The Watershed as a System
A watershed — the land area draining to a common outlet — is the fundamental unit of hydrology. Every drop of rain landing within its boundaries follows one of several paths: running off the surface, infiltrating into soil, evaporating back to the atmosphere, transpiring through plants, or percolating to groundwater. Integrated watershed models track all these pathways simultaneously to predict streamflow, water quality, and ecological health.
Partitioning Precipitation
The fate of rainfall depends on a hierarchy of factors: impervious surfaces shed water immediately; vegetated surfaces intercept and evaporate a fraction; soil absorbs water at a rate controlled by texture, moisture, and depth; slopes determine how long water has to infiltrate before flowing downhill. This simulation computes the annual water balance, showing how each parameter shifts the partition among runoff, ET, recharge, and streamflow.
Land Use and the Hydrologic Response
Converting forest to farmland increases runoff and reduces ET. Urbanization amplifies the effect dramatically — impervious surfaces bypass the soil entirely. Research consistently shows that crossing 10% impervious cover begins degrading stream health, and above 25%, stream ecosystems are severely impaired. The simulation visualizes this transition as you adjust the impervious fraction slider.
Toward Sustainable Watersheds
Effective watershed management balances human water needs with ecosystem requirements. Low-impact development (green roofs, rain gardens, permeable pavement) attempts to maintain pre-development hydrology. Watershed models inform these decisions by quantifying how proposed land-use changes will alter the water balance — enabling planners to evaluate tradeoffs between development and environmental protection before breaking ground.