Drought is viewed as a sustained and regionally extensive occurrence of below average natural water availability, either in the form of precipitation, river runoff, or groundwater. Droughts may be classified as meteorological or hydrological:
A Meteorological Drought occurs when there is below average availability of precipitation. The National Meteorological Service (NMS) subdivides meteorological drought into three categories: drought, extreme drought, and severe drought. Drought is defined as less than 60% (range 41-60) of normal rainfall over 8 consecutive weeks. Extreme drought is defined as less than 40% (range 21-40) of the normal rainfall over 8 consecutive weeks. Severe drought is defined as less than 20% (range 0-20) of the normal rainfall over 8 consecutive weeks. The normal rainfall is the thirty year mean.
A Hydrological Drought is marked by a deficit in the runoff of rivers where discharge (flow) remains below a low threshold or the rivers dry up completely and remain dry for a very long time and a significant natural depletion of aquifers (groundwater). The low threshold flow is the 10-year return period 7-day minimum mean flow.
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