كليدواژه :
In this paper the variation of Urmia Lake water level in last two decade based on morphological maps , meteorological data and satelite pictures are presented. These data with consedering the water balance equation for saline lakes is discussed and the effective factore in increasing evaporation rate and drying od lake is discussed.
چكيده فارسي :
Due to morphological conditions of the lake, variations of climatic conditions, and variations of water discharges to the lake, cause considerable variation in water level of the lake. The digital elevation map (DEM) of Urmia Lake (fig.1) indicate that more than 75% shoreline of lake is bounded by plains with very smooth dips. A satellite image of the lake is given in figure (2) that shows the boundary of the lake for various water levels in the years 1987, 1998, 2001, 2002. The reason for numerous spreading in area of the lake is the low slope of southern eastern and western regions of the lake. The image display that the most spreading of the lake has been occurred in the year 1998 and the least spreading in the year 2001and after that The water level and continuously has been decreased to present.
Fig .1- DEM of Urmia Lake, more than 75% of shoreline bounded by plain areas
The water balance equation for saline lakes may be described using the following equation of water volume variation with time (Wood and Sanford, 1990): dV/dt = Qp + QS + QI - QO - QE in which QP is the precipitation rate; QS is the average surface runoff; QI is the ground-water inflow (influent seepage); QO is the ground-water outflow (effluent-seepage); and QE is the evaporation rate. Measurement of water volume variation (dV/dt) in terms of Qp and QS is relatively easy to carry out. On the other hand, QI and QO are often estimated from the values obtained from other factors, including the evaporation rate. Evaporation rate (QE) from natural water bodies is a function of many variables, including salinity, relative humidity of the atmosphere, temperature contrast of air and water, wind speed and local turbulences. From decade of 1980 with increasing of dam buildings in Urmia Lake basin and construction of shahid Kalantari causeway essential changes have been occurred in water balance condition.. The width of this causeway is about 28m and its level is 3m above the level of the lake in the year 1983. From 1983 through 1993, some rock filling of 12 million m3 has been carried out from both sides of the lake for embankment construction. Meanwhile causeway is built in deepest and along narrowest part of the lake. With ending of causeway construction the water flowed to eastern and eastern plains and the area of lake is increased from 5200 km2 to about 6000 km2. Furthermore shoreline of lake increased about 25 km. Consequently
Fig. 2- variation of water levels in the years 1987, 1998, 2001, 2002, based on satellite images
the evaporation surface parameter in water balance equation increased. On the hand more than 40 dam is built on rivers in Urmia Lake basin and caused decreasing inflow by rivers. Additionally, construction of dams and deposition of water in dams was accompanied by change of sedimentation system which increased clarity of lake water. The increasing of water clarity is caused sunlight penetration to more depth and increasing evaporation rate. The increasing of evaporation has unavoidable effect on salinity of water. With increasing of salinity precipitation of hydrated mineral like Epsomite MgSiO4 together with halite (Sa´nchez-Morahe et al., 2002). Precipitation of hydrated mineral can be another factor of water loss, which in Urmia lake should be studied.
