Catchments as Organised Systems

Introduction to the main hypotheses and aspects in phase 1 of the project

Scope and objectives

The overall objective of the research unit is to provide a new framework for building hydrological models that allows a much more realistic representation of the surface and especially subsurface architecture of catchments at the lower mesoscale (10–200 km²).

The key methodology is to combine: a) recent observation and exploration technology from soil physics, geophysics, remote sensing and (tracer) hydrology. b) our understanding of landscape formation and soil structure formation. c) physically based process models as learning tools to assess novel information on surface and subsurface structures as well as on distributed process dynamics.

Key theoretical objective is to develop a model and mathematical framework that allows better integration of this information into the model identification process and thus facilitates communication between experimentalists and modellers. Research will be conducted in the hydrological observatory "Attert basin" that has been operated by the Gabriel Lippmann Research Institute in Luxemburg since 2003 and is among the best investigated basins in the World.

Main Hypothesis

A catchment is an organised system. Landscapes are characterised by typical patterns and structures which co-evolved over long time scales. This spatial organisation of the landscape controls space–time organisation of the water cycle and related processes.This means that there are a limited amount of of typical combinations of landscape attributes. When these typical combinations are known, only one of each need to be monitored intensively.

Conceptual Framework

We propose that organization of flow and transport across scales is compiled by classes of "Elementary Functional Units" (EFU), that interact within context dependent lead topologies along a hierarchy of driving potential gradients and that these interactions are mediated by a hierarchy of connected flow networks like macropores or pipe systems structures. Lead topologies is a set of EFUs that function in series depending on the prevailing context (rainfall or radiation driven conditions). An EFU is defined as a set of control volumes/landscape elements that is typical for a landscape and whose elements are statistically homogeneous (not uniform) with respect to their functioning:

Radiation driven case

In the radiation driven case, fluxes of water and energy are mainly vertical and macropores can be neglected. The lead topologies are typical combinations of soil profile classes and vegetation community classes.

Fig 1. Lead topology in radiation driven case

Rainfall driven case

In the rainfall driven case the driving gradientis downward. In the unsaturated zone flow of water and energy is vertical, while in the saturated zone flow is lateral towards the stream. Flow is mainly through macropores and matrix flow can be neglected.

Fig 2. Lead topology in rainfall driven case