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Water regime of soils

Water regime is a set of soil processes of water inflow, transfer, storage and discharge. Each of these processes separately is an element of water regime.

Water regime of soils is formed under the influence of a number of factors: climate, relief, water-physical properties of soils, water supply conditions, as well as human economic activity. Specificity of water regimes of specific zonal soil types is determined by amount of atmospheric precipitation and temperature regimes.

Water refers to terrestrial factors of plant life, in soil it is in liquid phase in form of soil solution. In soil it is in interphase equilibrium with soil, exchanging minerals with it. Part of the soil moisture is lost by seeping into deeper layers or by evaporation or runoff from the surface. The remaining moisture is retained by the soil and represents the soil solution. It is characterized by a number of important agrochemical indicators.

The importance of water in plant life

Soil scientist G.N. Vysotsky emphasized the exceptional importance of water in soil, comparing it to the blood in living organisms.

The role of moisture in plant life:

  • participates in the initiation of seed growth processes;
  • Is the medium for biochemical processes in plants;
  • transports mineral and organic substances in different parts of plants;
  • participates in thermoregulation and heat balance of soil;
  • supports vital activity of soil biota;
  • influences agrophysical indicators of soil: density, stickiness, resistance to
  • processing, ripeness – formation of aggregates and ability to crumble;
    determines agrochemical indicators of soil fertility: acidity of soil solution, availability of nutrients.

Table. Crop water consumption coefficients for the Non-Chernozem zone, m3/t dry biomass[1]Farming. Textbook for universities / G.I. Bazdyrev, V.G. Loshakov, A.I. Puponin et al. — Moscow: Publishing House «Kolos», 2000. — 551 с.

Crop
Years
Wet
Average
Dry
Winter wheat
375-450
450-500
500-525
Winter rye
400-425
425-450
450-550
Spring wheat
350-400
400-465
485-500
Barley
375-425
435-500
470-530
Oats
435-480
500-550
530-590
Maize
174-250
250-350
350-460
Potatoes
165-300
450-500
550-660
Beets
240-300
310-350
350-400
Flax
240-250
300-310
370-380
Perennial herbs
500-550
600-650
700-750

Seed requirement for swelling and translation of nutrients into digestible form for different plants (in % of seed weight) is as follows: barley, wheat – 50, oats, rye – 55-65, corn – about 40, flax, peas – 100, clover, sugar beet – 120-150. This figure for most plants is from 40 to 100% of seed weight.

Water makes up a significant part of plant weight: in seeds its amount is 7-15%, in stems, which include many dead lignified cells, – up to 50, in root crops, tubers and leaves – up to 75-93%.

Plants consume 200 to 1000 g of water to form 1 g of dry organic matter.

Normal activity of soil microorganisms is possible under sufficient moisture supply. For example, nitrogen-fixing bacteria (Azotobacter, nodule bacteria) require 25% soil moisture to reproduce. Lack of water reduces nutrient uptake by bacteria, and excessive excess leads to oxygen deprivation. The optimum soil moisture for bacteria and plants coincides at 60% of the total soil moisture capacity.

An excess of moisture in the soil, which exceeds the lowest field moisture capacity (FMC), has a depressing effect on plant growth and development. Although some of them react differently to overwatering.

In researches and practice on agriculture and crop production to account water consumption for crop creation water consumption coefficient is used. Water consumption coefficient is water consumption in m3 per one ton of crop, including productive, i.e. water consumption by cultivated plants, and non-productive expenses for evaporation from soil surface.

Transpiration

Transpiration – evaporation of water by leaves.

Transpiration rate is the amount of water a plant needs to form a unit of dry matter.

Plants use the soil solution of minerals in very small concentrations. Most of the moisture that enters the plants is not fully used. Thus, out of 1,000 parts of water that has passed through the plant, only 1.5-2 parts are used for nutrition, the rest of the water evaporates through the leaves.

Transpiration coefficients:

  • cereals of the second group – 200-400;
  • cereals of the first group, peas, long-fibred flax – 400-800;
  • perennial grasses – 700-900;
  • clover – 500-600;
  • alfalfa – 700-800;
  • corn – 230-300;
  • millet – 200;
  • winter wheat – 400-500;
  • sugar beet – 240-500.

Transpiration coefficient depends on illumination, temperature, soil and air humidity, provision with nutrients.

In Gelrigel’s experiments, in direct sunlight the transpiration coefficient was 349, in strong scattered light – 483, average – 519 and weak – 676.

Transpiration coefficient strongly depends on air humidity. In dry periods in such crops as millet, wheat, oats, corn it increases by 2 and more times compared to wet periods. In dry and hot regions water evaporation by plants is much higher than in humid and moderately warm regions.

Fertilizers can noticeably reduce the transpiration coefficient. For example, oats with a lack of nutrients has a transpiration factor of 483, with a sufficient supply of them – 372. Therefore, the use of fertilizers for arid areas of agriculture is important, since the plants are more economical in spending the limited supply of moisture.

Water consumption coefficient is the sum of transpiration water and water evaporating from the soil surface. It is expressed in m3 per 1 ton of yield. It varies, depending on moisture content, from 375 to 550 for winter cereals, from 240 to 400 for beet, from 170 to 660 for potatoes, and from 500 to 750 m3/t for perennial grasses.

The plants’ need for moisture is characterized by the transpiration coefficient, which approximately reflects the ability of the plant to consume a certain amount of water to create a unit of dry matter in the form of a crop.

Transpiration coefficient varies with weather conditions, soil fertility, and fertilizer. At low air humidity, strong heating of leaves and wind it increases. The latter factor especially increases water evaporation. K.A. Timiryazev wrote that even in low wind transpiration increases by 2 times, and in strong wind evaporation increases by 20 times more than in dry weather.

Less influence on transpiration coefficient is exerted by soil conditions: provision with nutrients, degree of moistening, value of osmotic pressure of soil solution.

The water requirement of the same plant depends on the growth phases.

Critical growth phase

The critical growth phase is the period of plant growth when water consumption is at its maximum.

For example:

  • in spike crops it is from ‘shooting’ to ear formation ‘heading’;
  • for sorghum and millet – from ear formation ‘heading’ to plumping;
  • in corn – during blossoming period – milk-ripe stage;
  • for sunflower – formation of a head – flowering;
  • cotton – blossom – capsule formation;
  • for potatoes – blooming, tuber formation;
  • for grain legumes and buckwheat – flowering.

Lack of moisture during the critical phase of growth weakens development, plants do not give good yields. Subsequent phases are less responsive to changes in water regime. Especially lack of water affects plants in the period of reproductive organs formation.

Drought leads to deterioration of soil conditions: the osmotic pressure of soil solution increases, which leads to the toxic effect of fertilizers, especially nitrogen ones.

Soil water types

Soil water has different mobility and properties depending on the nature of the relationship between water molecules, solid and gas phases.

Soil water is subdivided:

  1. by physical state into:
    • solid,
    • liquid,
    • vapor.
  2. according to the degree of mobility of water and the nature of connection with the solid phase into:
    • chemically bound,
    • solid,
    • hygroscopic (firmly bonded),
    • vaporous,
    • loose bound (capillary and gravitational).
Water of soil
Categories (forms) of soil water and soil-hydrolytic constants (by Rohde)

Chemically bound water

Chemically bound water is included in the solid phase of soil (chemically bound). It consists of constitutive (hydrate) and crystallization (crystalline hydrate) water. It is inaccessible to plants, immobile in soil, does not dissolve minerals.

Solid water

Solid water – water in the form of ice. It is formed by freezing of soil in the autumn-winter period (seasonal freezing) or is permanently at a certain depth, not thawing in summer, in the frozen soil column (permafrost). When thawed, it represents a potential reserve of liquid and vaporous water.

Solid water is inaccessible to plants, immobile.

Vaporous water

A vaporous water – water vapor in the soil air and sometimes reaches 100%. It is mobile, moving from places with high water vapor pressure to places with lower pressure and air flows.

It has little effect on the moisture supply of plants and is practically inaccessible to plants. In compacted crops, due to pulling of vaporous water by plants’ root system from root cavities, it plays an important role.

As the temperature drops and the dew point is reached, it condenses, transforming into a liquid form accessible to plants.

Hygroscopic (firmly bound) water

Hygroscopic (firmly bound) water is one of the forms of physically bound, or sorbed water. It is retained by the solid phase of the soil, mainly by colloidal particles due to sorption forces.

It forms a thin layer of water film 1-3 molecules thick around soil particles. The sorption forces result from the fact that part of the water molecule (on the oxygen atom side) is negatively charged, while the other (on the hydrogen atom side) is positively charged. This form of charge is called a dipole. Water molecules are attracted to and held by soil particles due to the strict orientation of the dipoles.

The freezing point of hygroscopic water is -4 … -7 °С, it does not dissolve water-soluble substances, has a density of 1.5-1.8 g/cm3 and higher viscosity.

It is inaccessible to plants.

Maximum hygroscopic capacity (MH) is the amount of water that soil can absorb and retain when placed in an atmosphere saturated to 96-98% with water vapor. The value of maximum hygroscopicity allows you to determine the water supply of plants. As a rule, the value equal to 1.5-2 times excess of the maximum hygroscopic capacity corresponds to the moisture content of the steady wilting of plants, or the so-called “dead stock” of water in the soil. It is taken into account in calculation of irrigation rates and productive moisture reserves. To calculate moisture content of plants based on value of maximum hygroscopicity, coefficient of 1.34 is used.

Dead moisture reserve is directly proportional to the content of silty (colloidal) particles and organic matter (humus) in the soil. Thus, sandy soil, poor in organic matter, contains only 1% of moisture inaccessible to plants, and heavy loamy and clayey soils rich in humus contain up to 15% of such moisture. In peat soils, the dead stock can reach 20-50%.

The amount of hygroscopic moisture directly depends on the granulometric (mechanical) composition of the soil and the content of organic matter. The greater the specific surface area of the soil particles (i.e., the smaller their size), the more hygroscopic moisture can be accumulated by the soil.

Loose bound (film) water

Loose bound water is the second form of physically bound (sorption) water, also called film water. It is formed due to additional sorption of water molecules to colloidal soil particles. Unlike hygroscopic (firmly bound) water, formed in an atmosphere of saturated water vapor, loosely bound water is sorbed by colloidal particles from the liquid phase, due to sorption forces sufficient to retain it.

Loosely bound water is sedentary and inaccessible to plants.

Capillary water

Capillary water – water that is in drip-liquid state in soil capillaries and is held by capillary forces. 

It is subdivided into capillary-hanged water, i.e. water coming into the soil from the upper layers due to precipitation or irrigation water, and capillary-prop up water rising by capillary forces from groundwater. Between the layers wetted by capillary-hanged and capillary-prop up water there is a gap of dry soil called capillary border.

Capillary (meniscus) forces that retain moisture and cause groundwater rise depend on capillary thickness. The smaller their thickness, the greater the force and, consequently, the height of water rise. For this reason, on unstructured and dense soils the arable layer dries up quickly, especially in the southern areas. Destruction of capillaries in the upper soil layer by loosening significantly reduces moisture evaporation. With a diameter of less than 8 mm the capillary forces are insignificant.

The effect created by capillary forces is also used in the opposite direction. When it is necessary to draw moisture from the lower layers, for example, when sowing seeds, especially in dry periods or arid places, the soil is reconsolidated by rolling.

Smallest moisture capacity (SMC) is the amount of capillary-hanged water retained by the soil after the expiration of excess liquid water.

The optimum soil moisture for plant growth and development is 70-100% of the smallest moisture capacity.

Moisture deficit – the difference between the value of the smallest moisture capacity and the actual soil moisture, is widely used in agriculture.

Gravitational water

Gravitational water is a liquid phase of water not bound to soil, which occupies large non-capillary voids between soil aggregates. It moves freely due to gravity. Displaces air from soil voids, negatively influencing air regime, creating anaerobic conditions.

Full moisture capacity (FMC) is the maximum possible amount of gravitational water that can be contained by the soil while filling all the voids.

The value of full moisture capacity is equal to porosity of the soil and ranges from 20-40 to 50-60%, sometimes reaching 80%.

Gravitational water is easily accessible for plants. In natural conditions it occurs under shallow groundwater table, under waterlogging of sites, bogs, swamps, marshes, prolonged precipitation, spring snowmelt.

Soil water regime indicators

The ability of soil to provide stable water regime is determined by its water properties:

  • water retention capacity,
  • moisture capacity,
  • water permeability,
  • water lifting capacity,
  • potential of soil water,
  • soil sucking power.

Water retention capacity

Water retention capacity is the ability of the soil to absorb and retain water in the profile, preventing runoff by gravity. It is quantified by the moisture capacity of the soil.

It arises mainly due to sorption and capillary forces.

Moisture capacity

Soil moisture capacity is the maximum amount of water retained by the soil.

The field (smallest) soil moisture capacity is the maximum amount of soil water that can be retained by the soil for a long time without inflow and its loss from evaporation. Characteristic of every soil, is an almost constant value. It is important in production activities, especially in irrigated agriculture in the calculation of irrigation rates.

At the level of the smallest moisture capacity the soil contains the maximum amount of moisture available to plants, with 60-80% of soil pores filled with water.

Clayey and humus-rich soils are characterized by high moisture capacity and low water permeability and a higher dead storage of moisture. Sandy and humus-poor soils, on the contrary, are characterized by low moisture capacity and high water permeability.

Water permeability

Water permeability of soil is the ability of soil to absorb and pass through its profile water coming from the surface.

It depends on density, granulometric composition, soil structure and degree of moistening.

Water lifting capacity

Water lifting capacity – the property of the soil to create a lift of water by capillary forces.

Moisture of steady wilting

Moisture of steady wilting or dead stock is the amount of moisture unavailable to the plant. It is determined by one and a half value of maximum hygroscopicity. As a rule, coefficient 1.34 is used for calculation.

The value of moisture of steady wilting depends on the content of organic matter and granulometric composition of soil, and varies greatly by soil type: in loamy sandy soils is 2-3%, in loamy – 5-6, in clay – 8-10, in humus-sandy and black earth – 14-16, in peat – up to 40-50% of the absolute dry weight of the soil.

Wilting of plants occurs as a result of lack of moisture in the soil, the so-called soil drought, or due to increased evaporation of moisture due to high dryness and high air temperature (atmospheric drought).

Water balance

Sources of water entering the soil are precipitation, groundwater rising due to capillary forces, condensation of water vapor from daily temperature fluctuations, and irrigation water.

Water balance is the amount of water entering the soil and its discharge.

Equation of soil water balance:

W0 + Wa/p + Wirr + Wg/w + Wc/m + Ws/i + Wl/i = Wev + Wtr + Winf + Ws/r + Wl/r + Wf ,

where W0 – initial water supply in soil; Wa/p – amount of atmospheric precipitation for a specific period; Wirr – amount of water supplied with irrigation water (under irrigation); Wg/w – amount of water supplied to soil with groundwater; Wc/m – amount of water supplied to soil due to condensation of atmospheric moisture; Ws/i – amount of water supplied to soil due to surface inflow; Wl/i – amount of water supplied to soil from intra-soil lateral inflow; Wev – amount of water evaporated from soil surface during specific period; Wtr is desiccation, the amount of water spent on evaporation by plants (transpiration); Winf is the amount of water lost due to infiltration into deeper soil layers; Ws/r is the amount of water lost due to surface runoff; Wl/r is the amount of water lost due to lateral runoff within the soil; Wf is the final water reserve in the soil.

Water reserves W (in m3/ha or in mm of water layer) in the soil layer of depth h, cm is calculated by the equation:

W = advh ,

where a – field moisture, %; dv – soil density, g/cm3.

Types of water regimes

According to Vysotsky-Rode’s classification, there are six types of water regimes depending on the amount of precipitation and the intensity of evaporation:

  • permafrost;
  • flushing;
  • periodically flushing;
  • nonflushing;
  • prop up;
  • irrigation.
Water balance
Water balance diagram for flushing (A), nonflushing (F) and prop up (B) types of water regime (by Rohde): 1 - evaporation from vegetation surface, 2 - surface runoff, 3 - evaporation from soil surface, 4 - intra-soil runoff, 5 - destruction

Permafrost type

The permafrost type of water regime is characteristic of permafrost areas. The permafrost layer serves as a waterproofing layer, which causes overwatering of the upper soil layer, which thaws during the warm season. For this reason, gleying of soils occurs, and therefore tundra soils are gleyed.

Flushing type

The flushing type of water regime is characteristic of areas with predominance of precipitation over evaporation during the year. As a result of excess moisture, a downward water flow is formed. It is found in taiga-forest zone, polesye, humid subtropics and tropics. In annual cycle of moisture turnover in wet periods, mainly in spring and autumn, there is a through washing of soils and parent rocks up to groundwater. Such intensive water flow leads to removal of soil formation products deeper beyond soil profile and formation of podzol type soils.

Periodically flushing type

The periodic flushing type is characteristic of areas with approximately equal amounts of water falling as precipitation and evaporating during the year. Alternation of dry and wet years causes alternation of non-flushing and flushing types of water regime. And the latter can take place once in 10 and more years. This type of water regime contributes to formation of gray forest soils, leached and podzolized black earths of forest-steppe zone.

Non-flushing type

Non-flushing type of water regime is typical for areas where annual precipitation is less than evaporation, and water does not reach groundwater. Soil thickness soaking in black earth steppe reaches 4 m, in brown and gray-brown soils of semi-deserts and deserts – 1 m. Between the groundwater and the upper wetted layer there is a layer with moisture approximately equal to the wilting moisture value.

Prop up type

Prop up type of water regime is characteristic of areas with unflushed type and close groundwater occurrence. In this case, there is an intensive rise of moisture by capillaries from groundwater to the surface and its evaporation, especially in the zone of semi-deserts and deserts. With high mineral content in groundwater, saline soils are formed, especially solonchak and solonetzic soils.

Irrigation type

Irrigation type is typical for territories with artificial irrigation. During a year water regime can change from flushed to non-flushed and even prop up regime depending on intensity and terms of irrigation.

Regulating the water regime of soils

Optimal moisture content in root layer at different stages of development of most plants is 60-80% of full field moisture capacity, during development of assimilative apparatus and intensive growth – 70-80%.

Methods of water regime regulation include: agrotechnical, hydroreclamation, agromelioration, forest-melioration and others.

Man has started to use different methods of water regime regulation since ancient times, based on consideration of biological features of crops and soil-climatic conditions of the territory.

In order to regulate water regime of specific lands, it is expedient to develop and use a farming system that takes into account peculiarities of water regime, and not to apply separate methods for its regulation.

Loose and structured soil absorbs significantly more moisture than compacted and unstructured soil. In compacted soil due to capillary forces there is a rapid lifting of moisture from the lower layers to the surface and its enhanced evaporation. Losses in the spring period under dry and windy weather can reach 50-70 t/ha on un harrowed land fall-plowed. Even shallow surface loosening, destroying the compacted top layer, dramatically reduces moisture loss from evaporation.

Mulching is a method of covering the soil surface with various materials (peat, straw-cutting, special films) to reduce water evaporation. As a rule, this method is used on small areas.

Control of weeds, as a factor of additional water consumption, also plays an essential role.

Introduction of clean fallows into crop rotation can also be considered as a method of water regime regulation.

Snow retention is an effective method of accumulating moisture in the soil at the expense of melt water. Melt water losses in regions of unstable or insufficient moisture in one year amount to 50-60 bln tonnes, taking into account that for each 100 t/ha of water (10 mm of precipitation) additional 200 kg/ha of grain for winter crops and 100 kg for spring crops can be gained. 

For snow retention on sloping lands, in addition to anti-erosion special methods of plowing (processing across the slope, slitting, intermittent furrowing), microbasin are arranged. 

Sowing of high-stem crops (sunflower, corn, mustard, sorghum) and leaving stubble after harvesting are widely used.

In addition to the snow-retarding properties, planting forest strips along the plots provides erosion control.

Techniques for regulating the water regime in arid areas

Irrigation – a technique used in intensive farming systems to regulate the water regime of the soil, consisting in irrigation. It is especially important in arid areas, where it allows to provide moisture to plants primarily in the critical phases of growth.

Placing crops in rotation with different root systems and water consumption allows the most efficient use of moisture of different soil layers.

Improvement of soil structure allows to prevent water runoff on the surface and reduce its evaporation.

Application of fertilizers by reducing the transpiration coefficient reduces water consumption by plants.

The development and use of drought-tolerant varieties with a reduced transpiration coefficient is a good means of rational moisture use.

Draining

Draining is a technique aimed at reducing excessive moisture in the soil.

Excessive moisture in the soil leads to soaking of plants, reducing their yields due to the creation of anaerobic conditions and deterioration of the air regime of the soil. Strong swelling when wetting clay soils with subsequent desiccation significantly compacts them, dense crust is formed on the surface.

Drainage methods include drainage, sowing on ridges, special methods of plowing, organic fertilizers, including green manure, surface treatment, leveling of micro- and middle lowlands, etc.

A network of closed or open drains allows to get rid of excessive moisture, and water supply by drains, on the contrary, gives an opportunity to regulate the water regime.

Sources

Fundamentals of agricultural production technology. Farming and crop production. Edited by V.S. Niklyaev. — Moscow: «Bylina», 2000. — 555 с.

Farming. Textbook for universities / G.I. Bazdyrev, V.G. Loshakov, A.I. Puponin et al. — Moscow: Publishing House «Kolos», 2000. — 551 с.

Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al. – M.: Publishing house of the All-Russian Scientific and Research Institute named after D.N. Pryanishnikov, 2017. – 854 с.

Plant breeding/P.P. Vavilov, V.V. Gritsenko, V.S. Kuznetsov et al; Edited by P.P. Vavilov. – M.: Agropromizdat, 1986. – 512 p.: ill. – (Textbook and textbooks for higher education institutions).