Type of Culminating Activity
Master of Science in Hydrologic Sciences
Alejandro Flores, Ph.D.
The stable isotope composition of water (δ18O and δ2H) can reveal important hydrometeorologic and hydroclimatic information. For instance, simultaneous measurement of the stable isotope composition of precipitation and stream water is used to estimate the distribution of hydrologic transit time in catchments, which can provide information about hydrologic flow paths, storage, and source water. However, in mountain watersheds characterized by large topographic relief, the spatiotemporal distribution of precipitation can vary dramatically, as can the isotopic composition of precipitation. Compounding this variability is the transition in precipitation phase from rain to snow, which can markedly affect isotopic compositions of precipitation. This study aims to improve the use of stable isotope methods for hydrologic investigations in complex terrain by investigating the hydrometeorological and spatial controls on the isotopic composition of precipitation, along with improving our understanding of the spatial variability and evolution of the isotopic composition of snow. Additionally, we constructed a seasonally weighted local meteoric water line (LMWL) for the Treasure Valley of southwestern Idaho, which is expressed by the equation δ2H = 7.40*δ18O – 2.17. The LMWL of the greater-Boise area is considerably influenced by the semiarid climate of southwest Idaho, yielding a slope and y-intercept lower than the global meteoric water line (GMWL: δ2H = 8*δ18O +10). Moderate to strong statistically significant (p < 0.05) correlations exists between several hydrometeorological variables (e.g., surface air temperature, relative humidity, precipitation amount, precipitation intensity) and the isotopic composition of precipitation from individual events. A very strong negative correlation (r = -0.98, p < 0.02) exists between the amount-weighted isotopic composition of precipitation and the elevation from individual collection sites (Altitude effect lapse rate: -2.2‰/km). Snow sampling campaigns revealed significant variability in the spatial distribution of the isotopic composition of snow. δ18O of bulk snow core samples varied between -16 and -19 ‰ in an area covering 256 m2at the Treeline study site in Dry Creek Experimental Watershed, which is greater than the variation of the annual δ18O of Dry Creek stream water. Revised methods for characterizing the input signal for transit time estimation (e.g., sampling snowmelt lysimeter water, applying Altitude effect lapse rate to isotopic composition of precipitation) will more accurately represent isotope tracers arriving in watersheds as precipitation and leaving as streamflow.
Tappa, Daniel J., "Isotopic Composition of Precipitation in a Topographically Complex, Seasonally Snow-Dominated Watershed: Hydrometeorological Controls and Variations from the Global Meteoric Water Line" (2013). Boise State University Theses and Dissertations. 369.