The fate of evaporated water from the Ganges basin

This research studies river basin moisture recycling rates in order to determine the atmospheric part of the water cycle and the influence of the land surface there on. For river basins in India (Ganges and Indus), the fraction of evaporation that falls again as precipitation in the same river basin (the moisture recycling) is determined. Furthermore, the seasonal variance of moisture recycling and the fraction of precipitation that originates from evaporation from the same river basin is quantified. Using a quasi-isentropic moisture tracking scheme, evaporation from land surfaces in India is tracked through the atmosphere until precipitation brings it back to the land surface. This scheme is forced with ERA-Interim reanalysis data from 1990 to 2009. With the information about the atmospheric paths of water vapor, the distance between evaporation and precipitation location is determined. To get an approximation of the influence of land use on the atmospheric moisture budget, the atmospheric paths of water vapor from two bordering areas with different evaporative regimes are compared. Results show a strong annual cycle in the recycling ratio. For the Ganges basin, the recycling ranges from 5% during the winter months (November–March) to 60% during the June–July–August season. The comparison of two focus areas in the Ganges basin with a difference in March–August evaporation shows that during the premonsoon months (March–May), up to 70% of the evaporation difference between the two areas recycles within the Ganges basin. Analysis of the soil moisture nudging terms in ERA-Interim compared to independent irrigation data strongly suggest this evaporation difference can be attributed to large-scale irrigation. The importance of basin moisture recycling for precipitation shows an annual cycle as well. An annual average of 4.5% of Ganges precipitation originates from water evaporating in the Ganges basin. During the dry winter monsoon, any precipitation originates from sources outside the basin. During March–April–May and October–November, 10% of the precipitation originates from evaporation within the basin. During the summer monsoon season, the large influx of moisture from the Indian Ocean dominates the precipitation, and recycling is 5% of precipitation