the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 19 Aug 2024
Variations in vegetation evapotranspiration affect water yield in high-altitude areas
Abstract. Global mountains and plateaus are the main water-producing areas on land. However, under the influence of climate change, the distribution of vegetation and the way water is utilized in these areas have undergone significant changes. As such, understanding the effects of evapotranspiration from high-altitude vegetation on precipitation and runoff is vital in addressing the uncertainties and challenges posed by climate change. Between 2018 and 2022, we conducted research in the northeastern Qinghai-Tibet Plateau, collecting data on precipitation, soil water, and Picea crassifolia xylem water to quantify the impact of vegetation transpiration and recirculated water vapor on precipitation. Our findings indicate that transpiration from vegetation accounts for the largest share of evapotranspiration within the entire forest ecosystem, averaging 57 %. Therefore, vegetation transpiration is the decisive factor in determining the water yield of inland high-altitude areas. The average contribution of local evapotranspiration to precipitation reaches 28 %, making it the main driver behind the increase in precipitation at high altitudes. The warming of global temperatures and human activities are likely to induce shifts in the distribution areas and evapotranspiration regimes of alpine vegetation, potentially altering water resource patterns in the basin. It is necessary to actively adapt to the changes in water resources in the inland river basin.
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RC1: 'Comment on egusphere-2024-2246', Anonymous Referee #1, 13 Nov 2024
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Comments on “Variations in vegetation evapotranspiration affect water yield in high-altitude areas”
The authors use isotopic analysis to estimate the fraction of transpiration in the total evapotranspiration of the Picea crassifolia forest in the northeastern Quingha-Tibet Plateau at sites with different elevations (from 2.5 to 3.4 km a.s.l.) in 2018-2022.
1/ During transpiration, the heavy water molecules (containing deuterium and 18O) tend to be left behind, thus the fraction of heavy isotopes in the xylem water increases. Comparing isotopic ratios in soil water, ambient water vapor and xylem water, it is possible to estimate the share of transpiration in the total evapotranspiration. It was estimated it at 58%.
2/ The authors also investigated the isotopic composition of precipitating water and indirectly estimated the isotopic composition of the water vapor brought by atmospheric advection to assess the ratio of local evapotranspiration to local precipitation. It was estimated at 28%.
3/ The third result was to compare directly measured local precipitation with local evapotranspiration retrieved from a global dataset (Table 4), with the overall conclusion that evapotranspiration greatly exceeds precipitation and hence “surface runoff cannot be collected in this area, which also proves that afforestation in this area will further enhance evapotranspiration, posing a threat to water distribution and utilisation”.
Evapotranspiration is a key process on land in several aspects. It is a proxy of biological productivity as well as an important process of energy conversion with significant implications for regional and global atmospheric moisture transport and climate. Evapotranspiration is difficult to measure directly. So all data on evapotranspiration are welcome and the authors appear to have performed much work to arrive at their first result. I have several relatively minor comments listed below that could clarify the significance of this result, but otherwise it is meaningful and can be published. I have to note though that I am not an experimentalist and could not follow all the procedures in detail, so perhaps an extra reading by another specialist could be helpful. I would also recommend the authors to explain briefly to the readers the physics and ecology of isotopic fractionation, e.g. following and/or quoting the excellent review by Farquhar et al. 2007 https://doi.org/10.1104/pp.106.093278 “Heavy Water Fractionation during Transpiration”
I have more reservations with respect to the second result, namely the estimate of how much the local evaporation contributes to local rain (moisture recycling). The problem is ultimately that of the inherent non-locality of moisture recycling.
In order to estimate moisture recycling, one has to know the isotopic ratio of the advected moisture. Then, knowing the isotopic ratio of precipitation and evaporation, it is possible to estimate the contribution of the latter to the former. However, we can’t measure the isotopic ratio of the advected moisture locally, where evaporation and precipitation take place. Because water vapor in the local air is a mixture of advected and locally evaporated vapor. To assess the isotopic ratio of advected vapor from the isotopic ratio of local water vapor and evaporated vapor, one has to know how big the contribution of evaporation is, but this is what we want to estimate.
To get around this problem, the authors measure the isotopic ratio of advected water vapor on a station upwind the location where evaporation measurements are taken. This upwind station is located at a lower elevation. As the moist air moves up and downwind towards the point of evaporation measurements, its moisture content declines. The isotopic composition of air as it arrives to the point of evaporation measurement is different from where it started.
The authors model this change using the so-called Rayleigh distillation (their Eq. 17). This equation assumes that the sole source of changes for the isotopic ratio is rainfall. This assumption allows one to estimate the local isotopic ratio of the advected water from the isotopic ratio upwind and the ratio of upwind and downwind water vapor concentrations.
However, this assumption is at best problematic. Because as the air moves downwind, its vapor composition is influenced not only by rainfall, but also by moisture recycling, i.e. by added evaporation over the area along which it is moving. Evaporated moisture has a lower content of heavy isotopes, so moisture recycling along the air pathway changes the isotopic composition of the water vapor. This is especially relevant when evaporation and precipitation are of the same order of magnitude as in the case in the studied locations.
A relevant study is that of Natali et al. 2022 https://doi.org/10.1016/j.jhydrol.2022.128497 “Is the deuterium excess in precipitation a reliable tracer of moisture sources and water resources fate in the western Mediterranean? New insights from Apuan Alps (Italy)”. It shows that there can be fundamentally different interpretations of changing isotopic ratios.
I would therefore request that the authors should provide a thorough justification for their assumptions behind the calculation of the isotopic composition of the advected moisture. Personally I cannot see a straightforward solution given the many unknowns and big uncertainties.
Regarding the third conclusion, that evaporation is several times higher than rainfall, my question is, how does this forest survive? I suspect that this conclusion could be an artifact of comparing evapotranspiration from a global gridded dataset to locally measure precipitation. Local evapotranspiration has very big uncertainties even on an annual scale. The authors refer to the study of Yao et al. 2022 https://doi.org/10.1016/j.agrformet.2017.04.011 “Improving global terrestrial evapotranspiration estimation using support vector machine by integrating three process-based algorithms”. Yao et al.’s Fig. 10, lower right panel, shows that estimated evapotranspiration can considerably exceed (by ~100 mm/year) the actual one derived from water balance measurements in river basins. Arguably local values on a small scale should be even more prone to errors. I would therefore urge the authors to find additional arguments to justify their conclusions which cannot be published as they now stand.
In summary, I recommend a major revision.
I would recommend to make a table listing and explaining all the notations used in the article.
Please also derive an explicit formula for f_re (defined in Eq. 10) using isotopic fractions delta, as it is your main result.
Please avoid reporting delta to the accuracy of four (!) digits. Apparently the data do not have this accuracy.
Minor comments (numbers are line numbers)
10: The abstract could better reflect the actual content of the study. Currently it does not mention the isotopic analysis at all despite that is the focus. The authors do not really study any climatological change, so referring to that in the abstract appears to be somewhat misleading.
10: “posed by climate change” – please consider adding “and anthropogenic transformation”, which is a major driver of vegetation changes
15: Even if local precipitation makes a 28% contribution to local rainfall, it does not automatically become “the main driver behind the increase in precipitation at high latitudes”. Again, changes in precipitation are not investigated in the paper.
20: The reference to Ault et al. 2020 seems to be missing from the reference list.
25: Please revise or provide supporting evidence to the statement “Notably, an increase in evapotranspiration and the content of inland advection water vapor contributes to a rising trend in global land water storage” There are different regimes in which increased evapotranspiration can increase or decrease moisture advection, see Makarieva et al. 2023 https://doi.org/10.1111/gcb.16644 “The role of ecosystem transpiration in creating alternate moisture regimes by influencing atmospheric moisture convergence”
40: “This phenomenon is more pronounced in high-altitude areas, which are characterized by greater vegetation coverage and favorable conditions for the confluence of water vapor, resulting in more abundant precipitation”. Do you mean “in those high-latitude areas where there is a greater vegetation coverage and favorable conditions…” Please check
75: Legend to Figure 1 “changes in meteorological conditions”. Do you mean seasonal variation of meteorological conditions? What time period do the data shown in the lower panel refer to?
120: Equation 2: please define epsilon^+
120: Please check this phrase “Defined by. about 2H and 18O are calculated as follows”
125: Please give a characteristic value for alpha^+. Is it large compared to 1?
130 and elsewhere: Please pay attention to get all the lower indexes right. Currently many of them are printed not as indexes but as plain text.
150: What is the difference if any between delta_A and delta_a?
155: picea should be Picea
165: Equation 10: please revise this equation or the preceding line, currently it is not the proportion of precipitation occupied by advective vapor but the proportion occupied by evaporation.
170: “This can be calculated” please clarify what is “This”
175: Please define “C-G”
175: Please check the phrase “are the stable isotopes in precipitating transpiration, transpiration, surface evaporation and advective vapour, respectively”. Should it be “water vapor in the precipitating column (?), transpiration, surface evaporation and advective vapour”?
What is delta_p in Eq. 13, is this for precipitating liquid water? Please explain.
180 and elsewhere: “steam” – please use “vapor” consistently everywhere
190: Please give more details about the HYSPLIT model
195: “as follow” “as follows”, “winds tation” “wind station”
200: Since rainfall is positively correlated with the surface vapor pressure of the whole study area (c=1.657e, where c is the water vapor content in mm, e is the surface vapor pressure in hPa, R2=0.94),
Do you mean the total water vapor content is correlated with surface vapor pressure? Rainfall has the units of mm per unit time, not mm. So correlation between total water vapor content and surface vapor pressure is not equivalent to correlation between rainfall and surface vapor pressure.
215-240: Please could you clarify in greater details how these figures (with the meteorological water line) are relevant to your main results. What parameters from these figures are you using and where?
235: please define “lc-access” and explain how it is relevant.
265: “in a distinct pattern” do you mean “opposite”?
230: In May and September, the evaporation rate of water vapor increases due to relatively higher humidity levels compared to other periods.
Please check this. Should not evaporation decrease when the humidity is higher?
315: The reference to Teo et al. 2021 seems to be missing from the reference list
325: In this case, the groundwater amount decreases gradually with the T value increase.
Were any measurements performed of this decrease?
In the reference list, all the journal names are missing
Citation: https://doi.org/10.5194/egusphere-2024-2246-RC1
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