So far, not much is known about the state of water resources in Didessa watershed. Thus, this book explores the temporal (monthly, seasonal and annual) variability in climate and streamflow within the watershed. Data from three meteorological and eight streamflow gauging stations were used. The observation period for the rainfall and temperature data spans from 1960 to 2004. However, the record for the streamflow was disrupted, and generally ranges from 1960 to 1984 or 1979 to 2004. Areal average rainfall for the watershed was derived by Thiessen polygon method. Meanwhile, its potential evapotranspiration (PET) was estimated by FAO Penman- Monteith method. The streamflow data at the outlet of the watershed was converted into its depth equivalent from discharge of water passing through a gauging station at m 3 /s to mm runoff so as to compare it with rainfall. Once these have been accomplished, data analysis followed. The data analysis methods included calculation of averages, percentages, and indices of variability, trends (using linear regression) and production of graphs.