Hydrology

Integrated watershed management relies on distributed hydrological models to simulate water transport processes and support decision-making. However, model reliability is often constrained by the resolution and quality of input data, particularly soil information. High-resolution soil datasets remain scarce in many regions of Sub-Saharan Africa, limiting the representation of spatial soil heterogeneity in hydrological simulations. This study evaluates the effect of detailed soil information derived using the Soil–Land Inference Model (SoLIM) on the performance of the Soil and Water Assessment Tool (SWAT) in the Sigi River watershed, a topographically complex watershed in northeastern Tanzania. Two model setups were compared: (i) a high-resolution SoLIM-based soil dataset and (ii) the coarser global ISRIC SoilGrids database. The SoLIM-informed model better reproduced hydrographs and flow duration curves and showed stronger parameter sensitivities, achieving superior calibration performance (NSE = 0.87, PBIAS = 8.7%) compared to SoilGrids (NSE = 0.86, PBIAS = 11.1%). Hydrological component analysis further revealed that SoLIM enhanced baseflow (181 vs. 60 mm/year) and percolation (349 vs. 135 mm/year) while reducing surface runoff (263 vs. 474 mm/year). These findings demonstrate that high-resolution soil data measurably improve the representation of subsurface processes and moderately improve streamflow performance, especially for baseflow and low-flow regimes; reduce model uncertainty; and improve the robustness of SWAT simulations, thereby supporting more effective watershed management in data-scarce and heterogeneous landscapes.

Coastal groundwater in monsoon-dominated regions faces compounding threats from seasonal hydrological extremes and seawater intrusion (SWI), yet the molecular-scale response of dissolved organic matter (DOM) remains poorly understood. We conducted a two-season investigation in Mannar District, Sri Lanka, integrating hydrochemistry, fluorescence spectroscopy, and Fourier-transform ion cyclotron resonance mass spectrometry to characterize DOM dynamics across shallow and deep groundwater. Dry-season chloride averaged 302 mg/L (shallow—5 to 12 m) and 505 mg/L (tube wells—20 to 30 m), then declined by 60–80% during monsoon recharge. Despite this freshening, DOM dynamics were decoupled from salinity: shallow wells showed dry-season DOC peaks (6.64 mg/L) driven by soil concentration, while tube wells exhibited wet-season enrichment (5.02 mg/L). Shallow aquifers maintained consistently high humification indices (around 0.70) and aromatic-rich DOM, indicating sustained buffering by soil-derived inputs. In contrast, wet-season recharge in tube wells appeared to stimulate microbial processing, as indicated by elevated protein-like fluorescence (C2: 26% to 36%) and a higher contribution of nitrogen-bearing formulas (CHONs: 31.4% to 37.1%). Tube wells also accumulated reduced, energy-rich DOM with correspondingly high molecular lability indices. Paradoxically, correlation networks suggested that these saturated aliphatic and halogenated structures persist due to kinetic protection under low oxygen, high-salinity conditions. These findings indicate that aquifer structure and redox conditions control DOM biogeochemistry in coastal groundwater systems. At the molecular level, DOM dynamics are influenced by aquifer depth and seasonal recharge, leading to a decoupling between salinity and organic matter transformation.

The impact of wind on disdrometer measurements has not yet been demonstrated through controlled reproducible physical experiments. This study aims to provide quantitative evidence of the deviation in raindrop trajectories approaching the sensing area of an optical disdrometer (the Thies Clima LPM) when immersed in a wind flow with a known velocity and direction relative to the sensor orientation. To this end, water drops with diameters between 0.9 mm and 1 mm were released in a wind tunnel and directed towards the instrument’s sensing area. Their trajectories were measured using a high-speed camera and compared with those expected in undisturbed conditions, as well as with the airflow field around the instrument body as measured in previous studies. This experiment provided the first direct measurement of the deviation in individual drop trajectories induced by wind near the Thies Clima LPM, a disdrometer commonly used in hydrological studies and applications. The effect of the non-radially symmetric geometry of the instrument on wind direction was observed, identifying the configuration most affected (parallel to the laser beam). The repeatability of the drop releasing system was checked by releasing multiple drops from the same position. This allowed attributing differences in the observed trajectories to a variation in the drop diameter. The collected dataset can be used to validate numerical models of the wind-induced bias of disdrometers and to develop adjustment functions for field measurements.