The Use of Stable Isotope-Based Water Age to Evaluate a Hydrodynamic Model

Abstract

Transport time scales are common metrics of the strength of transport processes. Water age is the time elapsed since water from a specific source has entered a study area. An observational method to estimate water age relies on the progressive concentration of the heavier isotopes of hydrogen and oxygen in water that occurs during evaporation. The isotopic composition is used to derive the fraction of water evaporated, and then translated into a transport time scale by applying assumptions of representative water depth and evaporation rate. Water age can also be estimated by a hydrodynamic model using tracer transport equations. Water age calculated by each approach is compared in the Cache Slough Complex, located in the northern San Francisco Estuary, during summer conditions in which this region receives minimal direct freshwater inflow. The model's representation of tidal dispersion of Sacramento River water into this backwater region is evaluated. In order to compare directly to isotopic estimates of the fraction of water evaporated ("fractional evaporation") in addition to age, a hydrodynamic model-based property tracking approach analogous to the water age estimation approach is proposed. The age and fractional evaporation model results are analyzed to evaluate assumptions applied in the field-based age estimates. The generally good correspondence between the water age results from both approaches provides confidence in applying the modeling approach to predict age through broader spatial and temporal scales than are practical to assess using the field method, and discrepancies between the two methods suggest aspects of both approaches that may be improved. Model skill in predicting water age is compared to skill in predicting salinity. Compared to water age, salinity observations are shown to be a less useful diagnostic of transport in this low salinity region in which salt inputs are poorly constrained.

Hydrodynamics drive pelagic communities and food web structure in a tidal environment

“…None EMV −30.2 ± 0.5 −29.7 ± 0.5 −29.8 ± 3.7 −27.4 ± 0.3 6.2 ± 1.7 8. often due to low exchange and higher water age in this region (Downing et al, 2016;Gross et al, 2019). The gradient in specific conductance and elevated turbidity near the maximum extent of the tidal excursion support our characterizations of the three sites as high, low, and no exchange.…”

“…Pelagic habitats along the terminal channel were structured by hydrodynamic processes, with differential exchange corresponding with variability in various habitat characteristics. Elevated specific conductance in upstream habitats is indicative of evaporation and is often due to low exchange and higher water age in this region (Downing et al, 2016; Gross et al, 2019). The gradient in specific conductance and elevated turbidity near the maximum extent of the tidal excursion support our characterizations of the three sites as high, low, and no exchange.…”

Consistent Boundary Conditions for Age Calculations

“…These expressions can also be obtained by setting K = 0 in ( 22) and ( 23) and dealing with the resulting equations in the sense of distributions. Calculating the age from the concentration and age concentration by solving the relevant equations under consistent initial and boundary conditions is generally much easier than evaluating the (mean) age from the age distribution function, which is why many studies relying on CART simply did so [39,47,65,69,[73][74][75][76][77][78][79][80][81][82][83][84][85][86][87][88]. However, we must bear in mind that this approach may veil some of the subtleties of the transport phenomena under study.…”

Application of Stable Water Isotopes to Improve Conceptual Model of Alluvial Aquifer in the Varaždin Area

“…Different methods are used, and one of them is an isotopic technique, which is often used successfully to help elucidate hydrological studies [1]. Knowledge about the isotopic ratios of oxygen (δ 18 O) and hydrogen (δ 2 H) in atmospheric precipitation and groundwater is important for hydrological, hydrogeological, climatological, and meteorological applications [1][2][3][4][5][6][7][8] because it can provide information on the mean recharge elevation of the aquifer, the mean residence time, water-rock interactions, etc.…”