Keyline Swales: Contour Water-Harvesting Simulator

Permaculture's keyline design lays shallow ditches — swales — along the contour of a sloped field so that rainfall is intercepted, slowed and given time to soak into the soil instead of rushing downhill as erosive runoff. This simulation renders a sloped 3D terrain with a real grid-based rainfall-infiltration-overland-flow model: rain lands on every cell, soil absorbs it up to a set infiltration capacity, and any surplus routes downhill toward lower total hydraulic head exactly as Manning's-equation overland flow does. Toggle the contour swales on and off, tune rainfall intensity, soil infiltration rate and slope grade, and watch a live water balance — infiltrated, still stored, and lost to runoff — show why a few contour trenches can turn a leaky slope into a moisture-retaining one.