3.11 DROUGHT

What is a drought?

While it is relatively easy to define what a hurricane or earthquake is, defining a drought is more subjective. Droughts do not have the immediate effects of floods, but sustained droughts can cause economic stress throughout an area. The word "drought" has various meanings, depending on a person's perspective. To a farmer, a drought is a period of moisture deficiency that affects the crops under cultivation—even two weeks without rainfall can stress many crops during certain periods of the growing cycle. To a meteorologist, a drought is a prolonged period when precipitation is less than normal. To a water manager, a drought is a deficiency in water supply that affects water availability and water quality. To a hydrologist, a drought is an extended period of decreased precipitation and streamflow.

What causes droughts?

A drought is a period of drier-than-normal conditions that results in water-related problems. Precipitation (rain or snow) falls in uneven patterns across the country. The amount of precipitation at a particular location varies from year to year, but over a period of years, the average amount is fairly constant.

When does a drought begin?

The beginning of a drought is difficult to determine. Several weeks, months, or even years may pass before people know that a drought is occurring. The end of a drought can occur as gradually as it began. Dry periods can last for 10 years or more. The first evidence of drought usually is seen in records of rainfall. Within a short period of time, the amount of moisture in soils can begin to decrease. The effects of a drought on flow in streams and reservoirs may not be noticed for several weeks or months. Water levels in wells may not reflect a shortage of rainfall for a year or more after a drought begins.

Why doesn't a drought go away when it rains?

Rainfall in any form will provide some drought relief. A good analogy might be how medicine and illness relate to each other. A single dose of medicine can alleviate symptoms of illness, but it usually takes a sustained program of medication to cure an illness. Likewise, a single rainstorm will not break the drought, but it may provide temporary relief.

A light to moderate shower will probably only provide cosmetic relief. It might make folks feel better for awhile, provide cooling, and make the vegetation perk up. During the growing season, most of the rain that falls will be quickly evaporated or used by plants. Its impact is short term.

A thunderstorm will provide some of the same benefits as the shower, but it also may cause loss of life and property if it is severe. Thunderstorms often produce large amounts of precipitation in a very short time, and most of the rain will run off into drainage channels and streams rather than soak into the ground. If the rain happens to fall upstream of a reservoir, much of the runoff will be captured by the reservoir and add to the available water supply. No matter where the rain falls, stream levels will rise quickly and flooding may result. Also, because the rainfall and runoff can be intense, the resulting runoff can carry significant loads of sediment and pollutants that are washed from the land surface.

Soaking rains are the best medicine to alleviate drought. Water that enters the soil recharges ground water, which in turn sustains vegetation and feeds streams during periods when it is not raining. A single soaking rain will provide lasting relief from drought conditions, but multiple such rains over several months may be required to break a drought and return conditions to within the normal range.

Considering all of the above, even when a drought has been broken it may not be truly over. 

[From The USGS Water Science School

https://water.usgs.gov/edu/qadroughts.html]

Causes of drought in INDIA

Drought, one of the most pervasive climatic phenomenon in India, is caused by a number of factors.

There are four major reasons for droughtin India- delay in the onset of monsoon/ failure of monsoon, variability of monsoon rainfall, long break in monsoon and areal difference in the persistence of monsoon.

Variability of monsoon rainfall

The variability in rainfall is a major cause of drought. The percentage of variability is inversely related to the total rainfall.

Areas like northeastern region and the western coastal plains and windward side of the Western Ghats, which receive the highest amount of rainfall in India, have low variability of rainfall. On the other hand, areas of low rainfall have a higher variability of rainfall. This covers the most of the interiors.

The areas most affected are interior Orissa, western Jharkhand, Chattisgarh, Madhya Pradesh, Rajasthan, Punjab, Haryana, Kathiawar and Kachchh in Gujarat, Vidharbha and Marathwada in Maharashtra, northern and eastern Karnataka and western Andhra Pradesh.

Delay in the onset of monsoon

Traditional farming and lifestyle patterns place heavy reliance on the timely arrival of monsoon. So even if the monsoon is delayed by a week, it causes a major upset in the farming communities and causes drought and famine.

These delays however are a fairly common occurrence in areas that get marginal rainfall.

The areas most affected are Punjab, Haryana, the Indo-Gangetic plains, Bundelkhand, eastern Rajasthan, eastern Gujarat, Madhya Pradesh, south Bihar, Jharkhand, interior West Bengal, Orissa, Andhra Pradesh, interior Karnataka and eastern Tamil Nadu.

Breaks in monsoon

Even within the monsoon if there are long dry periods, that too can cause drought in affected areas. This may happen due to a reduction in the speed of the rain-laden winds, or a diversion in the route. The damage to crops is usually extensive.

The areas most affected are the areas in the core monsoon affected areas.

Areal difference in monsoon

A deviation in the route of monsoon winds, or an early withdrawal of the monsoon too can have similar repercussions on an area. If the monsoon hangs over east Uttar Pradesh for a longer period of time, then west UP or Haryana are adversely affected.

[Source: https://www.hindustantimes.com/india/causes-of-drought/story-6I7AltPXsqnJ8SSlSRk7tI.html]

[Image source:https://timesofindia.indiatimes.com/india/Drought-hits-Gujarat-every-3-years-Report/articleshow/49264062.cms]