Response Of Green Water In Nanxiaogou Watershed To Land Use Change Based On SWAT Model

As the most serious area of water and soil loss in the world,the Loess Plateau is the key of water and soil conservation management and ecological construction in China.In recent years,water and soil conservation in the Loess Plateau has achieved remarkable results,but the overall ecological fragility has not changed,new problems have appeared along with the results.The shortage of green water resources has become the biggest restricting factor for sustainable ecological restoration.How to coordinate the relationship between vegetation and green water has become the key to vegetation restoration and reconstruction in the Loess Plateau.Centered on the relationship between vegetation and green water,the paper carries out data collection and experiments with "land use-green water" as the core,studied the ecohydrological mechanism and simulation method under the changing environment of small watershed,aimd at grasping the response rules of green water and land use mode,and provided basis for selecting reasonable land use mode and vegetation structure suitable for the development of watershed.To put forward suggestions for ecological protection and high-quality development of the Yellow River watershed.This paper selects Nanxiaohegou watershed,a typical watershed of soil and water conservation control in Loess Plateau,and carries out field water conservation experiment and comprehensive ecological hydrological observation.Through the observation data,the ecological hydrological SWAT model is established and the regional hydrological process is simulated.Based on the simulation results,the spatio-temporal distribution characteristics of green water in Nanxiaohegou watershed are analyzed,the response rules of green water and different land use modes are explored,and the vegetation allocation structure suitable for watershed development was predicted and discussed in combination with the national policy of returning farmland to forest and grassland by setting scenarios.The main research results of this paper include the following parts:(1)Using the data of land use,soil and meteorology from 1966 to 2020 in the study area,SWAT model of the Nanxiaohegou watershed was established.The watershed is divided into 27 sub-watershed and 169 hydrological response units.Sensitivity analysis and calibration of the model were carried out by SUFI-2 algorithm in SWAT-CUP tool.The top parameters were SCS runoff curve coefficient(CN2),basic flow regression factor(ALPHA＿BF),soil evaporation compensation coefficient(ESCO)and plant absorption compensation coefficient(EPCO).The model was verified according to the actual measured runoff data of Yangjiagou,Dongzhuang,Shibamutai and Huaguoshan reservoirs in the watershed,the results shows that NS,R2 are>0.5 and RMSE are<25 in the regular and validation period.The model generally meets the evaluation requirements of simulation and is suitable for Nanxiaohegou watershed,which can be used to simulate the hydrological process of the watershed.(2)From 1966 to 2020,the annual average green water volume in the Nanxiaohegou watershed is 504.79mm,accounting for about 90%of the water resources.Green water resources are abundant,including 402.11mm green water flow and 102.68mm green water storage.The change trend of green water flow and green water storage is basically consistent with that of rainfall.The fluctuation of green water flow is large and the green water storage tends to be stable,stabilizing at about 110 mm.The annual distribution is extremely uneven,mainly in May to September,accounting for 70.29%of the total green water resources.(3)The spatial distribution of precipitation,green water flow and green water storage is not uniform in the Nanxiaohegou watershed.The precipitation is abundant in the center,but small near the water distribution line.The rainfall in the sub-watershed is 102.75mm～115.81mm.The distribution of green water flow and green water storage shows a decreasing trend from southwest to northeast,upstream to downstream,and trunk to tributaries.Green water flow in sub-watershed ranged from 93.38mm to 130.79mm,and green water storage ranged from 36.03mm to 94.72mm.The contradiction between supply and demand of precipitation and green water resources is prominent in the molecular watershed.The phenomena of "high precipitation,low green water flow and low green water storage" exists in sub-watershed 1,2,5,6,7 and 8,which is necessary to attention to the occurrence of soil and water loss.The phenomena of "low precipitation,high green water flow and low green water storage" exist in sub-basins of 21,24 and 27.which is necessary to pay attention to the occurrence of soil desiccation.Irrigation measures should be properly implemented to alleviate the water potential difference between shallow and deep layers and form a soil dry layer that is difficult to reverse.(4)From 1985 to 2020,the main land use in the basin is cropland and grassland in the Nanxiaohegou watershed,Cropland accounts for more than 55%of the total area,and the area decreases year by year.Grassland accounts for about 30%of the total area.Secondly is woodland and impervious land,the area are increaseing year by year.The area of water is the least,and the area has no change over the years.In addition,some bare land appeared in the watershed,and the area increased dramatically.From 2012 to 2020,the conversion of land use in the Nanxiaohegou watershed mainly occurred between cropland,woodland and grassland.The conversion ratio between cropland and grassland was the largest,with the conversion ratio of cropland and grassland was 0.718 km~2 and grassland to cropland being 1.337km~2.Grassland is the main source of conversion of woodland and bare land,grassland and cropland are the main sources of impervious land.(5)The green water flow of each land use type(except water area)in Nanxiaohegou watershed was woodland>garden land>meadowland>high coverage land>cropland>bare land>impervious land.The green water flow generated by woodland is 478.02mm,accounting for 11.45%.Green water storage was cropland>grassland with medium coverage>woodland>high coverage land>bare land>garden land>impervious land.Green water storage was 159.35mm in cropland,accounting for 14.25%.Since the green water flow generated by woodland is larger than that of grassland,and the consumption of green water storage in garden land and high coverage land is higher.Therefore,the site conditions and characteristics of trees and grass need to be considered for the introduction and selection of vegetation in the process of water and soil conservation.The appropriate tree,shrub,grass and planting density should be selected based on the distribution of regional water resources,topographic and geomorphic conditions,distribution regularity of vegetation zones,bioecology and community characteristics of tree species.(6)According to the national policy and the development rule of land use type,the slope of Nanxiaohegou watershed is superimposed with the land use type and divided into 4 scenario models.At present,the area of cropland with slope over 15 degrees in Nanxiaohegou watershed is 1.633km~2,which can be returned to forest and grass in the later stage according to national policy.Compared with the initial scenario,the grassland produced more green water flow and less green water storage than the cropland.Therefore,grassland has better water retention effect and cropland has better water holding effect.In the high slope area,woodland has better water retention effect and grassland has better water holding effect,but comprehensive consideration of grassland is more ecological benefit than woodland.When high coverage land is converted into meadowland,the ecological benefits will be significantly improved.Therefore,it is not suitable to plant vegetation with high water consumption or high density on the slope,which will increase the ecological vulnerability of the region.Comparing woodland with high coverage land and garden land,woodland has better water retention and holding effect.In order to further promote water and soil conservation and increase woodland cover in the Nanxiaohegou watershed,it is suggested to carry out reasonable returning garden to forest and alfalfa to forest,which is conducive to the further improvement of regional ecological environment quality.