Development of monthly optimal flow regimes for allocated environmental flow considering natural flow regimes and several surface water protection targets

A key question in environmental flow (EF) allocation from reservoirs is the annual distribution of flow. Flow release from reservoirs can be partly supplemented or compensated for by natural runoff from the downstream (residual) part of the catchment. Here we present a new hydrological approach to estimate the optimal intra-annual flow regime for EF which also considers water inflow from the downstream residual sub-catchment. First, the standard intra-annual regimes at downstream target points are defined for reference. This annual hydrograph is then divided into two main parts: a variable hydrograph (related to upstream of the reservoir and the point of EF release) and a residual hydrograph (associated with downstream catchment runoff). Based on the magnitude of the residual hydrograph, the allocated EF is estimated by optimization. This method provides a first estimate of the intra-annual EF regime based on hydrological data, but biological, geomorphological, and other effective components can also be included in the optimization procedure. To test the method, the intra-annual EF regime was estimated for four different points of the Kor river in southern Iran. The results showed that the best fit between the EF regime and the natural river regime at the reference point depended on the magnitude of allocated flow and the allocation site in the catchment.