Work out catchment yield instantly with clear inputs, formula shown and shareable results.
Catchment yield is rainfall times area times the runoff ratio, which accounts for evaporation, transpiration and deep infiltration. A 45 square kilometre catchment under 900 mm of rain with a 0.35 ratio yields 14.2 million cubic metres a year - a mean flow of 0.45 m3/s. Runoff ratios span 0.1 in dry permeable catchments to 0.7 in wet upland ones.
Annual yield
volume = area x rainfall x runoff ratio
Mean flow
flow = annual volume / seconds in a year
Rainfall intensity and seasonality, soil permeability, vegetation and slope. Upland peat catchments run high, chalk catchments very low.
No. Reliable yield depends on storage and dry-period flows, and is typically a fraction of the mean.