Impact of irrigation and reservoirs on the flow regime of small agricultural basins

There are multiple conflicts between agronomic uses and environmental conservation. On the one hand, is the environmental flow that must be respected after building hydraulic structures, as reservoirs. The water regulation of Uruguay establishes the environmental flow by the 60% of exceedance probability of monthly flows. On the other hand, there is a return flow on irrigation systems that are not considered by the current regulation. The objective of this work is to assess the water cycle of the catchment under different irrigation strategies. For this purpose, interactions between atmosphere, plant, water, and soil are quantified with the SWAT model. The model was implemented for an agricultural basin at the north of Uruguay (tala catchment, 120 km²). The main crops are rice, soybeans, and corn that are irrigated by pivots and border irrigation, where the water is conducted by long dug channels. The water source is a reservoir 1200 Hm³ constructed with an earth damn and placed over vertisols soils. The swat model was cross-validated with water volumes in the reservoir and streamflows on the outlet of the catchment. Results show the effect of return flow on the natural flow regime of the catchment. Outcomes of this work could support new policies and water regulations.