The aim of the research is to consider the problem of reasonability and necessity for changing the present water demand for high-yield crops. Water demands of crops are established using the equation of water balance as a difference between evapotranspiration and natural moistening resources. Therefore, the study and establishment of quantitative terms of crops evapotranspiration are an urgent problem. It was pointed out that the yield of new cultivars of agricultural crops is significantly greater comparing to those studied before. While the water regime as well as the level of agrotechnology (with some changes in nutrition regime) is practically the same and there is no significant difference in the values of evapotranspiration. Numerous studies have found that for optimal water supply conditions evapotranspiration is determined by thermal resources. Gross evaporation is formed under the influence of agrophytocenosis radiative balance which is the difference between the quantity of the shortwave solar radiation being absorbed and effective long-wave radiation of the active surface. Radiative balance of the active surface irrigated increases. Under irrigation while the level of water supply increases, albedo decreases and the value of short-wave radiation absorbed increases. The changes caused by the increase of water supply level will lead to the increase of absorption coefficient of photosynthetically active radiation (PAR). However, for the same crop but for different cultivars at the same values of PAR the yield can vary in several times. It can be supposed that new high-yield crops will consume energy for photosynthesis much more than the crops zoned today.
Keywords: total water consumption, evapotranspiration, irrigation demand, radiative balance, gross evaporation, yield, thermal resources, photosynthetically active radiation (PAR), albedo.
