World Colleges Information

1. Lesson 1: Tamil Nadu Scenario and definitions of the terminologies used
2. Lesson 2: Causes for drought
3. Lesson 3: Impact of drought,
4. Lesson 4: Drought prediction and forecasting and present status
5. Lesson 5: Drought management and conclusion – Questions

Exercise 1:  Computing drought indices and interpretation:

Lesson 1: Tamil Nadu Scenario and definitions of terminologies used:

• Tamil Nadu has a total geographical area of 13 million hectares of land and out of which seven million hectares are under cultivation. In the total cultivable area, only 45 per cent are under irrigation and the balance 55 per cent are under dry land, Considering the irrigated area per cent age, the State Tamil Nadu could manage drought successfully with out affecting her economy for one to two years of continuous drought occurrence. This was witnessed from 2000 to 2003, wherein Tamil Nadu had failure of both South West Monsoon (SWM) and North East Monsoon (NEM) rains.

• The average annual rainfall of Tamil Nadu is 967 mm and the distribution is 36.7 mm during Cold weather period (January – February), 134.0 mm during Summer (March – May), 331.6 mm during SWM and 464.7 mm during NEM. These figures indicate that crop production is successful during NEM in all the 30 districts followed by SWM season in certain selected districts.  Both dry land and irrigated up/low lands are under cropping in all the 30 districts during NEM season, while during SWM, Dharmapuri, Salem, Namakal, Kanyakumari, The Nilgris, Vellore and parts of Villupuram, Cuddalore, karur, Thiruvallur and Perambalur districts get also benefited from this seasonal rain and hence, these districts have double cropping in dry lands.

• Though the total water availability of water in Tamil Nadu, based on geographical area and annual rainfall is 12.285 mhaM, the annual availability is only 4.74 mhaM and the balance goes as run-off with out any use. Out of 384 blocks in Tamil Nadu 287 blocks have been listed as ‘dark ‘ based on ground water availability indicating the need for focusing on water management.

• In 60 per cent of cultivable area, the red soil is dominant; while in the remaining area black soil and alluvial soils could be seen. Since the water holding capacity of the red soils are less (< 100 mm) as compared to black soils (200-300 mm) drought occurs very early in red soil under the same rainfall situation.

• Terminologies Used

• Drought: There is no universal definition for drought. It is a period of extreme water deficiency and is associated with prolonged and abnormal deficiency in rainfall over a place. As per India Meteorological Department (IMD), if a meteorological sub division receives seasonal rainfall less than 75 per cent of normal rainfall and then that area is considered to be affected by drought. 

• f the seasonal rainfall deficiency is between 25 and 50 per cent of the normal rainfall, then it is labelled as moderate drought, while the deficiency is more than 50 per cent of the normal rainfall, then it is called as severe drought. An year is considered to be a drought year when the area is affected by one of the criteria for drought, either individually or collectively and is more than 20 per cent of the total area of the country.

Types of drought:

1. Meteorological drought – It is a situation when rainfall deviates appreciably below normal for an extended period of time.
2. Hydrological drought – This drought follows meteorological drought in chain wherein below normal water level is seen in lakes, streams and reservoirs including in the ground water level.
3. Agricultural drought – It occurs when available soil moisture is inadequate to meet the crop water requirement and leads to moisture stress and wilting.
4. Socio-economic drought – A situation where water shortage ultimately affects the established economy of the region adversely.

In the case of agricultural drought, under ground situation the following droughts are common:

Early season drought – This is due to late onset of seasonal rain (sowing rain) from the normal onset. Crop establishment would be the biggest problem.

Mid season drought – It occurs in association with long gaps between two successive rain events in the mid of crop season and the stored soil moisture falls below the crop water requirement.  Some times this drought may be associated with inadequate rainfall in the growing season. The crop’s vegetative growth will be affected.

Late season drought – Early withdrawal of seasonal rain from the normal date would bring this drought. Crop’s productivity will be affected.

Apparent drought – Rainfall in a given season would be adequate for one crop and may not be for other crops grown under same soil condition.

Permanent drought –This drought is associated with inadequacy of soil moisture to meet the crop water requirement during most of the years in a region.

There are further two terminologies to indicate the relationship between area and drought and they are as follows:

Drought prone area – This is like permanent drought, where in drought is inherited in the area and drought could be seen in all the years of life.

Drought affected area – Drought occur once in a way with a periodicity of four years normal rainfall with one-year drought in a cycle of five years.


Lesson 2:  Causes for drought:

As discussed in the Lesson 1, the main cause for drought is inadequate seasonal rainfall below the level of requirement, irrespective of crop, dam stream etc.

The causes for low rainfall are as follows:

1.  Variation in atmospheric general circulation affects the development of the local rain producing disturbances. The causes of such circulation variations are not well understood, but links with sea surface temperature and snow cover have been established.

2.  Higher surface albedo would affect the radiation balance and hence drought occurs.

3.  The injections of large amounts of ash into the atmosphere by violent volcanic activity alter the Earth’s radiation balance and thereby create compensating circulation adjustments which include climatic fluctuations.

4.  ENSO (El Nino and southern oscillation)

5.  Geographical position of a region

6.  Global warming

7.  Deforestation

8.  Atmospheric pollution.

9.  Early withdrawal of seasonal rain

10. Late onset of seasonal first rain

11. Mid seasonal dry spell

Further improper management of rainfall and other water resources would bring drought very early to a region. Drought follows quickly under following rainfall and soil situations.

a. When the run-off is relatively lesser
b. When the rainfall intensity is lesser than the rate of infiltration
c. When volume of infiltered water is lesser than soil moisture deficiency

The causes for drought are still under debate and further research is still under progress to investigate this complex phenomenon.

Lesson 3:  Impact of drought:

In defining drought, it is necessary to consider both rainfall quantity and soil moisture storage together. Moderate rainfall in black soil may not bring drought forth with, but even with higher rainfall, there is greater possibility of occurrence of drought in red soils. Hence the impact would differ between these two soils. In general, the following impact would be seen in the drought-affected area:

1.   Crop’s failure and productivity loss
2.   Change in land use pattern and resource degradation
3.   Depletion in water resources
4.   Fodder non-availability
5.   Distress sale of animals
6.   Imbalance in rural economy
7.   Migration
8.   Discomfort environment
9.   Social disharmony
10.   Political instability
11.   spread of poverty
12.   Ecological imbalance
13.   Resource conflicts

Drought impact has to be monitored from the start of the season and to be assessed critically. In this context, drought intensity and its magnitude must be measured. Drought starts from one point and proceeds to nearby places. Under normal situation Revenue Department is given with the responsibility of assessing the drought. However, in order to assess the drought impact precisely, a multi disciplinary team has to be formed by the Revenue Department. The impact assessment must be made first at micro level followed by regional, national and global levels.

Lesson 4:  Drought prediction and forecasting and present status:

It is possible to reduce the adverse effects of drought by providing advanced or real time information on the possible occurrence of drought. However, it is very difficult to forecast the drought for operational purposes. Still research is in progress to forecast the drought. The existing ways are discussed to understand the drought occurrence in advance with little precision.

1. Long range forecast given by IMD for SWM
2. Seasonal climate forecast given by TNAU for SWM and NEM
3. Analysing long time climate records for drought probability occurrence
4. ELNino signal or ENSO signal
5. Seasonal initial probability rainfall analysis
6. Satellite derived charts
7. IMD weekly aridity anomaly chart
8. IMD weekly rainfall and agro-advisory bulletin
9. Agro advisory bulletin of medium range weather forecast field units

Few above items are discussed below.

1. Long range forecast:

IMD gives this forecast from 1988 for operational purposes for SWM of India. The forecast contains information on the rainfall quantity to be obtained in general for the four divisions of India viz, north India, south India, western India and eastern India. This forecast is given before the start of the SWM season. The deficiency of the forecast is lack of regional level rainfall information and also monthly distribution of rainfall with in the season. From this forecast it is possible to understand the possible occurrence of drought in general.

2. Seasonal climate forecast:

This is given by Department of Agricultural Meteorology, TNAU both for SWM and NEM for whole of Tamil Nadu with district wise information. This forecast provides real time rainfall information for total season and also for the months of each season. Accordingly drought could be monitored.

3. Drought probability analysis:

By taking long time rainfall data, (may be 30, 60, 90,120 years of seasonal rainfall) analysis can be done to identify the probability occurrence of below normal events. And from this, the periodicity could be assessed for the occurrence of drought.

4. ELNino signal:

If the information on ELNino is available for a particular year, there would be every probability of getting below normal rainfall during SWM and excess rainfall during NEM season for Tamil Nadu.

5. Satellite derived chart:

From the satellite data, Vegetation index (ratio between reflected radiation between visible band and near infrared band). These charts are available from National Remote Sensing Centre, Hyderabad. This chart gives real time information on when the drought initiates, where it starts, and its nature of spread, intensity and its magnitude.

6. IMD charts, Bulletins and others:

a. Agricultural Rainfall Index (ARI): This is one among the tools to be used to identify agricultural drought. Monthly Rainfall value (R) and monthly Potential Evapotranspiration (PET) values are taken on real time basis and the index is computed as follows:

ARI= R/PET; if the product is < 40, there is occurrence of drought in the concerned month.

b. IMD Weekly aridity anomaly index: This index provides information on drought hazard over a sizable area and periodic assessment of the current extent and severity of the drought over a region.

Aridity Index (Ia) = (PE-AE)/PE; where in Ia = weekly aridity index; AE = Actual-Evapotranspiration from water balance; PE = potential evapotranspiration

The departure of real time Ia from its long-term mean is expressed in percentage and this is called as aridity anomaly. From the computed aridity anomaly the Drought intensity can be measured as follows:
       

• Aridity anomaly
• Drought intensity
• Lesser than 25%
• Mild drought
• 26 to 50%
• Moderate drought
• Greater than 50%
• Severe drought

This is computed for 194 stations and anomaly chart prepared on weekly basis for Meteorological sub division of India and communicated to users.

c. IMD agro advisory bulletin: Weekly rainfall, its progressive total, departure from   normal along with other weather information is given along with agro advisories. This bulletin will be useful to find out the onset of drought, its progress, etc.

7. NCMRWF agro advisory bulletin:

This is given from different field units in India including six field units from Tamil Nadu. Anticipated weather information will be known to users on every Tuesday and Friday. This information may be used to monitor drought occurrence.

Lesson 5: Drought management and conclusion - Questions:

Prevention is better than cure. But this has less value in respect of drought prediction. In drought management the following approach may be adopted:

1. Analyse the historical rainfall data and understand the occurrence probability of the drought in that region.
2. Monitor the long range and seasonal climate forecast on real time basis and prepare a contingency plan for the concerned region covering all the activities of the existing population This can be done through group discussion with community, experts from different fields, Govt. Officials etc., The coping practices that were adopted in the earlier drought years may be documented and reviewed again for adoption during present drought year, with enough validation.
3. Prepare separate plan for sustaining agriculture, live stock, social net work and develop plan for local village cottage industries to maintain cash flow through employment generation against migration, which is common during drought years. The plan must have techniques of science relevance by properly mixing local indigenous practice with new cost effective scientific technologies.
4. Establish Village Knowledge Centres and Village Resource Centres at the appropriate places of the region with due support from concerned organisations and the community may be empowered to acquire updated knowledge.
5. The community may be made aware of natural disasters and should be prepared to meet with challenge at any time of their life.