A Vadose-Zone Water Fluxmeter to Measure Drainage Rates and Water Quality

GW Gee, AL Ward, and ZF Zhang(11/01/03)

We have designed, built, and tested a device called a water fluxmeter that is capable of directly measuring water flow in draining soils. The water fluxmeter has a large dynamic range, from less than 1 mm/yr to more than 1000 mm/yr. This means that the device is sensitive to drainage rates under almost all conditions, from dry deserts to wet tropical environments. The fluxmeter can be used in a variety of applications. It can be used in farming operations to measure and control irrigation during a cropping season. It can also be used to measure fertilizer losses from the root zone and help improve fertilizer application efficiencies. Similarly, it can be used at recreational facilities, such as golf courses, to measure and control excess water and nutrient losses. Also, it can be used at waste landfill sites to advise operators when drainage is occurring and where cover systems need to be improved.

Our water-flux meter is elegant in its simplicity. It consists of a soil column connected to a fiberglass wick, which controls the soil water tension at the bottom of the column. The column and wick drain into either a miniature recording tipping bucket or a mini-dosing siphon that can be monitored with a pulse counter. The water fluxmeter is generally augured into the soil and placed below the root zone. The depth of placement is only constrained by the auger depth and the fact that the bottom of the unit must stay above the water table. (One of the commercial units by Sledgeproducts, listed below, is specially designed to accommodate temporary perched-water conditions). The water-flux meter can be sized according to its specific application but is typically about 20 cm in diameter and up to 140 cm in length. The figure below shows the general features of our meter. The unit is self calibrated by periodically adding a known volume of water to a calibration line and checking the number of tips from the tipping bucket. The unit is connected to an electronic event recorder and drainage can be continuously monitored. Water chemistry can be obtained by withdrawing a sample from a collection vial located below the counting unit.

Our water-flux meter has been exhaustively tested in the laboratory and has been field-tested at more than a dozen sites throughout the world. Versions of the water fluxmeter are commercially available. Information about purchasing a water fluxmeter unit can be found at the following web addresses.

http://www.decagon.com/draingauge/

http://sledgeproducts.com/prod.html

References:

Gee, G. W., A. L. Ward, T. G. Caldwell, and J. C. Ritter. 2002. A vadose-zone water fluxmeter with divergence control. Water Resour. Res. 38(8)10.1029/2001WR00816,2002

Gee, G. W., Z. F. Zhang, and A. L. Ward. 2003. A modified vadose zone water fluxmeter with solution collection capability. Vadose Zone J. 2: 627-632.

Attached is a related article on water fluxmeters from WISPAS (A Newsletter about Water in the Soil-Plant-Atmosphere System) Published by the New Zeland HortResearch Nov. 2003 (Vol 86). ISSN 1176-2292.

After Fire Comes Rain: Drainage in Tonga

One of the main interests of the research in Tonga (see WISPAS 81, p. 1) is to quantify the drainage of water and solutes through the soil towards the groundwater. In Tonga intensification of agriculture, especially the growth of squash pumpkin for export to Japan, has led to an increase in the use of agricultural chemicals. Tongatapu is a lifted coral atoll. At our site a layer of soil of about 2.8 m rests on the permeable coral limestone and at about 23 m the fresh groundwater lens floats on denser salt water.

A gentle Tongan rainstorm of 50 mm!

Rainfall and drainage fluxes following a rainstorm.

Under the meters we put sand to ensure proper drainage. Both the Gee and Decagon meters have a wick inside of 50 cm length that provides suction and minimizes divergence, and have a self-siphoning sampling chamber. The HortResearch meter does not have a wick and thus only picks up gravimetric saturated transport of water and solutes. The HortResearch and Gee flux meters both work with a tipping spoon. The Decagon units operates with a capacitance reservoir. All water flux meters are connected to our CR10X logger, have a calibration relationship and a tube down to the sampling chamber that allows collecting the drainage for analysis.

Installation of the fluxmeters.

Our first results look promising.

Getting a closer look at the detail of the soil profile.

Last year we had our first trial with a non-suction water flux meter with some success. This year things are being greatly improved with the use of four additional water flux meters operating under suction and two improved versions of the non-suction water flux meters. Glendon W. Gee (Battelle, Richland, WA, USA) contacted us after reading of the Tongan research and proposed to install some of his new water flux meters (see WISPAS 83, p. 4). Two units were supplied by Sledge Sales (waynesledge@att.net) and two units (of slightly modified design) were supplied by Decagon Devices (http:// decagon.com). We gratefully accepted the donation to the project of the four drainage meters. WISPAS facilitated this, and is a good example of the efficiency and impact of fast electronic publications!

The installation was not without a hassle. First attempts to install the meters under a column of intact soil were without a success. Naturally though these weathered volcanic ash soils have a clay content up to 90%! Instead we installed the meters with a repacked column of soil on top (see drawing).

After the installation about 35 mm of rain had fallen until we got the first response of all our water flux meters after a heavy rain. It is nice to see the difference between the wicked water flux meters and the non-wicked ones. Three of the suction water flux meters initially responded similarly and at this moment the response of all four is evened out. These results allow us to quantify the fluxes of water and solutes in the rapid mobile and the immobile domain.

The squash crop approaching harvest.

Nice results and hopefully more scientifically exciting times ahead!