Resources for Specific Models:

American Society of Civil Engineers. Parameter Estimation of Streeter-Phelps Models.
http://cedb.asce.org/cgi/WWWdisplay.cgi?5014377

Michigan State University, Institute of Water Research. Revised Universal Soil Loss Equation (RUSLE) : an online soil erosion assessment tool using the soil loss equation (A = R * K * LS * C * P).
http://www.iwr.msu.edu/rusle/

Soil and Water Assessment Tool (SWAT)): a river basin scale model developed to quantify the impact of land management practices in large and complex watersheds. SWAT is a public domain model supported by the U.S. Department of Agriculture, Agricultural Research Service.
http://www.brc.tamus.edu/swat/

U.S. Department of Agriculture, Agricultural Research Service. A kinematic runoff and erosion model (KINEROS2): continuous an event oriented, physically based model describing the processes of interception, infiltration, surface runoff and erosion from small agricultural and urban watersheds.
http://www.tucson.ars.ag.gov/kineros/

U.S. Department of Agriculture, Agricultural Research Service. AGricultural Non-Point Source Pollution Model (AGNPS): continuous simulation surface runoff model designed to assist with determining BMPs, the setting of TMDLs, and for risk and cost/benefit analyses.
http://www.ars.usda.gov/Research/docs.htm?docid=5199

U.S. Department of Agriculture, Agricultural Research Service. Rangeland Hydrology and Erosion Model (RHEM).
http://www.ars.usda.gov/research/projects/projects.htm?ACCN_NO=406982&showpubs=true

U.S. Environmental Protection Agency. Better Assessment Science Integrating Point and Nonpoint Sources (BASINS 4): a free GIS tool for watershed analysis and monitoring.
http://www.epa.gov/athens/wwqtsc/html/basins.html

U.S. Environmental Protection Agency. River and Stream Water Quality Model (QUAL2K): a one-dimensional river and stream water quality model for a well mixed, vertically and laterally channel with steady state hydraulics.
http://www.epa.gov/athens/wwqtsc/html/qual2k.html

U.S. Environmental Protection Agency. Water Quality Models: This site includes information and guidance on several simulation models and tools for watershed and water quality monitoring.
http://www.epa.gov/waterscience/models/

U.S. Geological Survey. Surface Water and Water Quality Models Information Clearinghouse (SNTEMP).
http://smig.usgs.gov/cgi-bin/SMIC/model_home_pages/model_home?selection=sntemp