Timing of Drought Onset Controls Hydrological Drought Responses in Tropical Catchments

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We assess the degree of human impact on the natural catchment water balance by looking into the rainfall and runoff correlation.A number of 16 highly influenced catchments were discarded leading to a final of 82 catchments.
We also screen through the daily flow-duration curve of each gauge location to avoid catchments that are highly influenced by anthropogenic changes during the past decades.
Considering both surface and groundwater irrigation sources, the percentage area under irrigation in the individual catchments varies from 3-33%, with a median irrigated area of ~16%.The linear-circular dependency between the mean onset and deficit volume represents the temporal coherency.
Krishna river basin shows higher strength of dependency that varies between 0.6 to 0.92 (see figure 4, left panel).
To assess inter-regional differences in cluster properties (dependence strength between the mean onset and deficit volume), we use the Wilcoxon-rank sum test.
While localized assessment of Wilcoxon rank-sum test shows regions 2 and 3 are spatially different at 5% significance (p-value = 0.04), the Bonferroni corrected p-values for pair-wise  Region 2 is more drought-prone due to low soil moisture (see figure 5) and has a high weighted average deficit volume.
1.The spatial pattern of dependence strength between the time of onset and severity shows a median value of 0.64.A large median value suggests a significant nonlinear dependence between onset and deficit volume in observed streamflow droughts.
The basin geology and hydrometeorology condition of the catchment influence the dependence structure.
2. An apparent decrease in deficit volume of observed streamflow drought is accompanied by a delay in the time of onset -this is pronounced over catchments located in the core monsoon region (Region 1: 3. Despite high BFI, relatively low rainfall, results in a low soil moisture build-up at region 2, which controls high drought deficit volume in this regime.This explains the large variability in onset.
4. The rainfall variability plays a crucial role in deciding the onset and severity of the streamflow drought event together with catchment characteristics and soil properties.
A low BFI indicates rivers in this area are more flashy in nature indicating higher sensitivity of seasonal rainfall to streamflow (Region 3).This also indicates a short rainfall event during a prolonged dry period will result in a relatively slower rate of drought recovery owing to a smaller rise in streamflow as compared to a fast-responding catchment (Region 2: Figure 5; lower panel).
Soil moisture controls drought resilience at Region 3. Despite a low BFI, a high value of soil moisture results in a low deficit volume 5.The space-time coherency between drought onset and severity helps to identify three distinct regimes.
It aids in identifying drought vulnerable regions, forecast, and water resources planning.
Figure 1.Locations of Stream Gauges over the River Basins The Peninsular plateau covers nearly half of the Indian terrain; It includes the diverse topological and climatic patterns of South India.