Natural and anthropogenic influences on the recent droughts in Yellow River Basin, China.

Drought in human-dominated environments cannot be seen as a unidirectional hazard as its characteristics are derived and modified by both natural climate variability and human influences. In this study, we applied an observation-modeling framework to quantify the natural and human controls on drought characteristics based on simulated and observed hydrometeorological data from six sub-catchments of the Yellow River Basin in China. A calibrated Soil and Water Assessment Tool (SWAT) was applied to simulate the naturalized situation, whereas Standardized Precipitation Index, Streamflow Drought Index, and Soil Moisture Deficit Index were used to characterize drought at meteorological, agricultural and hydrological aspects. Furthermore, various statistical tools, i.e., bivariate correlation analysis, heat maps, and linear models based on multiple regression, were applied to find the statistical relationships between drought characteristics and the multiple influencing factors. The results revealed that the duration of precipitation's dry spells was important for agricultural drought duration, whereas hydrological drought severity and duration highly depended on soil moisture. Meteorological to agricultural drought propagation mechanism was primarily affected by land use/land cover change (LULCC), whereas meteorological to hydrological propagation was influenced mostly by direct human activities (DHA). The human modifiers were found to have both positive and negative effects on drought severity and duration. For instance, agricultural practices and afforestation intensified soil moisture drought, while grassland restoration had a positive impact on agricultural drought severity. Deforestation enhanced hydrological drought, while afforestation and grassland restoration had the opposite effect. Hydrological drought severity and duration were largely amplified by DHA but enhanced by irrigation return flow. Spatially, sub-catchments with high urbanization and irrigated cropland were found to have shorter and less severe droughts than those dominated by grassland.