Water Isotopes Analysis - USEPA/SuRGE GitHub Wiki

Methods

We used estimates of evaporation : inflow (E:I) and residence time based on water isotope data (O, H) collected as part of the EPA National Lake Assessment (Brooks et al. 2014). Briefly, an integrated sample from the upper 2 m of the waterbody was collected at the deepest part of the lake (where lake depth was measured), with most samples collected between June and August, but a smaller set collected in May and October. We compiled estimates across three NLA survey years: 2007, 2012, and 2017 and used mean values for each site for modeling purposes. We also used E:I to categorize reservoirs as either more run-of-river (E:I<0.2) or more storage (E:I≥0.2). Previous work identified a threshold E:I value of 0.4 for distinguishing open drainage basins (<0.4 E:I) from restricted drainage basins in the Peace-Athabasca Delta in Alberta, Canada (Wolfe et al. 2007). We chose a reduced threshold given that U.S. reservoirs generally have lower residence times than natural lakes (Hayes et al. 2017).

Results

We matched 112 of the study reservoirs to an estimate of E:I and residence time, with 81 of those sites having at least 1 (and up to 5) repeat visits. The mean evaporation to inflow ratio was 0.17 (ranging from 0 to 0.56), with an average standard deviation of 0.07 for repeat visit estimates. The mean residence time was 0.62 (ranging from 0.02 to 4.05) with an average standard deviation of 0.63 for repeat visit estimates. We classified 35 sites as “storage” and the remaining 77 as “run-of-river”.

References

Brooks, J. R., Gibson, J. J., Birks, S. J., Weber, M. H., Rodecap, K., & Stoddard, J. L. (2014). Stable Isotope estimates of lake water evaporation:inflow and residence time for lakes across the United States as a tool for national lake water quality assessments. Limnology and Oceanography, 59(6), 2150-2165. doi:10.4319/lo.2014.59.6.2150

Hayes, N.M., B.R. Deemer, J.R. Corman, N.R. Razavi, K.E. Strock. (2017). Key differences between lakes and reservoirs modify climate signals: a case for a new conceptual model. Limnology and Oceanography Letters 2(2):47-62. doi:10.1002/lol2.10036

Wolfe, B. B., Karst-Riddoch, T. L., Hall, R. I., Edwards, T. W. D., English, M. C., Palmini, R., . . . Vardy, S. R. (2007). Classification of hydrological regimes of northern floodplain basins (Peace -Athabasca Delta, Canada) from analysis of stable isotopes (O, H) and water chemistry. Hydrological Processes, 21(2), 151-168. doi:10.1002/hyp.6229