Andosols of Terceira, Azores: measurement and significance of soil hydraulic properties

Abstract

The Azores Islands are of recent volcanic origin and their soils, classified as Andosols, have characteristics peculiar to their formation and constituents. Several other areas in the world, including parts of Europe, have Andosols, but their hydraulic properties are not well known. To improve understanding of the hydrologic behaviour of these soils, two experimental sites were selected for in situ hydrological observations and sampling for laboratory analysis. The two studied soils are classified as Ferric Andosol and Haplic Andosol. Soil use at one site is permanent grassland and at the second there is a rotation of 5-year grass and 1-year silage maize. Water retention curves were measured using a sand box, a sand and kaolin box and a pressure membrane apparatus. Hydraulic conductivity curves were measured by the crust, hot air and two-step methods. Measurements in in situ soil monoliths were by the internal drainage and zero flux plane methods. The hot air and two-step methods overestimated the hydraulic conductivity compared with the field methods. These results suggest that soil physical properties of Andosols containing allophanes change when they are dried. Results for the crust method generally agreed with field data. Water retention values near saturation measured with the sand box tended to be greater than those observed in the field. Those estimated with the sand and kaolin box and with the pressure membrane apparatus agreed well with field observations. The water retention and the hydraulic conductivity curves fit the Mualem–van Genuchten model well (coefficient of determination 0.96).