Parameters that characterize the properties of peat are subject to considerable spatial variability, both between and within peatlands. Various parameters including hydraulic conductivity, porosity, water retention, and drainable porosity can be distinctly different in adjacent, visually similar hummocks. Awareness of the scales of variability of parameters governing the behavior of peat is important in predicting or interpreting its hydrology.

8.3.1 Hydraulic Conductivity

K_sat is commonly evaluated using the bail test method of Hvorslev (1951). To characterize the hydraulic conductivity in the acrotelm, a smaller slot length is generally required as discussed in Section 8.1, Well and Piezometer Installation and Use. This has the dual purpose of increasing the detail in the zone where the range of K_sat is highest and slowing the recovery rate, since this will likely be fast in the upper layers.

Hvorslev (1951) suggests slot length should be four times greater than pipe diameter to minimize error. Using this method with sufficient sampling depths generates a K_sat profile, that can be used to produce a transmissivity function (McCarter and Price, 2017c). A transmissivity function is important for calculating horizontal flow, given its sensitivity to water table elevation within the profile as discussed in Section 3.1, Saturated Zone Properties and Processes. Sometimes a well can be used to generate a transmissivity function (e.g., Price and Maloney, 1994). To achieve this, a bail test is required at the full range of water table elevations because the head recovery of a pumped well will depend strongly on the position of the water table, given the extreme vertical change in hydraulic conductivity in the acrotelm. In this case, the well will need to penetrate into the catotelm peat.

Because peat deposits are thin and near to the ground surface relative to most mineral aquifers, they commonly experience a large range in temperature, both seasonally and vertically within the peat profile. Head recovery can vary appreciably with temperature because temperature affects viscosity. Dynamic viscosity (μ; often expressed in units of Pascal seconds), which is a multiplier in the formula for relative permeability (k; often expressed in square meters), increases by 50 percent in water at 5 °C compared to viscosity at 20 °C, with a proportional reduction in permeability, thus hydraulic conductivity. While this is not relevant to the measured value of K_sat at one location in a peat profile, it becomes relevant when comparing peat properties down the profile or between peatlands, given their temperature differences. Comparing k, instead of K_sat, avoids this complication.

Laboratory determination of peat K_sat using a permeameter device follows the usual protocols used for mineral soils. However, core shrinkage or compression during sampling, transport, and preparation—including volume change on thawing—can lead to bypass flow down the inside wall of the permeameter. Paraffin wax (Hoag and Price, 1997) or

Parafilm^TM (McCarter et al., 2019) can be used to confine water within the peat sample during the tests.

Determining unsaturated hydraulic conductivity using fixed-plate pressure cells is problematic because peat shrinks away from the (upper) porous plate at lower (more negative) pressures (ψ). Using a peat sample between a floating upper pressure plate and fixed lower pressure plate (Price et al., 2008) can be used to control head differences, and thus calculate K_unsat at a range of pressure heads or moisture contents. Directing the water downward through the sample to generate a unit head gradient is the preferred method (McCarter et al., 2017c).

Alternatively, K_unsat can be determined using the evaporation method (Schindler et al., 2010). This requires the use of tensiometers to determine head (gradients). Using tensiometers in peat is common practice, but tensiometers do not work well in poorly decomposed peat (moss) because of poor contact between the peat (moss) and tensiometer cup. Tension infiltrometers provide an alternative approach that can be used in the laboratory or field. The tension infiltrometer releases water at a rate slower rate of seepage from ponded water by maintaining a small negative pressure on the water moving from a disk placed on a level peat surface.

8.3.2 Water Retention

Water retention experiments (including related tests such as for K_unsat) suffer from sample shrinkage at lower pressures. The common practice is to express the volumetric water content (θ_v) relative to the original (i.e., saturated) volume of the soil. However, the relation to the degree of saturation is non-linear and therefore not commonly established.

Estimating water retention at high pressure (high θ_v) requires a pressure plate or porous disk with high air entry pressure (e.g., Price et al., 2008).

For poorly decomposed (especially moss) samples, it is essentially impossible to measure ψ – θ_v points for |ψ| less than the sample length, since water drains immediately to the base of the sample (Golubev et al., 2021). As a consequence, many retention curves for poorly decomposed peat do not exhibit an air entry pressure as shown by Figure 16. For lower pressures (low θ_v), a pressure chamber can be used, but extreme shrinkage can occur.

8.3.3 Bulk Density and Porosity

Bulk density (ρ_b) is generally based on the dry mass of solids and original (field) sample volume. To avoid combustion, peat samples should be dried at 95 °C or less for 24 hours or until a stable mass is achieved. Total porosity (𝜙_t) is equivalent to the saturated water content (θ_s). This can be evaluated directly in the laboratory based on the saturated mass (minus dry mass, accounting for sample volume and water density). However, sample swelling at saturation can confound the volume, and may result in apparent 𝜙_t or θ_s > 1, which is impossible. In this case, 𝜙_t can be calculated on the basis of ρ_b such that 𝜙_t = 1 – ρ_b/ρ_p, where ρ_p is particle density. However, the range of particle density for peat can vary from ~0.9 to 1.5 g cm^-3 (Gharedaghloo and Price, 2021; Redding and Devito, 2006), and < 0.9 to ~0.7 g cm^-3 for undecomposed and lightly decomposed Sphagnum mosses (Whittington et al., 2021), so particle density should be assessed to acquire confidence in this method.

In peat, which comprises mobile (𝜙_mob), or immobile (𝜙_im) pores as shown in Figure 11, determining the distribution of porosity is methodologically challenging. McCarter and others (2019) found mobile porosity coincides with the drainable porosity at (ψ) = -100 cm, which is commonly measured during soil water retention experiments.

8.3.4 Specific Yield and Drainable Porosity

Specific yield (S_y) is the ratio of the volume of water that can drain by gravity from a saturated volume of material to the total volume of that saturated material. As such, S_y relates the change in water table elevation to change in storage and can be measured as the difference between the saturated moisture content and the moisture content following gravity drainage.

Specific yield is an important parameter in modeling saturated peat and estimating water budgets. Freeze and Cherry (1979, Section 2.10, subsection Transmissivity and Specific Yield in Unconfined Aquifers ) characterize S_y as a profile property rather than a property of a distinct layer. While many researchers have reported S_y for specific layers of peat, such values actually refer to the drainable porosity (𝜙_d). The drainable porosity within the acrotelm can range from 0.45 near the surface to 0.048 at the base (Rezanezhad et al., 2016). The specific yield of a peatland is thus the integrated value of a range of drainable porosities.

Determination of S_y can be done by comparing the amount of water added or lost from the peat profile (e.g., by measured rainfall or evapotranspiration) versus the measured water table elevation change. For example, the rain-to-rise ratio can be used, which is the amount of rain divided by the consequent water table rise (Dettmann and Bechtold, 2016). In the laboratory, the drainable porosity (𝜙_d) of a specific layer can be determined as the difference in the volume of water in a saturated, then drained, peat sample (typically drained for 24 hours), with respect to the total sample volume.