Infiltration, Runoff, and Sediment Yield in Response to Western Juniper Encroachment in Southeast Oregon

Abstract

Infiltration was measured in a western juniper (Juniperus occidentalis Hook.) watershed to characterize the hydrologic processes associated with landscape position. Infiltration rate, runoff, and sediment content were measured with the use of a small-plot rainfall simulator. Study sites were located in each of the four primary aspects (north, south, east, and west). Research sites were located in two ecological sites—South Slopes 12-16 PZ and North Slopes 12-16 PZ. Within aspect, plots were located in three juniper cover levels: high (> 22%), moderate (13%-16%), and low (<3%) juniper canopy cover. During rainfall simulation, water was applied at a 10.2-cm h-1 rate, levels comparable to an infrequent, short-duration, high-intensity precipitation event. Runoff was measured at 5-min intervals for 60 min. Comparing canopy cover levels, steady-state infiltration rates on control plots (9.0 cm h-1) was 68% greater than high juniper cover sites (2.87 cm h-1) and 34% greater than moderate juniper cover sites (5.97 cm h-1) on south-facing slopes. On north-facing slopes, no differences in infiltration rates were observed between juniper cover levels, demonstrating differential hydrologic responses associated with ecological site. Generally, all water applied to control plots infiltrated. Highest infiltration rates were positively associated with increased surface litter and shrub cover. In addition, depth of water within the soil profile was lowest in high juniper cover plots. This suggests that less water is available to sustain understory and intercanopy plant growth in areas with high juniper cover. Accelerated runoff and erosion in juniper dominated sites (high level) across east-, west-, and south- facing slopes can lead to extensive degradation to the hydrology of those sites. These data suggest that sustained hydrologic processes are achieved with reduced western juniper canopy cover.