Designing Watersheds for Integrated Development (DWID): Combining hydrological and economic modeling for optimizing land use change to meet water quality regulations

By combining information on nutrient output from the Soil & Water Assessment Tool (SWAT) and secondary data on local profits from different crop types, we devise a profit maximization problem subject to dynamic water quality constraints, which become gradually more restrictive over time. The solution aims to detect the optimal allocation of land parcels by crop type that maximizes the total net present value of landowner profits throughout the watershed. Over a nine-year time span, our model construct is applied to the Little River Experimental Watershed (LREW) in South Georgia. Water quality constraints involve the landowner adhering to specific permittable limits on numeric nutrient criteria recorded at the watershed outlet under various scenarios, including i) NO₃–N constraints, ii) total phosphorus (P) constraints, and iii) concurrent NO₃–N and P constraints. In the most extreme case, a reduction in aggregate profits of $24.1 million and $8.1 million was observed for combined NO₃– N and P constraints relative to commensurate solo constraints on NO₃–N and P, respectively. The Designing Watersheds for Integrated Development (DWID) model could support policymaking for ascertaining trade-offs between economics and water quality channelized through direct and indirect land use change considering environmental regulations in Georgia and beyond.