Drought is a deficiency in rainfall/precipitation over an extended period of time, usually a season or more. As per the India Metrological Department (IMD), a drought year is a year when overall rainfall deficiency is greater than 10% of Long average Value (LPA) and more than 20% area affected by the drought like conditions, either moderate or severe or combined moderate and severe. If the spatial coverage of drought is more than 40%, then it will be called as All India Severe Drought Year.
Declaration of Drought in India
In India, the State governments are the final authority to declare a region as a region affected by drought. The Union government has recommended four standard monitoring tools when declaring a region as drought affected. However, these guidelines are not binding on the state governments. The four parameters are:
- Rainfall deficiency
- The extent of area sown
- Normalised difference vegetation index (NDVI)
- Moisture adequacy index
These four parameters are used in a combination to determine a drought condition. At least, three of the above four factors need to be taken into consideration by the state governments before declaring of drought.
Area Under Sowing
An area is considered to be drought affected if the total sowing area by the end of August in the case of Kharif, and the end of November-December in the case of Rabi crops, is less than 50% of the total cultivable area.
Normalised Difference Vegetation Index (NDVI)
NDVI is an index showing the density of vegetation based on the reflection of visible and near infrared lights. The value of NDVI ranges from –1 to +1. if the value is negative, it means no vegetation. For declaring a drought, it is imperative on the part of the states to obtain NDVI values through the National Agricultural Drought Assessment and Monitoring System (NADAMS). However, the states cannot declare an area to be drought affected only by taking into consideration the NDVI values. They also need to take the other parameters into consideration.
It is the most important factor which is considered before declaring a drought. When there are excessive variations in the long term averages then an area is declared as drought affected. If the deficiency is between 26 and 50 per cent, the drought is labelled as “moderate” and if it is more than 50% it is termed as “severe”.
The state governments can declare a drought if the rainfall received in June-July is less than 50% of the average rainfall, or the total rainfall in the South-West monsoon(June-Spettempbr) and North-East Monsoon(December-March) is less than 75% of the average rainfall, as well as there is an adverse impact on vegetation and soil moisture, as measured by the vegetation index and soil moisture index respectively.
Moisture Adequacy Index (MAI)
MAI is a ratio of Potential Evaporation Transpiration (PET) and Actual Evaporation Transpiration (AET) expressed as a percentage. If the percentage of water balance is 76 and 100 then it is concluded that there is no drought; if it is between 51 and 75 then there is mild drought, and between 26 and 50 there is a moderate drought and below 25 there is a severe drought.
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