Effects of water stress on evapotranspiration of soybean
Based on the results of observations, it was specified that when decreasing soil moisture there is a disproportionate decrease in the average daily evapotranspiration (ET). Thus, in the range of soil moisture of 94-80% minimum moisture-holding capacity (MMHC) ET was 9,76 mm a day, and in the range of 70-62% MMHC - its value decreased by 3 times. When the soil moisture reached 58,5% MMHC, the value of ET did not exceed 0,5 mm a day, which is 20 times less than the initial one. It was determined that the decrease in soil moisture by 10% in the range of 90 - 70% MMHC occurs during 3 days, and from 70 to 60% MMHC and from 60 to 58% MMHC - during 8 days. When soil moisture is 70% MMHC and below, the actual evapotranspiration is less than ETo that proves the effect of water stress on soybeans ET. When calculating water stress coefficient (Ks), a mathematical model based on the dependence of Ks on soil moisture as a percentage of MMHC was obtained. The average absolute percentage error (MAPE) is 8,6%, which corresponds to the high accuracy of the obtained dependence. In the range of soil moisture from 58 to 80% MMHC, the water stress coefficient is calculated by the formula Ks =-0.0011·FC²+0.1925·FC-7,4541. When having soil moisture as 80% MMHC and above, Ks = 1. A comprehensive comparative assessment of existing methods for calculating waster stress coefficient Ks was taken and it was found out that the actual values of Ks when having soil moisture as 80-70 and 60-65% MMHC by 8-14 % and 72-32 %, respectively, less than Ks FAO 56, and by 35-40 % larger than those determined by Saxton method. It was proved the need of taking into account the reduction in evapotranspiration when calculating water balance under water stress of plants. The calculation of evapotranspiration (ETs) by the Penman-Monteith method, without taking into account the water stress coefficient, showed that the value of the actual and calculated water balance coincides only when soil moisture does not exceed 62% MMHC. With a further decrease in soil moisture, the estimated soil moisture was 20% less than the actual, which led to the errors in determining soil moisture after irrigation, because its actual value was almost 100% MMHC, and the estimated one was only 60% MMHC. It was proved that the determination of water balance by calculation methods without taking into account the water stress coefficient leads to significant errors.
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