Tracer and Darcy‐based Identification of Subsurface Flow, Bukit Timah Forest, Singapore

Abstract

The spatial distribution of soil hydrological properties and associated flow pathways within a single 0.01 to 10 km2 catchment are usually poorly defined by typical programmes of point‐scale measurements. This is due not only to “measurement uncertainty” caused by instrument error and inadequate spatial sampling but also to “conceptual uncertainty” resulting from a deficiency in the Darcy‐Buckingham‐Richards description of subsurface flow. This study examines these two sources of uncertainty in the identification of subsurface flow beneath the Bukit Timah forest, Singapore. The two distinct approaches of water‐tracing and Darcy‐Buckingham calculations are compared. Flow estimates resulting from quasi‐steady Darcy‐Buckingham calculations were found to be very sensitive to the magnitude and pattern of the saturated hydraulic conductivity, which itself was sensitive to the permeametry technique used. The use of a Hydro‐Physics ring permeameter in the estimation of subsurface flows gives rates that are far greater than the mean propagation rates of tagged water though the approach identifies the relative difference between the behaviour of the two slopes studied. Whilst the lumped differences between the two tropical slopes may be identified with the Darcy method, the poor correlation with the absolute velocity and direction of the tracer plume means that catchment or even slope scale simulation using parameters of the Darcy‐Buckingham‐Richards equation will be highly uncertain.

Contrasting flow pathways within tropical forest slopes of Ultisol soils

“…Sherlock et al, 1995Sherlock et al, , 2000. Other research has questioned the accuracy of the Darcy-Richards approach, particularly where natural soil pipes are present, when flow estimates derived from soil-water calculations have not matched measurements of throughflow or streamflow generation (e.g.…”

“…These estimates are, however, evaluated against direct measurements of the relative magnitude and direction of soilwater flow monitored with the use of artificial water tracers. Very few tropical studies have used artificial soil-water tracers (notable exceptions include Bonell et al, 1984;Cabellero et al, 2002;Reichenberger et al, 2002), and even fewer have compared hydrometric predictions against observed tracer paths (Sherlock et al, 1995;Sherlock, 1997).…”

“…The study catchment (1°21′ N and 103°46′ E) on Singapore Island is small (0·05 km 2 ), is located on the northwestern slopes of Bukit Timah hill, and ranges in altitude from 90 to 164 m. It is called the 'Jungle Falls catchment' by Sherlock et al (1995) and Sherlock (1997), but is referred to simply as the 'Bukit Timah catchment' in this paper. Slopes are typically gentle (<10°) in the hillslope crest zones, becoming very steep (>20°) close to the channel-heads.…”

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

“…Figure 1a,b shows the locations of the key hillslope hydrology studies (i.e., studies with at least permeability and soil-water measurements) published in the international literature. Six out of ten are on Acrisol-Alisol soils, the dominant soil of Southeast Asia (Sherlock et al 1995;Noguchi et al 1997;Chappell et al 1998;Dykes and Thornes 2000;Schellekens 2000;Kinner and Stallard 2004). A further two studies (Bonell and Gilmour 1978;Elsenbeer and Lack 1996) have results from a mix of Acrisol-Alisol and other soils.…”

“…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.…”