Applicability Evaluation Of ET₀ Calculation Formulas And Relationship Between Physiological Index And Evapotranspiration

Reference crop evapotranspiration (ET0) is the basic data for international general water resources'evaluation and crop water requirement's calculation. It also provides scientific basis for rational irrigation and water resources allocation. To estimate ET0 accurately has a momentous current and far-reaching significance for guiding water-saving irrigation and the development of water-saving agriculture. This article mainly takes measure of reference crop evapotranspiration, in the presumed conditions of PM formula(By defining the reference crop as the alfalfa with an assumed height of 12cm having adequate water supply, closely resembling the evaporation of an extension surface of green grass of uniform height, actively growing and adequately watered), researched the variation of canopy resistance and applicability of reference crop evapotranspiration calculation formulae and effect of stomatal resistance and canopy temperature on crop evapotranspiration.This experiment was conducted on the irrigation and drainage experiment field of key laboratory of agriculture soil and water engineering in arid and semi-arid areas by Ministry of Education in Northwest A&F University in Yangling,Shaanxi. Based on the theoretical analysis,the main results as follows:1.Rreference crop evapotranspiration was measured by cylindrical weighing lysimeter in the presumed conditions of PM formula, and performed experimental rating on canopy resistance. The results showe that variation of canopy resistance increases with growing processes, and it fluctuates at 70s/m in middle growth stage. The diurnal variation curve of canopy resistance takes the shape of"U", whose minimum point appears at noon(lower than 50s/m) and maximum point appears at 8:00am and 18:00pm(higher than 100s/m).2.Four internationally conventional ET0 formulas for calculated were evaluated by actual measurement reference crop evapotranspiration. The results showe that standard ASCE-PM is the optimum formula, after that is PM formula and MP formula, the poorest is national MP formula in Yangling, Shaanxi, semi-humid and easily arid in north.3.In the whole growth period, alfalfa's evapotranspiration decreases when stomatal resistance increases, in adequate water supply or in a little deficit condition. But stomatal resistance shows no correlation with evapotranspiration under a long-term larger deficit condition.4.In the whole growth period, the canopy-air temperature difference of alfalfa is negative with full irrigation, while positive with larger deficit. It is single-peak in the typical day for the shape of the diurnal variation curve of canopy temperature. Tr increases with the increase of Tc-Ta in full irrigation condition. Tr decreases when Tc-Ta is greater than 0℃in deficit condition.5.There is a typical logarithmic relationship between stomatal resistance and canopy-air temperature difference, and the equation is Rs = 2.4677 ln (Tc) - 3.7463, and the correlation coefficient is 0.6406. Under full irrigation condition, stomata open completely, and stomatal resistance and canopy-air temperature difference are smaller. Under soil water deficit condition, stomatal resistance and canopy-air temperature difference increase by partly stomata closure, and the canopy-air temperature difference is positive value.