Regional Trends and Physical Controls of Streamflow Drought Characteristics in Tropical Catchments

Globally around half of the population resides in the tropics. Understanding the regional trends and physical controls of streamflow drought in the present day helps us to recognize the future changes in the hydrological cycle impacting the freshwater supplies. Few studies have examined the climatic and catchment controls on the propagation of streamflow droughts in tropical catchments. This study unveils the regional pattern of drought onset and deficit volume and identifies their evolution by analyzing the dominant climatic and physiographic controls. Two crucial streamflow drought signatures, the time of onset and deficit volume, are extracted from daily observed streamflow records of 82 rain-dominated catchments from peninsular India (8-24º N, 72-87º E). A daily variable threshold approach with an 80% exceedance probability of the flow record is used to identify drought events. Despite a decreasing trend in deficit volume, we show a delayed shift in drought onset of about a week for more than 50% of catchments. However, droughts with mean onset time clustered around the summer (Mar-May) season show a sharp rise in deficit volume trends. We show that while dynamic (precipitation and soil moisture) factors influence the onset of droughts, both static (catchment, topographic, and soil properties) and dynamic properties play a significant role in deciding the deficit volume. Based on a statistical approach (Taylor skill score and non-parametric dependence metrics), we identify static features, surface (30 cm) and sub-surface (up to 1m) soil organic stock and cation exchange capacity (CEC) to be dominant soil controls, also topographic ruggedness index, slope, topographic wetness index, and curvature are found to be the main controlling catchment attributes. The derived insights add new avenues in understanding the causal chain of physical processes linking climatic and physiographic controls on streamflow drought mechanisms, elucidating tropical climate response to water availability in a changing climate.