IDENTIFYING PLANT WATER SOURCES IN AN ARID GRASSLAND WITH HYDROGEN AND OXYGEN ISOTOPES

We investigated the seasonal sources of water for plants in an arid grassland. The objectives were to: 1) establish a local meteoric water line, 2) determine the seasonal contribution of atmospheric precipitation to surface, ground and soil water, and 3) identify the relative supply of water sources for plants as water becomes limited during the hot, dry summer.

The study area is in the channeled scablands in eastern Washington where basalt bedrock and loess deposits were scoured by Glacial Lake Missoula floods during the Pleistocene. On average, two-thirds of the precipitation in eastern Washington falls as winter snow. To distinguish among different water sources for plants, we analyzed the hydrogen and oxygen isotopic ratios of atmospheric precipitation, surface water (lake, spring, and stream), and groundwater samples collected over one year. Water was also extracted from non-photosynthetic plant stems and soil (to a depth of ~100 cm) which were sampled in riparian and upland sites three times during the summer.

Preliminary data confirm that local meteoric water line (LMWL) for eastern Washington state (y = 6.5x – 15.6; r² = 0.95) is isotopically similar to lines previously established for central and southeastern parts of the state. From these data, the estimated mean δ¹⁸O of precipitation during our study period is -13.1‰, which represents a warmer-than-average winter. The actual mean ratio is likely closer to spring (δ¹⁸O = -13.9 ± 0.6‰) and ground water values (δ¹⁸O = -14.5 ± 0.2‰) which average precipitation over multiple years.

Over the summer, the isotopic ratios of water in the shallow vadose zone transitioned from being similar to the mean annual precipitation to displaying evidence of evaporation. Shallow-rooted grasses (Phalaris arundinacea and Lolium perenne) showed this same trend. On the other hand, the isotopic ratios of Pinus ponderosa xylem water decreased over the summer indicating a reduction in a shallow, soil water source and an increased relative uptake of groundwater. Because winter precipitation is a significant source of stream and plant water during the spring and summer, projected warmer and drier winters in the Inland Northwest are expected to reduce the water availability at the end of summer.