Application Of Dual Crop Coefficient Method And Water Environment Assessment In Apple Orchard On Loess Tableland

Since the Grain for Green project was carried out in the Loess Plateau,the land use has changed greatly and a large amount of cropland has been converted to forest and grassland.In order to restore the ecology and increase the income of local farmers,form a practical model of Grain for Green project,the new round of Grain for Green project clearly proposed to vigorously develop characteristic economic forest and other forestry industries,relying on its own geographical advantages,this area has become the world’s largest recognized apple eugenic area.However,some studies showed that the carrying capacity of vegetation on the Loess Plateau was close to the limit,water resources were undoubtedly the key factor for the sustainable development of apple orchards on the Loess Plateau.Precipitation is an important source of water in apple orchards in this region,most of which will eventually be lost in the form of evapotranspiration.Therefore,it is necessary to quantify evapotranspiration in apple orchards and clear soil avaliable water changes on Loess Tableland for providing a scientific basis for planting planning and production layout adjustment of apple producing areas.This study took the Changwu Tableland,a typical representative of the Loess Plateau,as the study area,selected 25 years apple orchard as the sample site for two-year experiment in 2020-2021.At first,each crop coefficients of dual crop coefficient method were calculated,and their accuracy was verified by measured values.Then evapotranspiration and its components were quantificated based on dual crop coefficient method.Finally,the water suitability was calculated based on precipitation,and the variation of available soil water in the main root layer was simulated to comprehensively evaluate the water environment of apple orchard.Specific research results are as follows:（1）Variation characteristics of crop coefficient in apple orchard on Loess Tableland.From 2020 to 2021,the K_cb of apple orchard increased firstly and then decreased,and the average growth period was 0.40 and 0.47,respectively.The initial growth period,rapid growth period,middle growth period and late growth period of K_cb were 0.13,0.37,0.53,0.40 and0.17,0.38,0.60,0.51,respectively.K_e had no obvious variation,and the average values of the whole growth period were 0.34 and 0.36,respectively.The average values of K_e were 0.34and 0.36 in the whole growth period.The initial growth period,rapid growth period,middle growth period and late growth period of K_e were 0.22,0.26,0.40,0.35 and 0.36,0.26,0.41,0.39,respectively.K_c had a similar trend to K_cb,with the mean values of 0.73 and 0.83 in the whole growth period,presenting a multi-peak curve.The initial growth period,rapid growth period,middle growth period and late growth period of K_c are 0.36,0.63,0.94,0.75 and 0.54,0.64,1.01,0.93,respectively.（2）Comparison between estimated and measured crop coefficients in apple orchard on Loess Tableland Two-year K_cb was 8.2%and 12.5%higher than K_cbm,two-year K_e was 4.8%and 9.1%lower than K_em,and two-year K_c was 4.2%and 1.4%higher than K_cm.There were no significant differences between K_cb and K_cbm,Ke and K_em,K_c and K_cm in different growth stages（P>0.05）.The evapotranspiration and its components of apple orchards in two years fitted well with the measured values and were significantly correlated（P<0.01）.Combined with MAE,RMSE,RRMSE and D results,it could be accepted that the dual crop coefficient method could deviate from the estimation of evapotranspiration and its components to a certain extent,and had high accuracy.（3）Changes of evapotranspiration and its composition in apple orchard on Loess Tableland.The range of transpiration was 0.1 mm d^-1-3.4 mm d^-1 and 0.1 mm d^-1-3.7 mm d^-1,the daily mean was 1.1 mm d^-1 and 1.3 mm d^-1,and the total transpiration was 278.4 mm and330.8 mm,respectively.Accounted for 56.0%and 58.7%of evapotranspiration in growth period,respectively.The range of evaporation during the whole growth period was 0.2 mm d^-1-3.0 mm d^-1 and 0.0 mm d^-1-3.1 mm d^-1,the daily mean was 0.9 mm d^-1,and the total evaporation was 219.2 mm and 232.6 mm,accounting for 44.0%and 41.3%of the total evapotranspiration during the growth period,respectively.The range of evapotranspiration was 0.2 mm d^-1-6.1 mm d^-1 and 0.1mm d^-1-6.8mm d^-1,respectively,during the whole growth period of two years.The maximum value of evapotranspiration occurred at the fruit expansion stage,and the daily mean value was 1.9 mm d^-1 and 2.3 mm d^-1,respectively.The total evapotranspiration was 497.6 mm and 563.5 mm,respectively.The water consumption of apple orchard in 2021 was higher than that in 2020.The main influencing factors of evapotranspiration and its components were rainfall,followed by temperature.（4）Water environment assessment of apple orchard on Loess Tableland.The water suitability was 0.89 and 1.18,respectively,which were higher than the optimal critical value.ASW decreased first,then increased,and then decreased.ASW loss was 18.2 mm in 2020,and gain was 14.3 mm in 2021.In 2020,apple orchard was subjected to drought stress for 65days,accounting for 25.3%of the total number of days.In 2021,apple orchard was subjected to drought stress for 66 days,accounting for 26.8%of the total number of days.Based on the analysis of ASW and water suitability,the water environment of apple orchard was medium in 2020 and poor in 2021.The water environment was better in germination,flowering,maturity and deciduous stages in 2020,but worse in young fruit and fruit expansion stage,indicating drought stress.In 2021,the water environment in bud,florescence,young fruit and deciduous stage is better than that in fruit expansion stage and mature stage,and there are drought stress and waterlogging problems respectively.The uneven distribution of precipitation between two seasons leads to severe water shortage in apple orchards during the key water demand period.