Evaluation of the «Penman-Monteith» model for determination of soybeans' evapotranspiration in irrigated conditions of the Steppe of Ukraine

Abstract

Shatkovskyi AP*, Romashchenko MI, Zhuravlov OV, Vasyuta VV, Melnychuk FS, Ovchatov IM, Yarosh AV and Semenko LO

The calculated equation of the Penman-Monteith method for determining evapotranspiration (ETc) is derived from the equation of energy balance of the soil surface, and the dependence of ETc on ETo reflects the culture coefficient Kc. Since the values of Kc (FAO) are typical values for standard climatic conditions, which are defined as sub-humid climate, it is necessary to evaluate this model for the conditions of the Steppe of Ukraine, whose climate is drier.

Thus, the aim of scientific research was the adaptation of the calculation method for determining the evapotranspiration of the ETc "Penman-Monteith" to the irrigated conditions of the Steppe of Ukraine on the soybean crop. The work involved conduction of short-term field experiments, generally accepted analytical research methods; the use of methods of variance, correlation, regression and variational analysis. The actual soybean irrigation regime, the reference evapotranspiration of ETo, the calculated and actual values of evapotranspiration of ET? were established using the digital Internet meteorological station iMetos® and the multisensor probe AquaSpy CTG-02. Based on the results of the field experiment, the crop coefficient Kc was adjusted according to the main phases of development of soybean varieties of different ripeness groups using deviations from standard conditions. The research confirms that the value of the actual crop coefficient Kc for soybeans under irrigation conditions of the Steppe of Ukraine differs from the typical Kc (FAO). It was found that at the initial and final stages of plant development, Kc (FAO) overestimates the actual values of ETc evapotranspiration to 42.8%, and in the middle phase, on the contrary, underestimates by 4.2-11.5%. The use of Kc (FAO) in the calculations of evapotranspiration for standard conditions leads to over-watering at the beginning and at the end of the growing season of soybeans and a deficiency of moisture reserves in the critical period of plant development (phase "beginning of flowering-poured beans"). It is recommended to use the “Penman-Monteith” method in the future to determine the actual evapotranspiration of ET? and operative control of the soybean’s irrigation regime on this basis, taking into account the adjusted values of the culture coefficients Kc.