Case study of Sonar and Bearma sub-basins of Ken basin for drought proofing analysis

Drought duration and severity (or cumulative deficit below a desired demand level) of the past events occurred in the Sonar and Bearma sub-basins were analyzed, and the availability of flow in these ephemeral streams during normal and lean years estimated using, respectively, 15 and 24 years of monthly data. Such a drought analysis for ephemeral streams does not appear to have been reported in literature. The probabilities of occurrence of monthly zero and non-zero flows and joint probabilities of non-zero flows were estimated to develop flow duration curves. These curves were used to determine the variable truncation levels at 70, 75 and 80 percentiles for (a) describing wet and drought periods and (b) determination of appropriate storage requirement for drought proofing. The study revealed that the hydrological droughts in these basins (a) usually began during August-October and (b) either terminated during the period between September and December or might continue till the onset of the next monsoon. Drought events starting during early monsoon months were more severe than those starting during the late or post monsoon months, and the variable truncation approach was efficacious in depicting both the drought and wet events and, in turn, in describing the drought duration and severity. The analysis further showed the availability of more than sufficient water in the considered basins for drought mitigation/proofing through a surface storage scheme.