AN EVALUATION OF THE INSTANTANEOUS PROFILE METHOD FOR IN SITU DETERMINATION OF HYDROLOGIC PROPERTIES OF LAYERED SOIL

Abstract

In the Abbotsford soil, which consists of 60 cm of silt loam over gravelly sand, the measurement of the partial unsaturated hydraulic conductivity characteristic Κ (ΨM) by the instantaneous profile (IP) method for the surface layer required hydrologic isolation of the experimental plot to prevent lateral flow within the surface layer. Lateral flow did not interfere with the measurement of the partial water retention characteristic θ (ΨM). Results, indicated wetting front instability at the interface between the two layers and the corresponding formation of "wetting fingers," which precluded calculation of Κ (ΨM) for the coarse layer. Tensiometer measurements indicated that wetting fingers had formed in the coarse layer around the tensiometer access tubes. The results point to difficulties inherent in the measurement of water potential in coarse layers with tensiometers and therefore in the application of the IP method. In the Delta soil, which consists of 30 cm of silt loam over fine to medium sand, difficulties were caused by irregular wetting of the Ap horizon which was observed to be related to its hydrophobic nature. Satisfactory θ (ΨM) curves were obtained for the sandy layer. No Κ (ΨM) functions were calculated for the sandy layer because of insufficient data.