Runoff vs Slope

Slope increase from 0% to 5% can decreases effective infiltration by over 300% on the same soil type. See studies below.

Some real data on Slope Vs Runoff (source: Runoff and Soil Loss Under a Center Pivot)

Also see Table 2–8 in Irrigation Guide

Also see Table 2 in Center Pivot Management For Forage Production

If we assume those numbers correlate to infiltration then slope decreases effective infiltration by around 300%

Its conceptually & nicely increases surface storage (table 3) based on slope and residue cover which i think is missing in a lot of models.

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