Surface erosion on pinyon pine (Pinus edulis Engelm.)-juniper (Juniperus spp.) dominated rangelands varied spatially and temporally due to the confounding effects of erratic climate, topographic changes, incongruities of soil and geologic substrate, and various other perturbations. Measurement of spatial variation was affected by plot size. Small plots (1 m2) were influenced by differences in soils, geologic substrate, and plant community structure. Therefore, many plots were needed to stratify vegetation, soil, and geologic differences. Runon and runoff processes between coppice dune and dune interspaces could not be measured. Large watershed size plots (a few hectares) were highly influenced by topographic features such as watershed slope, aspect, and shape. Plots that were 4 by 25 m could be located to minimize topographic changes, yet were large enough to include changes in soil, geologic substrate, and plant community structure.