Sensitivity of the Evapotranspiration Deficit Index to Its Parameters and Different Temporal Scales

Abstract

Sound estimates of drought characteristics are very important for planning intervention measures in drought-prone areas. Due to data scarcity, many studies are increasingly using less data-intensive approaches, such as the evapotranspiration deficit index (ETDI), in estimations of agricultural droughts. However, little is known about the sensitivity of this specific ETDI formula to its parameters, and to data at different temporal scales. In this study, a general ETDI formula, homologous to the specific ETDI formula, was introduced and used to test the sensitivity of the ETDI to its parameters and to data at different temporal scales. The tests used time series of remotely sensed evapotranspiration data in the Ruvu River basin in Tanzania. The parameter sensitivity tests revealed that ETDI is sensitive to its parameters, and different parameter combinations resulted in different drought characteristics. The temporal scale sensitivity test showed that drought characteristics, such as the number of drought events and the total drought durations, decreased as the temporal scale increased. Thus, an inappropriate temporal scale may lead to the misrepresentation of drought characteristics. To reduce uncertainty and increase the accuracy of ETDI-based agricultural drought characteristics, ETDI requires parameter calibration and the use of data with small temporal scales, respectively. These findings are useful for improving estimations of ETDI-based agricultural droughts.

Keywords

• agricultural drought
• drought characteristics
• evapotranspiration deficit index
• parameter sensitivity
• temporal scale sensitivity
• water stress anomaly