Soil water conditions and flow characteristics in the subsoil of a volcanic ash soil: Findings from field monitoring from 1997 to 1999

Abstract

Field experiments were conducted to investigate the effects of soil water conditions on water flow in the subsoil of a volcanic ash soil. Soil water content to depths of 30 and 100 cm and at 100-cm depth determined with time domain reflectometry (TDR), and matric potential at 90- and 110-cm depths determined by tensiometers were monitored every 30 min for a period of 3 years. Amounts of annual precipitation in 1997, 1998, and 1999 were 989, 1,530, and 1,214 mm, respectively. Annual mean values of volumetric soil water content to depths of 30 and 100 cm in 1998 were only 3 and 2% higher than those in 1997 although in 1998 the amount of precipitation was 1.5 times higher. Furthermore, differences in the volumetric soil water content at 100-cm depth among the 3 years were less than 1%. Annual deep percolation beyond 100-cm depth ranged from 227 to 762 mm depending upon the amounts of annual rainfall, and downward water flow larger than 1 mm d−1 occurred during 1 / 3 of a year on the average. Annual evapotranspiration ranged from 729 to 810 mm. Matrix flow based on Darcy's law accounted for about 75% of the annual deep percolation and the remainder was represented by the bypass flow that was not detected by the application of Darcy's law. Most of the values of the upward flow were less than 1 mm d−1 and 1.5 mm. d−1 was the largest value recorded at 100-cm depth. Changes in the amount of stored water to a depth of 100 cm based on the water balance equation could be neglected if the equation applied for 1 year, from May to October and November to April, due to the relatively small fluctuations in the amount of stored water compared to the amount of rainfall during the same period.