Abstract. Water resources, which are substantially affected by land use/land cover (LULC) and climate changes, are a key limiting factor for ecosystems in arid and semi-arid regions exhibiting high vulnerability. It is crucial to assess the impact of LULC and climate changes on water resources in these areas. However, conflicting results on the effect of the LULC and climate changes on runoff have been reported for relatively large basins, e.g., in the Jinghe River Basin (JRB), a typical large catchment (> 45000 km2) located in a semi-humid and arid transition zone on the central Loess Plateau, Northwest China. In this study, we focused on quantifying both the combined and isolated impacts of LULC and climate changes on surface runoff. It is hypothesized that under climatic warming and drying conditions, LULC change, which is primarily caused by intensive human activities, such as the conversion of cropland to forest and grassland program (CCFGP), will alter runoff markedly in the JRB. The Soil and Water Assessment Tool (SWAT) was adopted to perform simulations. The simulated results indicated that although runoff increased very little between the 1970s and the 2000s due to the combined effects of LULC and climate changes, LULC and climate changes affected surface runoff differently in each decade, i.e., runoff increased with elevated precipitation between the 1970s and the 1980s (precipitation contributed 88 % to the increased runoff). Thereafter, runoff decreased and became increasingly influenced by LULC change, with a 44 % contribution between the 1980s and the 1990s and a 71 % contribution between the 1990s and the 2000s. Our findings revealed that large-scale LULC under the CCFGP since the late 1990s has had an important effect on the hydrological cycle and that the conflicting findings on the effect of the LULC and climate changes on runoff in relatively large basins are likely caused by uncertainty in hydrological simulations.
Effect of land use/land cover and climate changes on surface runoff in a semi-humid and semi-arid transition zone in Northwest China