Contribution of groundwater fluxes to total river discharge

Analyzing the opinion of experts

Courtesy of Kevin Masarik.

It seems reasonable to say that an efficient management of water resources at the watershed scale may only be achieved if the overall properties and functioning of a given hydrosystem are correctly determined. Therefore, one of the core missions of water resource scientists and practitioners is to characterize the distribution of water over space and time. Ideally, the hydrologist wishes to estimate how much water is stored in each compartment of a catchment (soil, rivers, aquifers, lakes, ponds, etc.), for how long it stays there and how it circulates from one place to another. In order to establish a proper balance, somehow, it is always a matter of volume (storage) and dynamics (fluxes).

Here I want to tackle a very old question concerning the continental part of the water cycle and which still seems to be an unresolved problem: how much groundwater contributes to the total river discharge in a watershed? How do we estimate it best? Behind these two questions lies a more fundamental one: do we, hydrologists, correctly estimate flow partitioning between the surface and sub-surface of a catchment? Do we have a common strategy? Is it even possible to quantify this partioning with an acceptable uncertainty?

The overall objective of this work is to study how hydrology scientists or practitioners deal with the aforementioned questions and how they would answer them based on their own experience. Hence, the goal is not to seek an average estimate of groundwater contribution to total stream flow based on a knowledgeable crowd, but rather to evaluate how far from consensus we are regarding such fundamental questions, or whether we can detect different schools of thought for example.

Sample results from the study: answers to Q1, Q2 and Q3, « what would be the average long-term contribution of groundwater flow to total river flow in non-mountainous catchments at three different scales? » The scale, or watershed area, considered in each question is reproduced below the pie charts.

The full study can be downloaded in PDF format (~ 2 Mo):

Conclusion in short:

This study shows some evidence that the community of hydrologists does not favor one strategy in particular in order to tackle the question of groundwater contribution to total river discharge. Even if the community recognizes the role of watershed scale, no clear consensus arises on roughly what proportion of the total water export should come from groundwater, nor what method is to be preferred to estimate it. This may potentially result in large epistemic uncertainties in the determination of a crucial part of the water balance at catchment scale, namely the partitioning of flow between surface and subsurface. In turn, this means that performing a water balance as a whole is still an uncertain process in 2020, and we still haven’t fully resolved the most fundamental questions of our discipline. I argue that we need to better understand the role of our conceptualization of hydrosystems and the role of model construction on key balance estimates. We have to keep in mind that in a world were the water cycle is changed along with climate, buidling a resilient future for ecosystems and societies necessarily puts the management of water resources in a central position. A sound management will not be possible if we cannot correctly tell where and in which proportion the water goes.

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