Land atmosphere feedbacks and their role in the water resources of the Ganges basin

• The Ganges and Indus basins contain one of the largest contiguous areas of irrigation in the world.
• The processes and magnitude of feedbacks between the land surface and the atmosphere depend on spatial scale.
• Feedbacks into the Planetary Boundary Layer from irrigated land will reduce evaporation by as much as 31%.
• Increased cloud amount due to evaporation by irrigated areas may reduce evaporation by between 5 and 10%.
• Approximately 40% of the precipitation falling in the Himalayas originates from the irrigated areas in northern India and Pakistan.

The northern Indian subcontinent has frequently been identified as a hotspot for land atmosphere interactions. It is also a region with the highest concentration of irrigated land and highest (and increasing) population density in the world. The available water in the region with which to grow food depends on the Asian monsoon, groundwater and melt from Himalayan snows. Any changes or disruptions to these sources of water could threaten the food supply. It is therefore essential to understand how the land surface, and in particular irrigated land, interacts with the atmosphere. It is anticipated that the interactions will occur on many scales. To an extent the magnitude and form of these will depend on the depth of the atmosphere which is affected. Thus at the local, or micro, scale it is the surface layer (some 10s m deep) which is cooled and moistened by the evaporation of irrigated water, at the meso-scale the Planetary boundary layer (up to 1 or 2 km) will be modified — with possible atmospheric moistening, increased cloud and rain formation and at very large scales the whole dynamics of the south Asian Monsoon will be affected. This illustrates a strong interaction between the Asian monsoon and the regional topography. Of considerable significance is the finding in this paper that up to 60% of the evaporation from irrigated areas in the summer months is ultimately recycled to Himalayan rainfall and so feedbacks to river flows in the Ganges.