Monitoring the characteristics of meteorological droughts in Hashtgerd plain and investigating their impacts on groundwater level

Drought is a global phenomenon that is caused by the lack of precipitation and occurs in most landscapes. This phenomenon occurs in different climates and causes degradation of soils and natural resources (water and soil) and human life. Many of these drought effects are not directly caused by rainfall deficits, but are related to below-average storage conditions in surface water, reservoirs, and groundwater which are the consequences of the propagation of a meteorological drought into the hydrological system. Drought indices make transforming information of climatic anomalies easier and allow the scientists to assess the climatic anomalies quantitatively. For example, Nohegar and Heydarniya (2017), studied the evaluation of the effects of meteorological drought on groundwater table fluctuations. Bloomfield et al. (2019), considered the changes in groundwater drought associated with anthropogenic warming. In the absence of long-term changes in precipitation deficits, they infer that the changing nature of groundwater droughts is due to changes in evapotranspiration (ET) associated with anthropogenic warming. In this study, the seasonal variability of the groundwater level, and its relation with drought was studied for selected wells. The duration of drought events and lag times of groundwater responses, concerning the onset and termination of drought events, were also studied. The objective of this research is the analysis of drought conditions in Hashtgerd district, Iran, based on RDI and GRI. Meanwhile, the effects of Meteorological drought on groundwater resources have been studied as well.

The Hashtgerd Plane with a coverage of 1170 Km2 is located between 445667‘E and 508251’E longitudes and 3960289’N and 3996637’N latitudes in the southern part of Alborz province. The data was collected from 8 wells over the plain, while climate data were taken from Ghazvin weather station. The two well-known drought indices of RDI and SGLI were used. For determining the trend in time series, the Man-Kendall test was used. SPSS software was used for running Pearson correlation test and finding the correlation between groundwater level and drought indices. Finally, the simple linear regression between these mentioned time series was applied to determine the impact of different parameters on the groundwater level.

It should be mentioned that SGLI behavior is in accordance with groundwater. In some years, RDI is below zero lines, in which groundwater table depletion has occurred. In recent years, for most wells, the severity of the drought and its occurrence have increased. Increasing severity of droughts is equivalent to a decrease in the amount of SGLI, and increase in the number of years remaining in drought. According to the results, for the period of study, Qazvin Meteorological Station cannot be categorized in any of the drought classes (RDI values are more than -1). Also the correlation coefficient between drought indices has some fluctuations in 12, 24, and 48 months; however, NamakAlan well has the highest correlation with the index in 48 months’ period (0.7). The correlation between RDI and groundwater drought is the lowest for 1 and 3 months’ periods. Investigation of simple linear regression relation showed that 47% of the variance of average water level is affected by drought, and the remaining (53%) is the share of other factors. If the groundwater recharge in an aquifer is from the high elevated mountains (such as Hashtgerd Plain), expecting the short-term impact of a decrease or increase of precipitation on groundwater level in all wells in the region is practically incorrect. Therefore the contribution of other factors in reducing or increasing the water balance should be investigated. Given that part of the study area, plays the role of aquifer recharge, the selection of a few limited points to investigate the effects of precipitation, temperature and other meteorological variables (and even meteorological drought) on groundwater level will not yield desirable results, and small units should be selected and studied as far as possible.