Improved Prediction of Management‐Relevant Groundwater Discharge Characteristics Throughout River Networks

Abstract

Groundwater discharge zones connect aquifers to surface water, generating baseflow and serving as ecosystem control points across aquatic ecosystems. The influence of groundwater discharge on surface flow connectivity, fate and transport of contaminants and nutrients, and thermal habitat depends strongly on hydrologic characteristics such as the spatial distribution, age, and depth of source groundwater flow paths. Groundwater models have the potential to predict spatial discharge characteristics within river networks, but models are often not evaluated against these critical characteristics and model equifinality with respect to discharge processes is a known challenge. We quantify discharge characteristics across a suite of groundwater models with commonly used frameworks and calibration data. We developed a base model (MODFLOW-NWT) for a 1,570-km 2 watershed in the northeastern United States and varied the calibration data, control of river-aquifer exchange directionality, and resolution. Most models (n = 11 of 12) fit similarly to calibration metrics, but patterns in discharge location, flow path depth, and subsurface travel time varied substantially. We found (1) a 15% difference in the percent of discharge going to first-order streams, (2) threefold variations in flow path depth, and (3) sevenfold variations in the subsurface travel times among the models. We recalibrated three models using a synthetic discharge location data set. Calibration with discharge location data reduced differences in simulated discharge characteristics, suggesting an approach to improved equifinality based on widespread field-based mapping of discharge zones. Our work quantifying variation across common modeling approaches is an important step toward characterizing and improving predictions of groundwater discharge characteristics.

Continental-scale analysis of shallow and deep groundwater contributions to streams

“…The magnitude, spatial distribution, and source-flow path characteristics of groundwater discharge can control the physical characteristics of individual streams 8 , 13 , 14 and whole stream networks 15 . Characterizing the depth of contributing groundwater is particularly important for understanding broad-scale responses of stream ecosystems to land development and climate change 16 for three main reasons: first, groundwater depth is associated with annual thermal stability as natural surface temperature fluctuations are prominent within the shallow aquifer but quickly attenuate with depth 13 .…”

“…Thus, effective watershed management may have a different urgency depending on the depth of contributing groundwater. Also, naturally, deep and shallow groundwater tend to have different chemical profiles 24 – 26 , which has important implications for surface water quality and stream ecosystem function including delivery of legacy contaminants 15 . Third, shallow groundwater can be directly depleted via transpiration 27 , irrigation withdrawals 28 , and is more vulnerable to seasonal water table drawdown during dry periods while discharge from deeper groundwater sources is more seasonably stable 29 .…”

Advancing Process Representation in Hydrological Models: Integrating New Concepts, Knowledge, and Data

“…These papers are generally asking for more rigorous investigation of surface water -ground water interactions, while calling for careful evaluation of often accepted assumptions and conceptualizations. Barclay et al (2020) investigated the impact of various subsurface conceptualizations on model performance. Various models, if "well calibrated", can result in similar performance metrics while they exhibit different spatial properties.…”

Shallow and local or deep and regional? Inferring source groundwater characteristics across mainstem riverbank discharge faces

“…By efficiently identifying PDPs in the field using TIR, and monitoring temporal discharge flux rate patterns and constituents, groundwater sources can be inferred providing additional data with which to evaluate model predictions. Here we describe, implement, and evaluate an approach for assessing multiscale temporal discharge flux attributes over five extensive riverbank discharge faces to assess whether discharge faces represented common or convergent source groundwater flowpaths.Practically, if discharge faces along mainstem rivers tend to be dominated by common groundwater sources, sampling strategies can remain simplified to a few representative PDPs for the assessment of legacy contaminant transport and other management relevant characteristics as described byBarclay et al (2020). However, if discharge faces do indeed represent a focused convergence of a wide range of flowpath depths, lengths, and ages, numerous PDPs need to be evalu-ated across each discharge face to assess source groundwater characteristics.…”