Van Gölü'ndeki su seviyesi değişimlerinin hidrometeorolojik parametrelerle ilişkisi

Abstract

Van Gölü, ülkemizin en büyük, dünyanın 4. büyük gölüdür. Son yıllarda göl seviyesinde önemli değişiklikler olmuştur. Gölün su seviyesi giderek yükselmiş, kıyı şeridinde bulunan dinlenme tesislerine, iskelelere, tarım arazilerine ve ulaşım ağlarına zarar vermiş, kullanılmaz hale getirmiştir. Tarih boyunca sürekli değişken olan göl su seviyesinde son 70-80 yıl içinde önemli bir değişiklik olmamasına rağmen 1968, 1969 yıllarında gölde bir sıçrama olmuş ve 1 m. 'yi bulan bu yükselme geri çekilmemiştir. 1988 Yılından itibaren de çeşitli değerler alarak hep yükselmeye devam etmiştir. Van Gölü'ndeki seviye yükselmesi çeşitli nedenlerle izah edilmeye çalışılmışsa da elde somut sayılabilecek fazla bir bilgi yoktur. Bu çalışmada gölün hidrolojik parametreleri olan yağış akış, buharlaşma değerleri 1965-1975 ve 1985-1995 aralıklarında elde edilmeye çalışılmış ve bu parametreler arasındaki ilişkiler belirlenmeye çalışılarak bunların hangilerinin seviye değişikliğine neden olabilecek bir eğilim içine girdiği belirlenmeye çalışılmıştır. Devlet Meteoroloji İstasyonu'ndan (DMİ) alınan yağış, buharlaşma, sıcaklık değerlerinden yola çıkılarak, eksik değerler regresyon-korelasyon analizleri ile tamamlanmıştır. Akış değerleri için Devlet Su İşleri (DSİ) ve Elektrik İşleri Etüd İdaresi (EİEİ) akım yıllıklarına başvurularak, eksik değerleri yine regresyon- korelasyon analizleri ile tamamlanmış ve bu değerler seviye gidiş değerleri ile karşılaştırılarak etkin parametrenin bulunmasına çalışılmıştır.

Lake Van is the largest lake of our country, and the fourth largest lake in the world. The water level of the lake which is accepted to have an elevation of 1646 m. has rised in past years and reached over 1650 m. Due to their nearness to the lake, a number of facilities such as summer resorts, quays and transportation network together with farming fields can no longer be used as a result of the damage that came about following the sudden rise of water. Although there has always been fluctuations in the water level of the lake throughout the history, the records show no significiant change in the last 70-80 years except for a jump in the water level which added roughly 1 m. to it during the years of 1968 and 1969. It should also be noted that the water level in the lake has not gone back to its previous level ever since. Lake Van is a soda lake. The sum of bicarbonate and carbonate ions slightly suppresses the chloride content in mg/l. All the rivers reaching to the lake carry more bicarbonate than alkali earth metals; consequently, large amounts of the sodium are balanced in the rivers with bicarbonate rather than with chloride later in the lake. This is the reason why Lake Van has developed as a soda Lake. Lake Van is very alkaline, and that is, it contains absolutely more potassium and lithium and relatively more sodium than seawater. The basin of Lake Van looks very much like a mountainous region. Steep slopes and changing heights are typical. On the basin, elevations range from 1650 m. to 2600 m., and eventually reach up to an height of 4000 m. in the volcanic summits. In the west and northern parts of the region, there are only a few high mountains and the peaks are lower in general. On its west and north side there are Nemrut and Süphan mountains which constitute the highest mountains of the region with elevations 3050 m. and 4434 m., respectively. The eastern side of the basin is lower than the southern side of the basin, and the mountain rows are more spaced. There are narrow plains among these mountains. Surroundings of Lake Van get higher in the east, starting from a base about 1730 m. to further east, towards Turkish - Iranian border. These mountains on the eastern and southern sides of the Lake had strong breaks and curls, and these movements caused old layers to come out on to the surface in most places. The formations on western and northern sides of Lake Van are different from the southern and eastern sides by means of taking form and structures such that a few volcanos and neocene layers that surrender these volcanos take place instead of usually high rows of mountains. These table - like plateaus are 200-300 m. higher than the surface of the lake in most places. Winter is very severe in the region with three months having mean temperatures below 0° C. Precipitation in winter normally falls as snow. Precipitation can be maximum in early Spring. July, August and September are both the warmest and the driest months of the year. Precipitation increases in Autumn as temperature decreases. Zilan Brook, Deliçay; coming from north, Brook Bendimahi; coming from northeast, Karasu; coming from east, Hoşap Suyu; coming from southeast and Sufresor Suyu; coming from west are the most important water sources of Lake Van which has a total flow area of about 12000 km2. There is no flow out of the lake, and the statistical data about underground water is inadequate. This lake which was initially formed 60000 years ago, when Nemrut and Süphan mountains were active, has all the characteristics of an interior sea with a surface area of 3500 km2, water volume of 600 billions m3 and a depth of nearly 450 m. Lake Van is defined as a lake by seagraphic maps and world literature where- as common people often call it a "sea". Closely related to the drastic changes of the climate on earth in general and in the region in more specific terms, the water level in the lake has moved up and down from time to time in history, at one point marking 74 m. above and 250 m. below at the other, than what it is today. The encyclopedic data include that the Erciş town had a low elevation after the rising in 1800's. Today its elevation is 1175 m. XIV Many scientists claimed that the rising water level of Lake Van was due to the tectonic activities which have occurred in the lake basin but have not given way to serious earthquake. On the other hand, some of the scientists argued that the stains occurring on the sun had a lot to do with the level of water in the lake. Scientists have reached to the conclusion that such events all over the world are related to meteorological conditions. This study examines the happenings in the lake through the past 30 years in hydrological terms. The aim of this study was to obtain the hydrological parameters of the lake ; namely, flow, evaporation and precipitation values, thus to determine their effects on the lake's water level. This study started with the values for evaporation, precipitation and temperature which were measured at Ahlat, Tatvan, Erciş, Muradiye, Van stations;and were asked from The State Meteorology Station, but especially the deficiencies about evaporation values were tried to be completed by statistical ways. For these lacking values.regression and correlation analyses were used and by forming relations between the stations that are close to each other, the lacking values were tried to be obtained. To obtain the evaporation height of Lake Van,the Thiessen Polygon.which is drawn on a map with a scale of 1/200 000 was used. Every station's area of effect was found and by taking average area.monthly evaporation height was obtained. To calculate the amount of precipitation falling on the lake surface another Thiessen polygon was used, and precipitation height was obtained as monthly average values as before. To find the amount of water which goes into Lake Van as a surface flow and the flow height that this creates, flow values which were measured in Deliçay-Payköy, Zilançayı-Koçköprü, Sufresor Suyu-Kınalıkoç.Bendimahi Çayı-Gönderme Köprüsü, Karasu-Gölalan, Hoşap Suyu-Zemek Köprüsü stations were directly taken from Devlet Su işleri (DSİ) and Elektrik İşleri Etüd İdaresi (EİEİ) flow annuals. Data collected between years of 1965-1975 and 1985-1995 didn't show all the flow values. These missing values were obtained by regression-correlation analysis which is a statistical way. All the flow values were summed up and divided by 3500 km2 to find the height of the surface flow. XV By comparing the obtained values of the evaporation precipitation and surface flow heights with the level values measured in meteorological station of Van.their graphs could be drawn. As a conclusion, these graphs, the level of water of the lake rises in May, June, July every year,and falls down a bit in coming months. The variations of the water level of Lake Van, in other words any increase or decrease on the water levels depend on surface flow, precipitation and evaporation and the underground water. When the precipitation is inadequate and the evaporation is much during the dry years, the water level of the lake decreases. The level rises by more precipitation and surface flow. In the period of 1969-1995, the lowest value of the maximum water level values of the lake is 1647.71 m. and it has occurred in November 1972 The second lowest value is 1647.94 m. and it has occured in November 1987. The increase of the water level in 24 years between 1972-1995 is 2.75 m. increasing the level up to 1650.47 m. Because the values of flow height have usually taken their maximum values 2 months before the level becomes maximum., it became evident that there was a two months phase difference between flow and level. When we look at all the years one by one, we see that the most effective parameter on level is flow. The measurements taken around Lake Van suggest a long lasting policy to be dated and applied. In the folowing there are some suggestions about the subject:; -1) There is not enough knowledge about the underground water. 2) Lack of low scaled maps rendered all studies harder. 3) Water level in the lake became higher following the increase in the surface flow values. Accordingly, lower values in the level were obseved as the evaporation effects on the lake. This phenomenon shows that the flow height is the most effective parameter on the level differences of the lake. XVI 4) A connection between total annual values of precipitation, evaporation and flow height values and annual maximum water levels has been tried to be obtained ; however, a meaningful relationship could not be found. 5) A two months phase difference was determined between the flow height and level values. This phase difference showed that the water level in the lake was related not only to the surface flow but also to the seepage and underground flows. 6) The hydrometeorologic and tektonic measuring - following net which is very insufficient on the basin must be advanced and a healthy data base must be built. 7) In order to determine the effects of possible rising of the lake, wave action and sand bank erosions on the neighboring settlements, topographic maps must be drawn on a scale of 1/ 10.000 up to 1470 m. and on a scale of 1 / 1000 where significant construction takes place along the bank side line. Accordingly, middle and long term plans and maps indicating the use of the bank must be prepared. 8) The event of rising level on the lake must especially be examined in a whole system by Meteorological, Hydraulic, Hydrological, Geological, Geophysical, Geomorphological and Geodezical aspects. 9) Besides level rise of the lake "effect of wave and bank erosion" which are very important problems must be scientifically examined and suitable ways must be prepared for the banks and proges, which have a risk of potential bank erosion. 10) The needed administrative and technical decisions must be obtained for all these applications. 11) Snow depth observation in the region showed that snow depth is maximum while the lake water level rises. Increased snow fall has also been effective for increase of water level. 12) Related with the level rise of the lake, the flow conditions have changed by the regular water levels of the rivers which are flowing into the lake. So especially in XVII spring we must take measures to prevent the risk of overflowing in regular regions of river - beds. 13) The effect between watering and water structures that are planned or will be planned in the basin of the lake or neighborhood basins and the lake itself must be examined in more detail. Investments for dams and watering that are planned in the basin by DSİ must soon be completed.