Study Indicators of Environmental Flow and PHABSIM EcoHydraulic Model to Estimation of Ecological Optimal Release Flow Range from Latyan Dam Reservoir

The study of fish habitats is important for us to better understand the impact of reservoir construction on river ecosystems. Hydropeaking hydropower plants are the main source of renewable energy, meeting sub-daily peaks in electricity demand. Dam reservoir that regulates river water to supply different sectors demands such as drinking water municipal, agricultural, and hydropower generation and this ignores the need for water in the river as the first beneficiary. The development of a rational watershed planning and management plan (develop a suitable ecological flow scenarios) is significant for the survival, reproduction, and development of fish. Recently, several classical approaches were used to estimate ecological flow, including hydrologic, hydraulic, and a habitat suitability modeling approach. The hydrological approach determines ecological flow based on historical hydrological data, of which one representative method is the Tennant method. The hydraulic approach determines the ecological flow according to the wetted perimeter of the cross section of the river. Hydrological and hydraulic approaches are favored because of their simplicity and ease of calculation, but both of them lack the biological mechanisms and biological requirements. Fortunately, the habitat suitability modeling approach combines the knowledge of hydraulics and biology to establish the relationship between habitat and hydraulic factors. It has certain advantages in the evaluation of ecological flow and has attracted more and more researchers’ attention. The most classical habitat suitability modeling approach is the instream flow incremental methodology (IFIM) and its physical habitat simulation component (PHABSIM), which includes the suitability of habitat target species to a series of hydraulic factors such as flow velocity and water depth to build a habitat suitability index model. The habitat suitability modeling approach provides the best range of hydraulic factors such as flow for habitat target species, which has certain reference significance for guiding reservoir operation. The Physical Habitat Simulation Model (PHABSIM) is proposed to simulate the relationship between streamflow and physical habitat for various life stages of target fish species, and thus to determine the optimal ecological flow of the representative river reach.The classical PHABSIM includes two components, namely hydraulic simulation and habitat modeling. On the basis of that, one-dimension hydraulic simulation model is proposed to determine characteristics of the stream in terms of depth and velocity as a function of discharge. As for habitat modelling, the river sectors and aquatic species most vulnerable to the variation of streamflow should be identified initially. Then, the Habitat Suitability Index (HSI) is introduced to reflect the preferences of target fish species with regards to the flow velocity, depth, and channel properties. Basically, HSI was determined according to the number of fish population appearing at the target point. The maximum value of HSI is set at 1.0, and the rest of the HSI values are determined in terms of relative ratio to the maximum value. Then, the Weighted Usable Areas (WUA) is employed to reflect the amount of physical habitat that available for fish species at different flow condition, which can be calculated as an aggregate of the product of a composite HSI. In terms of ecological constraint, maintaining at least 75% of largest WUA is set as an example in optimization modelling, to describe the detailed process to incorporate the ecological demand into reservoirs operation under one certain recovery level.Based on the results, the environmental flow in the range of 2.5-18.55 m3/s was calculated as the amount of release flow needed to stabilize the fish species and habitat of Jajrood River. Also, the low flow months (July to November) compared to the high flow months (March to June), need to consider a higher proportion of the average monthly flow than the mean annual flow as the minimum required environmental flow. The reservoir operation makes flow series flat and the duration of a single pulse increases compared with flow series before construction. Overall, it is obvious that the hydrological parameters of natural flow have a lager alteration due to the construction and operation of reservoir, which may change the downstream riverine structure and threaten freshwater biodiversity. The combination of eco-flow indicators and hydrological regime alteration, which is capable of mirroring the key hydrological information and related ecological connotation, is prove to be an effective method to evaluate the eco-hydrological regimes. It was demonstrated that the operation of the hydroelectric power plant in the hydropeaking system is the cause of a large flow alteration in respect of the frequency and duration of low- and high-flow pulses and the rate and frequency of change in the flow. The change in the manner of operation of the hydroelectric power plant affected the reduction in the degree of transformation of most features of the flow. Trend of maximum and mean flow ratio and maximum and minimum flow ratio has attenuated in reservoir after condition.