistribution is determined by two factors (White, 2006; Shukla, 2023):

• Evaporation intensity (meteorological conditions);
• Soil hydraulic conductivity at a given moisture content.

Under weak evaporation and high soil moisture, the upward flux can be significant and compensate for water losses. Under strong evaporation and low moisture, the upward flux quickly becomes limiting, and the surface layer dries out (Shukla, 2023).

It is important to note that even during evaporation, water can move upward from fairly deep horizons if the capillary connection is not broken. Therefore, the depth of the water table critically affects the potential for capillary contribution (Marshall et al., 1996).

4.7. Role of Macropores in Redistribution

In real field conditions, macropores (cracks, root and worm channels, inter- and intra-aggregate voids) significantly affect the nature of redistribution (Weil & Brady, 2017; Radcliffe & Šimůnek, 2012).

Water entering macropores can:

• Move quickly downward to a significant depth, bypassing the main soil matrix (preferential flow);
• Create localized wetting foci in deep horizons;
• Accelerate the leaching of dissolved substances (salts, nitrates, pesticides) beyond the root zone.

During redistribution, macropores can either accelerate or slow down water movement depending on their orientation and connectivity. Horizontal macropores promote lateral redistribution, which is particularly important on slopes (Weil & Brady, 2017).

4.8. Practical Significance of Redistribution

Understanding redistribution processes is necessary for:

1. Predicting moisture reserves in the root zone. After rain or irrigation, water does not immediately become available to plants — it must redistribute within the profile. The depth and rate of wetting determine how effectively the moisture will be used.

2. Calculating irrigation rates and schedules. During irrigation, it is necessary to consider that part of the water will redistribute deeper, and not all of it remains in the active root zone. Therefore, the irrigation rate is calculated considering the wetting depth and the soil's water-holding capacity (Weil & Brady, 2017).

3. Assessing the risk of nutrient leaching (nitrates) and pesticides. Rapid redistribution through macropores can lead to their transport beyond the root zone and contamination of groundwater (Weil & Brady, 2017).

4. Designing drainage and irrigation systems. Understanding the directions and rates of redistribution helps in choosing drain depth, spacing, and irrigation water application schedules.

5. Combating soil salinization. If capillary rise brings mineralized water, it is necessary either to lower the groundwater table (drainage) or create a barrier to capillary rise (e.g., through deep tillage or application of organic materials) (Mukha et al., 2003; Weil & Brady, 2017).

Summary for Section 4

• Redistribution is the movement of moisture within the soil profile after infiltration ceases, driven by matric and gravitational potential gradients.
• During redistribution, zones of saturation, transmission, wetting front, and initial dry zone are distinguished in the profile.
• The rate of wetting front advancement is proportional to \sqrt{t} — it slows down with time.
• Capillary rise is the upward movement of water through capillaries, the height of which is determined by pore radius and texture.
• In layered profiles, a capillary barrier — the retention of water above a coarse-textured layer — is possible.
• Macropores create preferential flow, accelerating the downward movement of water and dissolved substances.
• Managing redistribution underlies many agronomic and reclamation practices, from regulating the water regime to combating salinization.

5.1. Definition and Purpose

Drainage is the removal of excess gravitational water from the soil to lower the water table, improve aeration of the root zone, and create optimal conditions for plant growth (Weil & Brady, 2017; Marshall et al., 1996).

In the context of our key question — how water comes, moves, is retained, and leaves the soil — drainage represents the forced removal of water that, for various reasons, cannot leave the soil naturally. It is a regulated process allowing the management of the water regime under conditions of excessive moisture.

The need for drainage arises when (Weil & Brady, 2017; Mukha et al., 2003):

• Water stagnates in surface horizons after rains or snowmelt (poor surface runoff);
• Groundwater rises close to the surface (less than 1–2 m), creating a zone of waterlogging in the root zone;
• The soil becomes waterlogged as a result of irrigation without proper drainage, leading to secondary waterlogging and salinization;
• A restrictive layer exists at a shallow depth, creating a perched water table — temporary or permanent saturation of the upper horizons.

Drainage is particularly important in regions with excessive moisture (humid zones), on floodplain and terrace lands, and on irrigated areas in arid and semi-arid regions, where it is a necessary reclamation measure (Mukha et al., 2003; Weil & Brady, 2017).

5.2. Types of Drainage

Depending on the method of water removal, two main types of drainage are distinguished: surface and subsurface (closed) (Weil & Brady, 2017; Marshall et al., 1996).

5.3. Principles of Drainage System Design

The main parameters of a drainage system — drain depth and spacing — are determined by several factors (Marshall et al., 1996; Weil & Brady, 2017):

• Depth of the restrictive layer or water table;
• Soil hydraulic conductivity (permeability);
• Water input intensity (rainfall or irrigation rate);
• Required depth of water table lowering for normal root development (usually 0.5–1.5 m from the surface);
• Soil type, structure, and porosity.

For calculating drainage parameters, theoretical and semi-empirical formulas based on Darcy's law and flow theory are used. The most well-known formula for steady-state conditions is the Hooghoudt formula for the case where drains are laid on or above a restrictive layer (Marshall et al., 1996; Weil & Brady, 2017):

where:

• s — drain spacing, m;
• K — soil hydraulic conductivity, m/day;
• h — height of the water column above the drain level midway between drains (i.e., the maximum height of the water table), m;
• D — depth of drains above the restrictive layer, m;
• d̄ — equivalent depth, accounting for additional flow below the drain level;
• ν — water input intensity (rainfall or irrigation minus evaporation), m/day.

In a simpler form for drains intercepting the restrictive layer (D = 0), the simplified formula is used (Marshall et al., 1996):

where z₀ is the water level in the drains (usually the depth to the drain bottom).

These formulas provide approximate values sufficient for practical design, as the accuracy of determining K and ν in the field is usually not high (Marshall et al., 1996).

When designing drainage, it is also necessary to consider:

• Drain slope — ensuring gravity flow to the collector (usually 0.5–1%);
• Pipe diameter — sufficient to carry the design flow without clogging;
• Filter bed — to prevent fine soil particles from entering the drains and to improve water inflow (Marshall et al., 1996).

5.4. Environmental Aspects of Drainage

Drainage of excessively wet soils has several positive effects (Weil & Brady, 2017; Mukha et al., 2003):

1. Improved aeration of the root zone — increased oxygen content in the soil air, stimulating root respiration and aerobic microflora.

2. Accelerated spring soil warming — drained soils dry and warm up faster, allowing earlier field operations and extending the growing season.

3. Reduced toxicity of reduced iron and manganese forms — in waterlogged soils, toxic reduced forms (Fe²⁺, Mn²⁺) form; drainage promotes their oxidation.

4. Increased root depth — due to improved aeration and reduced mechanical resistance.

5. Reduced risk of lodging and root rot diseases.

6. Possibility of using heavy machinery at earlier dates.

However, drainage can also have negative environmental consequences (Weil & Brady, 2017; Mukha et al., 2003):

1. Loss of soil organic matter — accelerated humus mineralization under aerobic conditions leads to fertility degradation and CO₂ emissions.

2. Land subsidence — especially on peat and organic soils, where organic matter mineralization causes surface settlement (up to 2–3 m over decades).

3. Increased leaching of nutrients (nitrates, phosphates) and pesticides into drainage waters and subsequently into rivers and lakes, contributing to eutrophication.

4. Destruction of wetlands — habitats for waterfowl, amphibians, and valuable plant species. Drainage of wetlands is considered one of the main causes of biodiversity loss (Weil & Brady, 2017).

5. Increased microclimate aridity in some cases, as evaporation and transpiration of moisture-loving plants decrease.

In this regard, modern practice recommends an integrated approach: drain only those lands where it is economically justified, with a mandatory assessment of environmental risks and compliance with legislation on wetland protection (Weil & Brady, 2017).

5.5. Drainage on Irrigated Lands

On irrigated lands, drainage is a mandatory element of the reclamation system. Without drainage, irrigation in arid and semi-arid regions inevitably leads to (Mukha et al., 2003; Weil & Brady, 2017):

• Secondary waterlogging — rising water tables due to the continuous input of irrigation water, creating a waterlogged zone;
• Secondary salinization — upward movement of mineralized groundwater to the surface and evaporation, accumulating salts in the root zone (see Section 7.3 on the effluent regime);
• Reduced yields due to oxygen deficiency in the root zone.

Drainage on irrigated lands can be both surface (discharge of excess water through furrows) and subsurface (horizontal pipe drainage). Particularly important is a controlled drainage system, allowing water to be supplied back through the drains to the fields during dry periods (dual-purpose drainage), which enhances water use efficiency (Weil & Brady, 2017).

The design of drainage on irrigated areas considers not only the removal of excess water but also the maintenance of the salt balance — the amount of salts removed with drainage water must equal the amount entering with irrigation water (Mukha et al., 2003; Weil & Brady, 2017). To achieve this, it is essential to ensure sufficient leaching capacity of the drainage and to prevent an effluent regime.

5.6. Examples of Drainage Systems and Their Design Features

Pipe drainage with a collector — the most common system in temperate and tropical regions. Drains (plastic or clay tiles) are laid parallel to each other at a specified spacing (usually 5–30 m). Water from the drains flows into a collector pipe (larger diameter) and then into an open channel or receiving water body. Depth of installation — 0.8–1.5 m (Weil & Brady, 2017; Marshall et al., 1996).

Mole drainage — used on heavy clay soils where pipe installation is difficult or expensive. A mole plow creates passages with a diameter of 7–10 cm at a depth of 40–60 cm, sloping towards open collectors. Service life is 2–5 years, after which re-moling is required (Marshall et al., 1996).

Vertical drainage (wells) — used on sites with a deep restrictive layer where horizontal drainage is ineffective. Wells are drilled to the aquifer, and water is pumped out. An expensive and energy-intensive method, used where land value is high (Marshall et al., 1996).

5.7. Controlled Drainage (Water Management)

The modern concept of controlled drainage allows for managing the water table level depending on the growth stage and weather conditions (Weil & Brady, 2017). For this purpose, control structures (flashboards, gates) are installed on collectors, enabling:

• During spring waterlogging — fully opening the drainage for rapid water level drawdown;
• During active plant growth — maintaining the water table at an optimal depth (e.g., 0.8–1.2 m) to provide capillary contribution while avoiding waterlogging;
• During dry periods — raising the water table or even supplying water back to the fields (subirrigation) (Weil & Brady, 2017).

Controlled drainage can significantly improve water use efficiency, reduce the transport of nitrates and other pollutants to water bodies, and conserve soil moisture between irrigations (Weil & Brady, 2017).

Summary for Section 5

• Drainage is the removal of excess gravitational water to lower the water table and improve aeration.
• Drainage is necessary under waterlogging conditions, with a shallow water table, on irrigated lands, and on restrictive layers.
• Surface (open ditches, grading) and subsurface (pipe, mole) drainage are distinguished.
• Drainage parameters (depth, spacing) are calculated using filtration theory formulas (e.g., Hooghoudt), considering hydraulic conductivity, water input rate, and required drawdown.
• Drainage has positive effects (aeration, warming, root growth) but can also cause negative impacts: organic matter loss, subsidence, leaching of substances, destruction of wetlands.
• On irrigated lands, drainage is mandatory for combating salinization and waterlogging.
• Controlled drainage allows managing the water regime throughout the season.

6.1. Definition and Essence of the Process

Evaporation is the physical process of water transitioning from the liquid state to the vapor state, occurring from the soil surface, water bodies, and plant tissues (Weil & Brady, 2017; White, 2006). In soil science and agronomy, evaporation is considered one of the main pathways of water loss from the soil profile — the process by which the water cycle is completed at the local level.

In the context of our key question — how water comes, moves, is retained, and leaves the soil — evaporation represents the main mechanism of water loss from the soil to the atmosphere.

Evaporation is not only a physical but also an energy-consuming process. To transition 1 gram of water from liquid to vapor at 20 °C requires about 2.45 kJ of heat (latent heat of vaporization). This energy barrier is the main limiting factor for evaporation: as long as there is no heat supply, evaporation does not occur (Weil & Brady, 2017; White, 2006).

In the soil system, two components of evaporation are distinguished (Weil & Brady, 2017; Shukla, 2023):

1. Physical evaporation (E) — the direct transfer of moisture from the surface soil layer and water surfaces to the atmosphere.

2. Transpiration (T) — the evaporation of water by plants through the stomata of leaves, stems, and other organs. This is a physiological-biological process closely linked to photosynthesis and gas exchange.

Evapotranspiration (ET) — the total water loss from both processes: ET = E + T (Weil & Brady, 2017; White, 2006). This term is widely used in hydrology, agrometeorology, and reclamation to assess the water consumption of vegetation cover.

6.2. Driving Forces of Evaporation

Evaporation from the soil surface and plants is determined by three main driving forces (Weil & Brady, 2017; White, 2006; Shukla, 2023):

6.3. Stages of Evaporation from the Soil Surface

Evaporation from the soil surface without vegetation cover (or with partial cover) passes through three stages depending on conditions (Shukla, 2023; Weil & Brady, 2017; White, 2006).

6.4. Evaporation from a Free Water Surface and Evapotranspiration

For comparison and assessment of potential evaporation, evaporation from an open water surface (E₀) is used. It is measured using evaporation pans (Class A pan) — standard water-filled containers placed in an open location (Weil & Brady, 2017; White, 2006). These pans provide an integrated assessment of the atmosphere's evaporative demand, as water is always available.

Potential evapotranspiration (PET) — the evaporation rate from a fully wet vegetated surface when water does not limit the process. PET is estimated from meteorological data using various empirical and physical models (e.g., the Penman-Monteith equation, Thornthwaite method) (Weil & Brady, 2017; White, 2006). PET is usually 0.6–0.8 of the evaporation from an open water surface (for herbaceous vegetation, the coefficient is about 0.65–0.85).

Actual evapotranspiration (AET) — the actual rate of moisture loss from a specific vegetation cover on a specific soil. It is always less than or equal to PET (under sufficient wetting, it is close to it; during drought, significantly less) (Weil & Brady, 2017; White, 2006). The difference PET – AET is called the moisture deficit — a key indicator of the aridity of conditions and the intensity of plant water stress.

6.5. Transpiration and Its Features

Transpiration is the evaporation of water by plants through stomata, regulated by physiological mechanisms. Unlike physical evaporation, transpiration is a biological process, closely linked to photosynthesis, gas exchange, and osmotic pressure in cells (Weil & Brady, 2017; White, 2006).

Main features of transpiration:

• Transpiration provides cooling for plants (evaporation from leaf surfaces lowers their temperature), which is critical in hot weather.
• It creates a transpiration stream of water and minerals from roots to leaves, essential for plant life.
• The transpiration rate is regulated by stomatal opening and closing, depending on the plant's water status, light intensity, and CO₂ concentration in the air (Weil & Brady, 2017).
• Under water stress (lack of soil moisture), plants close their stomata, reducing transpiration, but this simultaneously limits photosynthesis and growth.

Transpiration coefficient — the amount of water (in grams) consumed by a plant to accumulate 1 g of dry matter. It varies among different crops (Mukha et al., 2003):

The lower the transpiration coefficient, the more efficiently the plant uses water to produce organic matter.

6.6. Factors Affecting Actual Evaporation

6.7. Managing Evaporation in Agronomy

To improve water use efficiency in agriculture, it is necessary to aim for increasing the share of transpiration (T) in total evapotranspiration (ET) and reducing unproductive physical evaporation (E) from the soil surface (Weil & Brady, 2017; Mukha et al., 2003).

Main techniques:

1. Mulching — covering the surface with organic (straw, sawdust, crop residues) or synthetic materials. Mulch creates a barrier to vapor diffusion, reduces surface heating, and reduces physical evaporation by 30–50% or more. Organic mulch also improves soil structure (Weil & Brady, 2017; Mukha et al., 2003).

2. Harrowing and loosening the top layer — breaking capillaries in the surface layer reduces the upward flow of water from lower horizons and, consequently, evaporation. This is a classic "dry irrigation" technique, particularly effective in the steppe zone (Mukha et al., 2003).

3. Use of shade-providing crops (green manures, cover crops) — plants shade the soil surface, reducing its heating and physical evaporation. However, it must be considered that they also consume water through transpiration, so their use must be balanced (Weil & Brady, 2017).

4. Selecting crops with high water use efficiency — C₄ plants and drought-resistant varieties have lower transpiration coefficients.

5. Optimizing plant density — sparser stands increase soil evaporation; too dense stands intensify competition for water and can cause water stress.

6. Regulating irrigation (drip irrigation, sprinkler) — modern systems allow water to be applied directly to the root zone, minimizing physical evaporation and deep percolation losses (Weil & Brady, 2017).

7. Shelterbelts — reduce wind speed near the ground, decreasing turbulent exchange and physical evaporation (Mukha et al., 2003).

6.8. Evaporation Under Shallow Water Table Conditions

With a shallow water table (less than 2–3 m), capillary rise can maintain a continuous supply of water to the surface, making evaporation intense and prolonged (White, 2006; Weil & Brady, 2017). Under such conditions, evaporation can persist at Stage I for many days, especially in hot weather. This leads to:

• Significant water losses from the soil profile (up to several mm per day);
• Secondary salinization, if the groundwater is mineralized — salts are drawn to the surface and accumulate upon evaporation (White, 2006; Weil & Brady, 2017).

Therefore, on such lands, drainage is necessary to remove excess water and lower the water table to a safe depth (usually > 2 m), along with periodic leaching to remove salts.

Summary for Section 6

• Evaporation is the transition of water from liquid to vapor, the main mechanism of water loss from the soil.
• Driving forces of evaporation: energy (solar radiation), water vapor pressure gradient, and wind transfer.
• Three stages of evaporation are distinguished: potential (energy-controlled), falling (soil-controlled), and slow (diffusion). Major moisture loss occurs during the first stage.
• Evapotranspiration (ET) = physical evaporation (E) + transpiration (T). Potential evapotranspiration (PET) is the maximum possible under given meteorological conditions; actual (AET) is the real rate, limited by water availability.
• Transpiration is a physiological process providing plant cooling and substance transport.
• Managing evaporation is a critical agronomic task for improving water use efficiency: mulching, tillage, regulating plant density, crop selection, irrigation.
• With a shallow water table, evaporation can be intense and lead to salinization, thus requiring drainage.

7.1. Definition and Classification Principles

The soil water regime is understood as the totality of processes of water input, movement, retention, and expenditure in the soil, recurring regularly over time and determining the moisture content of the soil profile (Mukha et al., 2003; Weil & Brady, 2017). The type of water regime is a qualitative characteristic reflecting the predominant direction and intensity of moisture turnover in the soil over a year or a multi-year period.

The classification of water regime types is based on the ratio of two key indicators (Mukha et al., 2003; Weil & Brady, 2017):

1. Annual atmospheric precipitation (P) — the amount of water reaching the soil surface;

2. Annual evaporativity (E₀) — the maximum possible evaporation from an open water surface under given climatic conditions (determined by temperature, humidity, wind, and radiation balance).

The ratio P to E₀ determines whether the soil profile will be regularly leached by downward water flows, remain in a state of equilibrium, or experience a moisture deficit with a predominance of upward flows (Mukha et al., 2003; Weil & Brady, 2017).

Additional factors influencing the type of water regime include (Mukha et al., 2003; White, 2006):

• Depth and mineralization of groundwater;
• Soil water-holding capacity and permeability;
• Presence of restrictive layers in the profile;
• Topography (slope processes, moisture accumulation in depressions);
• Nature of vegetation cover;
• Anthropogenic impact (irrigation, drainage, agronomy).

7.2. Leaching Type of Water Regime

The leaching type of water regime is characteristic of territories where annual precipitation exceeds evaporativity (P > E₀) (Mukha et al., 2003; Weil & Brady, 2017).

7.3. Non-Leaching Type of Water Regime

The non-leaching type of water regime is characteristic of territories where annual evaporativity exceeds precipitation (P < E₀) (Mukha et al., 2003; Weil & Brady, 2017).

7.4. Periodically Leaching Type of Water Regime

The periodically leaching type of water regime occupies an intermediate position between the leaching and non-leaching types (Mukha et al., 2003; Weil & Brady, 2017).

7.5. Effluent (Exudative) Type of Water Regime

The effluent type of water regime develops under non-leaching conditions but with a shallow water table (usually mineralized) (Mukha et al., 2003; Weil & Brady, 2017; White, 2006).

7.6. Permafrost Type of Water Regime

The permafrost type of water regime is characteristic of territories with permafrost (Mukha et al., 2003; Weil & Brady, 2017).

7.7. Irrigated Type of Water Regime

The irrigated type of water regime forms on artificially irrigated lands (Mukha et al., 2003; Weil & Brady, 2017).

7.8. Relationship Between Water Regime Types and Soil-Forming Processes

Summarizing, the following relationship can be identified between water regime types and dominant soil-forming processes (Mukha et al., 2003; Weil & Brady, 2017):

Summary for Section 7

• The type of water regime is a qualitative characteristic determined by the ratio of water input and output and the predominant direction of water flows in the soil.
• Main types: leaching, periodically leaching, non-leaching, effluent, permafrost, irrigated.
• Leaching type — P > E₀, dominance of downward flows, leaching, formation of podzolic and lateritic soils.
• Non-leaching type — P < E₀, predominance of upward flows, accumulation of carbonates and salts, formation of Chernozems, Chestnut soils.
• Periodically leaching — P ≈ E₀, alternating regimes, transitional soils (Gray Forest, Leached Chernozems).
• Effluent — shallow mineralized groundwater, intensive evaporation, salinization and solonetzization.
• Permafrost — permafrost as a restrictive layer, gleying, cryogenic processes.
• Irrigated — artificial regime on irrigated lands, requiring control and drainage.
• The type of water regime determines the direction of soil formation, agronomic properties, and the need for reclamation measures.

8.1. Definition and Essence of the Process

Formation of soil moisture reserves is the process of accumulation and retention of water in the soil profile in a form available to plants, ensuring their life activity between wetting events (Mukha et al., 2003; Weil & Brady, 2017).

In the context of our key question — how water comes, moves, is retained, and leaves the soil — the formation of moisture reserves represents the result of the interaction of all the previously discussed processes: infiltration, redistribution, retention, and losses. It is the final indicator that determines plant water supply and agricultural efficiency.

The formation of moisture reserves is a dynamic process depending on:

• The volume of incoming water (precipitation, irrigation);
• The soil's ability to retain water against gravity (water-holding capacity);
• Wetting depth (depends on permeability and initial moisture);
• The intensity of losses to evaporation and deep percolation;
• Seasonal and weather conditions.

It is important to understand that not all water entering the soil becomes available moisture reserve. Part is lost to physical evaporation, part moves to deep horizons (beyond the root zone), and part remains in forms unavailable to plants (hygroscopic, chemically bound). The productive reserve is only the portion of moisture between field capacity and the permanent wilting point (Mukha et al., 2003; Weil & Brady, 2017).

8.2. Factors Determining the Formation of Moisture Reserves

8.3. Natural Sources of Moisture Reserve Replenishment

8.4. Agronomic Practices for Moisture Accumulation and Conservation

Managing the formation of moisture reserves is a critical task in agronomy, especially in zones with unstable or insufficient moisture. The set of practices aims to (Mukha et al., 2003; Weil & Brady, 2017):

1. Increase water input into the soil;

2. Reduce losses from evaporation and runoff;

3. Optimize water distribution within the profile.

8.5. Quantitative Assessment of Moisture Reserves

8.6. Relationship Between Moisture Reserves and Plant Productivity

Productive moisture reserves in the soil are one of the main factors determining yield. Even with sufficient nutrients and favorable temperatures, moisture deficit limits plant growth and development (Mukha et al., 2003; Weil & Brady, 2017).

Main patterns:

• Water stress occurs when moisture reserves in the root zone fall below 50–60% of field capacity (White, 2006);
• When reserves fall below 30% of FC, most crops stop active growth, photosynthesis decreases, and stomata close (Weil & Brady, 2017);
• When the wilting point is reached, plants lose turgor, and the recovery process after irrigation or rainfall can take several days (White, 2006).

Critical periods of water consumption for different crops (Mukha et al., 2003):

Therefore, managing moisture reserves is the basis for obtaining stable and high yields. This is achieved through a set of measures: from selecting drought-resistant varieties and optimizing plant density to using irrigation and agronomic practices for moisture accumulation and conservation (Weil & Brady, 2017; Mukha et al., 2003).

Summary for Section 8

• Formation of moisture reserves — the process of accumulation and retention of water in the soil, determining plant water supply.
• Moisture reserves depend on climate (precipitation, evaporativity), soil properties (water-holding capacity, thickness, texture), groundwater depth, and vegetation cover.
• Total moisture storage — all water in the profile; productive storage — water between field capacity and the wilting point (available to plants).
• Natural sources of replenishment: atmospheric precipitation, snowpack, groundwater (if shallow), condensation of water vapor.
• Agronomic practices: snow retention, regulation of meltwater runoff, harrowing, mulching, structure creation, shelterbelts, irrigation.
• Assessment of moisture reserves is carried out using generally accepted scales (for the 0–20 and 0–100 cm layers) and serves as the basis for decisions on field operation timing, irrigation rates, and reclamation measures.
• Productive moisture reserves determine potential yield, especially during critical phases of plant development.

Conclusion

We have examined all the main processes that constitute the soil water regime — from water input (infiltration) to its losses (evaporation, runoff, drainage), including internal redistribution and the formation of moisture reserves. The types of water regime that determine soil-forming processes and agronomic soil properties were identified.

Key conclusions of the entire lecture:

1. Water balance and water regime are two interrelated but different characteristics: the first is quantitative (how much water), the second is qualitative-dynamic (how water behaves over time).

2. Water in the soil exists in several forms, differing in availability to plants and mobility. The main source of plant water supply is capillary water.

3. Infiltration is the first stage of water entering the soil. Its rate depends on soil properties, rainfall intensity, and initial moisture content.

4. Redistribution is the movement of water within the profile after infiltration ceases, determining the distribution of moisture among horizons and its availability to roots.

5. Drainage — removal of excess gravitational water to improve aeration. Necessary on waterlogged and irrigated lands.

6. Evaporation — the main mechanism of moisture loss. Evapotranspiration includes physical evaporation and plant transpiration. Managing evaporation is the basis for improving water use efficiency.

7. The type of water regime is determined by the ratio of precipitation and evaporativity, as well as the depth of the water table. It determines the direction of soil formation and the agronomic properties of soils.

8. Formation of moisture reserves is the final result of all processes, determining plant water supply and potential yield.

Understanding the patterns of the water regime is essential for developing scientifically based farming systems, land reclamation, and soil resource protection.