Changes of vertical soil moisture conditions of a dry evergreen forest in Kampong Thom, Cambodia

Changes of soil water conditions in a soil profile were observed and estimated using a one-dimensional vertical soil water movement model for a dry evergreen forest area of Kampong Thom Province, Cambodia. The research site was in a dry evergreen forest where a meteorological observation tower had been established. Soil water matric potentials were measured at 20-, 50-, 100-, 150-, 200-, and 250-cm depths in an observation plot. Groundwater levels were observed at the site. Soil water matric potentials at each observation depth in a soil profile were simulated using a one-dimensional water movement model that was based on Richards' equation. Results of observations and simulation revealed the following. (1) The site's water-saturated zone was close to the ground surface during the rainy season. Water conditions in the unsaturated zone, which was above the groundwater level, were influenced strongly by groundwater. The groundwater level was 400 cm deep even in the dry season. The entire soil profile, from the surface to the bottom, never dried completely. (2) At the beginning of the rainy season, at the surface and subsurface depths of 20 cm and 50 cm, respectively, soil matric potentials were increased by rainfall events, which often supply water to the ground surface. Meanwhile, matric potentials at 200 and 250 cm depths were influenced directly by groundwater; they retained high potential values even during the dry season. In the middle zone, at depths of 100 and 150cm, the soil was quite dried; the minimum matric potentials in the rainy season resembled those of the surface zone. (3) At the beginning of the dry season, matric potentials at 20- and 50-cm depths decreased because of soil water loss by transpiration of trees and evaporation from the ground surface. Matric potentials at 100- 150-, 200-, and 250-cm depths were positive because the groundwater level, which was raised to 50 cm deep during the rainy season, remained 100 cm deep during that period. (4) Soil water movement along the soil profile at the observation site was simulated using a one-dimensional vertical soil water movement model. Results of the simulation accorded well with observations of soil water conditions in both rainy and dry seasons. The model based on Richards' equation was applicable to this research area, a dry evergreen forest area.