Investigation of groundwater level changes trend (Case study: Urmia plain)

In the world, groundwater resources are counted the second freshwater resources after polar glaciers. Investigating changes in groundwater levels in any region has an important role in planning and sustainable water resources management. Continuous decline of groundwater level has been observed in many places of the world in the past half-century. It has indicated the depletion of groundwater reserve in large scale. This reserve depletion has been caused either by over-exploitation or reduction of groundwater recharge. In the recent years, groundwater use has continuously increased to meet demand for expanding industrial, agricultural and urban water supply. Meteorological parameters changes exert extra pressure on further reduction of groundwater level. Since, until now, the comprehensive studies about trend changes of ground water level have not accomplished in Urmia region therefore the main goal of the present research is to investigate the changed trend of Urmia ground water level using Mann-Kendal non-parametric method.
The study area has located at 55° 10´ 26" longitude and 37° 4´ 12" latitude in northwest of Iran. For studied region, the mean annual precipitation is 304 mm and its climate is dry and cold. This plain located in west part of Urmia lake in west Azarbayjan. In the present research, trend analysis was conducted on groundwater level of 31 piezometeric stations in the monthly and annually scales in the period of 2002-2011 using Mann-Kendall non-parametric test. Data Analyze was carried out using the boxplot software in SPSS. Trend line slope was estimated by using Sen’s estimator for each time series. Homogeneity of trend was tested using the Van Belle and Hughes method.
The results showed that the trend of groundwater level for all stations was negative and the negative trend in 1% probability level was significant for 56 percent of the stations. The investigation of trend line slopes revealed that in average, the groundwater level of Urmia plain has been decreased about 19.9 cm/year. Also, the results of homogeneity test of trend in 5% probability level showed that the trend was non-homogeneous for different months and heterogeneous for various stations.
According to the obtained results from the present research, the changes trend of the region ground water level is negative. If the exploitation from ground water resources continues in the present form, social and economic crisis can be formed. According to the results of this research, it is necessary to control the ground water exploitation and ground water level will be increased by increasing agricultural water efficiency and artificial recharge.