Watershed‐Scale Simulation of Hydrology and Carbon Cycling With Explicit Lake Representation

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Lakes play a key role in regulating watershed hydrology and carbon transport, yet their effects are often simplified or omitted in hydrological‐biogeochemical models. This limitation is particularly evident in lake‐dominated basins, where lake can alter downstream water and carbon export. Here, we extend the Watershed‐based Integrated Simulator for the Environment (WISE) into a lake‐explicit framework that represents lake boundaries, river‐lake connectivity, lake‐local contributing areas, and hydrological‐biogeochemical processes. The model was applied to the Lake basin and evaluated using watershed outlet discharge, and dissolved organic carbon (DOC) observations at lake inflow and outflow locations. The model captured the main hydrological and DOC dynamics, with daily discharge KGE values of 0.65 and 0.64, and DOC KGE values calculated at discrete sampling dates ranging from 0.52 to 0.74 during calibration and validation, respectively. Simulations showed that lake modified watershed water and DOC transport by delaying downstream transmission, attenuating daily variability, and weakening concentration‐discharge relationships. The lake reduced annual variability of water and DOC fluxes by more than 50% in most years and reduced the DOC concentration‐discharge slope from 0.93 at inflow to 0.26 at outflow. Counterfactual scenarios showed that representing the lake as a river reach without recalibration altered the magnitude and timing of downstream DOC export relative to the lake‐explicit model. Although recalibration dampened some of the variability, the river‐reach representation still failed to reproduce the combination of peak attenuation and sustained release simulated by the lake‐explicit model. These results highlight explicit lake representation as a key direction for watershed DOC modeling.