Application Of SWAT Model In Green Water Assessment Of Nanxiaohegou Watershed

The gully region of the Loess Plateau is widely distributed in the middle and south of the Loess Plateau.It is an important grain production base and rain raising agricultural region.Influenced by the temperate continental monsoon climate,the rainfall is relatively low and the spatial and temporal distribution is very uneven,and the shortage of water resources has become the biggest restriction factor for the environment and development of the region.Green water is the primary water source for the soil-plant system in terrestrial ecosystems and plays a vital role in the region.Therefore,it is an urgent problem for regional water resources management to deeply understand the hydrological process of green water,master the formation and transformation mechanism of green water,make rational use of water resources,and put forward scientific suggestions for grain production and ecological restoration in the watershed.In this study,Nanxiaohegou watershed,a typical small watershed in the gully region of the Loess Plateau,is taken as the research object,Analyzed the trend,mutation and periodicity of precipitation and temperature.Establish SWAT model of Nanxiaohegou watershed,Parameter sensitivity analysis and calibration using swat-cup model.The applicability of the model is evaluated by the evaluation indexes and the simulation results of green water.On the basis,the SWAT model completed by calibration is used to assess the blue and green water resources under the future climate change scenario,and predict the future green water change trend.This research contributes to the understanding of the green water cycle and it provides a theoretical basis for ecological and environmental restoration,water and soil conservation,and water resource management in the Loess Plateau Gully Region.The main conclusions are as follows:(1)Based on the analysis of the trend,mutation and period of the annual precipitation and temperature values in 1955-2018 in Nanxiaohegou watershed,the corresponding statistical methods are adopted.It can be seen that the overall precipitation of the watershed shows a downward trend,which is not significant and has no significant mutation in the study period.During the calculation and statistics period,the precipitation of the watershed has three periodic changes:3-8a,9-15a and 18-25a.During the statistical period,the temperature showed an upward trend,and it did have a sudden change in 1996.Since 1997,the temperature showed a significant upward trend,It is obvious to fluctuate of temperature.The periodic changes were mainly divided into 4?8a,12?15a,20?23a and 25?30a.The regularity of annual average temperature periodic changes in the study area was weak.In the future,the maximum precipitation will occur in 2020s,and the temperature will rise gradually in the future.In the next three stages,the maximum temperature will appear in 2040s,and the temperature in RCP8.5 is higher than that in RCP4.5.(2)According to the water balance method,the average green water volume of the basin from 1955 to 2018 is 508 mm,which accounts for 95%of the total precipitation resources.The average potential evapotranspiration from 1955 to 2018 is 920.4mm by using micro meteorology method,and the average annual green water resources of Nanxiaohegou basin is 440.5mm by using Zhang model,accounting for 82.6%of the total precipitation resources of the basin.The change of green water is consistent with the change of precipitation.The calculation result of the micrometeorology method is closer to the actual situation of the basin.(3)In the study,precipitation and temperature data come from Xifeng station,Yangjiagou,dongzhuanggou and eighteenmutai station from 1955 to 2018,and runoff data are from eighteenmutai station.DEM data,land use data and soil data are all from the corresponding national websites.The model divides the basin into 25 subbasins and 217 hydrological response units.(4)When using swat-cup model for parameter sensitivity analysis,21 parameters were selected,it is related to Nanxiaohegou watershed.The results showed that the top three sensitive factors were SCS runoff curve coefficient f(cn2),basic flow ?-factor(ALPHA_BF)and soil evaporation compensation coefficient(ESCO),and the sensitive values were 42.32,36.42 and 26.32.The simulation value of swat-cup has the same change trend as the experimental observation value and the value is similar.Except that the observation value caused by extreme rainfall in some months is far greater than the simulation value,the observation value and simulation value in other months are in the range of 95 PPU,and the simulation value is greater than the observation value in most results.The narrower the 95 PPU range is,the higher the simulation accuracy of the model is.(5)SWAT-CUP model index evaluation mainly selected three,namely the efficiency coefficient Ens,the determination coefficient R2 and the error percentage Re.The evaluation showed that the periodic Ens,R2 and Re are 0.73,0.69 and 9.86%respectively,and the verification period Ens,R2 and Re are 0.76,0.72 and 10.11%respectively,so the parameter calibration results were reliable.The SWAT model completed by calibration is used to simulate the precipitation and green water volume of Nanxiaohegou watershed from 1955 to 2018 According to the statistical analysis,the average annual green water resources of the basin simulated by SWAT model is 431.5mm,accounting for 81.0%of the basin's precipitation resources.The simulation results of SWAT model were close to those of micro meteorology method,and SWAT model was reliable for the green water simulation results of Nanxiaohegou watershed.(6)According to the analysis of blue water resources in the future climate scenario of the basin,the increment in the future three periods under the RCP4.5 scenario is-9.90%,-14.77%and 4.36%respectively.Under the rcp8.5 scenario,the increment of blue water resources in the next three periods is 5.66%,4.68%and-19.40%respectively.In the future,the amount of blue water resources will decrease.(7)It can be seen from the analysis of green water resources in the future climate scenarios of the basin,Under the two climate scenarios,compared with the reference period,green water resources will increase in the next three periods.In 2020s,the increase of green water is the maximum in the next three periods.In 2040s,the increase of green water is the least in the three periods.The green water flow in the basin will increase in the next three periods,but the maximum value-added will occur in 2040s,and the minimum increase will be in 2040s.In the next three periods,the storage of green water will decrease,the lowest is 2030s,the highest is 2040s.