QUAL2K

QUAL2K, by Steve Chapra and colleagues, modernised the venerable QUAL2E framework for steady-flow river water quality: dissolved oxygen sag, BOD, nutrient cycling, algae, pH and pathogens along a one-dimensional river, with diel temperature and light. Its transparent, spreadsheet-adjacent implementation made it the standard teaching and TMDL screening model for a generation.

Development has largely wound down (the QUAL2Kw fork from Washington State Ecology added auto-calibration and continued longer), but the methods remain textbook-standard, and for a steady low-flow waste-load allocation it still does the job it always did.