Evaluation Of Water Supply And Demand Matching Degree In Malian River Basin Under Vegetation Restoration

In recent years,China has carried out a lot of soil and water conservation work to solve the serious problem of soil erosion in the Loess Plateau.Among them,the Grain for Green Project can effectively reduce the amount of yellow silt and ameliorate the regional ecological environment.The growth effect of artificial vegetation on the Loess Plateau is often restricted by water resources.So it is very important to research into the relationship between the supply and demand of vegetation moisture in the watershed after the implementation of the Grain for Green Project.In this paper,the Malian River Basin in the middle of the Loess Plateau is selected as a typical vegetation restoration area.The ecological hydrological model and the dual-source evaporation model are used to calculate the water demand and supply of vegetation and the spatial and temporal matching degree of the two.In order to evaluate the vegetation moisture after the implementation of the Grain for Green Project.The supplydemand relationship provides a theoretical reference for optimizing forest and grass restoration policies.The main results achieved are as follows:(1)An evaluation system has been established,which is used to evaluate the matching degree of water supply and demand for vegetation.Consider the water demand for a wellgrown vegetation as the demand side while the available water of the vegetation as the supply side in the conditions of the underlying surface in the study area.The space-time matching degree is used as evaluation index in the water supply and demand matching degree evaluation system.The evaluation system divides the matching degree of water supply and demand into five levels as “excellent”,“good”,“fair”,“poor”,“ extremely poor ”.The system can identify when and where vegetation moisture supply and demand is balanced or contradictory in the study area.(2)An ecological hydrological mode which is suitable for the Loess Plateau has been constructed.The RHESSys model constructed in this research is superior to the single hydrological model in the simulation effect and function of the typical watershed of the Loess Plateau.The regular Nash efficiency coefficient NSE is 0.763 and the determination coefficient R2 is 0.764;the NSE efficiency coefficient during the verification period is 0.705 and the determination coefficient R2 is 0.727.The model can more accurately simulate the ecological hydrological process of the Loess Plateau in the context of vegetation restoration and provide effective method support for the calculation of vegetation moisture matching.(3)The vegetation water demand and available water quantity in the Malian River Basin has been calculated.The spatial and temporal matching index is used to quantitatively evaluate the vegetation water supply and demand relationship and point out the problems in the Malian River Basin.The annual vegetation water requirement in the study area is 569.37 mm,with a large requirement in the south and a small requirement in the north.The average available water volume in the Malian River Basin for long time is 294.28 mm.The average available water accounting for 54.55% in summer and only 4.45% in winter with large difference during the years.From 2000 to 2018,the annual scale time matching degree of vegetation in the Malian River Basin is 0.62 while the monthly scale time matching degree is 0.63.The spatial matching degree in the sub-basin unit is 0.65 which is evaluated as “good”.In general,the water resources in the Malian River Basin can support the normal growth of vegetation under the current conditions.However,there are still some problems in the local area: The orchards and cash crops in the south grow well with large footprint resulting in excessive water demand for vegetation in the area.Regional water resources can barely support the normal growth of vegetation under the current conditions;the distribution of available water in the basin is extremely uneven during the year which is the reason of a poor matching assessment in winter in the northern part of the basin although the annual matching assessment is good.(4)Suggestions for forest and grass restoration suitable for the Malian River Basin are proposed,which is based on the evaluation of the relationship between water supply and demand.According to the assessment of the matching degree of vegetation water supply and demand and the meteorological,hydrological and current vegetation status of the watershed,the following suggestions are proposed:(1)Little water is available of the vegetation in the Malian River Basin in winter.Vegetation restoration should select drought-tolerant winter and low water demand throughout the year;(2)The degree of moisture matching in the Huan county and Huachi County is “good”.So the restoration of forest and grass can be improved;(3)The evaluation of the degree of matching of Heshui County,Ning County,and Zhengning County in the southern part of the basin is “general”.it is not recommended to plant new plantations in the region to maintain the current status of vegetation in the southern region;(4)The matching evaluation of Dingbian County in the northern part of the basin is “poor” in winter.It is necessary to adjust the planting structure.The planting area of economic crops in the northern part of the basin should be appropriately reduced.Switching to planting herbs with low water consumption in winter is suggested.