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.

  • $\begingroup$ Interesting information on this subject (including references), in video format: youtube.com/watch?v=Y3OWgb0Bv-A $\endgroup$ Commented Jan 25, 2015 at 14:58
  • $\begingroup$ 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. $\endgroup$
    – jamesqf
    Commented Jan 25, 2015 at 18:15
  • $\begingroup$ 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. $\endgroup$
    – cphlewis
    Commented Apr 2, 2015 at 23:52

2 Answers 2


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.

  • $\begingroup$ 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! :) $\endgroup$ Commented Jan 25, 2015 at 12:47
  • $\begingroup$ 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. $\endgroup$
    – naught101
    Commented Jan 25, 2015 at 13:09
  • $\begingroup$ @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... $\endgroup$
    – naught101
    Commented Jan 25, 2015 at 13:12
  • $\begingroup$ @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). $\endgroup$
    – dotancohen
    Commented Jan 25, 2015 at 17:09
  • 1
    $\begingroup$ @DavidMulder: I guess for a really simplified explanation, you could say that evaporation is just "evaporability" multiplied by surface area. Obviously leaves have a much lower evaporability than open water, but forests obviously have much more surface area dense rainforests might have a leaf area index of 5-6 $m^2/m^2$. $\endgroup$
    – naught101
    Commented Mar 2, 2015 at 22:56

Plants sweat. It's called transpiration. . Nearly 90% of this water exits the tree in the form of vapor through small pores called stomata on leaves. Atmospheric circulation moves water vapor around the globe, but oceanic evaporation meets most of it. It accumulates and falls as rain, which plants absorb, in high heat, they plants release water to dispose of heat and it accumulates again. COllecting to afix new humidity which fall as rain once again. enter image description here


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.