It is well known that precipitation derives in part from evaporation from oceans and in part from recycled moisture from terrestrial surfaces. In fact, a new model by van der Ent et al. (2010), which was highlighted in an International Centre for Research in Agroforestry (ICRAF) presentation demonstrates that up to 50% of rainfall in many areas is actually the recycling of water on land. Although external forcing and climatic parameters, such as solar radiation, aerosols and greenhouse gases affect sea surface temperature and, thus, determine oceans evaporation; terrestrial evaporation on the other hand is strongly influenced by vegetation and only in part by climate.
I found a recent paper by Keys et al., published in the February issue of Biogeosciences, which discusses precipitationsheds. They defined precipitationshed “as the upwind atmosphere and upwind terrestrial land surface that contributes evaporation to a specific location’s precipitation, e.g. rainfall” (Fig 1).They used the conceptual framework of precipitationsheds to illustrate how changes in land-cover in one region could affect evaporation, and thus precipitation, in a geographically separate region.
Firstly, they identified seven regions where rainfed agriculture is particularly vulnerable to reductions in precipitation, and then mapped their precipitationsheds. They then developed a framework to assess the vulnerability of precipitation for these seven agricultural regions. Their results showed that the sink regions have varying degrees of vulnerability to changes in upwind evaporation rates depending on the extent of the precipitationshed, the source region land use intensity and expected land cover changes in the source region.
As for the ICRAF presentation in Bonn, Henry Neufeldt said ICRAF created a backward model for the identification of “precipitationsheds” for defined areas. He presented modelled maps of the precipitationsheds for areas in the East and West Sahel, East Africa, and North and South China. Neufeldt showed the regional impacts of deforestation and land-use change in countries upstream on evapotranspiration within precipitationsheds, which cause downstream effects in countries in the precipitation zone.
During discussions, participants and panellists highlighted a number of further research questions, including: the impact of vegetation change on seasonal variation in precipitation; whether forest monocultures provide the same evapotranspiration benefits as natural forests; and the correlation between historical changes in vegetation cover in precipitationsheds to actual precipitation in precipitation zones.
Keys et al. said that future work using the precipitationsheds framework should aim to quantify how specific land cover changes (e.g. from forest to savannah) affect local evaporation, and downwind precipitation. This type of information, they added, may eventually provide dryland agricultural regions with enough information to adapt to significant changes in upwind land cover.
Keys, P. W.; Ent, R. J. van der; Gordon, L. J.; Hoff, H.; Nikoli, R.; Savenije, H. H. G. (2012). Analyzing precipitationsheds to understand the vulnerability of rainfall dependent regions. Copernicus Gesellschaft mbH, Gottingen, Germany, Biogeosciences, 2012, 9(2):733-746, many ref. http://www.biogeosciences.net/9/733/2012/bg-9-733-2012.pdf
van der Ent, R. J.; Savenije, H. H. G.; Schaefli, B.; Steele-Dunne, S. C. (2010). Origin and fate of atmospheric moisture over continents. American Geophysical Union, Washington, USA, Water Resources Research, Vol. 46(9), pp W09525, doi:10.1029/2010WR009127.