Determination of the actual evapotranspiration of tomatoes by calculation methods

Abstract

Goal. To determine the actual evapotranspiration of seedling tomatoes by calculation methods and, based on the obtained results, propose the optimal water regime of the soil depending on the phases of plant development. Methods. Short-term field methods, generally accepted analytical and statistical methods of experimental data processing. Results. Calculation of the actual evapotranspiration of tomatoes (ETf) by various methods confirmed that the process took place mainly from 7 to 8 p.m. and during that time 95–96% of the daily amount of moisture evaporated. The maximum values of actual and reference (ETo) evapotranspiration — 1.44–1.97 mm/h and 0.94 mm/h — were recorded from 12 to 3 p.m., and the minimum values — after sunset. The reaction of plants to changes in environmental factors was monitored. Thus, with a deficit of air vapor pressure of 5.2 mb, the speed of sap ascent through the xylem was 0.9 rel. units, and ETf depending on the method used — 4.4–6.6 mm/day. With a deficit of air vapor pressure of 16.2 mb, the speed of sap ascent through the xylem decreased to 0.5 v. units, and ETf, depending on the method used, increased to 9.6–14.2 mm/day. The correlation coefficient between the speed of sap movement through the xylem and ETf was 0.68–0.71. Under average meteorological conditions and the use of the Penman–Monteit, Monteit, and Budyko methods, the MARE error was 8%, 19, and 28%, respectively; in the case of adverse meteorological conditions and the use of the Penman–Monteit, and Monteit methods, it increased to 48–49%, and in the case of Budyko’s method, it almost did not change and amounted to 27%. The analysis of the daily dynamics of tomato evapotranspiration showed that all the selected methods responded equally to changes in climatic parameters. The errors of MARE and RMSE of determination of ETf by phases of tomato development were calculated. On average, during the growing season, the used methods gave a MARE error of 16.2–19.7% (good accuracy of ET determination), and RMSE — 2.0–2.3 mm. The smallest errors were observed using the Penman–Monteit method, and the largest — using the Monteit method. With the application of the specified calculation methods, the coefficients of tomato culture Ks were also calculated. On average, during the growing season, the MARE error, depending on the method, was 13.3–17.4%, and the RMSE error was 0.29–0.30. Conclusions. Based on the research results, the possibility of using the combined Penman-Monteit equation, Monteit, and Budyko equations to determine the actual evapotranspiration of seedling tomatoes without using additional coefficients was confirmed. It was found that under unfavorable meteorological conditions, calculation methods did not take into account the protective effect of the plant, and therefore overestimated the actual value of ETf by 30–60%. Comprehensive statistical analysis confirmed the good accuracy of ETf and Ks determination by the selected methods: the MARE error during the growing season was within 10–20%, and the RMSE error was 2.0–2.3 mm and 0.29–0.30, respectively.