# Where does the water from rain in rain forests come from?

When discussing a hypothetical world without oceans and just forest it occured to me that I wasn't so sure there would be any rain in the first place, due to the lack of oceans. Searching on Google however I found out that trees transpire a whole lot more than I thought, but I was unable to find a run down of how much water in rain forest rains come from the oceans and how much comes 'directly' from the rain forest itself.

• Interesting information on this subject (including references), in video format: youtube.com/watch?v=Y3OWgb0Bv-A – Willem Renzema Jan 25 '15 at 14:58
• Keep in mind that precipitation depends strongly on orthography. In other words, if you have a mountain range sitting across moisture-laden prevailing winds, it'll drop the moisture on the windward side and the leeward side will be dry. Think e.g. US west coast. Much of a world without oceans might look more like the interior of Asia, or the American Great Plains, than a rain forest. – jamesqf Jan 25 '15 at 18:15
• The Amazon rainforest recycles a lot of its rain before orography comes into it, though -- trees will put up particulates and maybe VOCs that help seed rainclouds. I wonder how the temperature would be buffered on a world without oceans, though. – cphlewis Apr 2 '15 at 23:52

Leaves, mostly. Photosynthesis requires carbon dioxide, and it gets it via stomata - small controllable pores in the leaf. When the stomata open, water goes out, because the air is generally drier than the interior of the leaf. Water in the leaves is drawn up from the roots, via the xylem, using the suction from the lost water in the leaves, and also acting as a capillary.

As for how much water comes from transpiration, Brustaert (2005) includes a list of estimates of global annual evapotranspiration from the land that range from 0.42-0.54 my$^{-1}$ (averaged over all 1.49×10$^8$ km$^2$), and 1.18-1.4 my$^{-1}$ for the ocean (3.61×10$^8$ km$^2$). That 6.22×10$^{13}$-7.99×10$^{13}$ tonnes of water or 62.2-79.9 petatonnes, compared to 425-505 petatonnes for the ocean.

So, for Earth, the land is something like an sixth of the total evaporation. Keep in mind that that includes desert areas. A rainforest planet would have significantly higher evaporation than that, per square meter. For example, Malhi et al. (2002) calculate evapotranspiration over the rainforest in the Amazon at 1.12 my$^{-1}$. That is much closer to ocean rates than average land rates.

• Brutsaert, W., 2005. Hydrology: An Introduction, Cambridge University Press.

• Malhi, Y. et al., 2002. Energy and water dynamics of a central Amazonian rain forest. Journal of Geophysical Research: Atmospheres, 107(D20), p.8061.

• Love these moments when you find out those diagrams from secondary school were so simplified one could nearly consider them wrong :P And thanks for the excellent answer! :) – David Mulder Jan 25 '15 at 12:47
• Hrm... I've seen a few good ones that include estimates of evapotranspiration from forests (or at least from land). Couldn't find any good free ones to include here though. – naught101 Jan 25 '15 at 13:09
• @DavidMulder: like this one from Encyclopaedia Britannica, which has similar ratios, but the totals appear to be out by 3 orders of magnitude - might be my calculations... – naught101 Jan 25 '15 at 13:12
• @DavidMulder: Those diagrams from secondary school were not intended to present an actionable understanding of the intricacies of the processes. They were intended to demonstrate that relationships exist and to provide a superficial overview of the relationships in order to facilitate further study for those so inclined, and basic comprehensions when the subject comes up in other contexts (such as the sidebar of my favorite computer programming website). – dotancohen Jan 25 '15 at 17:09
• @dotancohen: You realise that everything you just said I already had covered in one word in my comment: 'simplified'. I know what simplification is ;) All that I noted was that this simplification was far stronger than most. – David Mulder Jan 25 '15 at 18:14