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Analysis of Variability and Controls on the Pre‐Event Water Fractions in a Tropical Forested Experimental Catchment, Southern India

R. Sharma,M. Sekhar,K. Sreelash,D. Padmalal

Published 2026 in Hydrological Processes

ABSTRACT

Understanding rainfall partitioning and streamflow generation processes has become indispensable due to the increasing frequency of hydrological extremes globally. River catchments in the tropics are particularly vulnerable to changing characteristics of rainfall. Thus, decoding rainfall partitioning into pre‐event and event water becomes more critical in tropical catchments where runoff generation processes are often poorly understood. In this context, the present study aims to understand the runoff generation processes in the Vamanapuram River—a humid tropical forested experimental catchment in the southwestern Ghats, India—utilising high‐resolution (30‐min) hydrological measurements. Data from 78 and 76 storm events (during 2022 and 2023) were used for storm hydrograph separation using the conductivity mass balance method at Kallar River Gauging Station (KRGS) and Chettachal River Gauging Station (CRGS), respectively. Both KRGS and CRGS are found to be sub‐surface dominated catchments irrespective of season, with pre‐event water domination in 96% (KRGS) and 88% (CRGS) of the storm events. The averaged (volume‐weighted) pre‐event water fraction (PEWF(avg)) over the storm duration, and at the peak of the storm hydrograph (PEWF(peak)), are PEWF metrics used for understanding the dominant runoff generation processes. The seasonality in the mean Pre‐Event Water Fraction metrics showed higher values during the dry period and lower values in the wet period. The mean PEWF(avg) decreased from 0.78 (in the upstream KRGS) to 0.75 (in the downstream CRGS); similarly, the mean PEWF(peak) decreased from 0.76 to 0.73. Pearson rank correlation and dominance analysis showed that (i) the storm size and mean precipitation intensity are the key storm properties and (ii) 5‐day antecedent rainfall and initial groundwater levels are the key antecedent wetness conditions that control the variability in the PEWF.

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