Building Model Capability in Simulating Spatiotemporal Variations of Hydrology and Nitrogen Loading across Scales

Abstract:

Prairie potholes are shallow wetland basins that act like sponges, absorbing excess rainfall and releasing water slowly into nearby lakes and rivers. To meet the growing demands for food and biofuel, around 95% of the prairie potholes have been drained and converted into agricultural land, which substantially altered the hydrology and nitrogen (N) cycling in this area. Studies show that the N flowing out of Iowa contributes to near 50% of N entering the Gulf of Mexico in recent years. Process-based modeling is a powerful tool to study the comprehensive interactions between biogeochemical and hydrologic processes in a watershed. However, previous modelling work has limited performance in the prairie pothole region with a tile drainage system because monitoring data were mostly collected at field level or large watershed scale, and we have inadequate knowledge to inform models between these two scales. Here, we choose a representative catchment and a HUC-12 watershed in Iowa and run a process-based land ecosystem model to examine model performance against long-term monitoring data. The initial simulated discharge mismatched some peaks in winter and underestimated the base flow compared to the measurements. We have developed new features in the model to better represent snow dynamics, surface and subsurface water flows, and related N biogeochemical processes in tile-drained agricultural watersheds and areas with potholes. The results showed that these updates largely improved the modeling accuracy in tracking magnitude and variations of water flow and in-flow N concentrations. The introduced pothole mechanism and tile drainage system removed small peaks by temporally storing the water and increased subsurface flow through tile lines. We also found the simulated N loading is better than before. Our study indicates the importance of representing unique landscape composition and human-engineered projects that alter water flow and N movement for accurate modeling estimations.