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Soil moisture and groundwater dynamics in western juniper woodlands

TitleSoil moisture and groundwater dynamics in western juniper woodlands
Publication TypeConference Paper
Year of Publication2016
AuthorsOchoa, CG, Deboodt, T, Ray, G
Conference NameSociety for Range Management - Annual Meeting
Date Published02/2016
Conference LocationCorpus Christi, TX

The significant expansion of juniper (Juniperus spp.) during the last two centuries has disrupted important ecological functions and hydrologic processes occurring in many watersheds throughout the western U.S.  Our long-term research study in central Oregon has provided critical information regarding vegetation and hydrology interactions in winter precipitation-dominated watersheds. This paired-watershed study comprises an area of approximately 500 acres and includes one treated (~ 90% juniper removal) and one untreated watershed. The wet season in the study area occurs between September and April, with the majority of the precipitation occurring as snowfall.
The study site has been instrumented to record different hydrologic parameters including soil moisture, runoff, and groundwater. Also, different field campaigns have been conducted to assess vegetation features such as canopy cover and species frequency.  
In general, greater soil moisture content and an increase in shallow groundwater residence time was observed in the watershed where juniper tree density had been reduced.  Results from an intensive monitoring campaign (2014-2015) of top soil moisture showed there was a statistically significant, although relatively small, difference in soil moisture content in treated vs untreated watersheds. Also, vegetation data collected showed that canopy cover significantly affected soil moisture levels across dry and wet seasons. Perennial grass cover was positively correlated with changes in soil moisture, whereas juniper cover was negatively correlated with soil moisture content.
Shallow groundwater recharge during the winter season showed a 4 to 6 week delayed response in wells located in a downstream valley when compared to upland well locations. An isotope trace analysis showed similar signature for upland and valley well locations, indicating there are temporary hydrologic connections through the groundwater system.
Study results provide valuable information towards understanding ecohydrologic features of woody vegetation expansion in semiarid areas in the West.