Chris Taylor has alerted us to another very important paper. It is
D. V. Spracklen, S. R. Arnold, C. M. Taylor, 2012: Observations of increased tropical rainfall preceded by air passage over forests, 2012: NatureVolume:489,Pages:282–285 (13 September 2012)DOI:doi:10.1038/nature11390
- The abstract reads [highlight added]
Vegetation affects precipitation patterns by mediating moisture, energy and trace-gas fluxes between the surface and atmosphere1. When forests are replaced by pasture or crops, evapotranspiration of moisture from soil and vegetation is often diminished, leading to reduced atmospheric humidity and potentially suppressing precipitation2, 3. Climate models predict that large-scale tropical deforestation causes reduced regional precipitation4, 5, 6, 7, 8, 9, 10, although the magnitude of the effect is model9, 11 and resolution8 dependent. In contrast, observational studies have linked deforestation to increased precipitation locally12, 13, 14 but have been unable to explore the impact of large-scale deforestation. Here we use satellite remote-sensing data of tropical precipitation and vegetation, combined with simulated atmospheric transport patterns, to assess the pan-tropical effect of forests on tropical rainfall. We find that for more than 60 per cent of the tropical land surface (latitudes 30 degrees south to 30 degrees north), air that has passed over extensive vegetation in the preceding few days produces at least twice as much rain as air that has passed over little vegetation. We demonstrate that this empirical correlation is consistent with evapotranspiration maintaining atmospheric moisture in air that passes over extensive vegetation. We combine these empirical relationships with current trends of Amazonian deforestation to estimate reductions of 12 and 21 per cent in wet-season and dry-season precipitation respectively across the Amazon basin by 2050, due to less-efficient moisture recycling. Our observation-based results complement similar estimates from climate models4, 5, 6, 7, 8, 9, 10, in which the physical mechanisms and feedbacks at work could be explored in more detail.
In their conclusuions, they write
Our analysis explores the role of regional-scale vegetation patterns on precipitation. Through evapotranspiration, forests maintain atmospheric moisture that can return to land as rainfall downwind. These processes operate on timescales of days over distances of 100–1,000km such that large-scale land-use change may alter precipitation hundreds to thousands of kilometres from the region of vegetation change. Land-use patterns and small-scale deforestation may also alter precipitation locally, through changes in the thermodynamic profile and the development of surface-induced mesoscale circulations. Natural and pyrogenic emissions from vegetation can also have a role in rainfall initiation over tropical forest regions. The impact of cloud microphysical processes on precipitation is highly uncertain, and biogenic emissions could contribute to our observed relationship between rainfall and exposed vegetation. However, our water-balance calculations imply that cumulative increases in evapotranspiration over upstream forested regions more than account for the increase in downstream rainfall.
What this paper means is that the atmosphere is enriched by water vapor and convective potential energy as it is transported across a region of transpiring vegetation. The removal of this vegetation results in an atmosphere that is less conducive to precipitation. While the paper focuses on the effects of tropical deforestation, this would be expected to occur in locations where increases in transpiration occur such as due to irrigation. Several of our papers that have examined this issue include
Hossain, F., I. Jeyachandran, and R.A. Pielke Sr., 2010: Dam safety effects due to human alteration of extreme precipitation. Water Resources Research, 46, W03301, doi:10.1029/2009WR007704.
Degu, A. M., F. Hossain, D. Niyogi, R. Pielke Sr., J. M. Shepherd, N. Voisin, and T. Chronis, 2011: The influence of large dams on surrounding climate and precipitation patterns. Geophys. Res. Lett., 38, L04405, doi:10.1029/2010GL046482.
Woldemichael, A., F. Hossain, R.A. Pielke Sr., and A. Beltrán-Przekurat, 2012: Understanding the impact of dam-triggered land use/land cover change on the modification of extreme precipitation, Water Resour. Res., doi:10. 1029/ 2011 WR011684.
Pielke, R.A. and X. Zeng, 1989: Influence on severe storm development of irrigated land. Natl. Wea. Dig., 14, 16-17.
Pielke Sr., R.A., 2001: Influence of the spatial distribution of vegetation and soils on the prediction of cumulus convective rainfall. Rev. Geophys., 39, 151-177.
In Figure 9 in Pielke et al 2001, for example, we show the major impact on the potential for deep cumulonimbus clouds (and thus rainfall) of just an increasre in surface air dew point temperature by just one degree Celcius.
The new Sracklen et al 2012 is yet another example of why land use/land cover change is a first order human climate forcing.
Climate Science: Roger Pielke Sr. 
