Water consumption by crops means the water consumed on a certain area over the crop-growing season; it is expressed in m³/ha or mm. Water consumption is composed of water losses to transpiration and evaporation from the soil surface.

Water-use ratio (unit water requirement) is the water amount used over the growing season per ton of product (e.g. per ton of wheat for grain crops, per ton of fruit, etc.).

Water consumption is determined experimentally by using the water balance equation following long-term observations over precipitation, soil moisture storage, water losses, and so on.

Water consumption varies depending on environmental conditions, crop type, and level of agricultural technology. Combination of irrigation with using high-level agricultural technology, and application of fertilizers will result in maximal increase in crop yield with decrease of water-use ratio. Water-use ratio value fluctuates over a wide range depending on weather conditions in concrete years, soil type, and level of agricultural technology. Approximate water-use ratio values for the conditions of Central Asia, trans-Volga region, and Ukraine (according to A.N. Kostyakov) are as follows: for grain crops – 1100-550 m³/t; for cotton – 1800-900 m³/t (at crop yield of 3-5 ton/ha); for perennial grasses for hay – 1000-400 m³/t (at crop yield of 50-100 ton/ha).

Source: Wikipedia

It is possible to gain high assured yield of crops only with great water consumption by crops.

Water consumption E (evapotranspiration) implies the quantity of water used by the crop to gain planned yield.

Water consumption by the field under crops is composed of transpiration (Е_t) and evaporation from the soil (Е_s):

Е = Е_t + Е_s

Evaporation from the soil is influenced only by environmental factors, and transpiration depends on the interference of external and internal factors of crops. It is difficult to determine the shares of transpiration Е_t and evaporation from the soil Е_s in evapotranspiration (water consumption); therefore, as a rule they are determined as a single whole.

Evapotranspiration is in direct relationship to climatic, hydrogeological, and economic conditions, biologic crop properties, crop yield, hydraulic reclamation way, and plays an important role in the formation of field’s water balance, being the major expenditure item of balance sheet.

There are the following evapotranspiration estimation methods: direct field observations methods; computational methods; empirical dependences.

The water balance method (WBM) is based on a field water balance equation. It gives quite reliable data and is used for deep (5-10 m) groundwater table. In that case, the moisture exchange between groundwater and soil water can be neglected. This method allows estimating ten-day and monthly crop evapotranspiration for homogenous soils to an accuracy of 10-12%, and about 15% for heterogeneous soils. A disadvantage of this method consists in its laboriousness and inefficacy. It gives only an averaged evapotranspiration value without identifying its dependence on other factors.

Estimation of consumptive water use

There exist theoretical methods of the estimation of consumptive water use (total evaporation) that are based on physical laws of evaporation (methods of Penman, Turk, etc.), empirical methods based on the functional dependence of evaporation on yield, temperature, and relative air humidity (methods of Kostyakov, Sharov, Alpatiev, etc.).

Design institutes adopt total evaporation/water consumption (or evapotranspiration) values according to recommendations by research institutions or calculate by S.M. Alpatiev’s formula, that is a bioclimatic method is employed. According to A.M. Alpatiev, evapotranspiration is the function of air moisture deficit as follows:

E = C_b Σd

where:

Σd stands for the sum of average daily air moisture deficit for the target year, expressed in hPa (taken from the nearest weather station);

C_b stands for bioclimatic coefficient (determined experimentally in experimental stations under the conditions similar to those of the irrigation system being designed).

Evapotranspiration Е implies gross water discharge from a field under crops, i.e. total water loss to transpiration, evaporation from the soil, and evaporation from the vegetation surface after rainfall. It varies along the biological curve representative of each type of plants, and it is determined on an experimental basis by using the method of water balance foe each ten-day (target) period through the following formula:

C_b = E_td/Σd_td

where:

E_td is the actual evapotranspiration in experiments for a ten-day period, mm,

Σd_td is the sum of average daily air moisture deficit for the same ten-day period, hPa.

Average long-term bioclimatic coefficient C_b takes into account the plant development phase expressed as the sum of average daily temperatures beginning from the beginning of the growing season normalized to 12-hour daylight time, viz. with the correction factor I equal to 1—L: 12 (L is the astronomical length of daylight (photoperiod) in terms of hours, which varies latitudinally). Bioclimatic coefficient C_b is a region-wide value, therefore one should employ local materials when carrying out estimation.

Bioclimatic coefficient C_b for unmentioned crops can be taken similar to that for the crops with the same or close beginning and end of the growing season. For instance, fodder beet (mangel) and table (red) beet irrigation regimes can be calculated by resting on the curve of sugar beet, barley, or pea, or by using the curve of spring wheat taking into consideration its early maturation, and so forth.

With the formula-based approach, evapotranspiration is estimated according to air moisture deficit assessed at weather stations located, as a rule, in non-irrigated land areas. Therefore, it is valid for small irrigated oases against prevailing rainfed lands.

On large-area lands, air temperature is lower and the humidity is higher in comparison with the same measures on non-irrigated lands; this is why evapotranspiration on large-area irrigated lands is lower than on non-irrigated ones. The more the irrigation area, the lower becomes evaporability. Therefore, a microclimatic correction factor, which in every case is determined taking into account all the factors it depends on, is introduced to the design evapotranspiration. The correction factor varies within a range from 0.9 to 0.75 and can be taken equal 0.85 on average.

Source: Mse-Online.Ru