The way forward: Can connectivity be useful to design better measuring and modelling schemes for water and sediment dynamics?

Highlights

• We introduce a conceptual framework for modelling and measuring water and sediment fluxes.

• System phases and fluxes are differentiated to enable quantification of connectivity.

• The mutual benefits of combining modelling and measuring to understand connectivity is shown.

• Necessary data for measuring and/or modelling water and sediment transfer is identified.

Abstract

For many years, scientists have tried to understand, describe and quantify water and sediment fluxes, with associated substances like pollutants, at multiple scales. In the past two decades, a new concept called connectivity has been used by Earth Scientists as a means to describe and quantify the influences on the fluxes of water and sediment on different scales: aggregate, pedon, location on the slope, slope, watershed, and basin. A better understanding of connectivity can enhance our comprehension of landscape processes and provide a basis for the development of better measurement and modelling approaches, further leading to a better potential for implementing this concept as a management tool. This paper provides a short review of the State-of-the-Art of the connectivity concept, from which we conclude that scientists have been struggling to find a way to quantify connectivity so far. We adapt the knowledge of connectivity to better understand and quantify water and sediment transfers in catchment systems. First, we introduce a new approach to the concept of connectivity to study water and sediment transfers and the associated substances. In this approach water and sediment dynamics are divided in two parts: the system consists of phases and fluxes, each being separately measurable. This approach enables us to: i) better conceptualize our understanding of system dynamics at different timescales, including long timescales; ii) identify the main parameters driving system dynamics, and devise monitoring strategies which capture them; and, iii) build models with a holistic approach to simulate system dynamics without excessive complexity. Secondly, we discuss the role of system boundaries in designing measurement schemes and models. Natural systems have boundaries within which sediment connectivity varies between phases; in (semi-)arid regions these boundaries can be far apart in time due to extreme events. External disturbances (eg. climate change, changed land management) can change these boundaries. It is therefore important to consider the system state as a whole, including its boundaries and internal dynamics, when designing and implementing comprehensive monitoring and modelling approaches. Connectivity is a useful tool concept for scientists that must be expanded to stakeholder and policymakers.

Introduction

Earth scientists seek to understand, describe and quantify water and sediment fluxes, with their associated substances like pollutants, across landscapes at multiple scales (Wolman and Gerson, 1978; Howard, 1982). The aim of the researchers has a temporal and spatial approach: from hydro-geomorphological processes triggered by a single rainfall event to the geological timescale of landscape evolution (Howard, 1982; Wang et al., 2006); and from the particle and soil aggregate scale up to the continental scale (Kirchner et al., 2001; Renschler and Harbor, 2002). In the past two decades, a new concept called connectivity has been used by the scientific community as a means to describe and quantify the influences on the fluxes of water and sediment at different scales: pedon, location on the slope, slope, watershed and basin (Parsons et al., 2015). The concept of connectivity and measurement scales are interrelated, because water and sediment transfers change according to the scale at which they are observed as a result of changing connectivity (Cammeraat, 2002).

Understanding connectivity enhances our understanding of landscape processes and allows for developing better measurement and modelling approaches. These better measurement and modelling approaches, in turn, lead to a better potential of implementing this concept as a management tool. The connectivity-based approach provides the potential for holistic solutions, which are compatible with the implementation of key EU policy directives, such as the Water Framework, Bathing Waters and the future Soils Directives (Croke and Hairsine, 2006; SD Keesstra et al., 2016). The connectivity-based approach serves different disciplines, from soil science, to geomorphology, hydrology, geology, ecology and atmospheric sciences (Pauling et al., 2006; Urban and Keitt, 2001; Savenije, 2009; García-Ruiz et al., 1995).

The pioneer use of the term connectivity was by mathematicians (Whyburn, 1931) and later by physicians (Shimbel and Rapoport, 1948). Within the Earth Sciences, the concept of connectivity was pioneered by biologists (Leake and Anninos, 1976), and has been used to describe relations amongst species (Henein and Merriam, 1990). Connectivity also contributed to advances in meteorology in the 1950s (Munk, 1950), and was soon used by soil scientists to describe chemical interactions in the soil column (Webber and Jellema, 1965). Connectivity transferred to other areas of science such as geology (Clark, 1973; Bonham, 1980), archaeology (Bronson and Asmar, 1975), chemistry (Bahnick and Doucette, 1988; Gerstl and Helling, 1987), and geomorphology and hydrology (Grisak et al., 1980; Burt and Gardiner, 1982). Since the beginning of the 21st century the concept of connectivity has been further elaborated in hydrology (Bracken and Croke, 2007) and geomorphology (Brierley et al., 2006), where connectivity is seen to act on different spatial directions, i.e. longitudinal (river channel), lateral (hillslope/floodplain-channel), and vertical (surface-subsurface) connectivity (Ward, 1989; Brierley et al., 2006; Fryirs et al., 2007). In recent years, the concept of connectivity has become increasingly prevalent (López-Vicente et al., 2015; Buendia et al., 2015; Marchamalo et al., 2016) as a tool to understand the processes in Earth Science.

In the field of water and sediment transfer, scientists have been struggling to find a way to quantify connectivity. Most studies remain conceptual (Bracken et al., 2013), or use connectivity to describe and understand the system better a qualitative way (Bracken et al., 2015). Other uses of the concept include assessing connectivity through a relative index used to compare different compartments of a catchment (Cavalli et al., 2013; Heckmann et al., 2015). The current view on connectivity identifies structural connectivity, defined as the extent to which landscape units (at multiple spatial scales) are contiguous or physically linked to one another; and functional connectivity, defined as the way in which interactions between multiple structural characteristics of the system in question affect geomorphic, ecologic and hydrologic processes (Wainwright et al., 2011). In this study, the authors also acknowledge that there is an interaction between structural and functional connectivity, which causes problems in longer term studies, because structural connectivity becomes functional when the time span is long enough (López-Vicente et al., 2017). This in turn, causes problems defining the way the processes and finally connectivity can be assessed in the field or in a modelling effort. These issues call for a new approach to describe and use the concept of structural connectivity that can be applied to all temporal and spatial scales.

The aim of this paper is to review State-of-the-Art connectivity concepts and adapt the existing knowledge on connectivity better to understand and quantify water and sediment transfers within catchments. The objectives are: (i) to give an overview of the State-of-the-Art in the field of water and sediment connectivity; (ii) to propose a framework to describe catchment system dynamics from a connectivity point of view; (iii), to discuss the implications of this framework for measuring and modelling of water and sediment fluxes; and (iv) to identify priority research issues to advance our understanding of measuring and modelling approaches using the connectivity concept.