Comparison of the water balance in treated vs. untreated juniper-dominated watersheds in central Oregon, USA

The significant expansion of woody vegetation species such as juniper (Juniperus spp.) observed throughout the western United States over the past 150 years has been associated with negative changes in hydrologic processes such as reduced streamflow and increased erosion. This has led to increased concerns regarding water availability in multiple semiarid regions dominated by juniper. In this study, we aimed to characterize the hydrologic effects of western juniper (Juniperus occidentalis) removal in central Oregon, USA. We monitored multiple surface water and groundwater variables in a treated (90% juniper removed) and an untreated WS over a six-year period. We used the field data collected to quantify various water budget components including precipitation, streamflow, springflow, and change in soil moisture in each watershed. We estimated evapotranspiration (ET) using the Hargreaves-Samani equation from on-site and nearby weather station data. Due to the heterogeneous vegetation cover present at the study site and its influence on actual ET, remote sensing imagery was used to assess the characteristics of vegetation cover, to include the Normalized Difference Vegetation Index, at both watersheds. Deep percolation was calculated as the sink term in the daily water balance equation. In addition, seasonal groundwater level fluctuations were measured at six-well transects located at the outlet of each watershed. Preliminary results indicate that springflow ranged from 0 to 121 L min^-1 at the untreated WS and 2.6 to 189 L min^-1 at the treated WS. Further, streamflow rates were lower at the untreated WS (0 to 756 L min^-1) compared to the treated WS (0 to 1019 L min^-1). Potential ET was estimated to be approximately 750 mm yr^-1 and precipitation ranged from 250 to 350 mm yr^-1. Ongoing analysis include estimates of actual ET and comparisons of soil moisture storage. Results of this study help to improve the understanding of the ecohydrologic impacts of western juniper encroachment and inform land management actions, including juniper removal.