Small reservoirs have proliferated in the latest decades in many watersheds, often drawn by more frequent and severe droughts. Reservoirs decrease streamflows and may threaten river ecosystems, while the sustainability of their uses in a changing climate is questioned. At the catchment scale, several models of varying complexity are available to quantify the impact of numerous reservoirs and the efficiency of management rules (e.g., pumping restrictions, instream flows) or incentives (e.g., alternative crops), both now and for different scenarios of global change. Our objective is to develop an open source model which catchment managers could use for long-term planning and daily management. The model is the conceptual semi-distributed model airGRiwrm, a variant of the acknowledged rain-flow model GR. This work investigates how to include in the model small reservoirs used for irrigation, fish farming and leisure, to produce information for decision making. The study site, the Sèvre Nantaise watershed in Western France (2350 km²), has an oceanic climate and no large aquifers. We complemented the existing database of the watershed reservoirs, configured the model with a detailed description, especially for the irrigation demand for each farm dam, and successfully calibrated it over observed 2008-2020 flows on 13 subcatchments. We then assessed the impact on the calibration results for different hypotheses for linking reservoir surface, filling and volume, for aggregating reservoirs, and for the terms of their water balance, especially irrigation demand and evaporation. The results indicate the adequate accuracy for reproducing low-flows, a prerequisite for simulating management or climate scenarios.