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22 March 2014 [Publications]
Why we need to reassess the role of forests for water security

Makarieva A.M., Gorshkov V.G., Sheil D., Nobre A.D., Bunyard P., Li B.-L. (2014) Why does air passage over forest yield more rain? Examining the coupling between rainfall, pressure, and atmospheric moisture content. Journal of Hydrometeorology, 15, 411-426, doi:10.1175/JHM-D-12-0190.1. Abstract. PDF (1.5 Mb). Supplemental information.

The effects of forests on weather are generally viewed solely in terms of moisture recycling. That is, evaporation from forest returns moisture to the atmosphere where it can increase local rain. In our new paper, using a recent study of Spracklen et al. (2012) Nature 489: 282 as an example, we discuss deficiencies in this perspective.

Among other things, we argue that the existing global circulation models cannot be used for evaluation of the climatic effects of deforestation because of their inability to reproduce various phenomena that we have ascribed to a new mechanism we have termed the biotic pump -- by this mechanism forests generate large-scale pressure gradients that cause winds to flow and bring moisture from oceans to land.

Water cycle on land and biotic pump
Water cycle on land and the forest moisture pump

Moist air arrives to land in the lower atmosphere, rises and becomes depleted of moisture (as clouds form and rain falls). This dry air returns to the ocean in the upper atmosphere while the net imported moisture returns to the ocean via rivers (and other minor flows). Air circulation models describe the aerial component of the water cycle: i.e. how much moisture is carried by winds. Measures of river runoff provide an independent check on any such models' validity as this runoff must match what the winds bring in. (Unfortunately the models escape such a check in the oceans as there is no runoff to measure!)

Current air circulation models do not pass this check: inputs don't match outputs. For example, the Amazon models can only account for half the measured river runoff. Similar considerable discrepancies are common for all regional models and no fix to this problem has yet been identified (e.g., Hagemann et al. 2011).

In the new paper we provide new evidence for the biotic pump. We analyse the relationship between wind direction and surface air pressure in forested and deforested regions of the Amazon basin. This highlights how the intense evaporation from forest creates low pressure. Let's take a look -- schematically -- how this condensation-evaporation cycle works.

Stage 1: After rain the atmosphere is dehumidified

Immediately after rain the local atmosphere is relatively dry (water vapor has condensed and precipitated). Winds are negligible. The atmosphere slowly regains water vapor via evaporation.

Stage 2: Evaporation enriches the atmosphere with water vapor

Owing to the high cumulative surface of leaves, the forest enriches the atmosphere with water vapor more rapidly than does the ocean. Total air pressure in the area slowly grows reflecting the accumulating water vapor. (Our analyses show that rainy days in the Amazon forests are on average characterized by a slightly higher pressure than the rainless days. In the deforested region the opposite is true -- we discuss this is greater detail in the paper (see Section 4b,c), but the pattern is consistent with the biotic pump.)

Probability to observe rain within one hour of measuring a given CWV
Rainfall probability as a function of the total amount of water vapor in the atmosphere (CWV). As used in Fig. 5 of Makarieva et al. (2014) based on the data of Holloway and Neelin (2010).

Small differences in evaporation rates translate into large differences in the probability of rainfall due to the sharply rising relationship between water vapor content and the likelihood of rain. Thus rain is much more likely to start again over the forest.

Stage 3: Condensation begins over forest

Once sufficient water vapor has accumulated over the forest, condensation begins. Local air pressure starts to decline. In contrast to the gradual process of water vapor evaporation, condensation can occur quickly.

Stage 4: Condensation creates winds that bring moisture to sustain condensation

The resulting pressure differences now draw wet air from the ocean to the forest, this air rises and cools. Now moisture generated locally via forest evaporation precipitates on land together with additional moisture brought from the ocean. This additional moisture is what ensures land remains wet while rivers keep running back to the ocean.

Thus, rather than merely recycling moisture, forests actually drive the water cycle on land. Recognition of this role will lead to re-evaluation of the importance of natural forests and the need for forest conservation to prevent water scarcity. We urge a major reassessment of the role of forests in atmospheric dynamics.