Spatial And Temporal Evaluation Of Vegetation Water Suitability In Yanhe River Basin

Since the implementation of the project of the grain for green project on the loess plateau,the regional ecology has been restored well and the vegetation coverage has been greatly improved,which has played an important role in the sustainable development of the regional ecosystem.However,as the loess plateau has always been dry and short of water,and the spatial and temporal distribution of available water in the region is highly inconsistent,the contradiction between the high water requirement during vegetation restoration and the low supply of available water will be an important factor threatening the sustainable development of the vegetation system.Rainfall is the main source of available water resources in the region,but it can only be absorbed and utilized by vegetation if it is transformed into effective water(rainwater utilization potential).In order to answer whether the available water resources in the region can support the water requirement of vegetation restoration,it is necessary to conduct a quantitative exploration on the balance relationship between the amount of water resources that can be utilized by vegetation and the water requirement for vegetation growth.In this paper,the balance relationship between the rainwater utilization potential(RUP)and water requirement of vegetation(WRV)was defined as vegetation water suitability.The Yanhe River Basin,as a typical region of hilly and gully regions on the loess plateau,is characterized by serious soil erosion and unequal distribution of rainfall.Based on this,we selected Yanhe River Basin as the research area,using distributed hydrological model and the Shuttleworth-Wallace dual-source evapotranspiration model(S-W model)for quantitative evaluation of vegetation water suitability,aiming to provide a reasonable reference for the optimal allocation and sustainable restoration of vegetation in Loess Plateau.The progress of this study is as follows:(1)The simulation results of SWAT distributed hydrological model were calibrated and verified by using measured runoff data.In the calibrated period,the correlation coefficient R~2 and the Nash-Sutcliffe efficiency coefficient(NS)of the model are 0.84and 0.75,respectively,while the R~2 and NS coefficients in the verification period are0.78 and 0.72,respectively.According to relevant evaluation criteria,when R~2 and NS coefficient are both greater than 0.7,it indicates that the model performs very well,proving that the results simulated by SWAT model are available in this study.(2)Analysis of the temporal and spatial trends of rainwater resources potential on the interannual and annual scales.In the past 34 years,the RUP and rainfall in the study area have shown a slight upward trend.The RUP shows a decreases trend from southeast to northwest in terms of spatial distribution.According to the three types of hydrological years determined by the rainfall frequency,the annual RUP of these three years is mainly concentrated in June and September,and shows an increasing trend from northwest to southeast in space with the passage of time.(3)The change trend of WRV in different hydrological years was analyzed on temporal and spatial scales.The annual WRV in three hydrological years of Yanhe River Basin shows a very similar change trend,which is mainly from April to September.From 1996 to 2012,the WRV shows a trend of gradually increased from southeast to northwest,which could be attributed to the large-scale transformation of agricultural land into grassland and forest land in northwest China after 1999,resulting in the increase of water demand.(4)Calculated and analyzed the vegetation water suitability at different hydrological years on time and space scale.The monthly vegetation water suitability in dry year,normal year and wet year were 0.59,0.63 and 0.67,respectively.The average vegetation water suitability of each subbasin on the spatial scale of the three hydrological years was 0.60,0.74 and 0.77,respectively,and shows an increasing trend from northwest to southeast since the implementation of the grain for green project.The suitability of the central and northern parts of the study area was poor,while that of the southeast was always at a high level.(5)There is no good balance between available water for vegetation and water requirement of vegetation in the three types of hydrological years in the Yanhe River Basin,which means that the available water resources in the study area cannot fully guarantee for vegetation growth,it is necessary to reconsider water supply facilities and replanning the distribution of vegetation types.In the spring with low water content,irrigation can be used to supplement the water shortage of vegetation.In the northwestern where the rainwater supply is insufficient,grassland and other vegetation types with less water demand can be planted according to specific geographical conditions.