Use of hand-held radiometers to evaluate the cover and hydrologic characteristics of semiarid rangelands

The spectral reflectance characteristics of 11 rainfall simulator plots wer e measured with a hand-held radiometer on semiarid rangeland surfaces, of t he U.S. Department of Agriculture, Agricultural Research Service, Walnut Gu lch Experimental Rangel- and Watershed, Tombstone, Arizona. Rainfall simula tions were made in Map and June 1984 for plots characterized by natural veg etation, natural vegetation clipped and removed, and natural vegetation cli pped and rock fragments >5 mm removed. In 1994, reflectance and cover data were collected on the same plots to evaluate changes over time. Measurement s were taken at 38-40 degrees and 74-77 degrees sun elevation angles with a four-band hand-held radiometer (blue 0.45-0.52 mu m, green 0.52-0.60 mu m, led 0.63-0.69 mu m, and near-infrared 0.76-090 mu m). Correlation and regr ession relationships were computed between spectral reflectance and percent soil, rock, and vegetative cover; percent runoff; and eroded sediments. Hi ghly significant correlations were measured between vegetative cover and pe rcent runoff; relationships with soil-rock cover and eroded sediment were p oorly correlated. The normalized difference vegetation index (NDVI) was the best predictor of percent vegetative cover, with shrubs-forbs being most s trongly correlated to reflectance. The regression relationships between 198 4 and 1994 spectral reflectance and vegetative cover were very different, e ven though cover percentages were similar. The amount of standing live and dead biomass and the proportion of green biomass strongly affect spectral r eflectance, and the 1984 and 1994 conditions were quite different. Spectral reflectance data can be used to predict rangeland cover characteristics, w hich in turn, can be used to determine parameters needed for models that pr edict hydrologic processes on rangeland surfaces.