The influence of small farm reservoir network characteristics on their cumulative hydrological impacts

Abstract. In many regions of the world, the use of infrastructure to store runoff and stream water, such as small farm reservoirs, is the only way to enable irrigation and thereby secure and increase food production. The presence of multiple reservoirs in one catchment has cumulative impacts that are not necessarily the sum of the individual impacts. However, we still have little knowledge of the spatial factors that drive these cumulative impacts. In this work, the effects of the distribution of small reservoirs in a catchment on their hydrological impacts are investigated with a modeling approach. Our numerical experiment consists of randomly generating multiple small reservoir networks in the same catchment with realistic reservoir numbers, capacities, and spatial distributions and then comparing their hydrological impacts over a 20-year period. We focused on two variables, namely, the outlet discharge and the mean proportion of the network in low flow, which we computed annually and seasonally. We used the distributed agrohydrological model MHYDAS-small-reservoir, which represents small reservoirs and their links with the hydrological network and the irrigated plots. In our context and with current reservoir management rules, we found that the impacts of reservoirs are more important in summer, with discharges reduced by more than 20 % and up to 60 % compared with the reference situation without reservoirs. Moreover, low flow proportions are always higher than those in the reference situation. For these two indicators, the main explanatory factors are the number and distribution of reservoirs, with a limited effect of the storage capacity. The effects of the study factors on the seasonal and annual indicators were thoroughly interpreted with respect to the hydrological functioning of the catchment and the timing and amount of irrigation. This work contributes to a better understanding of the drivers of the cumulative hydrological impacts of small reservoirs. Although many questions remain, our results can help scientists and water managers choose the best representation of small reservoirs in their models to address their needs.