Soil water regime and water balance. The totality of all the processes of water inflow to the soil, water condition in the soil, and water consumption in the soil is called water regime. Quantification of all types of water inflow to the soil and discharge within a certain time interval is called water balance.

Soil water regime is the totality of all the phenomena of water inflow to the soil, its movement, change of its physical condition, and discharge from the soil.

The foundations of the theory of soil water regime were laid by G.N. Vysotskiy (1934) and A.A. Rode (1956). They have broken down soil water regime into six types and several subtypes. Later the works by I.A. Kachinskiy (1970), I.G. Minashina (1974), M.A. Kozin (1977), A.G. Bondarev (1996), O.I. Khudyakov (1988) et al. were devoted to the addressing of this problem.

Among water regime components are absorption, seepage, capillary rise, surface, down-going, and lateral flows, physical evaporation, water freezing, unfreezing, and condensation

Depending on the quantitative ratio between these phenomena, they distinguish the following types of water regimes (according to Rode):

1. Cryogenic regime is characteristic of permafrost areas. There is always water confining stratum (aquifuge). During frost-free seasons, the soil is saturated with moisture because of the formation of perched water. The precipitation-evaporation ratio PER > 1. It is characteristic of arctic and tundra soils.
2. Leaching regime is characteristic of the areas with precipitation-evaporation ratio PER > 1, i.e. total precipitation is more than evaporability. Atmospheric moisture transpierces through the entire soil stratum and reaches groundwater. The alkaline and alkaline-earth elements (podzols/spodosols, brown forest soils, krasnozem/red soils, zheltozem/yellow soils) are washed out. The boggy soil subtype develops at PER > 1, shallow groundwater, and presence of aquifuge (podzolic boggy and boggy soils).
3. Periodically leaching regime: PER = 1. It is characterized by the interchange of non-leaching and leaching water regimes in dry and wet years (grey forest soils, podzolized soils, and leached chernozems/black soils).
4. Non-leaching regime: PER < 1. Atmospheric moisture spreads to the upper soil horizons and never reaches groundwater. In steppe soil, moisture penetrates down 3-4 m deep; in desert soils (brown desert-steppe soils and gray-brown desert soils), to 1 m.
5. Exudational regime: PER < 1. It is characteristic of steppe, semi-desert, and desert areas with shallow groundwater. Vertical flows prevail there. With high groundwater salinity, soil becomes saline, alkali soils develop. At the desuctive-exudational regime, plants intercept water, and moisture evaporates through transpiration, salts are deposited in the stratum, rather than on the surface.
6. Irrigation regime is created when carrying out irrigation. It is characterized by the interchange of leaching and non-leaching or even exudational water regimes.

The water regime is expressed by means of water balance characterized by water input and output. According to Rode, the soil water balance for a particular period is expressed in the general form by the following equation:

M₁ = M₀ + (R + K + GW) - (T + E + FS + FL + FG)

Where:

M₁ stands for the moisture storage in the soil stratum at the end of the period under study

M₀ stands for the moisture storage in the soil stratum at the beginning of the period under study

R stands for total precipitation

К stands for moisture condensation

GW stands for the content of the moisture incoming to the soil from groundwater

Т stands for transpiration

Е stands for evaporation

FS stands for surface flow

FL stands for subsurface lateral runoff

FG stands for groundwater runoff (in mm or m³/ha).

Precipitation and groundwater are reckoned as the major source of water balance. While among additional sources of soil moistening are surface inflow and the moisture condensed from water vapor. The water output is composed of the physical water evaporation off the water surface, moisture lost during transpiration, the water lost in the course of surface and subsurface lateral inflows, and the water infiltrating soil stratum.

Soil water regime control

The basic methods for controlling water regime are land drainage, irrigation, bare fallow, and the ways aimed at reducing unproductive soil evaporation and retention of snow.

In arid areas, the most effective way to improve soil water regime is irrigation.

Number of water application processes, water application and irrigation rates should be adjusted depending on the current and expected weather conditions in order to make the most effective use of irrigation water the storage of which is limited.

In the subhumid zone, bare fallow facilitates moisture accumulation. By the time of autumn sowing on bare fallows in dry-steppe areas, productive moisture accumulates as much as 50-60 mm more than on nonfallow fields. Often this difference even comes to 70-100 mm. Early ploughing of fall-plowed field is of great importance for controlling soil moisture.

All the measures aimed at reducing non-productive evaporation and snow retention in the dry zone of steppe contribute to the optimization of the soil water regime.

Papers

Devkota M., Martius C., Gupta R.K., Devkota K.P., McDonald A.J., Lamers J.P.A. - Managing soil salinity with permanent bed planting in irrigated production systems in Central Asia (2015)