Determination of the Relationships Between the Meteorological and Hydrological Droughts in the Hashtgerd Plain

The Hashtgerd Plain of the Savojbolagh Watershed with an arid and semi-arid climate is one of the plains that have experienced a significant increase in groundwater exploitation in recent years. Over the past five years, the water table has dropped more than 5 meters. Therefore, the role of management factors in this plain is important. To find the effects of rainfall and climatic conditions on the groundwater level fluctuations, rainfall data from 6 rain gauge stations and 21 piezometric wells during the 1991 –2011 period were used. Meteorological and hydrological drought sequences were prepared by using the standard precipitation index (SPI) and the standard groundwater index (SWI) for six cumulative periods of these indices of 6, 9, 12, 18, 24 and 48 months. As the rainfall data series and groundwater level did not have a normal distribution, the Mann-Kendall nonparametric test was used to determine the trend of meteorological data and the water level, which showed that the slope of the groundwater level has been decreasing in all months of the period. Correlation analysis between the two indices indicates a 24-month lag between meteorological and hydrological droughts. According to the SPI6 index, the highest drought occurred during the dry season in the last quarter of of 1995 and fisrt qurter of 1996. In addition, drought zoning maps were prepared using the IDW method on a 24-months time scale, indicating severe droughts in the south of the basin. The results in some parts of the region show that there was a two-year lag between meteoroiogical and hydrological droughts. The perception of this phenomenon may help planners to manage the water resources efficiently.