ROCK COVER - Guidelines for Estimating Rock Cover in the Field

ROCK COVER - Guidelines for Estimating Rock Cover in the Field

RockCover-GuidelinesforEstimatingRockCoverintheField

Introduction: The RUSLE2 computer program has an input box on the Profile view screen for “Rock cover, %”. This document offers guidelines for making estimates in the field for the percent cover fromrock, rock fragments, or coarse fragments. Coarse fragments on the soil surface affect the Cover and Management factorin RUSLE2.Rock cover does not affect the SoilErodibility factor.

Caution - Use Good Judgment: Research data shows that the presence ofrock cover can significantly reduce soilerosion, and the RUSLE2 model accounts for this effect.However, users should be cautioned toexercise good judgment when developing conservation planning alternativesthatreflectthepresenceofsurfacerockfragments.For example, a rock cover entry inRUSLE2thatreducessoillosstoacceptablelevelsshouldbere-considered if the hillslope showsclearevidenceof severe,activeerosion.

RUSLE2 uses the Kf (rock free)soilerodibility factor.

If surface rock fragments are present and not entered, erosion is over predicted.

RUSLErockfragmentsaredefinedasthosegreaterthan3/8ofaninchinsize. The RUSLE2 Users Guide states, “Do not use rock cover values or rock content in the soil profile from the NRCS soil survey database to determine rock cover. The definitions of rock cover in that database do not correspond with RUSLE2 definitions.”The percent rock cover values from soil data could be significantly different from an actual on-site estimate. In addition, mixing of the surface layers with tillage operations changes the surface rock percentages. Growers can also pick up the rocks from the surface.

ThefollowingareguidelinestouseinRUSLE2, ifthesurfacerockfragmentcoverisnot measured in the field.

Surface Texture Modifier / Rock Fragment Content Range / Amount to use if no field measurement
NoTextureModifier / 0-15 / 3
channery, shaly, gravelly,
cobbly, cherty, flaggy / 15-35 / 15
very channery, very shaly,
very gravelly, very cobbly, verycherty,veryflaggy / 35-60 / 35