Rising trend of short but intense spells of rainfall is making usable water scarce in India
If you decide to ignore the news above as freak incidents you make a mistake. They are the order now. They are happening repeatedly all over the country. Farmers do not know why. Nor do they know how to tide over the crop-destroying downpours. Scientists and meteorologists have an answer to the first question. The root cause is global warming, they argue.
In 1997, K J Henessey, scientist at the Australia-based Commonwealth Scientific and Industrial Research Organization, used mathematical calculations and empirical evidence to predict climate warming would cause intense rainfall and increase risk of floods. It proved true for India.
Intense rainfall, above 150 mm a day, increased by 10 per cent per decade in the past 50 years, said scientists from Pune-based Indian Institute of Tropical Meteorology (iitm). The team led by the Institute director B N Goswami published their findings in the US journal Science on December 1, 2006.
The Centre for Mathematical Modelling and Computer Simulation, Bengalooru, in its report in March 2007 showed dry areas, like south Gujarat, north Madhya Pradesh and south Orissa, were also receiving extreme rainfall. So were the semi-arid regions of north India.
How could these rains be linked to global warming? A study found a strong statistical correlation between rise in sea temperatures due to global warming and increase in incidence of extreme rainfall.
It was carried out by researchers from the Tirupati-based National Atmospheric Research Laboratory, led by M Rajeevan, and published in the journal Geophysical Research Letters on September 20, 2008.
Sea turns its heat on rains
The unprecedented 944 mm rain in Mumbai on July 26, 2005 was the starting point for Rajeevan's quest. He and his team sifted through rainfall data from 1901 to 2004 to see if there was a pattern to the extreme rainfall events. Like Goswami, they found them increasing. But Rajeevan needed evidence to link increase in intense rainfall with global warming. For that, he looked at sea surface temperature of the past 104 years. He noticed sea surface temperature and extreme rainfall events followed a similar, increasing pattern (see graph A statistical connection).
"As the climate warms in response to increase in greenhouse gases, evaporation from the sea surface also takes place at a faster rate. This increases water vapour content in the atmosphere that condenses and falls as intense rain when the temperature cools down," said Rajeevan. His findings were reinforced by marine observations from around the world compiled as International Comprehensive Ocean Atmosphere Data Set (icoads) that showed a 25 per cent rise in water vapour content near the ocean surface in the past 50 years.
Increase in average wind speed over the sea surface too has contributed to temperature increase, the icoads data showed. The wind carries away the water vapour and creates space for more vapour. When this vapour condenses and falls as raindrops, the heat trapped within is released and warms the air close to the earth's surface, which has a compound effect leading to more vaporization. "This increased windspeed with high moisture causes storms and depressions," Rajeevan said.
His study showed a 14.5 per cent increase in the number of incidents of extreme rainfall every decade in the past 50 years. This was slightly more than the 10 per cent increase Goswami calculated for the same period. The variation could be because the two studies used rainfall data of different monitoring stations. Rajeevan and Goswami's studies also show that moderate rainfall is decreasing at the rate of 2.3 incidents per year.
The high intensity rains occur due to localized heating and high evaporation. "Such extreme events are difficult to predict due to their localized generation and occurrence," said Swati Basu, scientist with the Ministry of Earth Sciences.
Monsoons give India a miss
While parts of the country are receiving extreme rainfall, overall moderate rainfall that benefit crops, is decreasing. Monsoon rainfall is waning in two thirds of the country, according to a study by N Singh, scientist with iitm. For instance, in Kanker district of Chhattisgarh the average annual rainfall has decreased from 1,500 mm in 1927 to 900 mm in 2000, said another study by the Indira Gandhi Agricultural University, Raipur, in 2008.
This declining trend is attributed to southwest monsoon winds bypassing India. The number of days of more than 12 mm rainfall have decreased by 78 per cent in the last 53 years, said a study published in Current Science on August 25, 2005, by P V Joseph, scientist at the Cochin University of Science and Technology. "Active monsoon periods are created when the lower jet stream--the moisture-laden wind from the Arabian Sea and the Indian Ocean--passes through India. When these winds bypass the peninsula and flow south of it, there are breaks in the monsoon," explained Joseph. He added that these breaks are increasing and active monsoon periods are decreasing. This is bad news for farmers who rely heavily on monsoons.
S K Dash, atmospheric scientist at iit Delhi, said faster rise in sea surface temperature could be preventing the monsoon winds from moving towards land. Monsoon happens when the moisture carrying winds move from sea to land due to temperature difference (sea is cooler than land in summers). "Now if sea warms up faster in the summer, the temperature difference between sea and land reduces. This could be affecting the monsoons," said Dash. This reduction in temperature difference between land and sea surface has not been proven yet and requires further study. Decline in temperature difference near the surface may not have been proved but Goswami said he noticed the temperature difference decreasing at a height of two to 10 km above the land and sea surface.
While monsoons have decreased, moderate and severe droughts have increased in the last decade from one in three years to one in two-and-a-half years, said a study by Pratap Narain of Central Arid Research Institute at Jodhpur in Rajasthan. It was found that Barmer, also in Rajasthan, is the most affected with 20 severe droughts in the last century, four of which occurred between 1991 and 2000.
High extreme rainfall combined with decreasing monsoons is making rainfall more unpredictable. Lack of uniformity in temperature increase in different regions of the country could be making it more so.
Winters get warmer
Dash was instrumental in proving the lack of uniformity of warming in India. He said winters in northern India are warming faster than in southern India. Dash compiled temperature data of 102 years to prove significant variations in temperature increase in different regions. The minimum temperature (recorded during night) in winter in the northern part of the country increased 0.7C while in the south it increased only 0.3C in 102 years.
|As the climate warms due to greenhouse gases, sea water evaporates faster and increases air moisture that condenses and falls as intense rains
Director, NARL, Tirupati
||Lack of uniformity in temperature changes in India affects microclimates and makes weather predictions more and more difficult
S K Dash,
Atmospheric, scientist, IIT Delhi
||Increased intense rainfall with less of mode -rate rainfall could decrease groundwater recharge and soil moisture, affecting agriculture
B N Goswami,
Director, IITM, Pune
The findings were published in Current Science on September 25, 2007.
Cloud cover over northern India could be the reason for the variations in minimum temperature increase, said Dash. In northern India, where air pollution is higher, suspended pollutants absorb moisture particles to form aerosols. When the aerosols merge they form clouds that trap the heat radiated from the earth's surface, causing temperatures to rise. Again, the increase in the average temperature in the past two decades in the north was 0.7C, while in the south, the same temperature increase happened more gradually over a period of three decades.
Within the north and the south too some regions warmed faster than others in the past 10 years. The variations were due to local factors like biodiversity, soil characteristics and proximity to mountains or seas, said Dash. "This lack of uniformity in temperature changes affects microclimates and makes weather predictions more difficult," he added (see table Sun and snow). Lack of uniformity in temperature is also reflected in the unexplained heat and cold waves. For instance, in 2002-2003, northern India reeled under severe cold wave conditions. The following year it was scorched by a heat wave.
The temperature increase in different seasons was also different. Winter warming was faster than summers across the country. After the 1980s, winter warming accelerated. The maximum temperature increased 1.25C in winter. The increase in summer was relatively less 0.7C. The minimum temperatures, too, increased but not so sharply. The increase was 0.7C in winter and 0.3C in summer, according to the paper.
Warmer winters and hotter summers are bad news but sea temperature trends bring worse tidings. Dash's paper shows that in the last century, surface temperature of the Bay of Bengal and the Arabian Sea increased 0.8C, which is 50 per cent higher than global average rise in sea surface temperature. This could make weather more unpredictable in India.
There is however no one-on-one correlation between high temperature and very high rainfall. "Even though there are general relationships between sea surface temperature and very high rainfall, it is not necessary that the warmest year gets the highest rainfall," explained Rajeevan. 1993 witnessed the hottest summer till then and also the highest sea surface temperature but not very high intensity rainfall. That happened the next year when the temperature was lower. Forty incidents of very high rainfall occurred in central India in 1994.
Low on flow, high on floods
The effect of rising temperatures can be seen on the Himalayan glaciers. They have thinned considerably. Larger glaciers numbering 466 shrunk 21 per cent between 1962 and 2001, and 127 smaller glaciers 38 per cent, according to a joint study by the Ahmedabad branch of the Indian Space Research Organization, Himachal Pradesh Remote Sensing Cell, and the Government College in Dharamshala.
The study, published in Current Science in January 2007, showed glaciers feeding the Chenab, Sutlej and the Beas rivers have shrunk from 2,077 sq km to 1,628 sq kilometer.
This reduction in glacial cover in turn has affected the flow of rivers. Many Himalayan rivers have less waterflow throughout the year including monsoon. But they flood villages and towns more frequently when there is intense rainfall. Studies conducted on four rivers originating in the Himalaya--the Sutlej, Chenab, Beas and the Ravi--show a decrease in average flow in the Sutlej and the Beas.
But there is a sharp increase in the peak flow of all four rivers in the monsoon leading to high-magnitude floods. A paper published in the Current Science in September 2008 by Military Engineering College in Pune and the University of Pune, shows that average discharge in the Sutlej, Beas and the Ravi has decreased even in winter and monsoon, when the rivers have good flow. This is due to the gradual disappearance of glaciers that fed these river systems, said Mahendra Bhutiyani, one of the authors. The average discharge of Sutlej has decreased 33 per cent between 1991 and 2004, he said.
The Chenab is relatively unaffected as most of its glacial cover is nearer its origin as compared to the other rivers, the researchers said. The scientists used remote sensing maps of glaciers to arrive at this conclusion. The Beas and Ravi basins were found to have fewer glaciers.
"There was excessive melting of the glaciers before 1990. This has decreased water flow during winter though temperatures and rainfall have increased during this period," said the paper.
The flow in these rivers has decreased but frequency of floods has increased in recent years, the study said (see table Rivers of wrath). This increase in floods is definitely connected with extreme rainfall incidence," Bhutiyani said. The peak discharge in the Godavari and the Narmada are also increasing. But it is decreasing in the case of the Krishna and the upper Ganga though reasons are not known, said Vishwas Kale, co-author of the study.
The overall prediction for major rivers is grim. Six of the country's major river basins, including the Sabarmati, Cauvery and the Mahi, are severely stressed for water and are depleting. By 2025, five more rivers, including the Ganga, Tapi and the Krishna will be water stressed, according to a policy paper published by icar in 1995.
Fresh water from rivers will become more scarce.
Intense rains may sound like good news that will increase availability of water for crops. It is not. This is because soil does not have the capacity to hold this water and most of it is lost as run-off. "These rains neither recharge the groundwater nor increase ground moisture. It does not help agriculture but destroys crops," said Nityanand Singh, scientist with iitm. He said it was the long duration monsoon that is beneficial.
The effect of temperature rise on agriculture has been observed across the country. For instance, wheat production in India has fallen four to six million tonnes over the past eight years. This is due to an increase in the temperature in February and March, said a study by the National Centre for Medium Range Forecasting last year. L S Rathore, who coordinated the study, said, "Early warming in March is speeding up crop maturation. As a result there is insufficient biomass accumulation in the grain and the grain size is smaller." A rise in temperature by 1C reduces crop yield by 25 kg a hectare, said J S Samra, former icar deputy director general.
Rice production in the Gangetic plains has been decreasing in the past two decades, according to a paper published in the Indian Journal of Agricultural Sciences in November 2008. This is partially due to temperature changes, said P K Aggarwal who is coordinating the climate change network project of icar.
The network was set up four years ago to study the impact changing climate would have on agriculture. Aggarwal gave another instance from Himachal Pradesh where apple growing has shifted from Shimla and Kullu, where temperatures have risen, to Lahaul and Spiti.
Rise in temperature dries up the soil and increases water requirement of crops. And water from extreme rainfall is of little help. So farmers now depend more on irrigation and groundwater. In Anantapur district where farmers like Narayana, who lost his groundnut crop to freak rains twice in the past three years, water requirement for groundnut would increase 70 mm per hectare between 2005 and 2020, said a study conducted by Hyderabad-based Central Research Institute for Dryland Agriculture (crida).
Prabhjot Kaur Sidhu, agro-meteorologist at Punjab Agricultural University, said increase in water requirement made irrigation costlier.
Scientists are also studying the impact of carbon dioxide (CO2) on crops. "Initial increase of CO2 levels enhances photosynthesis and crops grow fast. But the temperature increase due to higher carbon dioxide level stalls growth of plants," said Maddi Vanaja, plant physiologist with crida.
No clear solutions
The scientists at crida are also monitoring changes in temperature, sunlight hours, and rate of evaporation through instruments like rain gauges and automatic weather stations. Scientist Subba Rao who is carrying out this study has reported change in rainfall patterns and increase in average temperatures (see map Hot trends).
For instance, in Raipur district of Chhattisgarh, the peak rainfall period has shifted from July to August and farmers have been asked to change their crop cycle and delay sowing. Scientists offer some solutions to the farmers like changing the crop variety depending on prevailing conditions.
But there is no certainty that Narayana in Anantapur district would be able to save his crops or Harmohan in Mahasamund district will harvest 55 sacks of rice the next season or the one after that. While scientists seek answers in weather patterns, the government urgently needs to change its agricultural policy to deal with acute shortage of usable water for farmers' fields.
|Forecasting weather for Maharashtra farmers
| Centre approves project for developing crop patterns according to changing climate
Farmers in 14 drought-prone districts of Maharashtra have suffered repeated crop failure in recent years. Ramachandra Sable, head of the climate studies department of Mahatma Fule Agriculture University, may be able to help them out. He has developed a mathematical model for accurately predicting rainfall patterns called the Sable Model that could help farmers decide what crops to grow. Sable was able to predict rainfall pattern for Pune in 2004 and the entire Maharashtra state for the period 2005-2008 with 90 per cent accuracy.
Sable has been asked to coordinate a project for developing crop patterns based on the changing climate and rainfall pattern in the 84 drought-prone tehsils. The three-year project was recently approved by the Union agriculture ministry and will be carried out jointly by the four agricultural universities of the state starting April, 2009. Rainfall and temperature data of each tehsil in the last 30 years will be analysed decade-wise to identify pattern changes. On the basis of this data, regions with similar rainfall patterns will be clubbed and yearly rain forecasts will be made for them. The forecast would include information on the arrival and departure of monsoon, the time and duration of dry spells and the quantum of rainfall in a region. Simultaneously, crop patterns will also be be evaluated and experiments will be carried out to assess yield levels. Crop pattern maps will be drawn for different regions and publicized through radio, television and print media, Sable said. He gave the example of Jalgaon district where cotton is the most important crop. If water availability at the boll formation stage is low, there may be crop failure.
With a proper forecast system, farmers can be warned, Sable said. The project would have to be sustained throughout as climate change is an ongoing process, he added.
Aparna Pallavi, Nagpur