Hydrology in the air: the influence of irrigation in India on the atmospheric branch of the water cycle

Large areas of India are heavily irrigated in order to create enough agricultural production to feed India's growing population. The supply of water needed for this irrigation is drawn from the major rivers (Ganges, Indus, Brahmaputra) and from ground water. The redistribution of this water onto large areas, first along irrigation channels and subsequently onto the irrigated field, increases the total evapotranspiration from the land surface.

Apart from the local meteorological effects of the changed land surface characteristics in India, such as decreased surface temperatures and increased cloud formation and precipitation (Tuinenburg et al. 2010, Journal of Climate, under review), non-local effects exist. Water that has evaporated from irrigated areas follows its way through the atmosphere and falls again as precipitation in another place: the atmospheric branch of the water cycle. Measures to quantify this part of the water cycle include the recycling ratio, in which the fraction of evaporated water that falls again as precipitation in a limited distance from the evaporation location. Factors that influence the recycling ratio are orography, wind speed and direction, land-atmosphere interactions, but also the size of the area under consideration. Usually, square (grid) boxes are used to determine the recycling ratios. However, in this study we use the area of the river catchment, because if water recycles in that area, it can potentially be reused at the location it evaporated from.

Using the ERA-interim dataset (wind, moisture, precipitation and evaporation fields), evaporated water is followed with a forward trajectory scheme (Dirmeyer and Brubaker, 2007, Journal of Hydrometeorology). After evaporation, parcels are tracked following the three dimensional wind fields. During this atmospheric path, water evaporates into the parcel and precipates out of the parcel. Just after the release of the parcel, all the water that evaporated into it (the original water) is still present. Along its path, evaporation into the parcel decreases the fraction of original water present in the parcel. The parcel is followed until the fraction of original water is less than 5%. For each location along the path the product of the amount of precipitation and the fraction of original water present gives the fraction of original water that precipitates at that location. The sum of this product for locations within the catchment gives the within catchment recycling ratio.

Results for the major river basins in India show a strong annual cycle in the recycling ratio. For the Ganges basin, the recycling ratio ranges from less than 5% during the winter months (Nov-Mar) to about 60% during the summer (JJA) season. For the Indus basin, the recycling ratio ranges from less than 5% in winter to about 40% in summer. For different regions in India, the spatial patterns of the atmospheric part of the water cycle differ. The downwind precipitation distribution due to local evaporation will be presented for different regions. Moreover, an analysis will be presented of how these recycling ratios differ for irrigated and non-irrigated areas, both for years with strong and weak monsoon precipitation.