Concepts in catchment hydrology - an overview and lessons for model development

Abstract:

Introduction

The formulation and investigation of scientific theories or hypotheses are key to scientific progress. While in catchment hydrology most hypotheses run the risk of being falsified in other catchments than they have been derived for and thus never can be seen as theories, concepts nonetheless play an important role in modern hydrology (Beven, 2001; Wagener et al., 2010).

Such concepts, being either thought models, hypotheses or paradoxes, largely determine not only the way we understand hydrological processes, but moreover the way in which we communicate our knowledge about hydrology to the future generation of hydrologists and model builders, namely our students. We argue that in spite of the progress in measurement techniques and field experiments, many of the concepts derived from observations have not yet found their way into most of the widely-applied (global) hydrological models and conceptual rainfall-runoff models at the catchment scale, thus potentially limiting the progress of hydrology as a science as opposed to hydrology as a branch of engineering.

In this contribution, we discuss several concepts or hypotheses that have been derived from catchment-scale observations, and which have shaped the way we view the functioning of catchments from an experimentalist point-of-view. It is foreseen that the list of concepts will grow as a results of discussions during the conference. We provide two well-known examples below and discuss their representation in hydrological models at the catchment scale.

Preferred states in spatial soil moisture patterns

A good example of a concept or hypothesis resulting from novel observations at the catchment scale is the concept of preferred states in spatial soil moisture patterns. Based on extensive semi-automatic observations of the spatial distribution of soil moisture (top 30 cm) on a detailed grid of 500 points over the 10.5 ha small Tarrawarra dataset (Western and Grayson, 1998), Grayson et al. (1997) showed that during the transition from wet to dry conditions, the spatial distribution of soil moisture suddenly changed from an organised state with catchment topography leading to organization of wet areas along drainage lines, to a state in which the pattern was more random and determined only by local soil and vegetation properties. In the wet state, horizontal fluxes were believed to dominate, whereas in the dry state vertical fluxes were most important in determining the spatial pattern.

While the concept of preferred states in surface soil moisture distribution is easy to understand, only few, if any, conceptual hydrological models are able to actually simulate or conceptualize the dynamics of these spatial patterns (with the exception of detailed grid-based models which allow for horizontal flow at the scale of the measurements). This is somewhat surprising given the importance of saturation along the drainage lines for saturation excess runoff and catchment-scale runoff thresholds. While TOPMODEL provides a useful conceptualization of landscape-driven saturated subsurface flow at the catchment scale, most implementations of the TOPMODEL concept use a root zone model to calculate recharge, thus preventing the top soil to saturate.

Higher ET from forested catchments

It has long since been known that land cover in a catchment has a large effect on streamflow response to rainfall. While such effects were mostly evaluated by so-called paired catchment studies (Brown et al., 2005), the increased availability of long-term streamflow data and meteorological observations for catchments around the world allowed Zhang et al. (2001) to conceptualize the average catchment ET as a function of partial forest cover and precipitation only across a wide range of climate conditions.

Whereas the resulting formula can be seen as a model in itself on timescales of multiple years, the concept or knowledge that forests have a higher evapotranspiration on average is often not considered when models are developed "bottom-up" to be applied at much shorter timescales. In this case the wrong choice of interception parameterisations and/or surface conductance parameters can lead to the opposite land use signal in models, i.e. a higher rather than lower water yield for forested catchments.

Other concepts, hypotheses and paradoxes.

Other well-known concepts that will be discussed are for instance the "old-water paradox" (Kirchner, 2003), which deals with the observation that water in streams often responds fast to rainfall while its chemical signature shows that this is mainly old water, the fill-and-spill hypothesis for the formation of subsurface stormflow (Tromp-van Meerveld and McDonnell, 2006), and the variable source area concept (Hewlett and Hibbert, 1967), and the capillary fringe-ridging hypothesis (Sklash and Farvolden 1979).

Conclusion and Outlook

Many different concepts on the hydrological functioning of catchments have been developed over the past century. Often, those concepts arose from observations using innovative techniques. It can be expected that with the continuous development of new techniques, new concepts on the functioning of catchments will be developed in the future. It is an ongoing challenge for the hydrological community to develop hydrological models at the catchment scale that are consistent with the generally accepted concepts and theories. With this contribution we would like to stimulate the development of new hydrological model concepts that are consistent with the concepts derived from more than a century of field observations, as well as the ongoing development of new concepts from experiments.

References

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