Drought and Its Classification

Drought and Its Classification

Definition:
Drought is a period of inadequate or no rain fall over extended time creation soil moisture deficit and hydrological imbalances.

Classification of Drought:

Drought on different basis is generally classified into three categories.

Drought classification

A. On the basis of source of water availability:
Drought is classified into three types on the basis of water availability.

1. Meteorological drought:
The meteorological droughts mainly indicate deficit rain of different quantum. The IMP classified this drought as follows from the rainfall departure.

1. Slight drought          : When rainfall is 11 to 25% less from the normal rainfall.
2. Moderate drought    : When rainfall is 26 to 50% less than the normal rainfall.
3. Severe drought        : When rainfall is more than 50% less than the normal rainfall

2. Hydrological drought:
It is defined as the situation of deficit rainfall when the hydrological sources like streams, rivers, lakes, wells dry up and ground water level depletes. This affects industry and power generation.

3. Agricultural drought:
This is the situation resulted from inadequate rainfall, when soil moisture falls to short to meet the water demands of the crop during growth. Thus affects crop may wilt due to soil moisture stress resulting into reduction of yield.

B. On the basis of time of occurrence:
Drought differs in time and period of their occurrence and on this basis Thormathwite delineated following three areas.

1. Permanent drought area:
This is the area generally of permanent dry, arid  p desert regions. Crop production due to inadequate rainfall is not possible without irrigation. In the these areas vegetation like cactus. Thorny shrubs, xerophytes etc. are generally observed.

2. Seasonal drought:
It occurs in the regions with clearly defined as rainy (wet) and dry climates. Seasonal drought may occur due to large scale seasonal circulation. This happens in monsoon areas.

3. Contingent drought:
This results due to irregular and variability in rainfall, especially in humid and sub humid regions. The occurrence of such droughts may coincide with grand growth periods of the crops when the water needs are critical and greatest resulting into severity of the effects i.e. yield reduction.

C. on the basis of medium:
 On the basis of medium in which drought occurs. Mexico (1929) has divided the drought into two types. 

1. Soil drought:
It is the condition when soil moisture depletes and falls short to meet potential Evapotranspiration of the crop.

2. Atmospheric drought:
This results from low humidity, dry and hot winds and causes desiccation of plants. This may occur even when the rainfall and moisture supply is adequate.
Strategy to mitigate drought OR How to overcome the drought:
1. Preventing and recycling of excess runoff
2. Deep tillage to absorb and hold maximum moisture.
3. Timely weed management to control water loss by ET.
4.  Planning for suitable cropping system.
5.  Selection of short duration and drought tolerant crops.
6. Contingency crop planning for abnormal weather situation.
7. Management of various inputs to suit the climate.
8. Conserving the soil moisture by agronomic practices like mulching use of antitranspirant on the crops to reduce ET.
9. To apply irrigation.
10. Reduction of plant population to reduce ET.
11. Timing of foliage to reduce ET.
Drought year:
The year is considered “drought year “when less than 75% of the normal rainfall is received. Drought   prone area: It is defined as one in which the profanity of a “drought year “is 20 to 40.
Chronic drought prone area:
 Is defined as one in which the probability of “drought year” is greater than 40%.