Fluid Boundaries: the hyporheic zone of a tropical tidal river

Abstract

While the hyporheic zone is increasingly acknowledged as an important component of the hydrological cycle, being critical to ground water and surface water interactions, knowledge of its hydrological functioning is still limited, particularly at local scales. Tropical and tidal hyporheic zones principally are underrepresented in academic studies, in spite of their geographical significance. The distinct seasonality of the tropics, combined with known tidal influences, such as tidal pumping, suggests different hydrological processes dominate in tropical and tidal systems to those of non-tidal or temperate rivers. This research is an initial attempt to articulate the complexity of local hyporheic zone characteristics of a site on the Bang Pakong River, Thailand. This characterization began with a literature-based conceptual model of the hyporheic flux paths and velocity and zone morphology of a generic site on the river that was then refined into a field model, using field observations and limited field data gathered during a dry-season field trip to a specific site. The resultant field model clearly demonstrated increased hydrological complexity. It was argued that this complexity was a response to local-scale features; features invisible to regional-scale research. For example, the conceptual model predicted hyporheic zone morphology included level layers, however the presence of a secondary channel at the research site suggested a concave morphology. With limited knowledge of the Bang Pakong River’s hyporheic zone, and tropical and tidal hyporheic zones generally, this research represents a modest, yet important, starting point to better understanding the hydrological function of these zones. Such knowledge contributes to an increased appreciation of hyporheic zone behaviour more broadly, as well as providing specific information to incorporate in policies and practices guiding the management of tropical tidal rivers.