During high flow periods, recharge tends to occur through the river bed whereas groundwater often contributes to low flows. Figure 3 is a conceptual diagram of flow through a river-supplied irrigation scheme. Figure 4 illustrates the links between surface and groundwater. Minimum demands from both existing and potential future users need to be clearly identified and assessed in relation to current and future low flows. The quality of low flows is also important.
Return flows are likely to have significant quantities of pollutants.
Low flows need to be high enough to ensure sufficient dilution of pollutants discharged from irrigation schemes and other sources such as industry and urban areas. A reduction in the natural river flow together with a discharge of lower quality drainage water can have severe negative impacts on downstream users, including irrigation schemes. Habitats both within and alongside rivers are particularly rich, often supporting a high diversity of species. It is particularly important to identify any endangered species and determine the impact of any changes on their survival. Such species are often endangered because of their restrictive ecological requirements.