Impacts Of Cascade Dam Construction On Land Use And Hydrology In A Small-scale Watershed

Dams and reservoirs are typical countermeasures for energy demand and disaster prevention.The construction of dam brings values to human's life,but also causes negative impacts.Dam may reduce the connectivity between different river sub-basins,cause stream flow impairment,destroy the composition and function of the ecosystem and lead to changes in land use pattern.Numerous studies have discussed the impact of dam on land use,hydrology,water quality and ecological environment security.However,these studies reveal the ecological effect of large dam on large-scale watersheds.The impact of small dam on small watersheds that is more widespread in China is poorly studied.This study explores the effects of dam construction on land use and hydrology in the upper Yashi creek watershed,a small-scale watershed with cascade dams which is composed of Su creek,Xiao creek and Longmen creek.Geospatial technology,watershed modeling,Indicators of Hydrological Alteration(IHA)and Range Variability Approach(RVA)analysis method are used for analyzing the changes in land use and hydrological regimes induced by cascade dam construction.The main findings are as follows:Aided by remote sensing and geographic information system,we proposed a viable method including image fusion,water extraction,unsupervised classification,and intensity analysis,which facilitates us to identify the watershed land use changes induced by dam construction.Through intensity analysis,it is found that during the period of construction and after the construction of the dams,the land use in the upper Yashi creek watershed has changed vigorously.The change of land use is mainly due to the strong increase of water and built-up and the decrease of woodland and farmland.The bare land has a steady increase in the construction period.Woodland and farmland are mainly flooded or transformed to water bodies.SWAT model was further used to look into the effects of damming on hydrologic alterations.Through scenario analysis,this study uncovered that reservoir can effectively reduce the annual stream flow of small scale watershed.The peak of extreme water conditions,the volume of stream flow during the flood season and normal water level period is reduced by dams.Dams can also postpone the drop of water in extreme water conditions.The effect of cascade dams on small watershed is remarkable,cause stream flow impairment.The results derived from IHA show that stream flow was significantly reduced in July and August after the construction of cascade dams in the upper Yanshi creek watershed.The arrival time of extremely high and low runoff become earlier.The frequency of high and low pulses of stream flow is reduced.The number of rising and falling times increase.Comparing the results of the RVA method and the minimum discharge method,it is found that the runoff may be lower than the minimum discharge standard in the low water period.The construction of dams brings risks of ecological environment safety in the upper Yanshi creek watershed.