River flows depend on precipitation in their catchments, where the flow is highly correlated with precipitation, among many climatic and geographic factors. The relationship between precipitation and runoff is of great importance in estimating flow changes in The HWAIZ basin that is located between The-Zrod and The-Gelani basins. The Al-HWAIZ Dam was built on the HWAIZ River with storage capacity of 16.5 MCM. The purpose of this study is to find a relationship between rainfall and runoff in The HWAIZ basin. This study depended on statistical analysis of rainfall and runoff data, and the analytical study of the annual rainfall data (1959-2011), to guess the trend of rainfall and its future changes and forecasting changes in the HWAIZ river flows. The study showed that the runoff coefficient values ranged between (0.007-0.66). A mathematical relationship was established that allows to estimate flow based on measured or predicted precipitation values, as well as appraise missing or lacking data with accepted level of accuracy.