Evaluation of RVA Method to Determine the Environmental Flow Values Of Palangver River for Preserving the Ecological Values and Comparing It with Hydrological Approaches

The effects of natural and human factors on changes in hydrological cycles in watersheds have threatened the safety and health of aquatic ecosystems and reduced biodiversity in rivers and wetlands. In recent years, in order to preserve natural ecosystems, functions and services related to them, increasing attention has been paid to the evaluation of environmental flows in these ecosystems. The present paper was carried out with the purpose of estimating the environmental water demand of Palangver river using simple ecohydrological methods that take into account the ecological characteristics of the river in addition to the hydrological characteristics.
Research

In this research, due to the existence of different ecological conditions and different habitat diversity in the upstream and downstream of Palangver river, the data of 2 hydrometric stations of Masjedpish in the upstream and Kolesar in the downstream were used at first. Then the effective meteorological parameters such as precipitation and the periods of wet and drought year were analyzed and by calculating the hydraulic regime of the flow, the flow rates in the river were analyzed in different months of the year. In the next step, the environmental flow of the river was calculated on a monthly basis using the variability curve method in minimum and maximum conditions and compared with the results of other hydrological methods such as Tennant, Texas, modified Texas and transfer of flow continuity curve methods.

The environmental flow values estimated by the RVA method in the Palangver River at two hydrometric stations of Masjedpish in the upstream and Kolesar in the downstream, in the conditions of minimum environmental flow were obtained 0.84 and 5.01 cubic meters per second, and in the conditions of maximum environmental flow were obtained 1.4 and 9.69 cubic meters per second, respectively. The estimated values of the environmental flow in minimum conditions at Masjidpish station show that the RVA method can completely maintain the ecological conditions of the river in a part of the year, because in the months of September to May, it covers a number between 60 and 80% of the monthly average. But in the months of June, July and August, when water withdrawal from the river is increased due to the agricultural uses, the RVA method has considered about 20-40% of the average flow; although this amount is far from the ideal conditions, it can be an acceptable amount to maintain minimum ecological conditions in the river considering the environmental conditions and water resource allocation management.
Also, the results of the estimated values of the environmental flow in the minimum conditions at Kolesar station in the downstream of the river showed that the RVA method can maintain the ecological conditions of the river because in the whole year, it has been able to include a number between 55 and 80% of the monthly average even in hot and critical months.

In the condition of lack of data, using the values estimated from the flow variability range method in minimum average conditions can maintain the ecological conditions of the river to a large extent by providing reasonable values. Since the main spawning time of fishes in the Palangver ecosystem is in the months of March, April and May, therefore the results of the RVA method have been able to maintain the suitable conditions for this important ecological function in the minimum conditions in both upstream and downstream stations and the full scientific guarantee for this ecological value is to maintain fish spawning operations in this area. It is also recommended to use RVA methods in the maximum conditions and FDC-shifting method in the cold months of the year, i.e. from October to May, due to providing values close to the average or even higher. But in the critical months of the year, it is recommended to use RVA methods in the minimum conditions, modified Texas method, Texas method and FDC-shifting method respectively.