December 15, 2009

Irrigation decreases, urbanization increases monsoon rains

Monsoon rainfall has decreased over the last 50 years in rural areas where irrigation has been used to increase agriculture in northern India, said Dev Niyogi, an associate professor of agronomy and earth and atmospheric sciences. At the same time, heavily urban areas are seeing an increase in heavy rainfall.

"In the rural areas, we're seeing premonsoon greening occurring two weeks earlier than what it did 20 years back as the demand for agricultural intensification to feed India's people increases," Niyogi said. "The landscape has also moved in some places from what was once a traditionally rural setting to large urban sprawls. Both of these phenomena have affected monsoon rains."

Niyogi used more than 50 years of rainfall data - spanning back to 1951 - collected by 1,803 recording stations monitored by the India Meteorological Department to determine different regions' average yearly monsoon rain totals. While the mean monsoon rainfall for the entire country remained stable, Niyogi found that rainfall averages in India's northwest region decreased by 35 percent to 40 percent from the historical mean during the past 50 years.

Analysis of soil moisture showed that before monsoon rains came, the northwest region had become as much as 300 percent wetter in recent years relative to the past 30 years, which has been attributed to irrigation from groundwater to sustain intensified agricultural production. This wetter surface causes cooling that weakens the strength of low pressure necessary for monsoons to progress into northern India.

Satellite data showed that northern India is greening sooner than it had in the past. That greening is creating a barrier for monsoons, which provide much-needed rain to replenish groundwater reserves being used for irrigation.

"In this case, you need a warm, dry surface to advance the monsoon," said Niyogi, whose findings were published in the journal Water Resources Research. "Because of increased irrigation, you now have a wet, green area, which does not allow the monsoon to reach far enough north."

Since that rain isn't reaching the region, more irrigation is needed to sustain agriculture there.

"Unless this is checked and controlled, the problem is going to become more and more severe," Niyogi said. "With more irrigation, we will have less monsoon rain. With less monsoon rain, you will need more irrigation, and the cycle will continue."

Urban areas, on the other hand, are being pounded with rain when it comes. Niyogi said there have been storms in some urban areas that drop as much as 37 inches of rainfall in a single day.

Analysis of the areas that have received increases in heavy seasonal rainfall, based on Indian Meteorological Department and NASA satellite data, showed that those areas were experiencing fast urban growth. Areas where seasonal rainfall decreased were determined to have slow or no urban growth.

"You only see these types of heavy rainfall events in those areas with heavy urbanization," said Niyogi, whose research on the urban effect was published in the International Journal of Climatology. "The more urbanization spreads in those areas, the more of these heavy rain issues we'll see and the more flooding will become a problem."

Niyogi said there are two theories on why that's happening. The first says that urban landscapes create heat, which extends into the atmosphere and energizes storms. The second theory is that pollution created in urban settings interacts with passing clouds and increases rainfall.

Niyogi said the results of his study could have land-use implications elsewhere.

"If urbanization is affecting the Indian monsoon season, it has the ability to affect patterns here in the United States," he said. "This likely isn't localized in India."

He added that India is hotter than the United States, and that may be exacerbating the issues. As global temperatures rise, other parts of the world could see similar climate changes - if they aren't already - based on how land is used and developed.

Chandra Kishtawal, of the Space Applications Center of the Indian Space Research Organization and a co-author on the papers, said he hopes the findings trigger discussions on the role of large-scale land-use planning in regulating climate change in India.

"These kinds of things are not sustainable," Kishtawal said. "They cannot continue in the long run."

The next step in this research is to examine landscapes in the United States to see if development has affected weather patterns historically.

Writer: Brian Wallheimer, 765-496-2050, bwallhei@purdue.edu

Source: Dev Niyogi, 765-494-6574, climate@purdue.edu

Ag Communications: (765) 494-8415;
Steve Leer, sleer@purdue.edu
Agriculture News Page

Using gridded daily rainfall observations and monthly satellite land surface datasets, the connection between land use change and monsoonal rainfall climatology is analyzed. A combination of statistical analysis involving genetic algorithm (GA), Empirical Orthogonal Function (EOF), and causal discovery algorithms (CDA) are used. Study objectives are to (i) identify regional trends in the observed precipitation data over the Indian summer monsoon region, (ii) investigate the relation between land use change/agriculture intensification and changes in rainfall, and (iii) explore whether land use change and agricultural intensification has caused change in the rainfall climatology. The satellite based vegetation dataset shows significant agricultural intensification over northern India. For the period just before start of the summer monsoon season (April and May), the normalized differential vegetation index (NDVI) shows an increase only over Peninsular India. The EOF and GA based analysis identified the strongest climatic signal for monsoon rainfall with an increasing trend over the east-central regions of India and a decreasing trend in monsoon seasonal precipitation over north/north-west India. The areas of decreasing rainfall coincided with regions of agricultural intensive land use, and are analyzed further. The correlation and the causal data analysis suggest that premonsoon (March-April) vegetation affects July month precipitation over peninsular India. In particular, a negative relationship exists between them. The results are more robust over Peninsular and northern India indicating that an increase in NDVI has possibly weakened the early monsoon rainfall in this region. The results of this study suggest that land use change associated with agricultural intensification could be reducing the summer monsoon rainfall over certain regions of India.

We assess the urbanization impacts on the heavy rainfall climatology during the Indian summer monsoon. While a number of studies have identified urbanization impact on local precipitation, a large-scale assessment has been lacking. This relation between urbanization and Indian monsoon rainfall changes is investigated by analyzing in-situ and satellite based precipitation and population datasets. Using a long-term daily rainfall dataset and high-resolution gridded analysis of human population, this study showed a significantly increasing trend in the frequency of heavy rainfall climatology over urban regions of India during the monsoon season. Urban regions experience less occurrences of light rainfall and significantly higher occurrences of intense precipitation compared to non-urban regions.  Very heavy and extreme rainfall events showed increased trends over both urban and rural areas, but the trends over urban areas were larger and statistically more significant. Our analysis suggests that there is adequate statistical basis to conclude that the observed increasing trend in the frequency of heavy rainfall events over Indian monsoon region is more likely to be over regions where the pace of urbanization is faster. Moreover, rainfall measurements from satellites also indicate that urban areas are more (less) likely to experience heavier (lighter) precipitation rates compared to non-urban areas. While the mechanisms causing this enhancement in rainfall remain to be studied, the results provide the evidence that the increase in the heavy rainfall climatology over Indian monsoon region is a signature of urban induced rainfall anomaly.

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