From Soil to Stream: Modeling the Catchment-Scale Hydrological Effects of Increased Soil Organic Carbon

Abstract. Droughts are increasingly threatening agricultural productivity. One potential adaptation is to increase the soil water retention capacity, which can be achieved by enhancing soil organic carbon (SOC) through agricultural management. We investigated how increasing SOC affects catchment-scale hydrology including extremes. SOC increases were implemented via adjustments to soil hydraulic parameters (ρ_b, θ_PWP, θ_FC, θ_Sat, K_sat) in a mesoscale hydrologic modeling (mHM) framework, following literature-reported effects. Our analysis focuses on the medium-sized, agriculturally dominated Broye catchment in Western Switzerland, wherein we evaluated five SOC increase scenarios of varying depth and magnitude. At the plot scale, SOC increases resulted in higher net soil water content (2.99–8.13 %) and slightly higher evapotranspiration (0.15–0.4 %), while subsurface runoff was reduced (0.28–0.72 % across all scenarios). These values represent overall net changes; while at shorter timescales, the magnitude and even direction of effects varied by season and location. Increased water retention meant more soil water was retained and latter evaporated and less was available for groundwater recharge and eventually as streamflow. At the catchment scale, streamflows were slightly reduced, with peak flows modestly attenuated. Low flow responses depended on catchment characteristics and timing. In warmer and drier subcatchments, low flow frequency increased in some years, whereas in cooler and wetter subcatchments, conditions in spring and early summer produced a beneficial effect, slightly reducing low flow frequency. Overall our analysis suggest that a large-scale increase in SOC, while benefiting agricultural productivity and peak flow attenuation, may also induce trade-offs by potentially reducing groundwater recharge and downstream water availability.