Spatio-temporal characteristics and driving factors of flash drought recovery: From the perspective of soil moisture and GPP changes

Flash drought recovery is significant to evaluate the impact of flash drought and the resilience of ecosystem. However, the definition of flash drought recovery stage and its mechanism are still unclear, and studies focusing on it are quite limited. This study proposes the definitions of flash drought recovery stage based on soil moisture change and gross primary productivity (GPP) change. The spatio-temporal characteristics, driving factors and differences between two identifications over the typical humid and semiarid basins, namely Wei River Basin (WRB) and Xiang River Basin (XRB), in China are explored. The main results are as follows:

(1) According to the spatial characteristics of flash drought recovery, XRB has a slightly longer recovery time and a higher recovery rate than WRB in terms of flash drought recovery with soil moisture-based definition, but in terms of flash drought recovery with GPP-based definition, the recovery rate of WRB is higher and the recovery time of two basins is similar. The temporal characteristics of flash drought recovery in two basins are basically similar. In terms of flash drought recovery with soil moisture-based definition, the recovery time and recovery rates of different flash drought events in the same basin vary greatly, and events with rapid recovery occur in both basins. In terms of flash drought recovery with GPP-based definition, the majority of events in both basins recover within 2*8 days, although a small number of events take up to 9*8 days to 11*8 days to recover.

(2) The evolution mechanism of flash drought recovery is closely related to meteorological conditions. The main driving factors for flash drought recovery with soil moisture-based definition and GPP-based definition are both increased precipitation (PRE), decreased average temperature (TEM) and decreased solar radiation (Rs). In terms of flash drought recovery with two definitions, regions with relatively higher PRE, lower TEM and lower Rs tend to have higher recovery rates and shorter recovery time, and regions with relatively lower PRE, higher TEM and higher Rs tend to have lower recovery rates and longer recovery time. Besides meteorological variables, land use is also a factor that impacts the recovery of flash drought with GPP-based definition. Grasslands tend to recover more quickly than forests from flash drought with GPP-based definition.

(3) Apart from the differences in spatio-temporal characteristics of flash drought recovery, the impacts of different identification methods have two other main points. One is that not all flash drought events identified by soil moisture change have an obvious impact on GPP. Some flash drought events have no recovery stages when using GPP-based definition, while all flash drought events have recovery stages when soil moisture-based definition is used. The other is that, soil moisture responds immediately to changes in meteorological conditions, but GPP responds with a lag of 8 days to 2*8 days. So the recovery stage with soil moisture-based definition begins when the changes of meteorological variables associated with flash drought recovery begin, but the recovery stage with GPP-based definition begins 8 days to 2*8 days after the changes of meteorological variables associated with flash drought recovery begin.