Technical Abstract: Dryland crop yields and forage production are limited by low and variable precipitation in the southern Great Plains. Large precipitation variation often involves great production risks, which can be reduced by better planning if probability distributions of precipitation, rainwater retention, and soil erosion under various production systems are known. The objective of this paper was to probabilistically quantify the impact o precipitation variations, land use, cropping and tillage systems on rainwater retention and soil erosion. Five 1.6-ha watersheds with 3 to 4% slopes, having a similar silt loam soil, were in native grass or winter wheat (Triticum aestivum L.) under different tillage systems. Daily runoff and soil erosion were measured with an H-flume and an automatic sediment sampler at the outlet of each watershed. Precipitation distributions varied greatly seasonally and between years, and rainwater retention distributions resembles those of precipitation. Cropping and tillage systems affected rainwater retention but to a much less degree as compared to precipitation variations. Available soil water storage, which was affected by ET, played a significant role in retaining rainwater. This indicates that cropping systems can be customized to precipitation variations for more efficient use of precipitation if reliable weather forecasts are available. Landuse, cropping and tillage systems had a much greater impact on soil erosion than on rainwater retention. Soil erosion risks were caused by a few large storms and were proportional to the level of tillage disturbance. Probability distributions derived in this paper provide information for evaluating production risks associated with insufficient rainwater retention and excessive soil erosion in the region.