According to the US EPA, a watershed approach is the most effective framework to address today’s water resource challenges. Watershed-scale computer models are used to develop total water quality standards and evaluate landuse scenarios to meet the standards in the US under the Total Maximum Daily Load program and in Europe under the Water Framework Directive. This oral session (with a supporting poster session) will describe recent changes to models designed to improve the simulation of variable source area hydrology, onsite septic systems, stormwater BMPs, ephemeral gullies, riparian buffers, and wetland processes. Auto-calibration tools and down-scaling climate change model data will also be discussed.