Runoff vs Slope
Some real data on Slope Vs Runoff (source: Runoff and Soil Loss Under a Center Pivot)
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Also see Table 2–8 in Irrigation Guide
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Also see Table 2 in Center Pivot Management For Forage Production
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If we assume those numbers correlate to infiltration then slope decreases effective infiltration by around 300%
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Its conceptually & nicely increases surface storage (table 3) based on slope and residue cover which i think is missing in a lot of models.