تحلیل هیدروکلیماتولوژیکی نوسان‎های سطح آب دریاچۀ ارومیه

Introduction
Lake Urmia, at the northwestern tip of Iran, is one of the largest permanent hyper saline lakes in
the world and the largest lake in the Middle East. The lake is located between Eastern
Azerbaijan and Western Azerbaijan, west of the southern portion of the similarly shaped
Caspian Sea. It extends as much as 140km from north to south and is as wide as 85km from east
to west during high water level periods. Because of being located in a dry and semi-dry region,
this region doesn’t have suitable water resources comparing with global average water
resources. Drought, climatic fluctuations, and shortage and disorder of rainfall cause many
problems with regard to food and water for people who live in this region. Urmia lake is also
one the most important and the largest aquatic ecosystems of Iran. The systemic and chain
changes in the lake will lead to great effects on the climate and economic, social and hydrology
conditions. Oscillations of the lake water levels and volume in recent years have attracted many
opinions and created apprehensions.
Methodology
Groundwater data, meteorological precipitation and temperature data were obtained from Urmia
station of Meteorology Organization for the period from 1981 to 2010. Then time series were
formed for temperature and precipitation in the form of annual, seasonal, and monthly files. The
time series of temperature, precipitation, rivers discharges, and water table levels and
oscillations of the lake water level were collected and adjusted for the periods 1981-2010. To
inform homogeneity and randomness of data and possibility of any trends in the time series, the
nonparametric test was used. In this study, precipitation, river discharges and water tables and
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∗E-mail: d.yarahmadi@gmail.com Tel: +98 9161654281
14 Physical Geography Research Quarterly, 46 (1), Spring 2014
temperatures were assessed as the independent variable and the water level as the dependent
variable. After reviewing the different parametric and nonparametric tests on the data in this
study, we eventually used a multivariable regression parametric test (Y=a+b1x1+b2x2) for
temperature and precipitation and these tests ultimately showed the ability to cover the analysis
of data and review of this study.
To determine the direction of the trend, type and time of changes based on a Man-Kendall
graphical and statistical test, the following formulas were used:
􀜷􀝅 􀵌
􁈺􀀃􀏃 􀀃􀝐􀝅 􀵆 􀀃􀜧􀝅 􁈻
􀎾􀀙􀂋
1)
􀜷´􀝅 􀵌
􀵆􁈺􀀃􀏃 􀀃􀝐´􀝅 􀵆 􀀃􀜧´􀝅􀀃􁈻
􀎾􀀙´􀂋
2)
Results and Discussion
In this study, the relationship between climatic factors and their effects on the hydrological
conditions such as the river discharge, water level of Urmia Lake and wells water table were
studied. For presence or absence of relationship between them, the Pearson correlation
coefficient was used. The highest correlation between the water table and lake water level was
0.71 which is significant at the level of 0.05. Among the four effective independent variables,
the lowest correlation was observed between climate change and the water level of the lake. The
coefficient for the river discharge and the water table was, respectively, 0.72 and -0.71. The
Pearson correlation test shows that linear gradient during the period is significant with time
increasing. The results indicate that the relationship between the precipitation and water level is
negative and temperature and the water table is positive. The regression gradient line at the
scatter plot shows that the precipitation increase raises the water level. The highest annual
decreasing rainfall is -2.56. Increasing temperatures and declining rainfall, snowfall reduction,
increasing evapotranspiration and reducing the water as input decreased the water level of the
lake. As a result, the lake water level trend was decreasing 0.18 mm in each year. The model
and the regression analysis were calculated due to the delayed effects of climate and
hydrological factors interference in each other. The coefficient determination indicates that
other factors remain constant; approximately 0.30 of the dispersion of the observed changes in
the lake water level is justified by temperature and precipitation variations. If we assume that
hydrology parameters are constant, we can say that lake water level increases 0.005 meters per
one mm rainfall and the lake water increases 1.672 meter per one cm river discharge.
Conclusion
By designing a hydrology model, it was determined that 42 percent of water level fluctuations is
due to changes in the river discharge of the region and groundwater and water table. By
examining regression models, we find that changes in hydrological parameters that are related to
human factors rather than climatic parameters have the most influential effects on the lake water
level fluctuations. The temperature increase affected the lake water level dropping more than the
Physical Geography Research Quarterly, 46 (1), Spring 2014 15
rainfall decrease. By examining Man-Kendall graphics, we characterized that leaping in
temperature started in 1993. Precipitation and discharge decreasedin1993-1994 and this caused
the rise of the trend water table and reducing of water level in Urmia Lake, happened with a
four-year delay which started from 1998.