Effects Of Landuse/Climate Variability On The Runoff And Sediment In Qingshuihe Watershed On The Loess Plateau, West Shanxi Province, China

The decreasing Yellow River streamflow during the past fifty years has been attributed to land use/land cover change and climate variability in addition to the uptake by water projects.Large scale re-vegetation based soil and water conservation deployed to combat the severe soil erosion since 50s of last century on the Loess Plateau of China have resulted in the dramatic change of land use and land cover during the past six decades or so.On the other hand,the region is considered one of the most sensitive aera for globle climate variability.Therefore,it is critically important to understand and predict the coupling effects of land use and cliamate variability on the runoff and sediment characteristics for integrated watershed management and ecological restoration.The study examined the trend of annual streamflow,sediment,precipitation,temperature and PET and their change points for a typical mesoscale Qingshui River watershed located on the Loess Plateau by using non-parametric Mann-Kendall test,Moving t-test technique,and hopped parameter analysis.A simple water balance and mass balance based methodology was used to separate the landuse change effects on the runoff and sediment yields from that of climate variability.To predict the future land use/land cover change effects on the runoff and sediment yield,the calibrated and validated at both annual and monthly scales SWAT was used to analyze the potential effects of land use/ land cover scenarios on the sediment and streamflow.The ultimate objective of model excercises was to provide the sound theoretical basis for vegetation restoration and management aiming at optimized configuration of vegetation across the watershed to improve the soil erosion control while reduced water use. Based on the land use data and the land use map which interpretated by aerial photo and TM images,the main transfer characters of land use/land cover is woodland increased,shrubland increased, grassland decreased,cropland decreased,orchard increased and urban rapidly increased.In the basin broed-leaved forests and coniferous forests are located in the high-altitude,shadow slope and bottom valley with slope of 20～40°;cropland and orchard are located in the low-altitude of gentle and sunny slope;shrubland and grassland are located in the altitude of 850～1550m.By using fragstats software analysis,the number of landscape patches decreased,the average area of patches increased and AWMSI decreased,which indicated that the fragmentation of landscape become lower,the landscape connectivity become better and the landscape heterogeneity become weaken.On the side of landscape diversity,the dominance of original dominance landscape reduced and the distribution of various patches were apt to homogenization.The results of MK trend analysis and change point test showed that precipitation and extreme precipitation had no significant trend,;while mean temperature,maximum temperature and minimum temperature exhibited significant increasing trend,the change points occurred in 1997,1994 and 1998. There was a significant upward trend in PET and its change point was 1997.Both runoff and sediment discharge exhibited downward trend and they shared the same change point of 1980.Based on the principle of water balance,it was estimated that climatic variables accounted for 48.17%of the reduction in mean annual streamflow,and the other 51.83%contribution to the change of land-use. Then by using USLE,it was found that precipitation contributed 9.89%to the sediment discharge relief and the contribution of land-use was 90.11%.Among the land-use changes,the woodland's change play an important role,so the increase of forest area appeared to be the main cause of the reduced streamflow and sediment discharge.The simulated results by SWAT model showed that:in the calibration and verifcation periods the efficient coefficient Ens of the average runoff and sediment discharge were over 0.5,the relative error were less than 20%and R~2 reached more than 0.7,which indicated that the model had good applicability in Qingshui River Watershed.According to the distribution of the precipitation,depth of runoff and sediment discharge,it was known that precipitation effected runoff and sediment discharge on both direct and indirect,the indirect showed the effects on the distribution of vegetation.On the other hand terrain factors played an important role especially in the slope,runoff and sediment discharge of sub-basins with the steep slope were higher than others.Uncertainty analysis results showed that runoff and sediment discharge were significantly in response to the resolution of the DEM, and exhibited increasing trend with the DEM resolution increased.The order of runoff calculated by different PET method was Priestley-Taylor＞Penmon＞Hargreaves.The runoff followed the order of shrubland＞grassland＞broad-leaved woodland＞orchard＞coniferous woodland under the extreme scenarios.Compared with the original landuse type,the shrub land and grassland made the runoff increase,and the rest of scenarios led runoff reduce.With different scenarios sediment discharge followed by grassland＞shrubland＞broad-leaved woodland＞orchard＞coniferous woodland,among the scenarios grassland made sediment diacharge increase while the others made sediment discharge reduce.Along with the forest cover increased,the runoff and sediment discharge decreased;meanwhile the sediment discharge had the maximum rate of descent with the forest coverage rate between 40-70%.For efficient using of water resources and reducing the soil erosion,the optimal landuse scenario was designed according to the simulated results of extreme landuse sceniaros and the characters of landuse with differential terrain.The simulated results of runoff and sediment diacharge with optimal scenario showed that:the depth of runoff and sediment modulus decrease16.34%and 94.71%.It seemed that the optimal scenario played an active role in efficient use water resources and lessen soil erosion.At the same time,the spatial distribution of runoff of optimal scenario had no change,but the spatial distribution of sediment modulus had a great change that high-value areas was from concentration to scatter which illustrated that the sensitivity of the factors affected sediment had changed,the sensitivity of vegetation was decreased and the sensitivity of precipitation and runoff was increased,while the terrain factor is still the most sensitive factor.