| Land use type is one of the important factors affecting the soil water cycle in Loess Plateau.In recent years,under the needs of economic transformation in the Loess Plateau,a large area of wheat and corn field has been transformed into apple orchard.However,the evapotranspiration of apple orchard is far greater than precipitation supply,resulting in the reduction of water resources that are available for apple trees.Then the normal growth of apple trees is limited,leading to the reduction of the yield and quality of apples.Therefore,it is necessary to analyze water consumption characteristics and the soil water conditions of continuous cropping apple trees,spring corn and winter wheat in the Loess Plateau under the conditions of land transformation and climate fluctuation.It will provide a theoretical basis for optimizing the management measures in the process of agricultural production and the sustainable utilization of soil water in the Loess Plateau.In response to the above problems,Changwu tableland,a typical representative of the Loess Plateau,were selected as the research area in this study.Apple orchards with different forest ages under the same management measures,corn field and wheat field were taken as the research objects.Then the field experiment and Root Zone Water Quality Model(RZWQM2)were combined to do the research as the objects as below:Analyzed the evapotranspiration characteristics of apple orchard under different time scales;Obtained the calculation method of evapotranspiration and the water consumption characteristics in corn and wheat fields under continuous cropping mode;Clarified the dynamic characteristics of soil moisture in the apple orchards with the increasing age(corresponding to apple tree ages of 7,8,9,10,17,18,19,and 20);Discussed the effect of apple orchard on soil moisture to the adjacent cropland land;And explored the feasibility of RZWQM2 in simulating soil water of apple orchard,and then analyze the water supply and demand of agroforestry ecosystem under the condition of climate fluctuation using the model.The main conclusions of this study are as follows:(1)The evapotranspiration characteristics and influencing factors of apple orchards at different time scales were clarified.During the vigorous growth period of apple trees,the transpiration composition of orchards is mainly determined by environmental factors.It is greatly affected by tree age after harvest.On the hourly scale,the sap flow velocity should be measured by TDP.On the daily and monthly scales,the water consumption of the whole plant and the whole stand can be calculated by monitoring temperature and humidity.(2)The calculation method and water consumption characteristics of cropland evapotranspiration in the Loess Plateau are clarified.When the meteorological data in the Loess Plateau is enough,ASCE-PM,FP17 and Pn can be used to calculate the daily reference crop evapotranspiration in this area,and the Taylor skill scores were 1.00,0.97 and 0.99respectively.When the meteorological data are incomplete,the radiation based Prs-Tylr formula and the temperature based Harg formula can be selected according to the actual situation to calculate the daily reference crop evapotranspiration,and the Taylor skill scores were 0.93 and 0.91 respectively.The two growth stages of winter wheat from returning to green stage to jointing stage and from filling stage to maturity stage are relatively short of water in the study area.So that the appropriate water replenishment and moisture conservation measurements are needed to ensure the normally yield from the seriously dry years.During the growth period of continuous cropping spring maize and winter wheat,the evapotranspiration of spring maize fluctuated for 2-3 years and 3-6 years.And the evapotranspiration of winter wheat has periodic changes of 2-3 years,3-5 years and 6-7 years.(3)The characteristics of soil water conditions in apple orchards with stand age were clarified.The soil water content in the deep soil layer(200-600 cm)is higher in the young orchard than that in surface layer(0-200 cm),but it is opposite in the elder orchard.There is no dried soil layer in the young orchard.The thickness of dried soil layer below 200 cm in elder orchard is 280,340,340 and 260 cm,respectively.(4)The soil water distribution characteristics of adjacent cropland in apple orchard was discussed.The change of land use type has caused significant changes in the process of soil water consumption and replenishment in agroforestry ecosystems.The soil water storage capacity of cropland is significantly higher than that of apple orchards.Apple trees can utilize soil water from adjacent cropland,so that apple trees can grow healthily.With the increasing age of apple trees,the water consumption of cropland intensifies.Under the long-term continuous cropping mode,it will affect the sustainable development of the agroforestry ecosystem in the region.Therefore,the proportion of orchards and cropland should be controlled,and the planting period of orchards should be adjusted appropriately to maintain the sustainable development of the agroforestry ecosystem.(5)The feasibility of RZWQM2 in simulating the water supply and demand characteristics of agroforestry ecosystems in the Loess Plateau was explored.The results showed that RZWQM2 can be used to simulate the growth dynamics of winter wheat and spring maize,and the soil water characteristics of apple orchard,corn field and wheat field in the Loess Plateau,which is under the condition of less precipitation(R~2>0.70,-3.86%<PBIAS<0.49%and D>0.89).Under the long-term continuous cropping mode,the consumption intensity of soil water in apple orchard is the largest,followed by corn field,and the consumption intensity of soil water in wheat field is the smallest.The water satisfaction rate of apple orchard decreased first and then increased with the increasing planting years,while it decreased with the increasing planting years in the corn and wheat field.For the adjustment of agricultural structure in the Loess Plateau.In a word,the water consumption of elder apple orchard is greater than that of young orchard in the Loess Plateau.The soil water content decreases first and then increases with the increasing age of the apple tree.In addition,apple trees will compete with adjacent cropland for soil water.Therefore,for the adjustment of agricultural structure in the Loess Plateau,it is possible to consider restoring the soil water of apple orchards after 22 years of apple tree planting by replanting winter wheat or strengthening apple tree canopy trimming,increasing surface cover and planting clover in the orchard to maintain the sustainable development of agriculture in the region. |
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