Spatial Downscaling Global Land Use and Land Cover (LULC) Change for Modeling Regional Water Management

Abstract:

Projected growth in both population and income globally will require changes in LULC to meet humanities’ need for food and fuel. There is frequently a need to downscale global LULC trends to smaller regions and at higher grid resolution to conduct impact studies on local areas of interest. The aim of LULC downscaling is to retain the impact of global drivers such as international food and fuel markets in determining net land use change while capturing factors salient to managing local resources in the spatial disaggregation process. In this paper we use a downscaling methodology called Demeter to downscale RCP 4.5 (aggressive mitigation) and RCP 8.5 (minimal mitigation) LULC change scenarios from the Global Change Assessment Model (GCAM). Both these climate mitigation scenarios project a large increase in biofuel production, entailing an increase irrigation demands. Water demands for biofuel crops may place additional stress on regional water supplies, especially when coupled with future impacts of climate to streamflow. The setting of our case study is the Columbia River Basin (CRB), where water resources are intensively managed for flood control, irrigation, hydropower production, and instream flow. Following downscaling of LULC change scenarios, we use VIC-CropSyst, a tightly coupled hydrology and cropping system model, and the RColSim reservoir management model to demonstrate tradeoffs in the CRB between irrigation, hydropower generation, and fish migration. We consider three levels of expansion in irrigated acreage for greater biofuel and food crop yields and evaluate impacts to those two other water uses. The scenario of maximum water rights expansion, which assumes all land suitable for irrigation has a water right, resulted in a 150% increase in irrigated acreage by 2060 compared to irrigated acreage in the same year under current water right conditions. The benefit of higher crop yields with greater irrigation was offset by reduced streamflow for the environment and hydropower generation. The projected large increase in biofuel production will bring a change in the composition of renewable energy as well as changes to water demands in the region. The methodology presented in this paper is applicable to other case studies focused on the impact of global LULC change on regional watershed management.