Assessing the cumulative impact of on-farm reservoirs on modeled surface hydrology

Abstract. On-farm reservoirs (OFRs) are essential water bodies to meet global irrigation needs. Farmers use OFRs to store water from precipitation and runoff during the rainy season to irrigate their crops during the dry season. Despite their importance to crop irrigation, OFRs can have a cumulative impact on surface hydrology by decreasing flow and peak flow. Nonetheless, there is limited knowledge on the spatial and temporal variability of the OFRs' impacts. Therefore, to gain novel understanding on the cumulative impact of OFRs on surface hydrology, here we propose a novel framework that integrates a top-down data driven remote sensing-based algorithm with physically-based models by leveraging the latest developments in the Soil Water Assessment Tool+ (SWAT+). We assessed the impact of OFRs in a watershed located in eastern Arkansas, the third most irrigated state in the USA. Our results show that the presence of OFRs in the watershed decreased annual flow on average between 14 and 24 %, and the mean reduction in peak flow varied between 43 and 60 %. In addition, the cumulative impact of the OFRs was not equally distributed across the watershed, and it varied according to the OFR spatial distribution, and their storage capacity. The results of this study and the proposed framework can support water agencies with information on the cumulative impact of OFRs, aiming to support surface water resources management. This is relevant as the number of OFRs is expected to increase globally as an adaptation to climate change under severe drought conditions.