Ways to Measure Water Loss Due to Evaporation

When evaluating the water loss due to seepage in a pond or lake it's very important to get an idea about the amount of loss due to evaporation as well as other environmental factors. Many times a lake appears to be "leaking" when in fact a higher than expected amount is really just evaporating or being drawn up by the surrounding vegetation.

There are several methods for establishing the amount of evaporation from a pond. These vary significantly in complexity as well as cost. Pan measurements, calculations based on evapotranspiration data, as well as using the Energy-Budget equation are common methods.

Pan measurements are good for estimating evaporation from a body of water. They are the least accurate, but relatively inexpensive of standard methods.

Several Equations based on the original Penman equation can be used to calculate evaporation, however determining the right equation is difficult and can result in significant errors should the wrong one be used.

The most accurate method is using the Energy-Budget equation. This method is both arduous and expensive. It involves gathering lake-surface temperature, air temperature, humidity, and net radiation data.

Using evapotranspiration data is a simple method, but still has errors inherent with using predetermined coefficients. Most states have meteorological stations scattered around. These stations usually publish evapotranspiration data for the area. This value can be used to extrapolate the approximate evaporation from a large body of water. Smaller bodies of water need to be adjusted with a coefficient depending on the season and region. This can be calculated as follows:

Evaporation from a body of water = ETo * (1/Ck)

For those of us that are really fine with getting an idea of how much water is being lost to evaporation we would propose the "5 gallon bucket and a ruler" method. Now before we start getting calls and emails about the degree of inaccuracy to this method, we do understand that we're making some compromises here. However, it works pretty well, it's very inexpensive, and even those us without a degree in engineering can calculate the results.

You'll need one or two 5-gallon buckets filled with your pond's water. We like to fill them to within about 0.5-1" of the top of the bucket in order to minimize splash effect yet also not completely eliminate the air movement above the water. Use a sharpie to record this water level. You will also need to place a metal ruler vertically into your pond. Record the current water level. You're now ready to start taking data. Every 24hrs for as long as you'd like to conduct your research take new measurements at all locations. If you want to feel official, record them into a lab book.

You'll need to consider rainfall, sprinkler runoff, etc. that may occur during this time because they can dramatically affect your results. You'll want to exclude those data points in your analysis. In the end, if you know how much water loss there was in the buckets and you compare that to the water loss from your lake, you should have a rough estimation of how much your pond is leaking.

Armed with the answers to these questions and your decision to proceed, feel free to contact Seepage Control, Inc (www.seepagecontrol.com) at 1.800.214.9640, or send us an email at info@seepagecontrol.com.