What are the hydrological drought definition?

The droughts are generally classified into four categories (American Meteorological Society, 2004),which include:1- Meteorological drought, 2- Agricultural drought, 3-Hydrological drought, and 4- Socio-economic drought.

Hydrological droughts are more related to water demands and they appear whenever a marked reduction becomes appreciable in natural streamflow or groundwater levels, plus the depletion of water storage in dams and lakes for water supply. Hence, hydrological droughts are very important and significant for urban areas or industrialized regions as well as for agricultural activities. Precipitation deficits over a prolonged period that affect surface or subsurface water supply, thus reducing streamflow, groundwater, reservoir, and lake levels, will result in a hydrological drought, which will persist long after a meteorological drought has ended.

The main impact of hydrological droughts is on water resources systems. Hydrological drought is not attached to the precipitation decrease only, but additionally decreases in the surface flow and drops in the groundwater levels provide joint impacts (Sirdas and Sen, 2003). Hydrological droughts are related to agricultural droughts by its effects on irrigation systems dependent on surface water and on river water quality. Hydrological drought is even further removed from the deficiency of precipitation since it is normally defined in terms of the departure of surface and subsurface water supplies from some average condition at various points in time.

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