A Short Viwe on Hydrological Data
Considering the climate of the region, Urmia Lake’s basin could potentially hold about 6800 Million cubic meters of water. Urmia Lake is supplied by direct precipitation, 17 permanent and ephemeral rivers and 39 intermittent streams mostly discharging into the lake south. Groundwater also supplies a very small fraction of the water inputs to the lake. All these resources directly depend on precipitation over the basin. Hence, the hydrological condition of the lake directly reflects the climatic conditions in the basin, particularly precipitation, temperature and humidity. The climate of the basin is characterized by cold winters and relatively temperate summers. The mean annual rate of precipitation over the lake is about 299 mm, and the mean annual evaporation rate is 1000 to 2100 mm. The long-term average water discharge to the lake was estimated to be about 5000 million m3 annually. River inflows are highest in spring as a result of snowmelt (Department of Environment, 2010). The flow circulation pattern alongside the lake is counter clockwise and the north- to-south and south-to-north flows through the causeway opening are almost equal in most of the year, except in spring when a south-to-north flow dominates as a result of considerable amount of freshwater entering the south part.
Fig. 1: Schematic river basin lake
Reviewing the existng data, three distinct periods are detected:
• The first period from 1968 to 1991, in which the average annual precipitation and inflow into the Lake has been 331 mm (compared with long term average of 320 mm.) and 4.2 BCM/y respectively. During this period the average water level in the Lake was 1275.9 (varying between 1275.4 and 1276.7), i.e. almost stable and variations are within the range of annual evaporation from the Lake’s water surface.
• The second period from 1992 to 1998 was a wet period in which an average of 6.5 BCM/y of flow (a peak of more than 11 BCM/y) entered into the Lake. During this period the average annual precipitation was 351 mm. Water level reached its maxima ever recorded, i.e. 1277.9. The average water level during this rather short period was 1277.2 m.
• The third period started in 1999 and is continuing till now. In this period, with the exception of year 2003 in which river inflows was slightly above the average, in the remaining years the inflows were below the long term average. During this period (up to 2006) the average annual precipitation has been about 275 mm/y and river inflows have been around 3 BCM/yr.
Data of river inflows into the Lake is missing for the years (2008-2011), partly because the field observations need quite a lot of calculations and processing before being available as river flow data. However precipitation data confirms that drought has continued, and it is estimated that during these years, much lesser flows than 3 BCM/y has entered the Lake. As a consequence, water level has progressively receded since 2000.
Fig. 2: Latest status of water resources (rainfall and runoff) in the basin of Urmia Lake
To sum up, Urmia Lake drying process started twenty years ago and during this period has declined by more than eight meters. In 1995, after recording the highest level of the lake (1278.48), water level has annually decreased in average 40 cm within last two decades. In September 2015, it reached to the lowest level and southern parts of the lake totally dried. Indeed, the Lake is a moderately shallow water body (average about 6 meter), there have been evidence of significant shrinkage in lake area and volume, during this period. The following lake long-term water balance fluctuation chart and satellite images indicates above mentioned.
Fig. 3: Long-term water balance fluctuation