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Pacific Northwest National Laboratory Richland, WA 99352
(glendon.gee{at}pnl.gov)
Received for publication October 20, 2004.
Dear Editor:
It is nice to see a paper in JEQ on improving soil water-flux measurements. Masarik et al. (2004) provide an interesting extension of previous work by Brye et al. (1999) related to the so-called AETL (automatic equilibrium tension lysimeter). The AETL is a device designed to automatically collect soil water at tensions similar to those found in the surrounding soil. Under such conditions divergence or convergence of flow is minimized and the collected water should be representative of true pore water concentrations. In theory, the drainage water is captured at flux rates similar to the actual soil-water flux. The major contribution of the Masarik paper is a proposed way to continuously monitor drainage, using an ECH2O (Decagon Devices, Pullman, WA) capacitance probe as a stage recorder placed in the collection pan of their AETL unit. As the water level in the collection pan rises, the capacitance probe output voltage changes in a predictable (near-linear) fashion. The ECH2O probe data can then be converted to a water volume (per unit cross-section) and subsequently reported in millimeters of water. Thus, for a given period of time, water flux can be estimated.
The authors suggest that their AETL units are superior to PCAPs (passive-capillary wick lysimeters) for drainage (water flux) measurements in the vadose zone for several reasons. First, PCAPs control the drainage at a nearly fixed tension, so if the soil is very conductive and drainage is occurring at some elevated tension, above that of the passive wick, flow can diverge around the lysimeter and the unit may undersample the drainage water. If, on the other hand, the soil-water tension in the PCAP is greater than the soil, the drainage water can converge and the PCAP may oversample. In contrast, AETL units are designed to control the tension between the collection zone and that external to the lysimeter so that the tension is essentially the same outside and inside the lysimeter. Second, Masarik et al. (2004) indicate that PCAPS have relatively small diameters (<1000 cm2) while the AETL units are generally larger (>1000 cm2) and thus will capture more water. Finally, they suggest that the PCAPs require backfilling of the soil, which they assert is a "serious flaw" in many studies that require drainage from undisturbed soil profiles, a problem that AETLs claim to avoid.
I wish to clarify several issues regarding PCAP-type water fluxmeters. First, the PCAP units that are commercially available from Decagon Devices (e.g., Drain Gauge) (www.decagon.com/draingauge; verified 6 Dec. 2004) or Sledge Sales Consulting (Dayton, OR) (http://sledgeproducts.com/about.html; verified 6 Dec. 2004) have collection systems that are superior in design to the proposed drainage-collection system for AETL units. While the Decagon Device unit employs an ECH2O probe to monitor the drainage in a fashion similar to the AETL unit, it does so in a much smaller volume (<60 mL for the Decagon Drain Gauge vs. more than 1500 mL for the AETL units), so that the sensitivity of the Drain Gauge is increased significantly over that of the AETL units. In the Drain Gauge, an auto-siphon drains the collection volume after approximately 45 mL and the process repeats itself. Data logging of the ECH2O probe captures both the stage and the auto-discharge events such that very precise measurements (i.e., much less than 1 mm water resolution) can be made. The water fluxmeter of Sledge Sales has a tipping spoon with approximately 5-mL resolution (<0.2 mm water).
The second issue is related to application of PCAP-type lysimeters to undisturbed soils. In a manner very similar to the AETL installation, PCAP lysimeters have been installed in soils for years (see Boll et al., 1992; Louie et al., 2000; Zhu et al., 2002). The Drain Gage water fluxmeter unit, in principle, can be installed in the soil in a manner similar to that of PCAP units and AETL units by caving out a hole in the side of a pit and jacking the unit into place. While many PCAP water fluxmeters have been installed in very coarse soils or waste sites, where disturbance is not a major issue, they have also been installed in undisturbed soils in a side-by-side comparison with AETLs. Depth of placement may at some sites be restrictive, but in areas where soils and water tables are deep enough to permit it, this should not be a serious limitation. It should also be mentioned that AETLs are not easily installed in soils with low structure (e.g., coarse sands or soils with high gravel contents) and installing any collection device including AETLs or PCAPs in "undisturbed" soil is still an art form.
Another issue is the size of the capture area. Passive-capillary wick lysimeters are not limited to 346 cm2, as stated by Masarik et al. (2004), but can be almost any size. The approximately 20-cm diameter has been simply a convenience rather than a particular restriction. As indicated by Gee et al. (2002)(2003), design considerations allow PCAP water fluxmeters to be as large as or larger than the present AETLs.
Finally, while AETLs likely capture drainage more accurately in the tension range beyond 100 cm than a standard PCAP unit, the design of the AETL may be inferior to PCAPs in the range from saturation to 100 cm. This is true because the heat dissipation units (HDUs) used by the AETL system for sensing tensions have a bubbling pressure typically above 100 cm so they are completely insensitive in this range in terms of tension control. In addition, hysteresis, which is significant with HDU devices, is difficult to account for and generally is not factored into the control scheme. It is possible that the apparent oversampling of drainage by Masarik et al. (2004)(Fig. 3), where drainage is shown to be initially greater than precipitation, could be due to the lack of tension control during wet (intense rain) periods. The wet-end problem might be overcome by using tensiometer-type controllers, but this adds complexity and cost to an already rather complicated system.
There is no perfect system yet for measuring soil water flux. Automatic equilibrium tension lysimeters, in principle, provide for reliable water flux measurements at tensions above 100 cm. However, they will need to be tested further in the wetter, high-flux range to evaluate their utility under these conditions. Passive-capillary wick lysimeters also have limitations. They work best in coarse soils and are relatively static in their tension control. However, they are robust, relatively inexpensive, and require much less sophistication in collection and control systems. Because of their simplicity in design, PCAP-type fluxmeters may prove to be a reasonable compromise between accuracy and cost. Under the right conditions they should provide a convenient and reliable way to measure water flux in the vadose zone.
REFERENCES
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