Optimal Coverage Of Robinia Pseudoacacia On The Loess Plateau

The Loess Plateau of China has severe soil erosion and fragile ecological environment.Robinia pseudoacacia is one of main afforestation tree species for vegetation restoration on the Loess Plateau due to its good adapation and drought resistance.The unreasonable plantation density of Robinia pseudoacacia resulted in the formation of a dried soil layer in the soil profile.Water is the main factor limiting vegetation construction.Based on the spatial distribution characteristics of precipitation resources,the optimal coverages of Robinia pseudoacacia for sustainable growth under different climatic,soil and topographic conditions were determined,which might be useful to guide vegetation restoration and reconstruction and sustainable development of ecological environment on the Loess Plateau.In this paper,we firstly determined the suitable area of Robinia pseudoacacia on the Loess Plateau.Secondly,the calibrated and validated SWAT model was used to simulate runoff and percolation of the whole Loess Plateau and calculated actual evapotranspiration based on water balance principle.Based on the relationships among leaf area index,actual evapotranspiration and the coverage of Robinia pseudoacacia,the spatial distribution map of the optimal coverage of Robinia pseudoacacia on the whole Loess Plateau was obtained by overlying the suitable areas of Robinia pseudoacacia.The main conclusions could be drawn as follows:（1）Based on the annual average rainfall and temperature data of 287 counties of the Loess Plateau,the suitable zone for Robinia pseudoacacia on the Loess Plateau were obtained using the Kriging spatial interpolation and overlying analysis.The suitable zone included most areas of Shanxi（except parts of Pianguan,Shenchi,Wuzhai and Kelan）,most areas of Shaanxi and Gansu（except parts of Yulin,Shenmu,Fugu,Dingbian,Huanxian,Weiyuan,Zhangxian,Dingxi,Yuzhong,Jingyuan and Baiyin）,and some areas of Ningxia,such as Jingyuan,Pengyang,Xiji,Haiyuan and Tongxin.（2）The SWAT model was calibrated and validated by using the measured annual runoff data from 1975 to 1986 and 2001 to 2010 at Wuqi hydrological station in the upper reaches of Beiluo River.The determinant coefficient（R²）and Nash-Sutcliffe coefficient（NSE）of the calibration period were 0.70 and 0.54 respectively,and the determinant coefficient(R²)and Nash-Sutcliffe coefficient（NSE）of the verification period were 0.88 and 0.72respectively.The results indicated that SWAT model can accuately simulate annual runoff in the upper reaches of Beiluo River.The validated SWAT model could be used to simulate annual runoff for the whole Loess Plateau.（3）The annual average surface runoff,percolation and actual evapotranspiration of the Loess Plateau presented obvious spatial distribution characteristics:a decreasing trend from southeast to northwest.The annual average surface runoff was mainly affected by rainfall and slope;percolation was affected by soil type,rainfall,slope and other factors;the actual evapotranspiration was mainly affected by precipitation.（4）The optimal coverage of Robinia pseudoacacia on the Loess Plateau decreased gradually from southeast to northwest,which was mainly controlled by precipitation.In the areas with annual precipitation between 350 and 450 mm,the optimal coverage of Robinia pseudoacacia ranged from 40%to 60%.In the areas with annual precipitation between 450and 550 mm,the optimal coverage of Robinia pseudoacacia ranged from 50%to 70%.The optimal coverage of Robinia pseudoacacia ranged from 60%to 80%in the areas with average annual precipitation greater than 550 mm.