Earthworms and tree roots: A model study of the effect of preferential flow paths on runoff generation and groundwater recharge in steep, saprolitic, tropical lowland catchments

Jianting Zhu,Fred L Ogden,Yanyan Cheng

doi:10.1002/2016wr020258

Abstract

AbstractPreferential flow paths (PFPs) affect the hydrological response of humid tropical catchments but have not received sufficient attention. We consider PFPs created by tree roots and earthworms in a near‐surface soil layer in steep, humid, tropical lowland catchments and hypothesize that observed hydrological behaviors can be better captured by reasonably considering PFPs in this layer. We test this hypothesis by evaluating the performance of four different physically based distributed model structures without and with PFPs in different configurations. Model structures are tested both quantitatively and qualitatively using hydrological, geophysical, and geochemical data both from the Smithsonian Tropical Research Institute Agua Salud Project experimental catchment(s) in Central Panama and other sources in the literature. The performance of different model structures is evaluated using runoff Volume Error and three Nash‐Sutcliffe efficiency measures against observed total runoff, stormflows, and base flows along with visual comparison of simulated and observed hydrographs. Two of the four proposed model structures which include both lateral and vertical PFPs are plausible, but the one with explicit simulation of PFPs performs the best. A small number of vertical PFPs that fully extend below the root zone allow the model to reasonably simulate deep groundwater recharge, which plays a crucial role in base flow generation. Results also show that the shallow lateral PFPs are the main contributor to the observed high flow characteristics. Their number and size distribution are found to be more important than the depth distribution. Our model results are corroborated by geochemical and geophysical observations.

Highlights

Hydrological processes in humid tropical lowlands are significantly different from those common in temperate climates [Mosley, 1979; Bonell, 1993; Sidle et al, 2000, 2001; Uchida et al, 2001; Bonell and Bruijnzeel, 2005]
The results indicated that runoff generation behaviors were not sensitive to the lateral preferential flow paths (LPFPs) number after it is above a certain threshold, after which Rs/R (Figure 7(c)) did not change much, while absolute Volume Error (VE) (Figure 7(a)) were all less than 4% and NSEsqr values (Figure 7(b)) were all greater than 0.65
The overarching objective of this study was to improve predictive understanding of how the preferential flow paths (PFPs) affect partitioning of rainfall into runoff, soil moisture, and groundwater recharge in the steep, humid, tropical lowland catchments with saprolitic soils

Summary

Introduction

Hydrological processes in humid tropical lowlands are significantly different from those common in temperate climates [Mosley, 1979; Bonell, 1993; Sidle et al, 2000, 2001; Uchida et al, 2001; Bonell and Bruijnzeel, 2005]. Metamorphic, or sedimentary bedrock this leads to the formation of deeper, saprolitic soils, while intense biological activity in these soils creates preferential flow paths (PFPs) by altering the geometric properties of the soil and leaving texture unaltered [Bachmair et al, 2009] These PFPs contribute significantly to catchment hydrological behavior by affecting the partitioning of rainfall into different hydrological pathways [Jones, 2010] but have not received sufficient attention [Beven and Germann, 1982; 2013], in the context of hydrological modeling. The role of macropores in both rapid runoff [Noguchi et al, 1999; Kinner and Stallard, 2004] and groundwater recharge [Cuthbert and Tindimugaya, 2010] is a fundamental research question [Beven and Germann, 1982] that has yet to be fully addressed within humid tropical hillslopes and basins [Chappell, 2010]

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Conclusion