Prediction of three key hydraulic properties in a soil survey of a small forested catchment

Abstract

Direct measurement of ψ(θ) and K(θ) relationships at all observation sites in soil survey is not feasible. Three key hydraulic properties — water content at field capacity (θ_{–5 kPa}), water content at wilting point (θ_{–1.5 MPa}), and saturated hydraulic conductivity (K_s) — can be used to derive K(θ) and ψ(θ) when combined with bulk density. These properties were measured in 'calibration' horizons in a soil survey in Yambulla State Forest in south-east New South Wales.

Pedotransfer functions (PTFs) for predicting θ_{-5 kPa,} θ_{–1.5 MPa}, and Ks from the physical and morphologic soil attributes are presented and evaluated here. Models for predicting θ_{–5 kPa} and θ_{–1.5 MPa} relied on per cent clay. An R² of 0.64 (for θ_{–5 kPa}) to 0.67 (for θ_{–1.5 MPa}) was obtained for linear regressions using only morphologic explanatory variables. An R² of 0.73 (for θ_{–5 kPa}) to 0.90 (for θ_{–1.5 MPa}) was obtained if laboratory-measured clay content was included as an explanatory variable.

K_s was measured in situ using well permeameters, and used for developing PTFs. Large cores were taken from a small subsample of horizons and measurements of K_s, K_{–0.1 kPa}, K_{–0.2 kPa}, and K_{–0.5 kPa} were made in the laboratory. K_s measurements from well permeameters were similar to K_{-0.5 kPa} from laboratory measurements.

Regression and tree models were used to predict K_s. The linear regression had an R² of 0.55, while the tree models accounted for approximately 40% reduction in deviance. Bulk density was the most useful predictor in all K_s models. The inclusion of per cent rock fragments, bulk density, and estimated percentage clay as useful explanatory variables demonstrated the utility of functional descriptors not routinely measured in soil survey.

The models are empirical and were locally calibrated for use in a soil survey. They may be applicable in target domains similar to the source domain (i.e. coarse-grained adamellite soils in similar climatic regimes).

surrogates, saturated hydraulic conductivity, K(θ), ψ(θ), Ks, pedotransfer functions, soil survey, soil morphology, PTF.