Evapotranspiration (ET) Lab
February 6, 2011
Estimation of ET from Multiple Methods
This lab has two objectives:
Compares estimates of forest evapotranspiration from several methods.
Estimate the effect of forest practices (harvesting, planting density) on a stand level water
Rainfall is partitioned onto several pathways after falling.
First, it may be captured by
the “architecture” of whatever ecosystem it falls on.
That may be the leaves, stems and branches
of a forest, or the roads and buildings of urban areas.
This can be between 10 and 50% of the
water budget, depending of factors like the amount and intensity of rainfall, and the type and
characteristics of the vegetation.
This water is generally evaporated back to the atmosphere, but
may also be stored or used.
Second, rainfall may fall through the canopy, either directly
(throughfall) or via stems (stemflow).
This water can either enter the soil where it can be taken
up by roots and transpired or runoff over the land surface.
The character of this partition is
dependent as before on the amount and intensity of the rainfall, on the infiltration capacity of the
soil (which can be dramatically modified by people) and by the amount of rainfall that has
preceded the present (so-called antecedent rainfall).
Any water that makes it into the soil starts
to fill the pore spaces and, if possible, percolate downwards.
As water passes through the soil,
plants can use it (transpiration) or it can be returned to the atmosphere abiotically (evaporation).
The sum of the interception that is evaporated, the evaporation of throughfall, and the
transpiration of soil water is
, and it is generally the biggest water loss
component, often by far.
The word evapotranspiration implies that it is a coupled process that
combines both biotic and abiotic mechanisms.
While there are techniques to disentangle the
components, it is generally modeled as one quantity, though one that is sensitive to the
vegetation at the site where it’s estimated.
Among the most important insight about the ET process is that it requires a LOT of
Remember the latent heat of vaporization?
That’s the energy required to convert liquid
water at 100 deg C to vapor water at the same temperature.
It takes 2.45 MJ (million Joules,
where the Joule is the SI unit of energy) to do that for 1 kg of water.
For comparison, the energy
in a liter of gasoline is about 31 MJ; that is 1 liter of gas can evaporate only roughly 13 liters of
This value (the latent heat of vaporization) is a function of temperature, but we’ll treat it
as a constant here (2.45 MJ/kg).
It’s also critical to remember that air has a capacity to hold water that is a function of
temperature of that air.
Specifically, the e
(saturation vapor pressure, measured in kPa, a
measure of pressure) is predicted by the following equation: