Vegetation Restoration And Its Impact On Hydrological Process In Typical Areas Of The Middle Loess Plateau

Land cover has played a key role on the hydrological process of watershed.Therefore,understanding the hydrological response to land use change can facilitate the development of sustainable water resources management.Since the end of the 1990s,large-scale ecological restoration projects have been implemented in the Loess Plateau,which significantly changed land use types and land cover.These projects enhanced ecosystem service functions and improved soil and water loss to a certain extent.More and more attention has been paid to the impact of these projects on the hydrological processes.In order to explore the problem of the unclear relationship between the changes of key hydrological elements and their relationships after Grain for Green Project(GFGP),the Yanhe River Basin(YRB),with a semi-humid climate in the middle of the Loess Plateau,was taken as an example.The Community Land Model(CLM)version 5 was used to simulate the impact of land cover changes on key hydrological elements and their relationships in this basin over 1980 through 2018.Firstly,in order to truly represent the characteristics of the underlying surface,a set of data collected from different sources are used to make the soil thickness and river channel distribution of Loess Plateau as the surface dataset input of CLM5.0.Secondly,a set of land use data set from two periods before and after GFGP are used to make the plant functional type as the surface dataset input of CLM5.0.Thirdly,the soil layers were divided according to soil thickness,and soil evaporation parameterization were improved according to actual evaporation status.Finally,based on the new surface dataset,the hydrological processes in YRB were simulated with the modified CLM5.0 under the different land covers.The results show that:(1)Significant changes have taken place in the land cover of the YRB.Grassland and cropland were distributed in a large area in the basin before and after the implementation of GFGP,although the proportion of this two land cover types in the total area of the basin showed a decreasing trend.The transfer of land cover types mainly occurred between woodland,cropland and grassland.After the implement of GFGP,52.40%and 5.77%of the cropland area were converted into grassland and woodland,resulting in an increase of the area of grassland and woodland.The change of grassland,cropland,woodland and shrub areas are affected by both natural and human factors,while the change of water bodies and urban areas is mainly affected by human factors.The change of land cover has a profound influence on the hydrological process of the YRB.(2)The modified CLM has more flexible and efficient features,and its improved model parameters and physical processes are representative in YRB.The construction of real soil thickness and channel distribution information reduced the simulation errors of the model in simulating the hydrological process,and the R~2 between the simulated and observed total runoff increased from 0.104 to 0.55,and the RMSE decreased from 14.5 mm to 12.61 mm.We tested four soil layering methods and found that CLM5.0 with at least 40 soil layers could produce rational simulations for both runoff and the vertical soil moisture profile.we changed the critical water content for dry surface layer formation to the field capacity to conduct new simulation.When compared for the simulations in the last section,R~2,RMSE and NSE between the simulated and observed total runoff changed to 0.61,2.1 mm and 0.54 respectively.The construction of surface data based on the true underlying surface characteristics and the improved CLM5.0 model can better describe the actual hydrological cycle process.(3)Land cover changes have led to a significant decrease in annual runoff and runoff coefficients of YRB.The fluctuation trend of annual runoff and precipitation is basically the same,but the variation of annual runoff is obviously greater than that of annual precipitation.After the implement of GFGP,the annual runoff decreased by 12.34%,and the annual runoff coefficient decreased by 11.46%of YRB.Through linear regression analysis,the contribution rates of annual precipitation to runoff change before and after the implementation of GFGP were 92.2%and 90.5%.In general,precipitation is still the dominant factor of runoff change.With the increase of human activities,its influence on runoff change has begun to become prominent.(4)Land cover changes have led to a significant decrease in monthly runoff and runoff coefficients of YRB,which has less impact on runoff in wet seasons than in dry seasons,and centralizes the distribution area of annual runoff.After the implement of GFGP,the average decrease rate of runoff and runoff coefficient was 8.72%in the wet season,and 29.5%in the dry season.Through linear regression analysis,precipitation and land cover change can explain 92.66%and 7.34%before GFGP.After the implement of GFGP,the contribution rate of precipitation to runoff change decreased by 0.63,the contribution rate of land cover change to runoff change increased by 0.66,and the contribution rate of dry season is significantly higher than that in wet season.As the largest area covered by continuous loess soils in the world,the Loess Plateau in China has complicated hydrological processes because of its extremely complex topography and unique soil types.A better understanding of the hydrological processes within the complex terrain and special soil types of the Loess Plateau is vital to improving the prediction of water resources in this region.In this study,the YRB was taken as an example,and multiple factors from vegetation to bedrock were comprehensively considered to construct a more realistic underlying surface data set.The improved model parameters and physical processes enhance the applicability of the CLM5.0 model in the YRB.The results of this study clarify the impact of land cover change on the hydrological process,and deepened the understanding of the impact of land cover change on the hydrological process in the area with deep soil and complex topography.To guide the rational use of the water resource and build a harmonious ecological environment in the Loess Plateau,understanding the hydrological process and its changing has a great theoretical value and practical significance.