Rain lands uniformly on the terrain and either infiltrates the soil or
runs downhill as sheet flow, accelerated by slope and slowed by vegetation
roughness. Terracing breaks a continuous slope into flat steps, giving water
more time — and flatter ground — to soak in between drops.
v_flow ≈ k·√(slope) · (1 − 0.6·veg_cover)
infiltration_fraction = base + veg_cover·0.4 − slope_factor
runoff_fraction = 1 − infiltration_fraction (terracing adds a flat-step bonus)
- Slope — steeper terrain accelerates runoff and cuts infiltration time.
- Vegetation cover — roots and leaf litter slow flow and increase soil absorption.
- Rainfall intensity — more water arriving per second raises total runoff even at fixed slope.
- Terracing — flat steps interrupt the slope, letting water pool and infiltrate instead of sheeting straight downhill.