Question No. 31

Dear Authors! Please explain what your grounds are when you state that forest evaporation exceeds oceanic evaporation. For example, in the book of V.V. Rakhmanov “Hydroclimatic role of forests” Moscow 1984 there is an overview of two centuries of measurements and calculations, according to which forest evaporation or transpiration does not in any way exceed oceanic evaporation, not being even close. Thank you in advance for your response.
Answered 30 January 2016.
Question author: Vladimir.
Asked 28 January 2016.

The statement that forest evaporation exceeds oceanic evaporation is based on consideration of the leaf area index, which in natural forests can be as high as 10 (on a regional scale - 6). This means that the evaporating surface (i.e. the cumulative surface of all leaves) per unit area occupied by the forest exceeds the evaporating surface of the ocean by several times. Therefore, other conditions being equal, forest evaporation can exceed oceanic evaporation by an order of magnitude.

Maximum LAI during peak green month
Figure 7b of Buermann et al. 2002

The multiyear average of the evaporation rate is constrained by the flux of solar radiation. On a global average about one half of solar radiation absorbed by the Earth's surface is spent on evaporation. Therefore, in the long term the forest cannot elevate its evaporation by more than twice as compared to the ocean. However, for the instantaneous rates, a significant excess of forest evaporation over oceanic evaporation is possible.

Since the rainfall probability rises sharply with increasing water vapor content of the atmospheric column, even a small and short-term difference between evaporation rates over the forest and the ocean can result in a predominant rainfall formation over the forest, see this mechanism in greater details here. Long-term mean forest evaporation does not significantly matter and can be either higher or lower than over the ocean. Note also that via biotic control of the atmospheric concentrations of substances serving as cloud condensation nuclei the forest can switch condensation on even when the water vapor content is relatively low. This is equivalent to control of relative humidity.

The book of V.V. Rakhmanov contains a lot of interesting and valuable information concerning the history of investigations of the hydrological role of forests. We would particularly mention the data on river runoff in forested versus deforested river basins. However, when it comes to the quantitative data on evaporation, the following statement on page 93 is the most relevant one: "Whether the cumulative evaporation of the forests is high or low, including in comparison with deforested lands, -- this question not only lacks a commonly accepted answer but the available opinions often directly contradict each other." Indeed, despite the ever increasing amounts of data, including satellite data and eddy flux towers in forests, the magnitude of evaporation on land and in the ocean continues to suffer from considerable uncertainties, especially in the tropical regions, see, for example, Trenberth et al. 2011.

As for the boreal forests, the available data testify unambiguously for a higher evaporation over the forest as compared to the adjacent oceans. The two figures below represent surface evaporation from the MERRA database corresponding to January and July of 2014. The blue color is low evaporation, green and yellow - high evaporation.

Evaporation in January and July 2014, MERRA
Monthly evaporation in July and August 2014 according to MERRA. The data can be downloaded from here (dataset MATMNXFLX tavgM_2d_flx_Nx)

These maps show that evaporation over the Russian boreal forest belt in summer, when the forest is biophysically active, significantly exceeds evaporation over all the oceanic regions adjacent to the eastern, western and northern coasts of Eurasia. In January, when the forest is inactive, the opposite is true. The reason is quite obvious: in summer land is warmer than the ocean, so if there is enough moisture in soil, evaporation will be higher on land other things being equal. If there is no vegetation and hence no soil moisture, evaporation will be absent independent of temperature. This is approximately the situation in Australia.