From Field to Catchment: Evaluating the Hydrological Effects of Soil Organic Carbon Increases with the distributed mesoscale hydrologic model mHM

Due to the increasing duration and magnitude of both agricultural and hydrological droughts, farmers face the problem of declining yields and reduced irrigation possibilities. In our recent study (Heinz et al. 2025, under review), we found that increasing soil organic carbon (SOC) could increase soil water retention and thus mitigate yield losses during a recent drought year. However, it is unclear how the accumulation of SOC in agricultural soils could affect hydrological processes on the catchment scale.Local- to regional-scale changes in land use, such as afforestation, or structural changes, such as terracing or check dams, on catchment scale hydrology have been widely studied (Farley et al. 2005; Deng et al. 2021). The effects of agricultural management adaptations at the field scale are less well understood. However, for example, the effect of switching to conversational tillage as a soil conversation measure is thought to reduce flood peaks and increase rise times (Samanta et al. 2023), highlighting the need for further research.  In this study, we address a major research gap by assessing the influence of increasing SOC on catchment-scale hydrology using the Mesoscale Hydrological Model (mHM). The study focuses on the mid-sized Broye catchment in western Switzerland, where mHM was applied with the novel subcatchment conservation module SCC, which significantly improved the simulations (Shrestha et al. 2025, under review).In the SOC-increase scenario, we assess the impact on key hydrological fluxes (e.g. evapotranspiration, percolation, groundwater recharge) and reservoirs (e.g. soil water storage), as well as the overall water balance and discharge dynamics.Preliminary results indicate that while SOC enhancement causes measurable changes in soil water storage and fluxes at smaller scales, its overall effect on catchment scale water balance and discharge is limited. These modest effects may be due to physical insensitivity of large-scale hydrological processes but may also be due to model limitations in parameterisation and representation of localised changes. This will be subject to further analysis, as will the assessment of the effect of increased SOC on peak and low flows.References:Deng, C., G. Zhang, Y. Liu, X. Nie, Z. Li, J. Liu and D. Zhu (2021). "Advantages and disadvantages of terracing: A comprehensive review." International Soil and Water Conservation Research 9(3): 344-359.Farley, K. A., E. G. Jobbágy and R. B. Jackson (2005). "Effects of afforestation on water yield: a global synthesis with implications for policy." Global Change Biology 11(10): 1565-1576.Heinz, M., M. E. Turek, B. Schaefli, A. Keiser and A. Holzkämper (2024). "Can adaptations of crop and soil management prevent yield losses during water scarcity? - A modelling study [Preprint]." EGUsphere 2024: 1-33.Samanta, S., S. Ale, D. K. Bagnall and C. L. S. Morgan (2023). "Assessing the watershed-scale effects of tillage management on surface runoff and sediment loss using a Curve Number-precipitation relationship based approach." Journal of Hydrology 625.Shrestha, P.K., Samaniego, L., Rakovec, O., Kumar, R., and Thober, S. (2025)(under review). “Enhancing Global Streamflow Modeling to Enable Locally Relevant Simulations”. Water Resources Research.