After analysing more than 50 years of rainfall data, Dev Niyogi of Purdue University in the USA found that although India’s mean monsoon rainfall has remained stable, rainfall in the northwest has dropped by 35–40 per cent. The decrease was linked to an increase in irrigation.

Soil moisture data suggest that, thanks to irrigation, the region is three times as wet before the monsoon as it was 30 years ago. The wetter surface causes cooling that weakens the low pressure systems required for the monsoon to move northwards. 

In contrast, urban areas are seeing increased rainfall levels. When Niyogi looked for areas that have received increases in heavy seasonal rainfall, he found that they were typically experiencing rapid urban growth. He suggests that this could either be due to the heat generated by urban areas energising storms or increased pollution interacting with passing clouds.

February 2010