Spatiotemporal responses of runoff to land use change in Wei River Basin

Change in land use impacts the runoff availability to a large extent, which in turn, influences the surface and groundwater characteristics in a drainage area, and hence easily leads to the occurrence of droughts or floods in the basin.Therefore, it is essential to study the runoff response to land use change of a drainage basin on sub-basin level in particular, which has significant importance for planning land use patterns rationally and controlling watershed extreme hydrological events effectively.The Wei River basin (WRB), a typical arid and semi-arid region in northwest China, was selected as the study area and divided into 5 sub-basins.The main goal of current study was to investigate the spatiotemporal runoff responses to land use change in the WRB at sub-basin level.The change characteristics of land use in 1985, 1995 and 2005 in WRB were firstly analyzed with the transformation matrix.The Soil and Water Assessment Tool (SWAT) was calibrated for a baseline period of 1978-1986.Based on the results of land use change, 5 land use change scenarios were set in this paper.Then the calibrated SWAT model was run under these land use change scenarios by holding other data (e.g., meteorological and soil data) constant.The spatial runoff responses to land use change at both yearly and seasonal scales were analyzed based on the SWAT outputs.The results showed that: 1) The SWAT model had suitable application in the WRB with R2 of higher than 0.6, Nash-Suttcliffe (Ens) of higher than 0.5 and relative error (Re) of lower than 20%, which indicated that the SWAT model could be used to study the impact of land use change on runoff variation in the WRB; 2) The main land use types of WRB were farmland, woodland and grassland which accounted for 98% of the whole basin area with high transfer frequency in space, while their area change percentages were all less than 10%; the total area of other land use types was small, but their change rates were all approximate to 40% and they were mainly transformed into cultivated land; 3) The contribution of land use change to runoff variation on decade scale in 5 subzones was less than 15%; the magnitudes of yearly discharge in 3 extreme land use scenarios in Beiluo basin were woodland > farmland > grassland, and farmland > grassland > woodland in other 4 basins; 4) For the conversion of farmland to woodland scenario, the yearly runoff in Beiluo basin increased while that in other 4 sub-basins decreased, and the conversion’s impacts on the yearly runoff in the mainstream were greater than that in the tributary; for the conversion of farmland to grassland scenario, the yearly runoff in 5 sub-basins was all decreased, and the yearly runoff in the middle and lower reaches of the mainstream as well as the Jing River basin among the tributaries was decreased more significantly than other sub-basins; 5) Generally, the response degrees of seasonal runoff in the conversion of farmland to woodland scenario were higher than that in the conversion of farmland to grassland scenario; compared to other 3 kinds of seasonal runoff (i.e.spring, summer and autumn runoff), the winter runoff showed the most sensitive variation in both scenarios; the winter runoff decreased in the Jing River basin but increased in other basins in the scenario of returning farmland to forest, while it decreased in the tributary but increased in the mainstream in the scenario of returning farmland to grassland.The research results can play extremely important roles on the improvement of the ecological environment and the control of hydrological extreme drought events, and can also help to provide valuable reference for the watershed land use pattern’s planning and integrated management in WRB.