Hydrological response to meteorological drought using the Palmer drought indices in Thessaly, Greece

This study evaluates the efficacy of Palmer drought indices to monitor hydrological droughts in river discharges and soil moisture in selected watersheds with varying geomorphologic characteristics in region of Thessaly, Greece. The Palmer four indices (PDSI, Weighted PDSI, PHDI and the moisture anomaly Z-index) were used as indicators of meteorological drought severity. The hydrological drought severity was evaluated from the outputs of the monthly UTHBAL conceptual water balance model. The UTHBAL model was calibrated with the available observed runoff data to extend, reconstruct and produce runoff and soil moisture timeseries for the hydrologic period 1960–2002 at the study catchments. The produced hydrologic variables were normalized through Box-Cox transformation and standardized to normal distribution. The standardized hydrologic variables were used as an indicator of hydrological drought severity and were compared with the Palmer drought indices timeseries estimated by basin-wide meteorological data. The results showed that, in general, the Weighted PDSI and the moisture anomaly Z-index were found to better represent river discharges and soil moisture, respectively, for all study watersheds irrespectively to their area, geophysical, and hydroclimatic characteristics. However, the results were quite variable in the identification of specific historical drought periods. Although, the Palmer indices were successful in the identification of drought severity of historical events, they failed to identify the drought duration.

This study analyzes the efficacy of the four Palmer drought indices to monitor droughts in river discharges and soil moisture. The study was carried out in selected small, medium and large watersheds with varying geomorphologic characteristics located in the Thessaly region of Greece where there is a high complexity of hydrological processes with a marked seasonal and interannual variability. The hydrological drought severity in river discharges and soil moisture was evaluated from the outputs of the monthly UTHBAL conceptual water balance model. The results showed that the Weighted PDSI and the moisture anomaly Z-index were found to better represent river discharges and soil moisture respectively, for all study watersheds irrespectively to their area, geophysical, and hydroclimatic characteristics. However, the response of Palmer’s drought indices to the identified historical drought episodes was quite variable. The assessment of hydrological drought severity was quite successful with the application of Palmer’s drought indices. However, different results were obtained in the evaluation of hydrological drought duration whereas the Palmer drought indices fail to represent satisfactorily drought episode duration. The above findings could be useful for forecasting and monitoring hydrological and soil moisture drought severity and in developing a drought preparedness plan in the region. To address this difficult task, the inclusion of other drought indices such as the SPI could help, complement and verify the outcomes of this study.