Hydrodynamic modeling of Beysehir Lake for 1998 water year

Bu çalışmada, Beyşehir Gölü’nün üç boyutlu hidrodinamik modeli kurulmuştur. Beyşehir Gölü Türkiye’nin en büyük tatlısu gölüdür. Kurulan hidrodinamik model; 1992-2001 su yıllarının ortalama akımlarını en iyi temsil eden 1998 su yılı için çalıştırılmıştır. Modelleme çalışmasında; Devlet Su İşleri’nden (DSİ) meteoroloji, hidroloji ve batimetri verileri, Elektrik İşleri Etüt İdaresi’nden (EİE) hidroloji ve Devlet Meteoroloji İşleri’nden (DMİ) de meteoroloji verileri günlük olarak temin edilmiştir. Bu çalışmada ELCOM (Estuary and Lake Computer Model) 3D modeli kullanılmıştır. Modele girdi olarak; günlük rüzgâr hızı, rüzgâr yönü, kısa dalga radyasyonu, hava sıcaklığı, bulutluluk oranı, nemlilik, hava basıncı, yağış, giren / çıkan akarsuların debileri ve giren / çıkan akışkanının su sıcaklığı verilmiştir. Ayrıca gölün batimetrisi modele, planda 103x85 ve düşeyde 10 parçaya ayrılarak oluşturulan 87550 grid yardımıyla dijital olarak tanımlanmaktadır. Model; sonlu hacimler metodu ile hesap yapmaktadır. Modelin çıktıları; su sıcaklığı ile akışkanın x, y ve z yönündeki hızlarıdır. Model sonuçlarını daha iyi analiz etmek amacıyla yazar tarafından Matlab dilinde programcıklar yazılmış ve bu programlar kullanılarak model sonuçları detaylı olarak irdelenmiştir. Modelden elde edilen su sıcaklık değerleri ile DSİ tarafından yapılan arazi ölçümleri kıyaslanmış, modelin yeterli doğrulukta olduğu gösterilmiştir. Kurulan model yardımıyla, göldeki su hareketi ve bu su hareketlerine etki eden parametreler belirlenmiştir. Ayrıca gölde modellenen hidrodinamik olayların yıl içi değişimi özetlenmiştir.  Anahtar Kelimeler: Beyşehir Gölü, hidrodinamik modelleme, ELCOM 3D, göl yönetimi. In this study the hydrodynamic model of Beysehir Lake is established. The prepared three dimensional hydrodynamic model is run for an average one year long period (1998 water year). In addition this paper includes the basic equations and modeling techniques used in the modeling studies. The model considers the pressure, air temperature, the turning effect of the earth, the tide force, the saltiness, the wind force, and the surface heat exchanges, the effects of the incoming and outcoming rivers. In the modeling study; the meteorological, hydrological and bathymetry data is provided by DSI whereas EIE delivered the hydrological data and DMI the meteorological data. As inputs for the model simulations; wind speed, wind direction, short wave radiation, air temperature, cloudiness,  humidity, air pressure, precipitation, in-and outflow discharges are considered in daily basis together with the digital bathymetry of the lake. The model is defined in the three dimensional cells. In the plan view from the top side there are 85 and 103 sections, horizontally and vertically, respectively. The lake is divided into 10 sections in depth; 8755 (85*103 = 8755) cells are formed and divided in the program. The selected grid dimensions satisfy the Courant-Friedrichs-Lewy (CFL) condition. The time interval is selected as 3600 seconds. The solution duration of the model becomes longer in case of decreasing the time interval and increasing the grid number. The longest river reaching the Beysehir Lake is the Sarisu River with Number 19-104. This river had the highest discharge on 30 March 1998 (10.5 m3/s). The discharge entries do not affect the lake in general. The mixtures are in the local level even for high discharge entries. The minimum and maximum water temperature differences obtained from the run of the program for the 1998 water year are shown in Figure 2. The stratification clearness is high parallel to the increase in the difference between maximum and minimum temperatures. Either water motions or stratification occurs because of the density difference caused by the water temperature change on the lake. For example, the stratification increases if the water in the upper layer cools and causes to a mixture if it gets warmer. These mixtures and stratifications may occur both horizontally and vertically. With the help of the model, the water motions on the lake and the parameters affecting the water motion have been determined. Besides, the change of the hydrodynamic events on the annual basis has been summarized.  The elements which form a lake, their characteristics, and the parameters affecting these elements are summarized. The loops in the lake, the stratification and the characteristics of the mixture are explained, the affecting external parameters are defined and the basic connections between them are presented. The validation tests are applied to the models and it is shown that the fit between the model and the data is sufficiently well. In addition the parameters effective in the models are investigated and their affects are summarized. The water temperatures obtained from the model are compared with the water temperatures measured by DSI. This comparison is presented in Figure 7. If the R2 value obtained by Equation 4 is close to 1 then the model results are close to the observed values (Bayazıt, 1996). In this study R2 value is found as 0.956. The results obtained can be summarized as follows: - The opposite stratification occurring in the winter is quite obvious, The stratification in the summer months is relatively weak, Beysehir Lake is a dimictic lake,The first three parameters affecting the model are; the precipitation, moisture and wind velocity,The river entries and the discharge abstractions are not effective in the whole lake,The shallow sections of the lake reacts with the external affects in a shorter duration, i.e. the changes in the air temperature cause the warming or cooling of the water. So vertical stratification occurs in the lake.  Keywords: Lake Beysehir, hydrodynamic modeling, ELCOM 3D, lake management.&nbsp