Monitoring Hydrological and Meteorological Drought in the North Karun Basin

Drought is a natural phenomenon that may occur in all areas of land under any climate conditions. Scientific study of drought is essential for water resource planning and management and for mitigating water shortages. Low rainfall is the main cause of the occurrence of drought. Precipitation reduction in a period of time than long term average of an area is defined as meteorological drought. Hydrological drought is defined as a significant reduction in available water of all forms in a landscape. Meteorological droughts lead to hydrological droughts with decreasing flow rates and aquifer discharge. In order to quantify drought phenomenon, some indicators on the basis of drought definitions or computational methods were provided. These indicators are calculated for a single point, but the extent of the drought and its severity vary in different parts of the basin and it is the key point of water resource management and planning for mitigation of drought crisis. The purpose of this study was to investigate the spatial and temporal variations of climatic and hydrologic drought in the North Karun Basin. In this study, some of the meteorological and hydrological indicators such as Deciles (DI), Z score, Standardized Runoff Index(SRI), Stream flow Drought (SDI), Standardized Water Index (SWI) and Groundwater Resource Index (GRI) were calculated to assess the status of the study area. In order to expand point information and convert it to spatial information, after calculating different indices, the meteorological and hydrological drought severity mapping was done using Kriging and IDW(with power 1, 2, 3 and 4) methods.
Result and The results of the meteorological drought indices showed that during the 30-year period (1974-2014), the studied area had experienced a severe drought only once between the years 2007 and 2008. The SDI index showed that the river flows had declined at the same time as the meteorological drought between the years 2007 and 2008. In addition, the river flow drought (2007-2008) is clearly obvious in the next water year. In recent years, the SDI has fluctuated from moderate to severe drought, and in general, all stations experienced a moderate drought. SWI indicator analysis showed that groundwater level drop in the North Karun Basin occurred in 2007 and 2008, that has been delayed by one year to the meteorological drought. The results of the GRI in the North Karun Basin showed that the hydrological drought began between 2007 and 2008. Due to the sharp drop in groundwater level, the severity of the groundwater drought in recent years has been dramatically increased in the plains of the study area. The analysis of the Z index map indicated that while the drought is more intense in the eastern parts of the basin, it is less severe and intense in its northern and southern parts. Groundwater drought zoning showed that the severity of groundwater drought in the northeast and southeast of the plains is high. The Shahrekord plain is one of the most important plains in the North Karun Basin, because it has the main concentration of agricultural and animal husbandry activities and has undergone a sharp decline in groundwater over recent years. The groundwater level of this plain reached its lowest level between the years 2013 and 2014. In general, the findings indicated that sever meteorological droughts in the time period of thirty years (1986-2015) occurred in two years (2000-2001 and 2007-2008). But its impact on surface flow and groundwater recharge is significant due to anomalies in the rainfall – runoff process and excessive withdrawals from the groundwater resources. The meteorological and surface water droughts periods in this basin often take one to two years. The duration and severity of droughts, especially in groundwater resources, have been significant in recent years. The results also showed that geostatistical method of kriging with exponential and gaussian model has a high ability for drought zoning. Also, it can be understood that the eastern part of the area has received fewer precipitation than the other parts. This result is also derived from the zoning of groundwater droughts that the eastern parts of the plains are more severely affected by drought. In general, hydrological droughts in the north Karun Basin are more intensive in the southern and southeastern regions. Simultaneous use of the meteorological and hydrological indicators can be a useful tool for the separation of the meteorological and hydrological droughts as well as the assessment of drought in the region.