Irrigation is supplied primary by sources of fresh water. However, irrigating with water that has been previously used for irrigation is very common. This paper presents some basic theoretical concepts for understanding the effects of multiple reuses of irrigation water on the global irrigation consumptive use coefficient (ICUC), water pollution and crop yield. Reusing agricultural water implies an increase of the ICUC but entails the degradation of water quality. Understanding the effects of the complex hydraulic connections in water reuse on the global ICUC, on the spatially distributed agricultural production and on the pollution of surface and ground waters is imperative for optimising the exploitation of irrigation return flows. Two case studies are presented. One in Northern California, where the connection between the drainage and irrigation networks allows partial internal reuse of the water. The second applies to an irrigation sector within a larger scheme in Southern Spain.