Inverse estimation of soil hydraulic properties under oil palm trees

Canopies of forested and agricultural ecosystems can significantly alter rainfall patterns into separate stemflow and throughfall areas. These two areas often have also different organic matter contents and soil compaction properties, and hence also soil hydraulic properties, thus causing further differences in the local infiltration rates close to and away from trees. In this study we analyzed possible differences in the unsaturated soil hydraulic properties of the stemflow and throughfall areas below an oil palm tree. Tension disc infiltrometer experiments were carried out underneath the canopy and in the interspace area of an oil palm tree plantation at successive tensions of 5, 2, and 0 cm. Soil hydraulic properties were estimated inversely from the measured data using the HYDRUS-2D/3D software package. Four van Genuchten soil hydraulic parameters (i.e., the residual water content, θ_r, the shape factors α and n, and the saturated hydraulic conductivity, K_s) were optimized. Saturated water contents, θ_s, were fixed at their laboratory-measured values. Initial estimates of the optimized parameters were set according to Wooding's solution, which ensured rapid convergence of the inverse solution. The stemflow and throughfall regions exhibited contrasting hydraulic properties as indicated by the estimated hydraulic parameters. Values of θ_s, α, n and K_s for the stemflow area were all found to be higher as compared to those of the throughfall area. The inverse solution using tension disc infiltrometer data proved to be very useful for rapid characterization of hydraulic properties of soil under the oil palm trees.