Monsoon drops in some areas, increases in others

THE AMOUNT of rain falling in various parts of India has changed over the years, decreasing in some places and increasing in others. K Rupa Kumar and his colleagues at the Indian Institute of Tropical Meteorology (IITM) in Pune report in a recent issue of the International Journal of Climatology that their analysis of data from 1871 to 1984 from 306 stations -- one from each of the districts in the plains -- points to a decrease in rainfall in 135 stations and an increase in 171 (see map). Thirteen of the decreasing trends and 18 of the increasing trends were deemed significant.

The IITM scientists reported the areas of decreasing and increasing trends were approximately equal in overall size; the excess or deficiency of rain occurred more often in the latter half of the monsoon, and the monsoon was concentrated in August, especially along the west coast and central India. On a more local level, scientists P Govinda Rao of the Union department of science and technology and K Krishna Kumar of IITM found an increase in the aridity in the Mahanadi river basin region from 1901 to 1980 (Current Science, Vol 63 No 4).

The increase in the annual aridity index of the region was progressive and during the 1960s and 1970s, exceeded the average on 10 occasions. On the other hand, the annual moisture index showed a downward trend and on five occasions during 1965-1980, the basin climate was more semi-arid than its normal dry sub-humid (see graphs).

The potential evapotranspiration (water need) of a place is the maximum water that would evaporate and transpire from a thickly vegetated area, when ample water is available. The annual rainfall was compared to this estimate of water need. Expressed as a percentage of annual rainfall, the difference when rainfall is excess gives the mositure index and when rainfall is less, the aridity index.

Scientists are not sure of the reasons behind these trends but say a pronounced warming over northcentral and northeast India and the decreasing trends observed in pre- and post-monsoon showers, could contribute to growing aridity.

The monsoon trends over India, warn scientists, are not continuous, but indicative only of an overall tendency to high or low rainfall situations. Rainfall fluctuations over time and across regions, they say, continue to be dominated by the highly variable nature of the monsoon.