Rainfall depth within small semiarid watersheds can have high spatial variability, but spatial variability in rainfall erosivity, a more direct determinant of sediment yield, has not been quantified. Using 12 tipping-bucket rain gauges within a 40-ha piñon [Pinus edulis Engelm)–juniper (Juniperus monosperma (Engelm.) Sarg.] woodland in New Mexico, we measured rainfall erosivity (EI30) and associated precipitation and erosion metrics for 14 convective thunderstorms. Spatial variability in EI30 had a median CV across storms of 22% (range: 9–73%), exceeded the median CV for rainfall depth (15%, range: 5–26%), and varied by up to a factor of five (5–25 N h−1) within 300 m. EI30 was better correlated with sediment yield measured in <0.1-ha microwatersheds (r2 = 0.67; p < 0.001) than rainfall depth (r2 = 0.43; p < 0.001). Our results highlight the potential importance for erosion related assessments of spatial variability in erosivity, which can be as great or greater than spatial variability in rainfall depth. The spatial variability in rainfall erosivity that we document here is relevant to erosion and contaminant transport issues near Los Alamos National Laboratory and may be applicable to other extensive semiarid areas.