From Rain to River
When rain falls on a catchment, only a fraction becomes streamflow. The rest infiltrates into the soil, evaporates, or is intercepted by vegetation. The rainfall-runoff process transforms spatially distributed precipitation into a concentrated discharge hydrograph at the basin outlet. Understanding this transformation is essential for flood prediction, dam design, and stormwater management.
The SCS Curve Number Method
Developed by the US Soil Conservation Service in the 1950s, the CN method condenses complex infiltration processes into a single dimensionless parameter. CN values encode soil permeability (A through D hydrologic soil groups), land cover type, and antecedent moisture conditions. The method computes maximum retention S from CN, then estimates runoff depth from the rainfall-retention balance equation — elegant simplicity that remains the global standard for ungauged basins.
Hydrograph Shape and Timing
The unit hydrograph concept, introduced by Sherman in 1932, assumes that a basin's runoff response to a unit of excess rainfall is consistent regardless of storm magnitude. Time to peak depends on storm duration and basin lag time, which itself depends on flow path length and slope. This simulation generates synthetic triangular hydrographs showing how changing CN or storm intensity reshapes the peak, volume, and recession limb.
Urbanization and Flood Risk
Converting forests and farmland to impervious surfaces dramatically alters the rainfall-runoff relationship. Urban basins with CN values above 90 produce flashy hydrographs with high peaks and rapid recession. A parking lot that was once meadow may generate five times the peak runoff for the same storm. Green infrastructure — bioswales, permeable pavement, retention ponds — attempts to restore pre-development hydrology by lowering effective CN values.