Most hydrological concepts have been developed based on experimental studies in temperate catchments; however, humid catchments have received comparatively less attention at the global scale. Therefore, the present study was conducted in the Vamanapuram experimental catchment, located in the humid tropical region of the southern Western Ghats, India. The monthly rainwater, river water, and groundwater samples collected between 2022 and 2024 were used to investigate streamflow generation and groundwater recharge processes. SWM rainfall experiences greater evaporation than NEM rainfall. A steeper GWL slope relative to the SWM LMWL indicates that groundwater during the SWM is partly sustained by slow-moving recharge derived from the previous year’s NEM rainfall. The seasonal variability observed in groundwater isotopic compositions suggests that recharge sources vary spatially across the catchment, with some wells dominated by local rainfall recharge and others influenced by delayed inputs from distant upland areas. Seasonal-scale hydrograph separation results closely mirror earlier storm-scale findings in the catchment, demonstrating that baseflow consistently dominates streamflow across temporal scales. The persistently high baseflow contributions throughout the catchment indicate a storage-dominated system, in which a substantial fraction of rainfall is retained within subsurface reservoirs (soil and shallow groundwater) and released gradually to the stream during and between storm events. The annual specific baseflow discharge in the catchment ranges from 2.09 to 2.44 MM 3. yr -1. Km -2. The relatively high specific baseflow discharge, unlike other regions is likely attributable to high annual rainfall, favourable soil and geological conditions that enhance recharge, and strong hydraulic connectivity that facilitates the delivery of older groundwater to the stream. Mean transit times of river water (45–143 days) fall within the range reported for humid catchments, with longer transit times associated with higher elevations and steeper slopes, underscoring the influence of topography on storage and release dynamics. Integrated isotopic, hydrochemical and hydrometric evidence highlights catchment-scale storage and release dynamics in data-scarce humid tropical environments.

Understanding rainfall partitioning and streamflow generation processes have become indispensable due to the increasing frequency of hydrological extremes globally. River catchments in the tropics are particularly vulnerable to changing characteristics of rainfall and decoding the 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 is aimed at understanding the runoff generation processes in Vamanapuram river – a humid tropical forested experimental catchment in the Southern Western Ghats, India utilizing 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. In 98% and 96% of the storm events respectively at KRGS and CRGS, pre-event water dominated the hydrograph.. The seasonality in the PEWF(avg) and Pre-Event Water Fraction at peak (PEWF(at peak)) has higher values in the dry period and lower values in the wet period. PEWF(avg) decreases from 0.82 to 0.80, and PEWF(at peak) from 0.76 to 0.73 from upstream to downstream reaches respectively. The beta distribution fits well the temporal variability in the PEWF(avg) and PEWF(at peak) at both river gauging stations. The effect of differential precipitation forcing in the subsequent year’s storm PEWFs has been studied, and it was observed that higher precipitation forcing results in higher PEWF at CRGS. However, the effect of precipitation forcing is not seen at KGRS, possibly due to significantly different geomorphic characteristics and higher forest cover. A more extended period of hydrological data in catchments with diverse climatic and geological settings will provide a better understanding on the effect of precipitation forcing on catchment rainfall partitioning and thus improve conceptualization of rainfall-runoff models.

The Vamanapuram experimental catchment has been established in a humid tropical setting, where the hydrological processes are poorly characterized and have received limited attention. The Monthly River water, groundwater, and rainwater samples have been collected from May 2022 to April 2024 to understand the spatial and seasonal variability of baseflow contributions. The Chemical (Electrical conductivity, silicate) and isotopic tracers (δ 18O, δ 2H) have been used to estimate the baseflow contribution (and associated uncertainty) and the mean transit time of river water. The two-component Classical Hydrograph Separation (CHS, mass balance-based) and the recent Ensemble Hydrograph Separation (EHS, correlation-based) have been used to quantify baseflow contributions. Although CHS and EHS resulted in almost similar baseflow contributions, but EHS is recommended due to reduced uncertainty. Estimated baseflow contributions using EC as a tracer are significantly lower than those estimated using other tracers. Estimates based on silicate, δ 18O, and δ 2H are also generally comparable, and isotopic tracers have a higher degree of uncertainty in estimates than chemical tracers, silica can be used as alternate tracer for baseflow estimation. The baseflow is found to be dominant contributor to streamflow (irrespective of season and location) with fraction greater than 0.5. Mean transit time of stream water ranges between 45 and 143 days, mainly controlled by topography (mean watershed elevation and slope). The groundwater wells found to be behave differently, indicating either seasonal replenishment by rainfall, or steady state or lack of replenishment and diminishing potential of wells. The Trend analysis of groundwater stable isotopic data, and levels indicated that parts of catchment are not getting replenished by rainfall, and groundwater resources are at risk, and needs immediate urgent attention.

Tropical rivers are undergoing significant changes in hydrochemical composition and solute fluxes due to alterations in monsoon patterns and increasing anthropogenic pressures. To better understand solute dynamics and hydrochemical interactions within a tropical river system, monthly water samples were collected from nested catchments representing upstream (Kallar, KRGS) and downstream (Chettachal, CRGS) segments. Concentration–discharge (C–Q) analysis revealed that the Kallar watershed functions as a chemostatic system with stable solute delivery, whereas the Chettachal watershed exhibits a more hydrologically responsive, chemodynamic behavior. Groundwater contributions, quantified using isotopic tracers, indicate that subsurface pathways play a major role in catchment-scale chemical erosion, accounting for approximately 55% and 70% of total solute fluxes at KRGS and CRGS, respectively. Silicate weathering emerged as the dominant geochemical process controlling solute dynamics, modulated by both seasonal hydrological variability and local anthropogenic influences. Strong seasonality was observed in silicate weathering rate (SWR) and CO 2 consumption rate (CCR), with annual mean SWR values of 21.93 and 26.67 t km -2 yr -1, and CCR values of 2.68 × 10 5 and 3.86 × 10 5 mol km -2 yr -1 at KRGS and CRGS, respectively. Correlation between groundwater hydrochemistry and water table depth further indicates distinct geochemical responses of shallow and deep aquifers to rainfall inputs. Insights from this nested catchment study provide a clearer understanding of tropical river hydrochemistry and highlight the importance of integrating hydrological and geochemical observations to elucidate runoff generation and surface water–groundwater interactions across monsoonal cycles.