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首页> 外文期刊>Hydrology and Earth System Sciences >Aggregation in environmental systems – Part?2: Catchment mean transit times and young water fractions under hydrologic nonstationarity
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Aggregation in environmental systems – Part?2: Catchment mean transit times and young water fractions under hydrologic nonstationarity

机译：环境系统中的聚集第二部分：水文不平稳条件下的集水区平均运输时间和幼水分数

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Methods for estimating mean transit times from chemical or isotopic tracers(such as Cl?, δ18O, or δ2H) commonly assumethat catchments are stationary (i.e.,?time-invariant) and homogeneous. Realcatchments are neither. In a companion paper, I showed that catchment meantransit times estimated from seasonal tracer cycles are highly vulnerable toaggregation error, exhibiting strong bias and large scatter in spatiallyheterogeneous catchments. I proposed the young water fraction, which isvirtually immune to aggregation error under spatial heterogeneity, as abetter measure of transit times. Here I extend this analysis by exploringhow nonstationarity affects mean transit times and young water fractionsestimated from seasonal tracer cycles, using benchmark tests based on asimple two-box model. The model exhibits complex nonstationary behavior,with striking volatility in tracer concentrations, young water fractions,and mean transit times, driven by rapid shifts in the mixing ratios offluxes from the upper and lower boxes. The transit-time distribution instreamflow becomes increasingly skewed at higher discharges, with markedincreases in the young water fraction and decreases in the mean water age,reflecting the increased dominance of the upper box at higher flows. Thissimple two-box model exhibits strong equifinality, which can be partlyresolved by simple parameter transformations. However, transit times areprimarily determined by residual storage, which cannot be constrainedthrough hydrograph calibration and must instead be estimated by tracer behavior.Seasonal tracer cycles in the two-box model are very poor predictors of meantransit times, with typical errors of several hundred percent. However, thesame tracer cycles predict time-averaged young water fractions (Fyw)within a few percent, even in model catchments that are both nonstationaryand spatially heterogeneous (although they may be biased by roughly 0.1–0.2at sites where strong precipitation seasonality is correlated withprecipitation tracer concentrations). Flow-weighted fits to the seasonaltracer cycles accurately predict the flow-weighted average Fyw instreamflow, while unweighted fits to the seasonal tracer cycles accuratelypredict the unweighted average Fyw. Young water fractions can also beestimated separately for individual flow regimes, again with a precision ofa few percent, allowing direct determination of how shifts in a catchment's hydraulicregime alter the fraction of water reaching the stream by fast flowpaths.One can also estimate the chemical composition of idealized "young water"and "old water" end-members, using relationships between young waterfractions and solute concentrations across different flow regimes. Theseresults demonstrate that mean transit times cannot be estimated reliablyfrom seasonal tracer cycles and that, by contrast, the young water fractionis a robust and useful metric of transit times, even in catchments thatexhibit strong nonstationarity and heterogeneity.
机译：估算化学或同位素示踪剂（例如Cl ？，δ 18 O或δ的平均穿越时间的方法> 2 H）通常假定流域是固定的（即，时间不变）并且是均质的。实际流域都不是。在同伴论文中，我表明，根据季节性示踪剂周期估算的流域平均运输时间极易受到聚集误差的影响，在空间非均质流域中表现出强烈的偏差和较大的分散性。我提出了幼水部分，它在空间异质性下几乎不受聚集误差的影响，可以更好地度量渡越时间。在这里，我使用基于简单两箱模型的基准测试，探索非平稳性如何影响平均追踪时间和从季节性示踪剂循环中估计的年轻水含量，从而扩展了这一分析范围。该模型表现出复杂的非平稳行为，其示踪剂浓度，年轻水份和平均运输时间具有明显的波动性，这是由上，下箱体的混合比快速变化引起的。高流量时河口的渡越时间分布逐渐偏斜，年轻水的比例显着增加，平均水龄降低，这反映了较高流量时上部水箱的支配性增加。这个简单的两盒模型具有很强的均等性，可以通过简单的参数转换部分解决。但是，过渡时间主要由剩余存储量确定，而残余存储量不能通过水位计校准来约束，而必须通过示踪剂行为来估算。 两箱模型中的季节性示踪剂循环对于平均过渡时间的预测性很差，通常误差百分之几百。但是，相同的示踪剂周期可以预测时间平均的年轻水分数（ F yw ）在百分之几以内，即使在非平稳且空间上非均质的模型集水区中（尽管它们可能是在强降水季节与降水示踪剂浓度相关的地点，偏差约为0.1-0.2。与季节性示踪剂循环的流量加权拟合准确地预测了流量加权平均 F yw 的入流，而未加权拟合于季节示踪剂周期的准确预测了未加权平均 F < / i> yw 。还可以针对各个流态分别估算年轻水的比例，精度也只有几个百分点，从而可以直接确定集水区水力状况的变化如何通过快速流路改变到达河流的水的比例。利用年轻的水馏分和不同流态下的溶质浓度之间的关系，理想化了“年轻水”和“老水”末端成员。这些结果表明，无法通过季节性示踪剂周期可靠地估算平均渡越时间，相反，即使在表现出强烈的非平稳性和非均质性的流域，幼水分数也是渡越时间的一种强大而有用的度量。

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