No abstract

[1] A high-frequency, field-deployable liquid water isotope analyzer was developed that is capable of quantifying d 18 O and d 2 H to better than ±0.17 and ±0.32%, respectively, on over 90 samples/d. The instrument was deployed for 4 contiguous weeks in the H. J. Andrews Experimental Forest Long-term Ecological Research site in western Oregon, where it was used for real-time measurement of the isotope ratios of precipitation and stream water during three large storm events. We were able to document fine-scale changes in rainfall composition and damping effects in the stream channel continuously through these periods. We also performed a rain-on-snow experiment where we sampled leachate from a melting snow core continuously at 2 min intervals for 5 h. These data show remarkable fine-scale patterns of internal rain-snow mixing, patterns that would not have been detected without such high-frequency sampling. These two preliminary applications show proof of concept of the new field analyzer, a device that will ultimately provide hydrologists with insight into water flow dynamics with unprecedented frequency over long time scales.

No abstract

Headwater groundwater subsidy, defined here as out‐of‐catchment groundwater flow contribution from a headwater catchment to its larger parent watershed (i.e., higher‐order stream), can influence the water quality and quantity of regional water resources. But the integrated flow and transport modeling approaches currently being implemented to quantify this subsidy are limited by an absence of critical field observations, such as water table dynamics and groundwater age that are required to test such models. Here we couple tracer (and hydrometric) observations from the well‐studied 4.5‐ha M8 headwater catchment in the Maimai experimental watershed with a new semianalytical free‐surface integrated flow and transport model. Our main research goals are to quantify the magnitude, age, and flow paths of the headwaters groundwater subsidies at the Maimai experimental watershed. Additionally, we explore through virtual experiments the effects of watershed slope, watershed active thickness, and recharge rate on the age, flow path, and magnitude of out‐of‐catchment headwater groundwater subsidies versus within‐catchment (or local) groundwater flow contributions. Our results show that more than 50% of groundwater recharged in the Maimai headwaters subsidizes their parent watershed. The relative proportion of headwaters groundwater subsidies is inversely proportional to recharge rate and/or directly proportional to slope angle. Our results also show that the age of the headwater groundwater subsidies is more than 500 years, almost 9 times older than the age of within‐catchment groundwater flow contributions. These findings highlight the need to consider headwaters groundwater subsidies in groundwater management area considerations.