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

<p>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.</p>

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

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.

Water flux in animals: analysis of potential errors in the tritiated water method

Laboratory studies indicate that tritiated water measurements of water flux are accurate to within -7 to +4% in mammals, but errors are larger in some reptiles. However, under conditions that can occur in field studies, errors may be much greater. Influx of environmental water vapor via lungs and skin can cause errors exceeding +/- 50% in some circumstances. If water flux rates in an animal vary through time, errors approach +/- 15% in extreme situations, but are near +/- 3% in more typical circumstances. Errors due to fractional evaporation of tritiated water may approach -9%. This error probably varies between species. Use of an inappropriate equation for calculating water flux from isotope data can cause errors exceeding +/- 100%. The following sources of error are either negligible or avoidable: use of isotope dilution space as a measure of body water volume, loss of nonaqueous tritium bound to excreta, binding of tritium with nonaqueous substances in the body, radiation toxicity effects, and small analytical errors in isotope measurements. Water flux rates measured with tritiated water may be expected to be within +/- 10% of actual flux rates in most situations.

Oxygen isotope abundance measurements in fine size fractions of Luna 16 and 20 soil

Determination of 8 18 O values in fine size fractions of Luna 16 and 20 soil appears to indicate that a simple monotonic increase in 18 O abundance with decreasing grain size does not exist. The weighted average 8 18 O values for Luna 16 and 20 soil are approximately + 6.33% and + 6.65% relative to the s.m.o.w. standard. These averages (within errors of about ± 0.2 %0) fall inside the range of 8 18 O values found by other workers for lunar soils. Three possible interpretations of the patterns of 18 O-enrichment with grain size are suggested. (1) The generally accepted explanation is that the heavy isotope enrichments reflect fractional evaporation and/or fractional condensation processes resulting from bombardment of the lunar soil by nuclear particles in the solar wind and/or micrometorites. Thus the fines at each locality could be a mixture of fractions with a specific range of grain sizes each with a different exposure history. The grain size fractions could each have been derived from impact events at specific distances from the Luna sample localities. (2) The low 8 18 O values ( ca , + 5.1%0, s.m.o.w.) obtained for two fractions are difficult to explain, but could indicate concentrations of minerals with low 18 O abundances, such as ilmenite or olivine, in these grain sizes. Similarly the relatively enriched values obtained for other fractions could indicate concentrations of phases enriched in 18 O and/or susceptible to 18 O-enrichment in these grain sizes. (3) At least some of the results obtained could be artefacts resulting from the exposure of the samples to the atmosphere of various laboratories.