Dynamic Groundwater Age Distributions: Exploring Watershed Scale Subsurface Systems

地下水系统中的停留时间是指地下水年龄，并用于诊断自然和人为影响的地下水行为。 环境示踪剂作为代理估计地下水年龄，但他们的应用几乎总是假设一个稳定的，稳定的流动。 瞬变流的影响和重要性没有考虑到，也是未知的。 该项目通过考虑天气和气候强迫的时间变化对地下水流动和年龄迁移的影响，解决复杂流域的非稳定流动问题。 它采用了一种双重方法。 合成系统将使用五维（5-D），广义地下水年龄模型（3-空间，1-时间，1-年龄）建模，以建立对地下水年龄分布（GWAD）的控制过程的理解。 在瞬变流中，GWAD是观测时间和观测到的水进入系统的时间的函数。 初步的理论结果表明，瞬变流带来的过程在所有尺度上，具有显着的长期记忆的影响。 然后将研究科罗拉多和新墨西哥州上格兰德河的两个仪器化和监测的自然流域系统。 其中一个地点位于新墨西哥州中北部的Jemez山脉，这个分水岭是NSF资助的临界区观测站（CZO）的一部分。 新的化学和同位素示踪剂测量的类型，时间和位置将根据其初步的5-D地下水年龄模型进行优化。 这些应用到自然流域将涉及本科生，链接模型与数据，测试的基本理论及其应用，并阐明如何最好地使用现有的一套示踪剂方法在非平稳系统中的GWAD估计。 将开发和测试5维广义年龄方程的创新数学求解方法，包括使用年龄的拉普拉斯变换以减少维数。 预计这些概念和方法将扩展到其他水文应用中的停留时间分布，包括流生态学，以及在环境示踪剂和环流模型方面遇到类似问题的大气科学和海洋学。

Residence times in groundwater systems are referred to in terms of groundwater age and used to diagnose both natural and anthropogenically impacted groundwater behavior. Environmental tracers are employed as proxies to estimate groundwater age, but their application almost always assumes a steady, stationary flow. The effects and importance of transient flow are not accounted for and are unknown. This project addresses non-stationary flows in complex watersheds by accounting for the impacts of temporal variability of weather and climate forcings on groundwater flow and age transport. It uses a two-fold approach. Synthetic systems will be modeled using five-dimensional (5-D), generalized groundwater age models (3-space, 1-time, 1-age) in order to build understanding of controlling processes on groundwater age distributions (GWAD). In transient flows, GWADs are functions of both the time of observation and the time at which observed water entered the system. Preliminary theoretical results show that transient flows bring in processes at all scales, with significant impacts on long-term memory. Then two instrumented and monitored natural watershed systems in the upper Rio Grande of Colorado and New Mexico will be studied. One of these sites is located in the Jemez Mountains of north-central New Mexico and this watershed is part of a Critical Zone Observatory (CZO) funded by NSF. The type, time, and location of new chemical and isotopic tracer measurements will be optimized based on their preliminary 5-D groundwater age models. These applications to natural watersheds will involve undergraduates, link models with data, test the underlying theory and its application, and elucidate how best to use the available suite of tracer methods for GWAD estimation in non-stationary systems. Innovative mathematical solution methods to the 5-D generalized age equations will be developed and tested, including the use of Laplace transforms on age in order to reduce dimensionality. Extensions of these concepts and approaches to residence time distributions in other hydrologic applications, including stream ecology, and to atmospheric science and oceanography where similar problems are encountered in the context of environmental tracers and circulation models, are anticipated.