Decrease in evaporation over the Indian monsoon region: implication on regional hydrological cycle

Using surface observations from 58 widely distributed stations over India, a highly significant (99.9 %) decreasing trend of pan evaporation (E-pan) of 9.24 mm/a/a is calculated for 1971 to 2010. This constitutes a similar to 10 % reduction of E-pan over the last four decades. While E-pan is decreasing during the wet summer monsoon season (JJAS), as well as during the dry rest of the year, the rate of decrease during the dry season is much larger than that during the wet season. Apart from increasing solar dimming, surface winds are also persistently decreasing over the Indian sub-continent at the rate of -0.02 m/s/a resulting in similar to 40 % reduction over the last four decades. Based on PenPan model, it is shown that both the above factors contribute significantly to the decreasing trend in E-pan. On a continental scale, annual mean potential evaporation (E-p) is larger than rainfall (P or E-p-P > 0, moisture divergence) indicating that India is water-limited. However, during wet monsoon P > E-p (or E-p-P < 0, moisture convergence) indicating that India is energy-limited during this season. Long term data shows that annually E-p-P follows a significant decreasing trend indicating that water limitation is decreasing with time. This is largely due to stronger decreasing trend of E-p-P during the dry season compared to weaker increasing trend of E-p-P during the wet monsoon season. The scatter plot of E-p-P versus E-p also conveys that the decrease in E-p leads to increase in moisture convergence in wet season and decrease in moisture divergence in dry season.