Environmental flow estimation for susurluk, gedi̇z and euphrates-tigris basins

As the civilization level of humankind increases, urban, agricultural or industrial energy and water needs are increasing. Because of limited resources, it is very important to ensure a balance between the socio economic benefits provided by the rivers to ecological needs of aquatic life. Therefore, determining the needs of the aquatic ecosystem and the application of the environmental flow that can respond to these needs are indispensable for sustainable water resources management. There are many environmental flow assessment methods in the literature. These are grouped under four categories; Hydrological, hydraulic rating, habitat simulation and holistic methods. In this study, environmental flows were estimated using hydrological methods, which are based on analysis historic streamflow data. Hydrological methods are the most popular technic because of low cost. Daily flow data of 4, 9, 11 flow monitoring stations in Susurluk, Gediz and Euphrates-Tigris river basins, respectively, were used in the study. It has been studying with data of at least 46 years in the Susurluk Basin. The shortest observation period of stations in Gediz Basin is 14, the longest is 52 years. The shortest observation period in the Euphrates-Tigris Basin is 25 years. This study was carried out to contribute to the planning of sustainable management of water resources in Gediz, Susurluk and Euphrates-Tigris river basins. For this reason, natural flows of several rivers in these basins have been studied. The monthly driest, wet and median stream hydrographs of the rivers were generated. These hydrographs were used to determine the difference between monthly maximum, minimum and median values. In addition, dry, wet and median years are determined according to the annual total water volume. Hydrographs that compared the driest, wet and median years of flow stations were plotted. In this way, the similarities between dry and wet years of rivers and the difference between median years of wet and dry years are examined. Then, environmental flow forecasts were made by hydrological methods. By examining monthly median flow hydrographs, it is understood that the wet season is December to April for Susurluk Basin. It is observed that in wet season, monthly maximum and median flow values rises in similar way. However, the behavior of monthly minimum flows is independent from the dry and wet periods. In other words, it has been observed that the minimum value of the wet period may be smaller than the minimum value of the dry period. All in all, April to December is Gediz Basin's wet period. During the dry season, the median flow values are very low, especially in summer. The monthly minimum flow values of the rivers are observed near-zero or zero in summer. Euphrates-Tigris Basin's rivers, as distinct from Susurluk and Gediz, are fed from snow water. Accordingly, the wet period is not seen in the winter months, but in the spring and early summer, when the snow melts. High flow period for Euphrates-Tigris Basin is March to June. Monthly maximum and minimum flows increase in the wet period, but there are also exceptional cases in this regard. The months, when lowest monthly flows of Euphrates-Tigris Basin rivers are observed, mostly August, September or October. The driest month changes from river to river. In the Susurluk Basin, the monthly flow values of the wet year are very different from the values of the median year in the winter months (from the December to the March), but this difference is very small in the summer and autumn (from November until July). The monthly flows difference, between median and dry year, is maximum at February and March. It has been observed that between July and November the flow values of the median and dry years at similar levels. The most common wet year observed in the Susurluk Basin is 1963. Drought years are different for each station. Monthly flows of wet years of Gediz Basin rivers have reached very high levels for one or two months. Apart from these months, they have been observed similar levels with the median years monthly flows. Generally, in Gediz Rivers wet, median and dry years highest flow values are very different from each other. For D05A021, D05A041, E05A014 and E05A024 2001, for D05A028 and E05A023 1989, for D05A026 and D05A039 2007 have been the driest years. There are no common years between wet years. In the Euphrates-Tigris Basin, the flow differences between the median years and the wet years suddenly rise in March, April and May. These months apart, median and wet years monthly flows are very close to each other. The difference between the median years and the dry years becomes apparent in the wet season, March, April, May and June. In July, August, September, October and November the wet, median and dry years and monthly flows are very close. In many basins throughout the basin, 1988 was the wet year. 2010 is the wet year for stations E21A049 and E21A054. 1983, 1989, 2001 and 2008 are common dry years. The years, which daily flow data are missing, have been completed by appropriate methods. Statistical flow-frequency analysis method is calculated as the lowest flow for seven consecutive days within 2, 5, 10 year return periods. Estimates were made by using 2 parameter log-normal, Weibull and 3 parameter log-normal, GEV probability distribution functions. In order to test distribution functions according to observed flow data, correlation analysis, q-q plot, root mean square error and bias have been used. Flow duration curve shows percent of time during which discharges were equaled or exceeded in time. Streamflow estimates Q50, Q70, Q80, Q90, Q95, Q99 were computed for 50, 70, 80, 90, 95, 99 percent probability of exceedances respectively. Tennant method that defines several percentages of the average annual flow according to aquatic life living conditions is also applied. In addition, considering that 10% of monthly median flow values may be suitable for environmental flow, the method has also been included in the calculations. By using Basic Flow Method, the basic flow which is minimum discharge value for all year and monthly maintenance flow regime have been calculated. It has been observed that the calculated main current regime for the Euphrates-Tigris basin is close to the 7-day minlow flow with 2- year return period even in the dry months. The monthly flow values of the basic flow regime in the Euphrates-Tigris Basin are the largest of the environmental flow estimates for almost the entire month of the year. For all basins studied in this study, Basic Flow Method is determined as appropriate hydrological environmental method for Susurluk and Euphrates-Tigris Basins. Tennant method is most suitable for Gediz Basin. The following results were obtained for environmental flow estimation for Gediz, Susurluk and Euphrates-Tigris river basins. i. The probability distribution that is determined as most suitable for Susurluk Basin is 3-Parameter Log Normal. ii. For the other two basins, it is concluded that GEV is most successful distribution about prediction of 7-day low flows. iii. 7 day low flow with 2, 5 and 10 year return period and Q80, Q95 and Q90 flows that are calculated by flow duration curve and 10% of annual average flow offer one environmental flow value for entire year. iv. Tennant Method is to use seasonal time step. Tennant Method recommends proper environmental flows for wet and dry seasons of year. v. The Basic Flow represents minimum flow that should flow in a river along the year. For determine environmental flow regime, Maintenance Basic Flow is to use monthly environmental flow prediction. Thus, temporal variability of environmental flow is procured. The environmental flow regime is found by using this method, follows similar patterns than natural flow regime. vi. In addition, 10% of the monthly average flows are calculated to compare with other monthly methods. vii. When comparing three basins, minimum Basic flows were obtained for Gediz Basin. Generally estimated values are very close or grater than 7 day low flow with 2 year return period. viii. Environmental flow estimation methods that can respond to ecological needs are The Basic Flow Method for Susurluk and Euphrates-Tigris basins, fair or degrading condition range of Tennant Method for Gediz basin. With this study, the need for environmental flow of Susurluk, Gediz and Fırat-Dicle basins has been approached more extensively. As things stand, it is important to esitimate environmental flow that correspond ecological needs. Different environmental flow methods have to be studied and compared in order to be able to respond to the ecological need in the most accurate way.