<p>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.
<a href="https://i.sstatic.net/y1O5K.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/y1O5K.jpg" alt="enter image description here" /></a></p>

CC BY-SA 4.0
2020-09-26T03:29:41.353Z
2020-09-26T03:29:41.353Z
LazyReader
<p>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.</p>

<p>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 <strong>0.42-0.54 my$^{-1}$</strong> (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 <strong>62.2-79.9 petatonnes</strong>, compared to 425-505 petatonnes for the ocean.</p>

<p>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. </p>

<ul>
<li><p>Brutsaert, W., 2005. <em>Hydrology: An Introduction</em>, Cambridge University Press.</p></li>
<li><p>Malhi, Y. et al., 2002. Energy and water dynamics of a central Amazonian rain forest. Journal of Geophysical Research: Atmospheres, 107(D20), p.8061.</p></li>
</ul>

CC BY-SA 4.0
2020-10-07T18:07:13.040Z
The question is how would such rainforest develop without sources of large bodies of water to begin with?
JeopardyTempest
CC BY-SA 4.0
2020-10-07T18:11:05.127Z
And without large sources, any drier regions (plus the loss of water into and [away from](https://earthscience.stackexchange.com/questions/9488/how-much-water-is-the-atmosphere-losing-to-space) the Earth) would be sinks gradually reducing water availability globally without replenishment? The idea that transpiration could support a lush planet without ocean-like areas seems like theorizing greenhouse gases could somehow keep a planet warm without a sun/other heat source. "Putting the cart before the horse". Fun idea, but plants seem a good way to hold/recycle water but not a real source of it
JeopardyTempest
CC BY-SA 3.0
2015-01-25T12:47:03.880Z
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
CC BY-SA 3.0
2015-01-25T13:09:05.470Z
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
CC BY-SA 3.0
2015-01-25T13:12:34.360Z
@DavidMulder: like [this one from Encyclopaedia Britannica](http://www.britannica.com/EBchecked/topic/278858/water-cycle), which has similar ratios, but the totals appear to be out by 3 orders of magnitude - might be my calculations...
naught101
CC BY-SA 3.0
2015-01-25T17:09:15.617Z
@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
CC BY-SA 3.0
2015-01-25T18:14:40.113Z
@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
CC BY-SA 3.0
2015-03-02T22:56:55.203Z
@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 $m^2/m^2$.
naught101
CC BY-SA 3.0
2015-01-25T12:23:09.427Z
2015-01-25T13:14:34.277Z
2015-01-25T13:14:34.277Z
naught101
<p>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.</p>

CC BY-SA 3.0
2015-01-25T14:58:32.377Z
Interesting information on this subject (including references), in video format: https://www.youtube.com/watch?v=Y3OWgb0Bv-A
Willem Renzema
CC BY-SA 3.0
2015-01-25T18:15:02.670Z
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
CC BY-SA 3.0
2015-04-02T23:52:37.347Z
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
<water><land-surface><forest><water-vapour>
Where does the water from rain in rain forests come from?
David Mulder
# Where does the water from rain in rain forests come from?

(Asked by: username_0 on 2015-01-25)

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.

**Tags:** water, land-surface, forest, water-vapour

> **Comments:**
> **username_1**: Interesting information on this subject (including references), in video format: https://www.youtube.com/watch?v=Y3OWgb0Bv-A  
> **username_2**: 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.  
> **username_3**: 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.  

---

## 2 answers

### Answer by username_5 on 2015-01-25. Score: 10 | Accepted answer

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.

> **Comments:**
> **username_0**: 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! :)  
> **username_5**: 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.  
> **username_5**: @username_0: like [this one from Encyclopaedia Britannica](http://www.britannica.com/EBchecked/topic/278858/water-cycle), which has similar ratios, but the totals appear to be out by 3 orders of magnitude - might be my calculations...  
> **username_7**: @username_0: 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).  
> **username_0**: @username_7: 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.  
> **username_5**: @username_0: 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 $m^2/m^2$.  
> **username_6**: The question is how would such rainforest develop without sources of large bodies of water to begin with?  
> **username_6**: And without large sources, any drier regions (plus the loss of water into and [away from](https://earthscience.stackexchange.com/questions/9488/how-much-water-is-the-atmosphere-losing-to-space) the Earth) would be sinks gradually reducing water availability globally without replenishment? The idea that transpiration could support a lush planet without ocean-like areas seems like theorizing greenhouse gases could somehow keep a planet warm without a sun/other heat source. "Putting the cart before the horse". Fun idea, but plants seem a good way to hold/recycle water but not a real source of it  

---

### Answer by username_4 on 2020-09-26. Score: 0

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](https://i.sstatic.net/y1O5K.jpg)](https://i.sstatic.net/y1O5K.jpg)


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