Determining water sources for summer stream flow and riparian plants in an arid grassland using stable isotopes of hydrogen and oxygen
We used stable isotopes of oxygen and hydrogen to track sources of water for stream flow, soil pore water, and plants in riparian and non-riparian portions of a semi-arid grassland in eastern Washington state (U.S.A.). Our study area receives ~ 40 cm of precipitation annually, with about 65% falling during the cool season. Soils are generally frozen and saturated through winter with runoff during spring thaw. As temperatures warm and precipitation decreases, stream flows decline to base flow and soils dry. In early fall, Turnbull National Wildlife Refuge (TNWR, Spokane Co., WA, U.S.A) has very low soil moisture (<5 %). However, the relative contributions of different water sources to stream base flows and plants during the summer are not known. We sampled stable oxygen and hydrogen isotope ratios of precipitation, groundwater, lake and stream water on TNWR from June 2014 through June 2015. We also collected soils and plants from 2 riparian and 2 upland locations 3 times during the summer. We analyzed xylem water from 3 ecologically important plant species: Pinus ponderosa, which may transport water from deeper groundwater, and the invasive grasses Philaris arundinacea (riparian) and Lolium perenne (upland). Additional samples collected summer 2011 were also analyzed.
Preliminary results indicate that stream water was derived primarily from a small lake outflow in summer 2011 and early 2014 and shifted to reliance on groundwater later in summer 2014. In early summer (2014), stable hydrogen and oxygen ratios of xylem water extracted from Pinus ponderosa were similar to those of moderate soil depths (>20 cm) at the locations where they were collected, while hydrogen and oxygen ratios of grass xylem were less negative, indicating water sources that had experienced greater evaporation than sources for pine. Some soil water pore samples collected in June had stable isotope ratios of both hydrogen and oxygen that were more negative than precipitation collected in May and June, indicating storage of precipitation from winter or early spring. Analysis of precipitation from throughout the year, as well as soil pore water from late summer will allow us to better determine the sources of stream, soil, and plant water that sustain this ecosystem during the driest portion of the growing season.