Assessing the potential impacts of climate and land use change on water fluxes and sediment transport in a loosely coupled system

Climate and land use change are the two primary factors that affect different components of hydrological cycle as well as sediment transport in the watershed. Quantifying potential impact of these two stressors enables decision makers to formulate better water resource management strategies to adapt to the changing environment. To that end, we have developed an integrated modeling framework employing an Agent-based approach to simulate land use conversion that then serves as input to the Soil and Water Assessment tool (SWAT) in a loosely coupled fashion. The modeling framework was tested on the Neshanic River Watershed (NRW), 142 km² area in central New Jersey that contains mix of urban, agricultural and forested lands. An ensemble of 10 different global climate models (GCMs) for two different greenhouse gas emission scenarios including representative concentration pathways-4.5 and 8.5 (RCP-4.5 and 8.5) were employed to model future climate from 2020 to 2045. Land use conversion for 2040 was developed based on six driving factors including distance to residential lands, agricultural lands, roads, streams, train stations, and forest using three land use transition potential models and further, the best transition potential model accompanied with some local land use restrictions. The study evaluated various components of hydrological cycle and sediment transport for the three different scenarios one-at-a-time including climate change alone, land use change alone, and combined climate and land use change. Results indicate that the changing climate will have a larger effect on the hydrologic cycle than intensifying urban land uses in the study watershed. The climate change scenarios, either alone or in composite with land use change, predict higher streamflow (32% and 36% increase over baseline, respectively), overriding the effect of land use change which predicts a decline of 5% in streamflow. The increase in streamflow results in an increase in sediment loading, presumably due to an increase stream downcutting. Conversely, the effect of land use change (in this case the conversion of agricultural land to low density residential uses), is predicted to decrease sediment load. When modelled in composite, the effect of changing land use (in this case the conversion of erodible agricultural fields to suburban development) appears to override the adverse effect of climate change, enhancing watershed resiliency by reducing sediment load and thereby improving health of the downstream aquatic ecosystems.