Low path dimensionality and hydrological modelling

Abstract

Increasingly, research is indicating that subsurface flow paths govern ion transport within river catchments. Distributed prediction of these solute flow paths in typically heterogeneous catchments must inevitably be highly uncertain without some identification of a spatial structure relating small-scale measurements of soil properties to flow predictions distributed over large catchments. To date, the evidence for profile and catenal structure within soil hydrological properties and resultant solute flow paths is not fully embraced by the hydrological community. As a consequence soil parameters are often poorly distributed within catchment-scale distributed models. This paper seeks, first, to generalize the disparate sources of evidence of parameter and flow path structure within the profile-ward and catena-ward dimensions. Second, to outline how much of this structure has been incorporated into previous hydrological simulations using distributed models, and third, to examine the physical basis of attempts to simplify parameter and flow path dimensions using pedological classifications. The available evidence suggests that a considerable number of world soils show profile-ward structure within their hydrological properties and resultant flow paths. Changes in profile-ward patterns along catenal sequences remain uncertain. The Plynlimon region of mid-Wales has been the focus for many detailed studies of solute flow paths, catchment-scale model simulations, soil property characterizations and soil classification. Comparison of these studies suggests that most model simulations and hydromorphic classifications of soil taxa fail to distinguish adequately between soil horizons and soil types with markedly different property distributions. Preliminary analysis, however, suggests that by using a catena based criterion to classify the hydromorphic characteristics of soils, soil elements with distinct patterns of properties and flow paths can be identified. This might suggest that the accuracy of distributed predictions of ion movements within river catchments could be greatly improved by the derivation of profile-specific patterns in soil properties. These profile-specific effective parameters need to be derived from measurements over a range of scales, including individual layers, profiles and complete catenas.

Soil pipe distribution and hydrological functioning within the humid tropics: a synthesis

“…As soils within the humid tropics often exhibit a predominantly monotonic decline of Ks with depth (Chappell and Ternan, 1992;Elsenbeer, 2001;Chappell and Sherlock, 2005), a lower Ks in the strata immediately below a lateral pipe is likely. Figure 6 shows the vertical distribution of Ks of soil profiles containing piping at La Cuenca (Elsenbeer and Lack, 1996;Elsenbeer and Vertessy, 2000), Bukit Tarek (Ziegler et al, 2006) and Danum (Chappell et al, 1998).…”

“…For hillslopes containing these extreme preferential pathways, the actual patterns of subsurface flow may bear little relation to those indicated by small core-based (or auger hole-based) Ks tests. Rather than flow responding to the patterns in Ks linked to tropical soil horizons (Elsenbeer, 2001) or a tropical soil catena (Chappell and Ternan, 1992), patterns may be overwhelmingly a function of the spatial distribution of the lateral pipes network (Kirkby, 1988;Bonell and Balek, 1993, p. 198;Bonell, 2004 p. 317). Indeed, the tropical tracer study of Chappell and Sherlock (2005) provides evidence that this may be the case with subsurface flow direction being little related to the vertical distribution of core-based Ks values.…”

Runoff Processes in Southeast Asia: Role of Soil, Regolith, and Rock Type

“…As yet, neither method provides a perfect solution. Maps of soil type, for example, are classified primarily by a range of edaphic and pedological factors, not just factors controlling runoff processes (FAO-UNESCO 1990;Chappell and Ternan 1992;Soil Survey Staff 1999). Equally, different runoff pathways can give similar hydrograph forms, so that is not easy to identify which are the dominant runoff pathways purely from hydrograph analysis (Chappell 2005b).…”

“…These hillslope experiments have been conducted at South Creek, Australia (Bonell and Gilmour 1978;Bonell et al 1981Bonell et al , 1983, Reserva Ducke, Brazil (Nortcliff and Thornes 1981), Fazenda Dimona, Brazil (Hodnett et al 1997), Ulu Belalong, Brunei (Dykes and Thornes 2000), Bukit Tarek, Peninsular Malaysia (Noguchi et al 1997), Danum, East Malaysia (Sinun et al1992;Chappell et al 1998Chappell et al , 1999aChappell et al ,b, 2004cChappell and Sherlock 2005), Luquillo, Puerto Rico (Schellekens 2000), Lutz Creek, Panama (Godsey et al 2004;Kinner and Stallard 2004), La Cuenca, Peru (Elsenbeer and Lack 1996), and Bukit Timah, Singapore (Sherlock et al 1995(Sherlock et al , 2000Chappell and Sherlock 2005). Others have attempted to generalise the runoff pathways illustrated in these studies (Chappell and Ternan 1992;Elsenbeer 2001;Bonell 2004), particularly in the upper layers of the ground; we attempt to add to this work.…”

“…Hydrological characteristics affecting runoff pathways, notably permeability, porosity, and soil moisture status, are only one set of the many factors. Thus, a perfect correlation between hydrological characteristics and soil type should not be expected (Chappell and Ternan 1992). Note also that the classification of soil type is often incorrectly confused with the soil textural class, which is simply the mix of different particle sizes that make up each soil horizon.…”