Why (maybe) it always seems to rain in town

By Owen Taylor, Editor
AgFax.Com - Your Online Ag News Source

A Purdue University scientist contends that man-made changes to the landscape have affected Indian monsoon rains, and he suggests that land-use decisions play an important role in climate change. Sprawling and packed urban areas are seeing an increase in heavy rainfall, according to the researcher, Dev Niyogi, an associate professor of agronomy and earth and atmospheric sciences.

Consider Houston, Texas, if you want a closer example of this effect.Every summer it seems that parts of the city flood, and the rainfall is generally unrelated to hurricanes. The flooding also can play havoc with crop production around the city.“All the asphalt, concrete and buildings accumulate heat, which rises into the atmosphere and creates a kind of thermal wall,” a Texas Extension worker noted last summer during one of those downpours. “When cool air carrying moisture hits that warm wall, it releases the moisture, and you end up with flooding rain in parts of the metro area and in rural areas where the 2 air masses meet.”

In India, this effect is clearly evident, Niyogi says. Some storms in urban areas have dropped 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, which also describes Houston.

"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."

Meanwhile, back in the Sierras...

Weekend storms have increased California’s Sierra snowpack – excellent news for farmers in the state’s Central Valley, who depend on winter accumulations for irrigation water in the spring and summer.

California Department of Water Resources officials say it is a good start to the season, the state’s Farm Bureau noted today on its web site. “However, they add this does not mean the drought has ended,” the report emphasized. “The overall snowpack for the Sierra is 95% of average for this date. That compares favorably to the 17% of average last year at this time. The Pacific Ocean El Nino current gives hope to farmers and urban dwellers alike for above-average precipitation, at least in the Central and Southern Sierra.”

A reduced snowpack going into the 2009 crop season meant severe limitations on water allotments in parts of the San Joaquin Valley, which makes up the southern half of the Central Valley and accounts for nearly all of the state’s cotton production area.

Growers and their crop advisors indicate that they will significantly increase cotton acreage in 2010, providing water is available again. Their projections are based on a strong price for Pima cotton and a falloff in other commodity prices, especially for tomatoes and dairy feed.