A two‐soil‐moisture‐reservoir model is developed to improve the estimate accuracy of a runoff‐volume‐prediction model. Soil moisture accounting in the two reservoirs is an intermediate step to runoff prediction. A decay‐type function describes the moisture depletion between days of rainfall. The moisture depletion constant in the function varies by season with soil moisture, pan evaporation, and mean daily temperature. The runoff‐prediction equation relates runoff to rainfall and soil moisture at the beginning of the storm. Computed runoff volumes are compared with values observed on a 3‐acre native grass‐meadow watershed for an 11‐year period. Accumulated computed amounts for the period agree within 1% of the accumulat