The propagation dynamics and causes of hydrological drought in response to meteorological drought at seasonal timescale

According to widely accepted definition of drought, meteorological and hydrological droughts originally develop from rainfalls and runoffs deficits, respectively. Runoffs deficit is mainly derived from rainfalls deficit. Therefore, hydrological drought is essentially propagated from meteorological drought, which is critical for agricultural water management. Investigation of the propagation from meteorological to hydrological drought is important for drought early warning, preparedness and mitigation. Nevertheless, the characteristics and dynamic of drought propagation in spatiotemporal scale remain unresolved. To this end, the characteristics and dynamic of drought propagation in different seasons and their linkages with key forcing factors are evaluated. In this study, the meteorological drought and hydrological drought are characterized by Standardized Precipitation Index (SPI) and Standardized Runoff Index (SRI), respectively. The propagation time is identified by the corresponding timescale of the maximum correlation coefficient between SPI and SRI. Then, a 20-year sliding window is adopted to explore the propagation dynamic in various seasons. Furthermore, the multiple linear regression model (MLR) is established to quantitatively explore the influence of meteorological factors, underlying surface features and teleconnection factors on the propagation time variations. The Wei River Basin (WRB), which is a typical Loess Plateau watershed in China, is selected as a case study. Results indicate that: (1) the propagation time from meteorological to hydrological drought is shorter in summer (2 months) and autumn (3 months), whilst that is longer in spring (8 months) and winter (13 months). Moreover, the propagation rates exhibit decreasing trend in warm seasons, which however show increasing trend in cold seasons; (2) a significant slowing propagation in autumn is mainly caused by the decreasing soil moisture and precipitation, while the non-significant tendency in summer is generally induced by the offset between insignificant increasing precipitation and significant decreasing soil moisture; (3) the replenishment from streamflow to groundwater in advance prompts the faster propagation from meteorological to hydrological drought in spring and winter; (4) teleconnection factors have strong influences on the propagation in autumn, in which Arctic Oscillation (AO), El Niño-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) mainly affect participation, arid index and soil moisture, thereby impacting drought propagation.