193 years of data shows it’s raining less in India

193 years of data shows it’s raining less in India
Comment E-mail Print Share
First Published: Mon, Aug 18 2008. 12 35 AM IST

Updated: Mon, Aug 18 2008. 12 35 AM IST
New Delhi: It’s raining less in India.
Or, to be scientifically exact, it’s increasingly raining less in at least 67% of India.
That’s the conclusion of a study that analysed rainfall trends between 1813 and 2006, and the results could have a significant impact on agricultural and economic planning and cropping patterns (or what is grown where, and when).
The study—which shows that there is a decreasing trend in rainfall received by at least two-thirds of India, and that the contribution of the monsoon months, June to September, to annual rainfall has been falling—could also explain why it has been raining more in certain states and less in others—changes that are usually attributed to erratic weather patterns.
PRECIPITATION PATTERNS (Graphic)
The study, conducted by scientists at the Pune-based Indian Institute of Tropical Meteorology, or IITM, used the widest ever data set of monsoon records and will, the people who worked on it claim, help farmers, policy planners and meteorologists better understand—and predict—rainfall trends in India.
That is critical in a country where agriculture accounts for almost one-fifth of the economy and provides employment to around 60% of a population of a little more than one billion. It is even more critical in a country where most agricultural land is “rain-fed”, a term that means that the primary source of irrigation is rainfall.
The study shows that except for the east coast regions of Tamil Nadu and Andhra Pradesh, south Gujarat, West Bengal and parts of Orissa, the country is showing a gradual decrease in rainfall levels and, while the overall annual decline, between the representative averages of 1944-1969 and 1970-2006, is only 4.1%, the shortfall is magnified at a microscopic level.
ALSO SEE
For instance, in the plains of Kerala, peak rainfall declined from at least 3,700mm (received in the 1920s and 1930s) to around 2,800mm in the 1960s, a fall of 24%, which means significant changes in irrigation planning and cropping patterns.
In the plains of Punjab, there has been a similar fall—from 1,100mm in the early years of the 20th century to around 687mm now, a decrease of 37.5%.
“The decline began in the 1960s, but why (it happened) hasn’t been very well understood,” said Nityanand Singh, senior scientist at IITM and one of the people behind the study.
“Global warming may be a factor but, studies suggest, there’s actually a cooling in the upper atmosphere near the Tibetan plateau. The atmospheric ‘warming’ stops 3-4km above. And, this cooling affects something called the Tibetan anticyclone that’s closely linked to India’s monsoon systems. We still don’t understand a lot of this,” he added.
According to Singh, the most important reason why his team undertook the study was to make India’s understanding of the monsoon more contemporary, “beyond the 1900s”.
Singh’s articulation of the benefits of the study and the choice of data across 193 years highlights all that is wrong with current weather forecasting techniques and resulting policies. Much of contemporary weather forecasting here relies on narrow data sets, sometimes just “20-30 years”, according to Singh who proffers the example of Maharashtra to indicate just how this affects policy planning.
In July, Maharashtra declared near-drought conditions in the state because it received low rainfall. “The last four-five years (that has averaged around 900mm of rainfall) have just been an unusually good run for the state,” said Singh. “Look back from the 1900s and it’s frequently around 750mm.”
The 30-year spike
The model also appears to have busted a few myths surrounding the monsoon.
Meteorologists here refer to a 30-year cycle to explain rainfall patterns. According to this cycle, rainfall increases for a period of 30 years, aided by extremely high rainfall, even floods, in various parts of the country. Then, for the next 30 years it decreases, aided by near-drought conditions, even full-blown droughts. And then the cycle is repeated. Singh said his study disproves this.
A longer-term view shows “nearly 60 years of consistently low rainfall”, he said.
The study’s findings become especially significant when transposed on an irrigation map of India with many of the states and regions experiencing a decline in rainfall levels not having developed their irrigation networks to the extent they should have. According to data from the ministry of water resources (which is updated only till the 9th Plan), only 11 states have developed their irrigation facilities to between 60% and 90% of their potential, with most states having developed them only to between 40% and 60% of their potential.
“There’s also a limit to the extent of irrigation facilities that can be developed, because of restrictions such as sustainability of structures and subsequent water salinity issues,” said Srinivas Jain, researcher at the Indian Institute of Technology, Delhi, who specializes in water management studies. “Then, there are inter-state water disputes, all of which means that access to water is a major problem and rainwater harvesting is our only way out.”
Pattern recognition
Madhavan Rajeevan, senior scientist at the National Atmospheric Research Laboratory, an Indian Space Research Organisation body, who was part of a similar study conducted in 2006 by the India Meteorological Department, or IMD, that analysed rainfall data between 1901 and 2003, said some regions such as Kerala and Madhya Pradesh showed declining trends in their rainfall.
“It’s an interesting and useful study, in that it’s the first time pre-1990 data is used to study the climate then, but the new study doesn’t substantially change our understanding of rainfall trends,” Rajeevan said.
The new study and its results come at a time when Indian meteorologists have moved away from historical data-based models because of their limited success (perhaps on account of considering limited data sets).
Most scientists here have moved to so-called general circulation models (GCM) for forecasting rainfall. These models rely more on simulating the atmosphere and physical conditions on a given day and then extrapolate them to see how the atmosphere behaves.
“GCM models are still not very reliable to predict Indian monsoon and rainfall trends, but a vital test for any model would be to run it backwards and check how well it has agreed with historical records,” said Rajeevan. “That’s where this data would be useful.”
Not everyone is convinced.
Ajit Tyagi, director general at IMD, said that though trend analyses of Indian monsoon and rainfall have been done before, their use is limited. “It’s easy to look back in the past and look for patterns, but very hard to judge when those patterns cease to be.”
Still, he said, the department “will be looking into this report closely”.
Comment E-mail Print Share
First Published: Mon, Aug 18 2008. 12 35 AM IST