Monitoring of matrix flow and bypass flow through the subsoil in a volcanic ash soil

Daily matrix flow at 1-m depth in a volcanic ash soil was calculated during a period of one year using Darcy's law. Unsaturated hydraulic conductivity of undisturbed core samples could be expressed as a unique function of the soil water content. Hydraulic gradient obtained from soil water suction by a tensiometer installed at 90- and 110-cm depths, and hydraulic conductivity converted from the soil water content by time domain reflectometry (TDR) were monitored every 30 min throughout a year in a maize (Zea mays L.) Chinese cabbage (Brassica pekinensis Rupr.) field. The matrix flow obtained by this method was substituted for the water balance equation to estimate the bypass flow, and monthly and annual evapotranspiration. Annual rainfall in 1997 was 989 mm and evapotranspiration was estimated to be 730 mm. Net matrix flow at 1-m depth was 164 mm downward even though upward matrix flow occurred during half of the year. Downward flow determined by the water balance method exceeded the downward matrix flow during two heavy rain events in the year and the difference between the two flows was 63 mm, which was considered to correspond to a bypass flow. The bypass flow accounted for only 6.4% of the annual rainfall. Matrix flow was well monitored by the application of unsaturated Darcy's law in a field, and monthly evapotranspiration and bypass flow could be quantified by the introduction of the water balance equation.