Experimental Calibration Of Reference Evapotranspiration And Determination Of Crop Coefficient In High-elevation Areas Of Tibet

Reference evapotranspiration?ET₀?is the theoretical basis accepted internationally for evaluating hydrological resources and calculating crop water requirements.It is also the basis for formulating water law,allocating international river water resource and water environment assessment.On this basis,crop water requirement can be calculated combining with crop coefficient?Kc?,which is the most common study in the research of water conservancy planning and design,water resources management evaluation and forward-looking scientific research.With an average elevation of more than 4000 m,Tibet is known as the "roof of the world".The unique meteorological and geographical conditions concluding low pressure?lower than 2/3 of the sea level?,strong radiation?annual solar radiation of 6,000-8000 MJ/m2?,long sunshine time?longer than 3000h?and great change of temperature and humidity lead to the particular calculation method of ET₀ and Kc.The FA056 Penman-Monteith?PM?equation was recommended by the International Food and Agriculture Organization?FAO?as a standardized method for ET₀ calculation and evaluating other methods in areas lacking of measured data.However,in Tibet with high elevation and complex meteorological conditions,the application of PM equation as semi-theoretical and semi-empirical formula based on assumptions is needed to be evaluated.In particular,ET₀ calculation methods and parameters based on measured data has long lagged behind in Tibet and even in high altitude areas around the world?especially above 4000m above sea level?.Therefore,based on the measured ET₀ data for pruning alfalfa and ET₀ data for barley by lysimeter,this project aims to evaluate the applicability of the PM equation recommended by FAO in Tibet and determine the appropriate crop coefficients for each breeding stage of barley,which is of great significance to fill the research gap of measured ET₀ in high altitude areas,promote the standardization and universal development of ET₀ calculation,and has great guiding significance and application value for formulating reasonable irrigation system and water use plan in high altitude areas in the future.The main conclusions are as follows:1 The regression constants of as and b,and the coronal reflection coefficient?a?in the energy conversion formula?Angstrom?are important parameters for estimating energy conversion during the calculation of ET₀ by the PM formula.In this study,the recommended monthly and daily a values were determined by the dynamic properties of alpha.At the same time,this study found that the as in Tibet is lower than the recommended value by FAO,which indicates that when the sunshine hours at high altitude are zero,the zenith radiation reaching the ground is very small as lower than the average at low altitude.bs is higher than the recommended value by FAO,which indicates that the amount of solar radiation reaching the surface at the top of the earth's atmosphere in high-elevation areas is higher than that at low-elevation areas under same latitude condition.2 Based on the modified PM equation,the daily and hourly scale ET₀ were compared and analyzed using the measured weight data of pruning alfalfa.The results show that the ET₀ and modified ET₀ values based on the sunshine hours vary slightly with meteorological elements changing and the ET₀ values based on the measured radiation data are closer to the measured ET₀.Then in order to calculate ET₀ more accurately in Tibet,it is proposed to calculate ET₀ using ET₀ values obtained by the measured radiation data multiplying by reduction coefficient of 0.82.In addition,it is proposed to calculate ET₀ using ETO and modified ETO based on sunshine hours multiplying by 1.49 and 1.37,respectively.3 Based on the daily meteorological data for 20 years from 15 typical stations in Tibet,the Hargreaves?HS?model was improved by introducing elevation and correcting temperature constants to obtain a simple and accurate method for ETO evaluation in high altitude.ET₀ HSE =10-5?aH2+bH+c??Tmean+36.6??Tmax-Tmin?0.5RaIn most areas above 2000 m,a is 8 × 10^ 6;b is 0.07 and c is 5,which improves the practicality and accuracy of simplified ETO calculation.4 Against the background of global warming,the temperature in Tibet has shown a marked upward trend and precipitation shows a significant downward trend.The ETO in most regions except for Shigatse region showed an upward trend and it in showed an insignificant downward trend in Shigatse region.The overall decreasing trend from west to east of yearly ET₀ appears in Tibet.But there are three relatively prominent high-value areas such as the eastern region of Tibet with Zuogong and Baju as the center,the south-central Tibet with Shigatse as center and the southern Tibet and the middle reaches of Yalu with razi county as center.5 Based on the measured weighing experiment of barley by Lysimeter,the crop coefficients in different growing stages of barley corresponding to two ET₀ calculation methods based on radiation data and sunshine hours data in high-elevation region of Tibet are given:Calculate ET₀ using the revised PM equation:?1?When calculating ET₀ based on radiation data:Kcini=0.63,Kcmid= 1.15,Kcend=0.83;?2?When calculating ET₀ based on sunshine hours data:Kcini=0.56,Kcmid=0.97,Kcend=0.62.Calculate ET₀ using the revised PM equation:?1?When calculating ET₀ based on radiation data:Kcini=0.51,Kcmid=0.95,Kcend=0.69;?2?When calculating ETO based on sunshine hours data:Kcini=0.83,Kcmid=1.44,Kcend=0.93.