Comparison of Temporal and Spatial Changes of Groundwater level in Isfahan-Borkhar, Najafabad and Chadegan

Isfahan province is one of the driest provinces in central part of the country center where two-thirds of its water requirement is supplied by groundwater resources. In recent years with population growth and the increase of water demand in different sectors of agriculture, industry, drinking, and health, groundwater resources have faced a lot of pressure. So, Understanding the behavior of the groundwater body and its long term trends are essential for making any management decision in areas. Mann–Kendall statistical test has been commonly used in many researches to assess the significance of trends in hydro-meteorological time series. As well as, geostatistical analysis has been theoretically defined and applied by many researchers, they have emphasized that geostatistics is a management and decision support system tool for analyzing spatial and temporal variations of groundwater level fluctuations. Geostatistical approaches can provide more helpful, reliable and efficient tools to increase the number of measurement points at unsampled locations, and variogram analyses for examining structural relationship of data (Rakhmatullaev et al. 2011).

In this study, the temporal and spatial variations of groundwater level for Isfahan-Borkhar, Najafabad and Chadegan plains with a 25 years period (1990-2015) investigated. Monthly water level data have been collected from 29, 40 and 8 observation wells in Isfahan-Borkhar, Najafabad and Chadegan plains, respectively during the study period. Precipitation data from synoptic, and climatology stations.were used in this research.
The trend analyses were done at monthly, seasonally and annual scales using Mann-Kendall Test and Sen's slope estimator nonparametric methods by Matlab 7.11 software. The Mann-Kendall test is a non-parametric method used for trend analysis of time series data. In this method, the presence of a statistically significant trend evaluated using Z value (equation 1).
(1)
A positive value of Z indicates an increasing trend and negative value indicates decreasing trend.
In this research, null hypothesis (H0) was that “there is no trend in time series of groundwater levels” is rejected if the absolute value of Z is greater than ±1.96 and ±2.58 in 5% and 1% level of significance, respectively. If value of Z is out of this range, null hypothesis is accepted and there isn’t any trend in time series (Yin et al. 2015). The groundwater level hydrograph was drawn also for each plain during 25 years (January 1990 to October 2015) in Excel software.
Prior to the geostatistical estimation, its require a model that compute a semivariogram value for any possible sampling interval. The experimental semivariograms were fitted with various theoretical models like spherical, exponential, Gaussian, linear and power in GS+ software. The underground water level zoning maps was prepared by Ordinary Kriging method for every year of the study period. Spatial zoning maps were prepared and ultimately, iso-falling map was provided at the beginning and end of the study in Arc GIS 10 package.

The Spatial distribution of trend by the Mann-Kendal test for annual groundwater level data indicated a general trend in all plains where most piezometers groundwater level is negative. the piezometer wells with positive trend were so low and the most positive trend is related to Isfahan plain. In Chadegan plain there wasn’t any positive trend in piezometers level.
The investigation of trend line slopes revealed that in average, the groundwater level of Isfahan-Borkhar, Najafabad and Chadegan plains has been decreased about 0.468, 1.12, 0.638 m y-1, respectively. The general trend of the groundwater level hydrograph shows that the level and storage of water continuously decreased in the last years.
Gaussian variogram model was the best for spatial structure of these data. Generally, the spatial pattern of zoning Groundwater maps indicated that direction of groundwater flow is northwest to southeast in all study plains. As well as, iso-falling map showed that groundwater level has dropped in most parts of the plains during the study period, Which was in agreement with this research (Rahmati et al. 2014, Azareh et al, 2014, De Brito Neto et al. 2016). The cuase of this agreement could be vulnerability of arid region ecosystems into human interference and other stressful factors.

Due to the lack of rainfall trend in the most of the stations across study area, the major cause of reduced groundwater could be related to human factors. overexploitation, growth of population, and increasing number of wells are the most important factors in decreasing of groundwater table. So the decision to management groundwater in the study area is required. it is necessary to immediately practice water conservation methods and water harvesting systems in the study area to prevent the causes of more damages to the available water resource