| The Loess Plateau(LP)region is one of the most serious areas of soil erosion in China.In order to control severe soil erosion,the Chinese government has implemented a series of ecological restoration measures in the region.These ecological restoration measures have resulted in significant land use/cover changes that have profoundly affected the hydrological processes of the LP.Evaluating and predicting the response of hydrological processes to land use/cover change is important for water resources management,especially in arid and semi-arid regions.This paper takes the Beiluo River Basin,a typical watershed on the LP where large-scale ecological construction has been implemented,as the object of study.Based on the analysis of climate and land use/cover changes in the watershed,the stability and long-term variability of streamflow components were analyzed using watershed comparison,ANOVA and Mann-Kendall trend test.We applied the similar habitat method and the principle of suitable trees in the area to simulate the future vegetation restoration potential and suitable areas for afforestation in the vegetation transition zone,and to predict the land use/cover changes and the their efffcts on hydrological processes caused by the future ecological restoration plan.Considering the complex and nonlinear characteristics of the hydrological processes in the basin,we try to use the Geodetector model to explore the potential influence mechanisms of changing environmental elements on runoff distribution,and to provide important references for future ecological restoration and water resources management on the Loess Plateau.The main conclusions are as follows:(1)Climate and land use/cover changes in the Beiluohe River Basin over the last60 years were analyzed.The annual average rainfall and potential evapotranspiration in the Beiluo River Basin from 1958 to 2019 are 570.22 and 984.46 mm,respectively,and the rainfall is spatially increasing from northwest to southeast,but the overall variation of rainfall is moderate,with a non-significant decrease in annual rainfall at a rate of-0.76mm/a.The high value of potential evapotranspiration is located in the middle reaches of Huangling and Zhuangtou,and the low value is in the upper reaches of Beiluo River Basin,while the potential evapotranspiration from 1958 to 2019 is weakly variable,with a nonsignificant increasing trend at the rate of 0.29mm/a.During 1970-2020,the land use change is significant,especially since the 1990 s,the main types of changes are the conversion of cropland to forestland and grassland,and the locations of the changes are mainly located in Wuqi,Zhidan and Liujiahe region in the upper reaches of the Beiluo River Basin and Zhangcunyi,Jiaokouhe and Zhauangtou region in the middle reaches.In addition,the spatial distribution of vegetation cover varied significantly from 1980 to 2020,among which the vegetation cover in the upper reaches of Beiluo River Basin was lower in Wuqi,Zhidan and Liujiahe region,with 34.27%,47.56% and 51.62%,respectively,while the vegetation cover in the middle reaches of Zhangcunyi,Huangling,Jiaokouhe and Zhuangtou region was the highest,with annual average vegetation cover of 74.68%,81.93% and65.72%,respectively.The vegetation cover in the upper and middle reaches of the watershed has increased significantly since the beginning of "Grain for Green" program.In contrast,the vegetation cover of the plain area in the downstream was relatively stable from 1980 to 2020,which was about 55.06%.(2)The stability and variability of runoff and its components under ecological restoration in the Beiluo River Basin were studied at multiple time scales.Mean annual streamflow totals for the ~60-year study period were 25.07?34.22 and 21.55?48.02mm/a for the three less forested watersheds(LFWs)and two forested watersheds(FWs),respectively.Average contributions of baseflow to total streamflow were 32?44% for the LFWs and 58?61% for the FWs.M-K tests showed significant decreasing trends for annual streamflow and stormflow(0.23?0.54%/a)from the LFWs throughout.Mean streamflow from the LFWs between 2000 and 2019 decreased by 58%compared to the pre-2000 period,while the average contribution of stormflow decreased from 66% to 35%(2010s).However,winter baseflow increased with time since start of restoration.Conversely,streamflow(components)for the FWs showed only slight fluctuations in decadal,annual and seasonal trends.Both total streamflow and stormflow exhibited strong power relationships with vegetation coverage,illustrating a tendency towards stable flow regimes for vegetation coverage >60-70%.(3)Future land use/cover changes and their effects on runoff processes in the Beiluo River Basin were simulated.The maximum recoverable vegetation cover of the hilly-gully area(Liujiahe watershed)in the Beiluo River Basin was 61.90%,where still 9.29% potential for the vegetation cover and the vegetation of 36.4% of the area were continued to improve in the next 20-30 years.By contrast,the area with the vegetation to be improved was accounted for about 11.5% in the earth-rock mountain area.The maximum recovery potential was achieved in the vegetation cover in the gully and terrace plain areas after20 years of restoration.Therefore,there was no room to improve in the future.The scenarios simulation was conducted to determine the plantation suitability using the principle of "Matching Tree Species with Site".The suitable planting area in the gully and terrace plain area was 279.16 and 233.73km2,respectively.A large area of returned cropland in the hilly-gully area was suitable for the natural restoration.This will result in cropland will decrease by 47.40%,and grassland and forestland will increase by15.77% and 0.71%,respectively,in the hilly-gully area.Similarly,this will also result in cropland will decrease by 22.95%,and grassland and forestland will increase by35.98% and 0.22%,respectively,in the earth-rock mountain area.Using the SWAT model to simulate changes in runoff processes within the Liujiahe watershed where large-scale vegetation restoration has been implemented,the results show that vegetation restoration between the 1980 s and 2020 s has already reduced the annual mean runoff and soil water content by 42.03% and 43.87%,respectively,while evapotranspiration has increased by 12.56%;Land use/land cover changes in the next30 years will further reduce runoff and soil water content by 15.64% and 11.11%respectively,and increase evapotranspiration by 1.13% in this region.(4)Integrated mechanisms of soil,land use/cover,and climate influences on spatio-temporal variation of runoff in the Liujiahe watershed from 1980 to 2020 were identified.The annual average value of soil bulk density in the Liujiahe watershed was 1.20g/cm3,which showed a decreasing trend between 1980 and 2020,with a spatial trend of low in the southeast and high in the northwest,especially since 1990,the soil bulk density in each subwatershed generally decreased,especially in the southeast;while the annual average values of total soil porosity,saturated hydraulic conductivity,soil organic matter and soil water-stable agglomerates The annual average values of total soil porosity,saturated hydraulic conductivity,soil organic matter and soil water-stable aggregates were 51.49%,0.24mm/min,5.74 g/kg and 32.06%,respectively,and the annual average values showed a gradual increase,especially after 1990,but the spatial variation was the opposite of soil capacitance,with a spatial trend of high in the southeast and low in the northwest.The results of the Geodetector model show that the influence of soil hydrological properties such as total porosity,soil saturated hydraulic conductivity and soil organic matter on runoff is significantly greater than that of land use/cover and climate,and is the main driving factor affecting the spatial distribution of runoff in the Liujiahe watershed from 1980 s to 2020 s,and the influence on runoff has been gradually enhanced since the implementation of the "Grain for Green" program in the 2000 s,although the influence of vegetation cover and forest-grassland area ratio and potential evapotranspiration have also gradually enhanced their influence on runoff after reforestation,the influence of soil hydrological properties on runoff has been firmly in the first place.The results by interaction detection also showed that the interaction of soil properties with land use/cover and climatic factors showed bivariate enhancement or nonlinear enhancement,which could significantly enhance the explanatory power of the spatial distribution of runoff.In conclusion,long-term trends in rainfall and potential evapotranspiration from1958-2019 are not significant,but land use/cover changes are drastic,especially after1990.Over 20 years of ecological restoration on the Loess Plateau has changed the streamflow regime primarily by regulating stormflows.The trends and proportions of streamflow components for the LFWs ultimately approached those for the FWs,which suggests that the current vegetation cover in the LFWs is likely to produce a stable hydrological regime in the near future.Nevertheless,there is still there is still 9.29%potential for vegetation cover to increase in the Liujiahe watershed in the upper of the Beiluo River,but the area suitable for suitable for future plantation is limited,and large areas of retreatable land are suitable for natural restoration,while future land use/cover changes will have further impacts on its hydrological processes.In addition,the improvement of soil hydrological properties by ecological restoration becomes the main driver of spatio-temporal variation of runoff in the Liujiahe watershed from 1970 to 2020. |
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