Multi-decadal assessment of hydrological drought using PHDI in an urban basin of Northeastern Nigeria

ABSTRACT

Hydrological drought poses a significant challenge to water-resource sustainability in semi-arid regions, yet its persistence and recovery characteristics remain poorly quantified in many data-scarce urban basins. This study assesses multi-decadal hydrological drought dynamics in the Gombe Metropolis Basin, northeastern Nigeria, using the Palmer Hydrological Drought Index (PHDI) for the period 1990–2020. Monthly basin-averaged precipitation (CHIRPS) and temperature (ERA5) data were used to compute Palmer drought indices, and hydrological drought events were analyzed in terms of severity, frequency, persistence, recovery, and long-term trends. Results reveal pronounced interannual and decadal variability in hydrological conditions, with monthly PHDI values ranging from −4.89 to 8.18 and a near-normal long-term mean (−0.07). Although extreme drought months were infrequent, PHDI revealed increasing drought persistence over time, with the longest event (32 months) occurring between 2016 and 2019. Modified Mann–Kendall trend analysis indicates a weak, non-significant decline in hydrological conditions alongside a statistically significant increase in potential evapotranspiration. Comparison with Palmer Z-index demonstrates strong hydrological memory in PHDI. These findings highlight the importance of hydrological drought persistence over frequency as an indicator of hydrological vulnerability and emphasize the need for drought monitoring frameworks that incorporate hydrological indices to support urban water management in semi-arid environments.