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$t->title = "ISUSM - Evapotranspiration Details";

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<ul class="breadcrumb">
    <li><a href="/agclimate/">ISU Soil Moisture Network</a></li>
    <li class="current">ET Details</li>
</ul>

<h3>Evapotranspiration Details</h3>

<p>Evapotranspiration (ET) is a term that describes the transport of water from
    the ground surface and soil to the atmosphere.  This includes processes like
    evaporation of liquid water and transpiration done by vegetation.  It is
    an important part of the Water Cycle and knowing the amount of ET happening
    during the growing season is important for usage in modelling, irrigation,
    and crop management.</p>

<h4>So lets 'observe' ET!</h4>

<p>Directly measuring ET is not practical as you would have to somehow account
    for every molecule of water vapor and know its exact source to know if it
    contributed to the local ET.  The most common approach to directly
    measuring ET is very expensive as it involves tracking air flow and
    moisture in 3 dimensions. Another approach is through the usage of a
    lysimeters, which attempt to keep an accounting of soil water.</p>

<p>So when all else fails, we can produce model estimates based on more
    commonly collected data.  That is what the legacy ISU AgClimate Network
    and now ISU Soil Moisture Network has done.  This page attempts to fully
    document the equations used and the application caveats of the data.</p>

<h3>Estimating Evapotranspiration</h3>

<div class="alert alert-info">The estimation techniques described should apply
        for both the legacy ISU AgClimate Network and the current ISU Soil
        Moisture Network.</div>

<p>Hourly estimates of ET are made by a set of equations found on the Campbell
        Scientific Data Logger.  These hourly values are then summed to produce
        calendar day totals to avoid confusion with having slightly conflicting
        ET values for a given day.</p>

<p>The hourly ET is estimated by the <a href="https://data.longpaddock.qld.gov.au/static/silo/pdf/ASCE_Standardized_Ref_ET_Eqn_Phoenix2000.pdf">ASCE Standardized Reference Evapotranspiration Equation</a>
        , which assumes a reference tall crop (alfalfa).  The equation is as follows,
        taken from the aforementioned reference.

<br /><img src="images/eqn2_et.png" alt="Reference ET Equation" /></p>

<p>The terms are defined as:
    <table class="table table-striped table-sm table-bordered">
<thead><tr><th>Symbol</th><th>Description</th><th>Units</th></tr></thead>
<tbody>
        <tr>
        <td>&Delta;</td>
        <td>Slope of the vapor pressure-tempertaure curve, estimated by using
        the mean temperature for the day or hour.</td>
        <td>kPa &deg;C<sup>-1</sup></td>
        </tr>

        <tr>
        <td>R<sub>n</sub></td>
        <td>Net Radition.  This is the combination of received shortwave minus
        the amount reflected by the crop (albedo) and longwave transmitted by
        the soil.</td>
        <td>MJ m<sup>-2</sup> d<sup>-1</sup></td>
        </tr>

        <tr>
        <td>G</td>
        <td>Ground Soil Heat Flux.</td>
        <td>MJ m<sup>-2</sup> d<sup>-1</sup></td>
        </tr>

</tbody>
</table>

<p><strong>References:</strong>
<ol>
    <li><a href="https://en.wikipedia.org/wiki/Evapotranspiration">Wikipedia's Evapotranspiration</a></li>
    <li><a href="https://data.longpaddock.qld.gov.au/static/silo/pdf/ASCE_Standardized_Ref_ET_Eqn_Phoenix2000.pdf">ASCE Standardized Reference ET</a></li>
    <li><a href="https://www.fao.org/4/x0490e/x0490e00.htm">FAO 56</a></li>
    <li><a href="https://github.com/woodcrafty/PyETo">Python Implementation</a> of FAO 56 and other equations</li>
</ol></p>

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