Mike Regan

ABSTRACT

Soils of the eastern Palouse region of northern Idaho commonly contain hydraulically restrictive fragipans or argillic horizons. Under a xeric moisture regime, perched water tables (PWT) develop above these restrictive horizons during the winter and spring. The objectives of this study were to monitor soil moisture content and identify the hydrologic processes controlling the PWT dynamics on a landscape scale. A 1.7-ha north-facing catchment containing a well-developed fragipan was chosen as a study site. The catchment was instrumented with 135 automated monitoring wells spaced on a 10-by-15m grid pattern. Monitoring of PWT height was performed during two seasons, 1997-98 and 1998-99. A water budget was developed for the catchment during the winter and spring of 1999. The components of precipitation, catchment outflow, potential evapotranspiration (PET), PWT declines, and net soil moisture storage were quantified. Saturated hydraulic conductivity (Ksat) of the restrictive layer and overlying horizons were measured using a Guelph permeameter. Soil moisture content was monitored for each soil horizon during the PWT season. Results indicate a relationship between PWT height and depth of the fragipan surface within the catchment. Surface topography has a large influence on the duration of saturated conditions. Saturation persists longer at lower landscape positions and slopes with a NE aspect vs. a NW aspect within the catchment. Moisture inputs from precipitation lead to a rapid increase in PWT height. PWTs increase and decrease rapidly in Ap, Bw, and BE horizons with less than 5% change in soil moisture content, while E horizons tend to remain saturated during most of the water table season. Results indicate that 36% of precipitation received from November until June was accounted for as catchment surface outflow. Approximately 36% of this outflow can be accounted for by the quantity of water lost when PWT levels decline. Catchment outflow ceased when average PWT height was in the E horizon. Maximum catchment outflow, 2.6 cm day-1, occurred during March when average PWT height was located in the Ap horizon. An increase in cumulative PET led to a gradual disappearance of the PWT. A majority of the precipitation received when PWTs are present is lost via surface runoff processes, thereby increasing potential for agrichemical transport and decreasing potential recharge to groundwater aquifers.