Sumit Sen

Uncertainty in hydrological modelling is often addressed through parameter estimation, while uncertainties from model inputs and structure remain underexplored. This limitation is critical in mountainous catchments, where sparse monitoring and high spatial heterogeneity amplify uncertainty. This study assesses precipitation input, parameter and model structure uncertainties in the Alaknanda River Basin, a partially glacierized Himalayan catchment with snow‐dominated upstream and rainfall‐driven downstream sub‐basins. Precipitation input and parameter uncertainties were evaluated using the SWAT+ model through Latin Hypercube Sampling and the Sequential Uncertainty Fitting algorithm, while model structure uncertainty was assessed using 47 conceptual hydrological models implemented in the MARRMoT framework. The precipitation input assessment shows that differences among precipitation datasets substantially affect simulated streamflow, particularly in areas with high precipitation variability. Parameter sensitivity analysis indicates that snow‐related parameters exert strong control on simulated streamflows in the upper basin, with parameter uncertainty producing the widest prediction uncertainty band at the glacier‐fed Joshimath station ( r ‐factor 0.85). The model structure assessment shows substantial variation among conceptual model structures, with wider prediction bands at the downstream rainfall‐ and baseflow‐dominated sub‐basins ( r ‐factor 0.63 at Rudraprayag‐A). Structural analysis further indicates that snow storage, soil moisture and routing stores are important for reliable simulations, whereas additional storage components do not consistently improve performance. Overall, the three uncertainty assessments reveal distinct spatial patterns associated with the dominant hydrological processes across the basin. These findings highlight the need for process‐specific calibration, data collection and model selection to improve hydrological predictions in complex mountainous catchments.

West-central Himalayas have experienced an extensive loss of its Banj oak- (Q. leucotricophora) dominated forests to native Chir pine (P. roxburghii) forests, a transition frequently associated with declining watershed hydrological yields, shaping the narrative for Chir pines as a relatively resource heavy species. However, direct and comparable observations of tree transpiration of these species are rarely available to evaluate this proposition. We present 2 years of in-situ sap flux measurement-based estimates of transpiration from 15 pine and 14 oak trees and show a possibly opposite hydraulic functioning in these species than conjectured earlier. In the two-year average, oak trees transpire at a higher rate (Tf, L day-1) than Chir pines with similar diameter at breast height. A higher flux density of transpiration (Fd, mL m-2 sec-2) facilitates a higher Tf in oaks despite their relatively smaller sapwood area. Moreover, younger oaks have the highest Fd of all oak and pine trees sampled, which can be consequential for succession dynamics after disturbance with their higher transpiration demand. However, during the first few anomalously dry months of the observation period, oak trees show a decrease or downregulation of transpiration, even encompassing the downregulation displayed by Chir pines. This relatively stronger sensitivity to background environment in oaks is also displayed by significantly larger regression coefficients of intra-seasonal daily Tf in oaks with hydro-meteorological variables. Overall, the study provides context for a deeper investigation of attribution for hydrological impacts of forest-cover change in the Himalayas. Inferences can be strengthened with climate controlled comparative measurements from the same forest. Challenges in measurements of sap fluxes in oak trees have also been discussed.

Over the years, methods for measuring groundwater‐surface water (GW‐SW) interaction have improved significantly. The use of heat to quantify GW‐SW interaction has evolved with technical advancements. Recently, the use of temperature probes embedded with compact self‐recording temperature loggers has increased for measuring GW‐SW fluxes. However, these loggers are often expensive, making it difficult for researchers to use, especially for the dense networks. Eventually, this can be a hinderence for research in resource‐constrained scenarios. This work presents an affordable instrument (TemPro), designed for monitoring temperature at discrete depths. While the base TemPro system has a fixed initial cost ($89), the marginal cost of adding each additional sensor is approximately $3.48, whereas the total cost of commercially available logger‐based systems increases linearly with each additional sensor. The instrument was tested in the field and validated against one of the widely used sensors for environmental applications. The instrument was tested for consistency, agreement and reliability using different statistical metrics (NSE, R ² , RMSE, MAE and ICC). Furthermore, we have presented a case study where GW‐SW fluxes were estimated using both the reference instrument and the new temperature probe. The estimated fluxes closely matched those from the reference sensor, with a very low MRE (<2.5%). Therefore, this study demonstrates the potential to enhance GW‐SW monitoring in a spatially robust and cost‐effective manner using the developed instrument.

In recent years, the frequency and severity of extreme rainfall events have increased in the Himalayas, triggering landslides and debris flows often as a cascading hazard. Understanding the interactions between rainfall, initial soil moisture, and the triggering of landslides or debris flows is essential for mitigating risks to communities and infrastructure. However, the limited availability of observed hydrometeorological data poses serious challenges for accurate hazard assessment and early warning. To address these, numerical modelling-based reanalysis of rainfall-induced cascading hazards are chosen as a good choice. This study examines the influence of key hydrometeorological parameters, particularly rainfall and initial soil moisture, on the initiation and progression of shallow landslides and runoff-generated debris flows within a small mountainous catchment in the Himalayas, utilizing a basin-scale numerical modeling approach. We utilize multiple precipitation data sources, including reanalysis products, satellite-based retrievals, and outputs from numerical weather prediction models, to conduct a retrospective analysis of a historical cascading hazard event. This approach enables us to assess how these parameters impact the timing and severity of individual hazards, such as landslides and debris flows, within the cascading hazard chain. Our findings reveal distinct temporal patterns and triggering mechanisms for shallow landslides and runoff-generated debris flows, shedding light on their cascading behaviour in data-scarce, topographically complex regions. We also observe that initial soil moisture has a strong influence on hazard severity, and understanding its connection with rainfall is crucial for reliable hazard assessment.

The west-central Himalayas are experiencing a major ecological shift with widespread, apparently water-conserving, Banj oak (Quercus leucotricophora) forests increasingly replaced by monoculture stands of Chir pine (Pinus roxburghii). This transition has been associated with declining streamflow and weakened hydrological services, yet the role of forest transpiration—an essential component of watershed hydrology—remains insufficiently understood.This study reports findings from a two-year measurement campaign quantifying tree-level (Tf) and stand-level (Ts) transpiration in a Chir pine–dominated watershed. We analyze seasonal variability in transpiration, its age-related differences, and sensitivity to hydrometeorological drivers. We also compare this with a few months of transpiration data collected at a nearby Banj oak forest. At the tree scale, Tf increases markedly with tree age and peaks during the post-monsoon autumn season. At the stand scale, mean annual Ts is ~1.1 mm day⁻¹, with a maximum (~1.3 mm day⁻¹) in autumn and minimum (~0.85 mm day⁻¹) in summer, along with suppressed transpiration during the monsoon. Tsin the pine forest is consistently higher than that in the oak forest during the monsoon (~0.3 mm day-1) and post-monsoon autumn seasons (~0.2 mm day-1) with similar values in early autumn. Oaks also display a stronger regulation of transpiration post-monsoon with a sharper decline in Ts as compared to nearly stable values in the pine stand. Ts is also strongly controlled by solar insolation in the post-monsoon autumn and winter seasons, signifying energy-limitation, which may not have been earlier reported for these forests. However, pines still opportunistically utilise available water resources during this period with a stable Ts. The forests also seem to be water-limited during the dry summer season, which requires supporting evidence from direct measurements. However, again, pines have an opportunistically high transpiration rate during this period, apparently utilizing watershed storage.These sap-flux measurements align well with complementary measurements of forest evapotranspiration (ET), made with the Bowen ratio method. Ts percentage contribution in ET varied with seasons, with the lowest in monsoon (24% and 40%) at the oak forest and pine forest, respectively. ET show stronger transipraive regulation in the oak forest, suggesting a conservative impact on regional hydrology. Annually, the pine forest evapotranspires nearly 168 mm more water as compared to the oak forest with a negative impact on watershed storage as shown by complementary discharge measurements. Overall, our study shows that the ongoing land cover change may have significant consequences for regional hydrological resources.