Stormflow generation involving pipe flow in a zero-order basin of Peninsular Malaysia

Abstract

Hydrological responses in a zero-order basin (ZOB), a portion of whose discharge emerged via preferential flow through soil pipes, were examined over a 2-year period in Peninsular Malaysia to elucidate primary stormflow generation processes. Silicon (Si) and specific conductance (EC) in various runoff components were also measured to identify their sources. ZOB flow response was dependent on antecedent precipitation amount; runoff increased linearly with precipitation during events >20 mm in relatively wet antecedent moisture conditions. Runoff derived from direct precipitation falling onto saturated areas accounted for <0.2% of total ZOB flow volume during the study period, indicating the predominance of subsurface pathways in ZOB flow. ZOB flow (high EC and low Si) was distinct from perennial baseflow via bedrock seepage (low EC and high Si) 5 m downstream of the ZOB outlet. Pipe flow responded quickly to ZOB flow rate and was characterized by a threshold flow capacity unique to each pipe. Piezometric data and pipe flow records demonstrated that pipes located deeper in the soil initiated first, followed by those at shallower depths; initiation of pipe flow corresponded to shallow groundwater rise above the saprolite-soil interface. Chemical signatures of pipe flow were similar to each other and to the ZOB flow, suggesting that the sources were well-mixed soil-derived shallow groundwater. Based upon the volume of pipe flow during storms, the combined contribution of the pipes monitored accounted for 48% of total ZOB flow during the study period. Our results suggest that shallow groundwater, possibly facilitated by preferential flow accreted above the saprolite-soil interface, provides dominant stormflow, and that soil pipes play an important role in the rapid delivery of solute-rich water to the stream system. Copyright © 2006 John Wiley & Sons, Ltd.