Assessment of meteorological and hydrological drought (Case study: Zohreh river)
Introduction:
Drought is a weather phenomenon that causes much damage every year. Kohgilouyeh and Boyer Ahmad province is located in the Southwest of Iran. Since it provides a substantial portion of the water of Karoun, Maroon, and Zohreh, which are three important rivers in this province, and evaluation and prediction of drought in this province seems necessary. This study aims to evaluate the compliance of meteorological and hydrological drought of the Zohreh river basin is in this province.
Using SPI (Standardized Precipitation Index) and SWI index (Standardized Water level Indicator), the relationship between meteorological and hydrological drought was evaluated, and the comparison between the occurrence of meteorological and hydrological droughts was performed. The SPI was calculated for 10 meteorological stations with 30 years of data, and SWI for 11 piezometric wells for the whole statistical data available. The trend of the river streamflow and piezometric wells' groundwater levels was assessed using the Mann-Kendall test for 5 hydrometric stations and the piezometric wells. The Pearson correlation coefficient was used to assess the relationship between rainfall, discharge, and groundwater level in different time lags.
Results showed that in negative SPI durations, the groundwater level decreased with some delay. The delay increased when the drought scale increased. In recent years, the duration and intensity of drought have increased. Maximum volume of 3, 6, 9, and 12 months precipitation drought of Nazmakan station occurred in 1378-78, the maximum intensity of 3, 6, and 9 months in 1383, and 12 months occurred in 1388. When the drought scale increased, drought duration and intensity increased, and the drought frequency decreased. Results for the other stations were nearly similar. SWI of Basht well showed that 6, 12, 24, and 48 months droughts continued nearly 48 months, from the end of 2006 to the middle of 2010. The intensity of 48 months drought was maximum and 12, 24, and 6 months were in the next level of intensity. This trend shows the long time accumulating effect of meteorological trends on the decreasing level of groundwater, although pumping withdrawal of groundwater is also effective. The trend for other wells was nearly similar. The Mann-Kendall test did not confirm the trend of stations discharge, but the decreasing level of groundwater in all wells was statistically confirmed. The Pearson’s coefficient of precipitation-discharge showed the maximum correlation with the same month data. It could be from the surface water's direct relation with groundwater, and the decreasing amount of snow precipitation in recent years, which lead to a delay between them. The maximum Pearson’s correlation of the groundwater level -precipitation, and groundwater level -discharge was observed with two months delay. It could verify the groundwater recharge with surface water, and the time needed for infiltration and movement of water in the aquifer.
The results could be used for predicting the effects of meteorological drought on hydrological drought. The properties of meteorological and hydrological droughts could be used to manage drought and water resources, supply water for agriculture, industry, and livestock, and supply drinking water for humans in the province.
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