The Influence of Hydraulic Nonequilibrium on Pressure Plate Data

Abstract

ards, 1948, 1965 Richards and Ogata, 1961), has been the method of choice for determining water-retention Pressure plates are used routinely to measure water-retention charcharacteristics for literally thousands of soil samples acteristics of soils. Plates of varying porosity are used, depending on the pressure range of interest. For applied pressures up to 1.5 MPa, during the past 50 yr (Clapp and Hornberger, 1978; 15-bar porous ceramic plates with fine porosity are used because of Rawls et al., 1982). Specific details for use of pressure their high bubbling pressure (Ͼ1.5 MPa), which limits airflow through plates for matric suction measurements are provided by the plate. The typical saturated hydraulic conductivity of the 15-bar Richards (1965), Klute (1986), Topp et al. (1993), and plate is Ͻ3 ϫ 10 Ϫ11 m s Ϫ1 . Low plate conductance coupled with decreas- Townend et al. (2001). Typically, soils are tested over ing soil hydraulic conductivities at high pressures strongly influence the pressure ranges from 0 to 0.1 MPa, from 0.1 to 0.5 equilibrium times, which theoretically may extend to months or years.MPa, and from 0.5 to 1.5 MPa. The soils are tested onWe measured the soil water pressures (suctions) for three soils, a plates that have bubbling pressures (air-entry values) sand, a silt loam, and a clay, placed on 15-bar pressure plates for 10 Ͼ0.1 MPa, Ͼ0.5 Mpa, and Ͼ1.5 MPa, respectively. Table d or longer, with and without static loads and with and without using 1 lists characteristics for plates that cover the pressure a kaolinite slurry to improve plate contact. Total matric suctions, inferred from peltier psychrometry data, were always Ͻ1.0 MPa. range from 0 to 1.5 MPa.

Plant-available soil water capacity: estimation methods and implications

“…Several samples are allocated on the plate and if they are very heterogeneous, there is a possibility that not all will reach equilibrium at the same time, when the samples are removed from the chamber. The comparison of results using WP4 and the Richards chamber with disturbed samples is useful because it eliminates the porosity configuration effect, since the higher water content observed in the Richards chamber could have occurred in function of the soil water extraction having been incomplete, not reaching equilibrium (Gee et al, 2002;. Klein et al, 2006), due to the conductivity of the water being very low at this potential (Angelotti Netto & Fernandes, 2005).…”

Determining Soil Hydraulic Properties

“…Since equilibrium time grows with the square of the length of the sample (Miller, 1980), equilibration times of large samples become prohibitively long for high suctions. Nonequilibration is in particular a problem for sands, where higher tensions can only be reached by equilibration via the gas phase (Gee et al, 2002). For the dry range of a retention curve, there are not many alternatives.…”

Measuring Soil Water Potential for Water Management in Agriculture: A Review

“…Repeated measurements at increasing pressures are performed to obtain several measurements of water content as function of the applied pressure (water potential). Several authors [38][39][40] showed that measurements with pressure plates are in error at potentials below −100 to −500 J kg −1 .…”