2.3. Simulation of hydrological processes and blue water resources
We used the Agricultural Policy/Environmental eXtender (APEX) model to simulate the basin's hydrological processes. The APEX model (Williams and Izaurralde, 2006) was developed by the Blackland Research and Extension Center (Temple, Texas, USA). It is a flexible, dynamic, and physically based distributed model that can be used to simulate the impacts of land use and management on watersheds. The applicability of the APEX model has been tested for the Huangqihai River Basin in previous studies (Liu et al., 2014 and Wang et al., 2014). The results show that the APEX model is appropriate to simulate the hydrological processes throughout the Huangqihai River Basin.
The basin's BWR is the sum of the surface runoff, lateral subsurface flow, and percolation below the root zone. The APEX model simulates these flows for each sub-basin and for the entire basin on a daily basis. Surface runoff volume was simulated using a modified Soil Conservation Service (SCS) curve number technique described byWilliams (1995), and the lateral subsurface flows and percolation were computed using the storage routing and pipe flow equations of Gassman et al. (2010). We used the average BWR within the simulated period (1982–2011) in this study.
The APEX model required a digital elevation model (DEM), soil data, land cover data, and climate data. The DEM data were obtained at a resolution of 90 m (http://www.gscloud.cn). Soil data were obtained at a resolution of 1 km from the