A review of the evolving perceptual model of hillslope flowpaths at the Maimai catchments, New Zealand

Abstract

The Maimai catchment has been the site of ongoing hillslope research since the late 1970s. These studies have facilitated the development of a detailed perceptual model of hillslope hydrology at Maimai. This perceptual model has grown in complexity beyond analytical description; nonetheless it provides a very useful case study of hillslope hydrological processes and encapsulates much of what field hydrologists have come to recognize as the dominant hillslope runoff processes in steep, humid catchments. No single research approach has resolved the complexities of streamflow generation in this highly responsive catchment. Yet, each data set reviewed in this paper adds to the cumulative understanding of catchment behavior by providing alternative (and sometimes conflicting) interpretations of hillslope subsurface flow. Initial dye tracer studies of macropore flow provided insight into hillslope flow processes, but suffered from the limitations of a single-method approach. Subsequent water isotopic tracing studies showed clearly that stored soil water and groundwater comprised the majority of channel stormflow; notwithstanding, isotope-oriented approaches did not enable the development of a mechanistic understanding of hillslope processes. An integration of tensiometer recording and tracer techniques was required for later reconciliation of different process interpretations concerning the role of macropores and old/new water ratios. Although single throughflow pits continued to be the indicator of subsurface flow timing and magnitude for several published studies at Maimai, subsequent whole hillslope trench studies showed that flow varied widely across a slope section—making the single pit observations of the previous studies suspect. Most recent observations demonstrate that small depressions in the bedrock surface may exert a significant control on water mobility and mixing. In particular, the bedrock topography appears to determine spatially the pathway of rapid saturated subsurface water flow and tracer breakthrough at the hillslope scale. The Maimai catchments in New Zealand provide a historical perspective on the issues faced by hillslope/small catchment hydrologists since the mid 1970s and highlights the advantages of multiple repeat experiments for testing hypotheses and improving our mechanistic understanding of subsurface flow.