Simulation Of Soil Desiccation Effects And Production Water Footprint Of Apple Orchards In Loess Plateau

As one of the largest apple-producing areas in the world,the Loess Plateau is faced with great challenges due to the shortage of water resources and climate-related problems.Rainfall is scarce in this region,and groundwater recharge is difficult.More than 80%of apple orchards are dry orchards without irrigation conditions.As a deep-rooted and high-water-consuming crop,long-term soil water deficit is bound to cause the phenomenon of soil drying,which threatens the high and stable yield of orchards.The dynamic change of effective soil water content and the formation of dry soil layer in apple orchards on the Loess Plateau are closely related to precipitation,planting density and tree age.Unreasonable planting density is not only detrimental to the high yield of orchard,but also aggravates the drying degree of soil.Therefore,it is the key to the long-term and stable development of orchards in the Loess Plateau to know the soil drying effect and apple growth in different climatic regions,choose the planting density reasonably,and predict the soil conditions and fruit tree growth characteristics in different regions in the future climate change.In order to find out the water consumption characteristics and growth characteristics of deep soil in apple orchards in different climatic regions of the Loess Plateau,four typical apple planting regions,Fengxiang,Luochuan,Changwu and Mizhi,were selected,and Win EPIC model was used to quantitatively simulate and analyze the dynamic change of soil moisture in apple orchards at 0-15m and the evolution law of apple production water footprint,and the optimal planting density of fruit trees in different climatic regions and its response to future climate changes were obtained,which provided a scientific basis for the water cycle and the healthy and stable development of apple industry in this region.The main conclusions are as follows:（1）RMSE of observed and simulated apple yield and soil water content values in different climatic region were all<3 t·hm^-2,RRMSE<15%,R² were all>0.8.The simulation effect was good and the simulation accuracy was high,which could meet the long-term simulation study of soil desiccation effect and production water footprint of dry apple orchard in different climatic region of the Loess Plateau.（2）The annual output of mature orchards in different climatic regions shows an"S"curve trend.During the simulation period from 1980 to 2020,the average annual output values of Fengxiang,Luochuan,Changwu and Mizhi were 25.05,26.29,22.17 and 18.19 t·hm^-2,respectively,which showed that the annual average value of apple production in areas with high precipitation was higher,and the annual average values of evapotranspiration and water consumption were longer.The effective soil water content of fruit trees in Fengxiang 1～25,Luochuan 1～19,Changwu 1～21 and Mizhi 1～18 shows a significant fluctuation decreasing trend.Compared with Mizhi in the semi-arid area,the effective soil water content in the other three places has a buffering stage in the early stage,and the initial dry soil layer is generally formed and reaches a stable time later in areas with high precipitation.When the soil water is in a deficit state for a long time,an irreversible dry soil layer will eventually be formed.The water footprint of the four apple orchards was low in the early stage and high in the late stage,and the average annual water footprint values were 0.193,0.176,0.202 and 0.195 m³·kg^-1,respectively.Apple yield and production water footprint are affected by precipitation.In the Loess Plateau,where it is difficult to obtain water,It is suggested that the best useful life of apple trees should be 18～25 years,and the maximum should be no more than 25 years.（3）During the simulation period from 1980 to 2020,under the planting densities D1 to D6,the evapotranspiration and water over-consumption of orchards increased with the increase of planting density,but the increment was not significant.However,the average annual yield of fruit trees is not always increasing,but there is a maximum.Compared with low-density planting,the apple yield under high-density planting declines earlier.At the initial stage of the growth of simulated fruit trees,the soil water storage capacity is enough to meet the growth needs of young trees,but there is no drought stress in Luochuan and Mizhi orchards.With the increase of fruit age,drought stress occurs in different density orchards one after another,and the time of drought stress comes earlier with the increase of planting density.Compared with Luochuan in semi-humid area,under the same planting density,the drought stress time of rice fat in semi-arid area appears earlier,and the annual drought stress days are longer.The higher the planting density,the earlier the appearance of the dry soil layer,the earlier the stable dry soil layer,and the more severe the soil drying effect.The annual average water footprint of apple production in Luochuan is 0.209,0.199,0.187,0.176,0.177,0.180m³·kg^-1,and the Mizhi is 0.196,0.198,0.194,0.195,0.193,0.193 m³·kg^-1,respectively.（4）During the simulation period from 2025 to 2065,under RCP4.5 and RCP8.5 scenarios,the soil effective water content of 0～15m soil layer in Luochuan was higher than rice fat,and under RCP4.5 scenario it was higher than that under RCP8.5 scenario.During the growth period of fruit trees,if the precipitation is always at a high value at a certain stage,the soil water will seep downward to supply during this period,effectively alleviating the soil water content of 3～5m soil layers in apple orchards at this growth stage.Compared with 1980-2020,the average annual yield of Luochuan and Mizhi orchards is increasing,but compared with RCP4.5 scenario,RCP8.5 scenario has a larger yield fluctuation and greater variability.The annual average water footprint of apple production in two orchards is 0.179 and 0.178 m³·kg^-1 in Luochuan and 0.194 and 0.199 m³·kg^-1 in Mizhi respectively.To sum up,suitable planting density of apple trees should be selected in different areas to achieve the best harvest yield.For the sustainable and healthy development of apple industry,it is suggested that the best utilization period of apple trees should be 18～25 years.At the same time,precipitation is still the main factor restricting the phenomenon of soil drying in apple orchards in different areas,in the future,we should consider increasing water supply in the first place in production management,and appropriately adding water in the growth process of fruit trees to increase fruit tree yield and delay the degree of soil drying.