retardation factorretarding reservoirretentionretention forcesretention furrowretention periodretention terrace bankretentive goalretreat velocityretrievalretrograde erosionretrogressionretrogression of levelsreturn flowreturn overland flowreturn periodreturn waterreverse flowreverse osmosisreversible adiabatic processrevetmentreynods numberreynolds numberrhizobia phizobiumrhizosphereribbon lightningrichardson extrapolationrichardson numberridgeridge plantingridgingrift valleyright overbankright runnigrigid modelrigionalrillrill erodibilityrill erosionrill fanrill patternrill washrilling testrimerime fogring valveripeningrippleriprapripsnow