According to Wikipedia, the Hydrologic Unit Modeling for the United States (HUMUS) is a national-scale hydrologic modeling system developed by the USDA Natural Resources Conservation Service to evaluate water quantity and non-point source pollution across the United States. It integrates the Soil and Water Assessment Tool (SWAT), Geographic Information Systems (GIS), and extensive environmental databases to simulate watershed processes within 8-digit hydrologic units. HUMUS supports water resource planning, conservation, and environmental impact assessments by analyzing the effects of land use, climate, and management practices on watershed hydrology.