Frank Joseph Wambura

Sound estimates of drought characteristics are very important for planning intervention measures in drought-prone areas. Among many drought indices used in estimation of drought characteristics in many parts of the world, evapotranspiration deficit index (ETDI) is increasingly used to estimate agricultural drought. However, in most studies ETDI has been computed using the specific ETDI formula. Thus, there is no clear information about sensitivity of ETDI to its parameter and temporal scales. In this study, the general ETDI formula homologous to the specific ETDI formula was introduced and used to test sensitivity of ETDI to its parameters and temporal scales using time series of remotely sensed evapotranspiration data in the Ruvu River basin (Tanzania). The parameter sensitivity test revealed that ETDI is sensitive to its parameters. Different parameter combinations resulted into different drought characteristics. In order to reduce this uncertainty, the general ETDI formula might require parameter calibration. On the other hand, the temporal scales sensitivity test showed that drought characteristics such as number of drought events and the total drought durations decreased as the size of temporal scales increased. Thus, inappropriate temporal scales may lead to misrepresentation of drought characteristics. In order to increase accuracy of drought characteristics derived from ETDI, small temporal scale data are highly recommended. Therefore, this study has provided useful information for improving application of ETDI in estimation of agricultural drought characteristics.