Evidence for the Activation of Shallow Preferential Flow Paths in a Tropical Panama Watershed Using Germanium and Silicon

Abstract

In humid tropical watersheds, the hydrologic flow paths taken by rain event waters and how they interact with groundwater and soil matrix water to form streamflow are poorly understood. Preferential flow paths (PFPs) confound storm infiltration processes, especially in the humid tropics where PFPs are common. This work applies germanium (Ge) and silicon (Si) as natural flow path tracers in conjunction with water stable isotopes and electrical conductivity to examine the rapid delivery of shallow soil water, the activation of PFPs, and event water partitioning in an experimental catchment in central Panama. We employed a three‐component mixing model for hydrograph separation using the following end‐member waters: (i) base flow (high [Si], low [Ge], and low Ge/Si ratio), (ii) dilute canopy throughfall (low [Si] and low [Ge]), and (iii) shallow (<15 cm) soil matrix water (low [Si], high [Ge], and high Ge/Si ratio). These three end‐members bounded all observed Ge/Si streamflow ratios. During small rain events (<∼24 mm), base flow and dilute canopy throughfall components dominated stormflow. During larger precipitation events (>∼35 mm), we detected the third shallow soil water component with an elevated [Ge] and Ge/Si ratio. This component reached its maximum during the hydrograph's receding limb coincident with the maximum event fraction, and increased proportionally to the total storm rainfall exceeding ∼35 mm. Only shallow (<15 cm) soil matrix water exhibited elevated Ge concentrations and high Ge/Si ratios. This third component represents rapidly delivered soil matrix water combined with shallow lateral PFP activation through which event waters interact with soil minerals.

Land cover effects on soil infiltration capacity measured using plot scale rainfall simulation in steep tropical lowlands of Central Panama

“…Our observations of soil pipe flow add to the growing body of evidence that PFPs play a major role in flashy storm runoff behaviour in ASP catchments (Cheng et al, ; Gardner et al, ; Krishnaswamy et al, ; Litt et al, ; Ogden et al, ) and the greater humid tropics (Putty and Prasad, 2000; Chappell, ). Although this present study's soil pipes activated under very large applied rainfall rates, Litt () observed natural soil pipe flow in the upper 20 cm within the same pasture as PAS‐A/B/C during a relatively small rain event with rainfall intensity below 30 mm h −1 .…”

“…Past soil infiltration studies in ASP and PCW have inferred catchment‐scale infiltration‐excess overland flow responses based on small‐scale soil core observations (Hassler et al, ). Yet the present study and numerous field observations, storm runoff, geochemical tracer, and modelling studies (Cheng et al, ; Crouch, ; Gardner et al, ; Hendrickx et al, ; Litt, ; Ogden et al, ) point to spatially disperse PFPs dominating storm runoff rather than widespread overland flow. Furthermore, rapid subsurface flow path responses to high‐intensity rain have been evidenced across the globe (Chappell, Jones, Tych, & Krishnaswamy, ).…”

“…At the much larger catchment scale, both geochemical data and numerical modelling exercises have also identified preferential flow as an important ASP runoff process. Gardner et al () showed how rainfall in pasture can become rapid interflow in the upper 15 cm and acquire a distinct Ge/Si signature. This rapid, shallow soil pathway contributes significantly to storm runoff, particularly during storm events with more than 35 mm of rainfall.…”

“…Hassler et al, () inferred that given such low median infiltration rates infiltration‐excess overland flow should be a common flow mechanism. However, catchment scale runoff generation evidence from hydrometrics and natural geochemistry at ASP does not point to overland flow as a common runoff mechanism but rather PFPs (Gardner, Litt, Lyons, & Ogden, ; Litt, Gardner, Ogden, & Lyons, ). Macroporous soils can exhibit considerable scale dependence of infiltration rate when measured at small scales up to 700 cm 2 , whereas infiltration rate measurement accuracy improves at larger plot scales (Sarkar et al, ).…”

Land Use‐Dependent Preferential Flow Paths Affect Hydrological Response of Steep Tropical Lowland Catchments With Saprolitic Soils

“…Results by Cheng et al (2017) showed the role of lateral PFPs in contributing to quick flow and the role of vertical PFPs in controlling groundwater recharge. These findings were supported by geochemical tracer studies (Litt et al, 2015) and shallow subsurface flow path activation identified using Ge/Si as a tracer (Gardner et al, 2017).…”