Impacts of irrigation and anthropogenic aerosols on the water balance, heat fluxes, and surface temperature in a river basin

Changes in both land cover and the atmosphere have impacted the heat fluxes of south Asia in ways that may have altered the timing and magnitude of the monsoon. Century-long budgets of water and energy in the Krishna Basin (258,948 km²) in southern India demonstrate that irrigation impacted the sensible heat flux of the land surface (H) as much as or more than did the atmospheric brown cloud (ABC) over 1960-2005. Annual discharge of the Krishna River fell from 226 mm during pre-irrigation land cover (1901-1960) to 64 mm by 1990-2005, when 14-20% of the basin area was irrigated. Over the same period, annual evaporation increased by 166 ± 32 mm (+28%) causing H to decrease by 12.7 ± 2 W m^-2 (-18%) compared to a decrease of 11.2 ± 1.8 W m^-2 caused by the atmospheric brown cloud (ABC). The rate of change in H during irrigation expansion (1960-1990) was between -3.4 and -5.0 W m^-2 per decade (da^-1) due to irrigation expansion and -1.8 to -2.3 W m^-2 da^-1 due to the ABC. The trend in H caused by irrigation was negligible over 1990-2005 as irrigated area and evaporation stabilized. Previous work using the Parallel Climate Model estimated that the ABC decreased the latent heat flux by 2.4 W m^-2; this decrease was more than offset by irrigation, resulting in a net increase in the latent heat flux of 12.9 W m^-2. The maximum surface air temperature (Tmax) either decreased or remained the same in areas experiencing irrigation expansion but increased in a majority of unirrigated areas during the post-monsoon season. The results provide observational evidence that irrigation changed both the basin-scale sensible heat flux and surface air temperatures.