In much of the western United States, riparian zones are shifting to invasive grass dominated ecosystems that may reduce stream flow. Compared to the woody species they are often replacing, dense grass stands may have similar or higher productivity and evapotranspiration rates, yet may access shallower water sources and thus alter stream hydrology. In eastern Washington, many streams experience low flow during the growing season that degrades water quality, concentrates pollutants, and reduces habitat. Most of these streams’ riparian zones have extensive stands of reed canary grass (Phalaris arundinacea), an aggressive invader that creates monotypic stands in riparian areas through much of the northern United States. Our goal is to determine the effect of reed canary grass on stream hydrology in semi-arid eastern Washington. We conducted vegetation, groundwater, and stream flow surveys at nine sites along four watersheds to determine community composition and flow regime. The ratio of hydrogen and oxygen isotopes in plant stem water, groundwater, and stream water were used to determine water source of several dominant riparian species. Evapotranspiration rates of reed canary grass and other riparian species were compared throughout the growing season and across geomorphic surfaces.
Results/Conclusions
We measured reed canary grass evapotranspiration rates per leaf surface area ranging from 0.012 to 0.031 mM of H20/cm2/s, which are similar to other riparian plants we measured. However, reed canary grass produces more biomass and leaf surface per ground area than other riparian species, increasing its potential to draw soil water. When averaged over 1 m2 plots, the evapotranspiration rates of reed canary grass were much higher than other species (reed canary grass 59.55 g H2O/m2/hr, snowberry - Symphoricarpos albus - 1.79 g H2O/m2/hr, rose - Rosa woodsii - 4.26 g H2O/m2/hr, wheat grass - Elymus repens - 18.36 g H2O/m2/hr). Our preliminary isotope data indicates differences in the hydrogen and oxygen isotope ratios in groundwater and stream water. Further isotope analysis is being undertaken to determine water sources of dominant vegetation. Our results may implicate reed canary grass as a major factor in the regional low stream flow during the growing seasons. If reed canary grass is altering stream hydrology, then aggressive management of this invasive grass should be implemented to improve water quality and quantity in this semi-arid region.