An assessment of the piezometer method for measuring the hydraulic conductivity of a Cladium mariscus – Phragmites australis root mat in a Norfolk Fen
Andrew J. Baird(1), Ben W.J. Surridge(1), and Russ P. Money(2)
Sheffield Wetlands Research Centre (SWeRC), (1)Department of Geography, University of Sheffield, Winter Street, Sheffield S10 2TN, UK and (2)Department of Animal and Plant Sciences, University of Sheffield, Western Bank, Sheffield S10 2TN.
Hydrological Processes 18: 275-291.
Despite their common global occurrence, little work has been done on characterising the hydraulic properties of root mats, particularly their hydraulic conductivity (K). A common method for estimating K in the field is the piezometer slug test. In some root mat systems, the piezometer slug test may be the only technique, field or laboratory, which is able to provide reliable estimates of K. We installed 17 piezometers in a Cladium mariscus L. (Pohl) and Phragmites australis Cav. (Streud.) root mat in a UK fen. Tests were initiated by both slug insertion and slug withdrawal. Repeat tests were conducted on some piezometers to check for test consistency. Rapid recoveries after both slug insertion and withdrawal were found, with 90% recovery reached for all tests within 1500 s and for many within 300 s. We found significant and highly reproducible differences between recovery after insertion and recovery after withdrawal, with the latter being more rapid than the former. We invoke a reversible pore blocking mechanism to explain this behaviour. Many of the piezometers showed recoveries after withdrawal broadly consistent with the assumption of a rigid porous medium in which Darcy´s law holds. Tests on four further piezometers were used to examine the impact of piezometer `development´, different slug sizes and the use of `clean´ water on test behaviour. The results from these test piezometers were difficult to interpret. Nevertheless, it appears that replacing `dirty´ water in the piezometer standpipe with clean water leads to more rapid recoveries and reduces the difference between recovery after insertion and recovery after withdrawal. The K of the root mat, as estimated from withdrawal tests on 14 piezometers and standardised to 20C, ranged between 0.00144 and 0.03022 cm s-1. This appears to be at the very top end of K values reported for fen peats and suggests that water can flow readily through this medium given sufficiently large hydraulic gradients.
Figure shows differences in rates of recovery to equilibrium water level between three tests initiated by slug injection and three tests initiated by slug withdrawal. The values of hydraulic conductivity subsequently derived would differ significantly depending on which test mode is employed.