Scarcity of water is the most severe constraint for traditional agriculture in semi-arid areas of Ethiopia. Precipitation is extremely variable, thus water harvesting is crucial for ensuring improved crop production. A study was carried out to identify potential rainwater harvesting systems for improved crop production under climate variability in Adulala watershed, central rift valley of Ethiopia. Primary and secondary data together with other relevant information through a well-structured questionnaire were collected and analyzed to charachterize climate variability and water harvesting structures and to estimate runoff and crop water requirement.The inter annual rainfall variability showed a significant (p<0.05) increasing trend of 1.86mm per year. The variability in the start of the season was non significant while increasing at a decreasing trend of 0.042days per year. In the watershed, there are 38 water harvesting structures of which 34 are hemispherical and 4 are rectangular with storage capacity of about 90 and 320m3 each respectively.The average monthly and annual surface runoff were found to be 3.05 and 36.6 mm respectively. The total irrigation volume required to supplement both major crops and vegetables per farmer was found to be 3285.9m3 to cover 2 hectares .Considering the situation, additional storage structures for supplementary and full irrigation are necessary. Irrigation for small vegetables could be encouraged with the current storage volume and use of early maturing varieties should be considered under variable climate.