What is a hydrological drought short note?

Drought is a climatic anomaly, characterized by deficit supply of moisture. Droughts can be classified on the following basis.

a. Meteorological Drought: It is a situation where, there is more than 25% decrease in precipitation from the normal precipitation over an area.

• • The drought is classified as moderate if the seasonal deficiency is 26% to 50% and the drought is said to be severe if this deficiency is more than 50% of the normal value.
  • A year is said to be drought year in case the area affected by severe or moderate drought is more than 20% of the total area of the country.
  • If the drought occurs in an area with a probability of 0.2 ≤ P ≤ 0.4, then the area is classified as a drought prone area.
  • If the probability of occurrence of drought at an area is greater than 0.4 then such an area is called chronologically drought prone area.

NOTE: In India about 33% area is drought prone.

b. Hydrological Drought: Hydrological drought means below average value of stream flow contents in lakes, ground water etc.

Hydrological drought has 4 components:

• Magnitude (Amount of deficiency)
• Duration
• Severity (Cumulative amount of deficiency)
• Frequency of occurrence.

In the studies on hydrological drought, different techniques have to be adopted for study of (i) surface water deficit, and (ii) groundwater deficit. The surface water aspect of drought studies is essentially related to the stream and following techniques are commonly adopted.

• Low-flow duration curve
• Low-flow frequency analysis
• Stream flow modelling

c. Agricultural Drought: This is mainly characterized by deficiency of rainfall. Agricultural drought can be defined with the help of different indices, for example-Aridity Index, Palmar Index and Moisture availability index etc.

NOTE:

• PET refers to the amount of water consumed by the plant consumptively if sufficient moisture is available.
• AET refers to the actual amount of water consumed by the plant under existing conditions.

Al anomaly	Severity Class
Zero or negative	Non-arid
1-25	Mild arid
26-50	Moderate arid
>50	Severe arid

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