ET_Lab_11 - Evapotranspiration (ET) Lab February 6, 2011...

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Evapotranspiration (ET) Lab February 6, 2011 Estimation of ET from Multiple Methods Overview This lab has two objectives: 1) Compares estimates of forest evapotranspiration from several methods. 2) Estimate the effect of forest practices (harvesting, planting density) on a stand level water budget. Introduction 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 evapotranspiration , 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 energy. 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 water. 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 s (saturation vapor pressure, measured in kPa, a measure of pressure) is predicted by the following equation: [1]
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Where T is temperature in degrees C, and e s is in units of kPa. The vapor deficit is
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ET_Lab_11 - Evapotranspiration (ET) Lab February 6, 2011...

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