عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Desertification is one of the global phenomena affecting natural resources which occurs not only in arid and semi-arid regions but also in some of the areas in semi-wet regions. Evaluating desertification in a region with vulnerable climate faces unpredictable and damaged conditions plays a significant role in evaluation of the environmental capabilities, prevention of desertification, and restoration and rehabilitation of degraded areas.Destroying the water resources, both their quality and quantity, can lead to decrease in biomass and resulted in desertification. The decreasing levels of ground water resources and deteriorating water quality are considered among desertification processes.
Nowadays, groundwater exploitation in arid and semi-arid regions for agricultural lands has increased and subsequently over-exploitation of these sources has degraded groundwater quality. Therefore, investigating the groundwater quality and its effects on the increasing rate of desertification intensity is a necessary task. In this research, the effects of groundwater quality on the desertification in Mehran plain have been investigated by employing water quality parameters including electrical conductivity, total dissolved solids, sodium adsorption ratio, and chloride using the Iranian model of IMDPA.
In the IMDPA model, a weight ranging from 1 to 4 was assigned to each layer based on its influence on desertification according to literature renews and researches performed by other researchers and geographical condition of the Mehran plain. Value 1 and value 4 indicate the best and the worse conditions of desertification, respectively. Numerical value of produced indices were transformed to thematic maps in ArcGIS Desktop 9.3. In order to determine the current quality of water in Mehran Plain, the data obtained from 9 quality wells located in the area have been used and analyzed over three time periods (2001-2004, 2005-2008, and 2009-2012). Then, to investigate the desertification intensity of Mehran Plains based on the IMDPA model, the criterion of water quality was used. After determining the numeral value of each parameter, the standard numerical value is determined based on the geometrical average of the selected criteria.
The information layers related to each index and criterion for all the three periods were prepared using interpolation and weighting methods in GIS. After preparing the related layers for the criterion of water quality in selected wells, the measures were combined in Arc GIS 9.3 software application to obtain the final map for the desertification status of the region for all the three selected time periods. Finally, they were classified in desertification classes of low, medium, intense, and severe intensification.
The results show that the indices of chloride and sodium absorption ratio duo to their locations in the low desertification class and quantitative value of less than 1.5 are not very effective on the intensity of desertification in the region. Based on the results, the amount of chloride and the sodium absorption ratio over the entire study region during the recent years have not fluctuated significantly, so that many of the areas are classified as low desertification and the effects of this index are not readily visible in this region. The total dissolved solids show a greater change compared to chlorine and sodium absorption ratios in the last period in so that based on this index, during the two first periods, the region is classified as low desertification while in the last period, due to higher concentration of these substances, the region is classified as mild desertification. The desertification intensity map of Mehran Plain based on the index of electrical conductivity shows that the intensity of desertification from the beginning of the period has been increased towards the end of the period in a way that 8.95 percent of the region in the first period is classified as intense desertification and in the last period, the extent of this area reaches 14.47 percent of the entire region. Moreover, the desertification map based on the indices of ground water quality during these periods shows that almost 72 percent of the region is classified as low and negligible desertification.
Decreasing of ground water levels caused by digging multiple wells has led to an increased electrical conductivity of the water from these wells as well as their salinity. Intense decline of water level due to uncontrolled withdrawal and other factors such as climate change, drought, and irrigation methods have increased the water salinity, which in turn increases the salinity of the soil in the study area.
The results obtained from evaluating the average weight of the quantitative values of the selected indices for the water quality showed that in the studied region, the average weight of the criterion of water quality is in negligible and low desertification class. Among the indices assessed for water quality, the obtained value (the score) for the electrical conductivity is higher than those for other measures, so, it has more effects on increasing the desertification intensity in the region regarding the quality of water criterion (i.e. it has caused a rise in the final amount of desertification since the value of each criterion is obtained from the geometric average of the numerical values and the value of the total desertification score is obtained based on the geometric average of the numerical value of the criteria). Hence, the electrical conductivity index is the most important index in the desertification of the region.
Considering the fact that one of the reasons behind higher electrical conductivity is the low level of water, it is recommended that uncontrolled withdrawal be avoided in this region as well as similar regions all over the country by making the wells smart using devices such as electronic cards for water withdrawal. In other words, after withdrawing a predefined quantity, there must be a power outage so that nobody can withdraw any more water. At the moment, unfortunately there are several unauthorized wells withdrawing water in all regions in the country which lead to decrease in levels of ground water and caused aquifers go dry. Hence, it is recommended that these unauthorized wells must be closed since preventing uncontrolled withdrawal is one of the effective ways to solve the water crisis.