Measuring actual evapotranspiration with weighing lysimeters

In my Watershed Hydrology class this semester, we conducted student-designed experiments with potted plants and bare soils to study the factors affecting evapotranspiration. The procedure we used is a simplified version of a weighing lysimeter.

How a lysimeter works

Weighing lysimeters are one of the very best ways of measuring the actual evapotranspiration from a small area of land, because they use mass balance (i.e., changing weight) to give us the combined total of plant transpiration, soil evaporation, and interception losses over time. Our class potted plant experiments were in the laboratory, but in order to understand the effects of environmental variability on evapotranspiration rates, most weighing lysimters are installed in the outdoors – what we called “in the field.”

Here’s how a weighing lysimeter works: A large soil core (called a “monolith”) is carefully removed from the ground and put inside a cylinder or box (A in the image above). The monolith-in-a-cylinder is then placed back into the ground and vegetation is planted. The cylinder usually will have a drain in the bottom so that water that percolates downward through the soil can leave the cylinder and be accounted for as a deep drainage loss (C in the image above). The cylinder sits on a load cell, which is like a giant scale that continuously records the weight of the soil (B in the image above). Decreases in weight are associated with evapotranspiration and deep drainage, but since deep drainage is often measured separately, it can be split off and the mass lost because of evapotranspiration can be accurately measured. Some weighing lysimeters may also have a way of accounting for surface runoff generated in the cell as shown in D in the image above, some will have sensors embedded in the soil measuring moisture content and other soil properties, and most lysimeters will have a nearby weather station including a rain gauge.

Lysimeter math

Weighing lysimeters take advantage of the water balance equation:

Input – Output = Change in Storage

The construction of the lysimeters simplifies the inputs and outputs, by removing complications like lateral groundwater flow.  The equation becomes:

Precipitation – Evapotranspiration – Deep Drainage = Change in Storage

[Sometimes surface runoff will also be included, as shown in the figure at the top of the post. However, this is avoided where possible.]  Rearrange the equation and solve for evapotranspiration:

Evapotranspiration = Precipitation – Deep Drainage – Change in Storage

Precipitation and deep drainage are measured directly, with a rain gauge and the collection device shown in the image. The changing weight of the lysimeter measures the change in storage of water in the soil and plants. Taken together, that means the only unmeasured term is evapotranspiration, so the numbers can be put into the equation and voila…evapotranspiration.

And because most lysimeters constructed these days have digital load cells and data loggers, they collect high frequency data. This means we can calculate evapotranspiration at the time scale of minutes, hours, days, … or whatever timescale is of interest.

Lysimeter data

Let’s look at some data from a real lysimeter. This lysimeter is operated by the Desert Research Institute in Nevada. This lysimeter is one of three lysimeters, each 2.2 m diameter and 3.0 m depth filled with soil from the Mojave Desert. In the graph below, you can see the mass decrease during each day as evapotranspiration removes water (and mass) from the system. At night, there is a very slight increase in mass coinciding with dew or frost formation on the soil. This is a telltale sign that mass decrease is evapotranspiration and not drainage – we wouldn’t expect drainage to stop at night or have a fairly constant rate day after day.

On February 22nd, the site got some rain, and you can see a near instantaneous increase in the mass of the soil. After the rain ends on about February 23, you see a rapid decline in mass up until the end of the graph. This rapid decline may represent a period when there is deep drainage of water from the soil monolith. Note how much faster it is compared to the evapotranspiration losses earlier in the graph.

You can read more about the Nevada lysimeters here and look at all of the great real-time data they post to the web.

Look at some real lysimeters

As you can see, a real-life weighing lysimeter is quite a bit more sophisticated than the periodic measurements of the weight of a potted plant that we’ll be doing in class this week. But the concept is the same, and weighing lysimeters come in a range of sizes. You can read more about how two large lysimeters helped California scientists learn more about orchard and vine crop water use here. The California link has some great photos the lysimeter construction, old-school measurement techniques, and the sort of analyses that can be done with the data.

The videos below show the installation of a small weighing lysimeter and the construction of a series of rather larger ones. (The second video is in French but has English captions.)

This video (in German) shows the underground laboratory where the lysimeter data and water samples are collected.

Obviously, large lysimeters are expensive to construct and are used for a lot more than just measuring evapotranspiration. Two of the videos above show how lysimeters can be used to gain insight into water quality problems. Also, sometimes it’s not necessary – or practical – to preserve an intact soil core or monolith. However, disturbed or constructed soils won’t have the same soil pore structure or other characteristics of natural soils – at least until they’ve had a chance to evolve for a few years. But regardless of the additional data collected or the way they are constructed, weighing lysimeters are unparalleled in their capability of fully closing the water balance and getting accurate measurements of actual evapotranspiration.