| The Loess Plate.au Gully Region of China has long been considered a very fragile area that suffers from severe drought and water erosion.With the proceeding of global climate fluctuation and regional implementation of soil and water conservation measures such as returning farmland to forests and grasslands,it is of great significance to effectively reveal the eco-hydrological response to land use change and climate variation.In this study,a typical watershed of the Loess Plateau Gully Region-Yanwachuan was taken as the study area and the theory of blue-green water in eco-hydrology was selected as theoretical basis.Based on the analysis of land use change and climate variation from 1981 to 2016 and the respective prediction of future change trend,the influence of changing environment on eco-hydrological process was discussed by using traditional hydrological analysis method.Furthermore,distributed hydrological model MIKE SHE and soil erosion prediction model RUSLE were used to simulate the eco-hydrological process in Yanwachuan basin,and to predict the eco-hydrological process under the changing environment in the future period,therefore,the response mechanism of eco-hydrological process to land use change and climate variation was further revealed.The main conclusions are as follows:(1)In different land use periods,the area of cultivated land in Yanwachuan basin was the largest,followed by grassland,and the area of water area was the smallest.As a whole,land use in Yanwachuan basin has changed greatly,mainly reflecting in the reduction of grassland and residential land and the increase of forest land.The land use quantity and spatial change was very small during 1981-1995,and then became relatively obvious during 1995-2000,however,the land use change reached a climax during 2000-2016.The prediction results of CA-Markov model showed that the order of land use area in different periods in the future was:cultivated land>grassland>forest land>residential lan.d>water area.Forest land was still the fastest growing land use type.Furthermore,the cultivated land would reduce while grassland area would increase compared with 2010.These were all related to the land use driving forces of the ecological restoration measures.(2)The average annual precipitation in Yanwachuan basin was 535.8 mm during 1981-2016,presenting a non-significant increasing trend with an average annual rate of 1.351 mm,and there was no sudden change.The annual average daily temperature,daily maximum temperature and daily minimum temperature were 9.3,14.4 and 5.4 ℃,respectively.These three factors all showed a very significant increasing trend(p=0.01),and their mutation points occurred in 1997,1994 and 1997,respectively.In addition,the average annual potential evapotranspiration was 1172.1 mm,showing a very significant increasing trend(p=0.01)with an average annual rate of 4.121 mm,and the mutation point occurred in 1993.Based on the Pearson correlation analysis results,the increase of temperature and the decrease of relative humidity were the main factors contributing to the significant increase of potential evapotranspiration in Yanwachuan basin.The forecast results of future precipitation and potential evapotranspiration showed that the maximum precipitation period occurred in 2030s,followed by 2040s,and the minimum precipitation period was 2020s;the maximum potential evapotranspiration period occurred in 2040s,followed by 2020s,and the minimum period was 2030s.However,the precipitation and potential evapotranspiration in the future period would increase compared with the mean values of 1980-2009 regardless of climate scenarios.(3)The annual average green water,blue water and sediment transport in Yanwachuan basin during 1981-2016 were 514.2 mm,21.6 mm and 1619.3 t km-2,respectively.There was no significant increase in the annual green water(1.865 mm·a-1),while the annual blue water(-0.118mm·a-1)and sediment transport(-25.825 t km-2·a-1)exhibited a significant decreasing trend(p=0.05).In addition,the results of mutation test showed that there was no mutation in annual green water,while the abrupt change in annual blue water and sediment transport both occurred in 1998.The contribution rates of precipitation variation,potential evapotranspiration variation and underlying surface change to the increase of green water were 65.9%,13.4%and 20.7%,respectively,while the corresponding contribution rates to the reduction of blue water were-39.2%,54.5%and 84.7%,respectively.Furthermore,the contribution rates of climate variation and land use change on sediment transport reduction were 12.8%and 87.2%,respectively.It was clear that precipitation variation was the main factor causing the increase of green water,while the influence of land use change played a key role on blue water and sediment transport reduction.(4)MIKE SHE model could reproduce the dynamic change of daily runoff in Yanwachuan basin.According to the results of model adaptability evaluation,the correlation coefficient and Nash efficiency coefficient of the MIKE SHE model during base period were 0.836 and 0.629,respectively,and the corresponding coefficients during change period were 0.781 and 0.589.According to the output results of MIKE SHE model,the average annual green water in Yanwachuan basin was 509.2 mm.The variables of vegetation transpiration,soil evaporation,vegetation interception and soil water storage were 227.1、252.3、28.4和 1.4 mm,respectively.Green water resources could meet the water demand of vegetation’s basic survival under the restriction of minimum ecological water demand.Under the restriction of suitable ecological water demand,green water resources could basically maintain the normal growth of grassland and cultivated land,but it could not meet the water demand of normal growth of forest vegetation(the average profit and loss was-23.7%).(5)The average annual soil erosion modulus was 3543.7 t km-2,presenting a significant decreasing trend(p=0.05)with an average annual rate of 47.928 t·km-2.Compared with the base period,soil erosion in the change period varied from higher erosion intensity to lower erosion intensity.However,there may be some limitations in areas where erosion was very intense such as gully areas.Grassland had the largest proportion of soil erosion in all land use types,and the largest slope of soil erosion occurred during 15-25°.Therefore,the grassland and 15-250°slope area should be the key area for soil erosion control in the future.In addition,land use change and climate variation contributed 33.2%and 66.8%to the reduction of soil erosion,respectively,which indicated that climate variation played a leading role in the reduction of soil erosion.However,this conclusion was based on ignoring the impacts of engineering measures such as reservoirs and silt dams,the contribution rate of land use change to the soil erosion reduction may be reduced.(6)Green water resources could meet the basic water demand of vegetation survival under the restriction of minimum ecological water demand in the future.And the green water resources could basically maintain the normal growth of grassland and cultivated land(water shortages would occur mainly in April,May and June)under the restriction of suitable ecological water demand,however,it could not meet the water demand of normal growth of forest vegetation at all.Ecological water shortage would occur in forest land in most months.The profit and loss of green water in 2020s,2030s and 2040s could reach-21.3%,-15.9%and-19.3%in RCP8.5 scenario,respectively.The future soil erosion response laws in RCP 4.5 and RCP 8.5 scenarios were similar.Taking RCP 4.5 scenario as an example,the average soil erosion modulus of Yanwachuan basin in 2020s,2030s and 2040s were 3701.0 t·km-2,3930.2 t·km-2 and 3780.6 t·km-2,respectively.Compared with the current average soil erosion modulus(2251.5 t·km-2)during 2006-2016,the average soil erosion modulus increased by 64.4%,74.6%and 67.9%,respectively.Although watershed management would continue to reduce the intensity of soil erosion in the future,the intensity of soil erosion in Yanwachuan basin showed an obvious increasing trend under the effects of precipitation changes. |
PDF Full Text Request