Cross-scale interactions between land cover/land use, climate, and river water quality: A case study of the Manawatu River catchment, New Zealand
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The interaction of climate, geomorphology, and land use dictates catchment sediment production and associated river sediment loads. This case study in the Manawatu River catchment in the Lower North Island of New Zealand was a decade-long examination of the short- and long-term effects of an extreme storm event on sediment supply and exhaustion in the landslide-dominated Oroua and Pohangina subcatchments. Indicators of Hydrologic Alteration (IHA), a program developed to characterize hydrologic regimes, was used to analyze daily suspended sediment records over a period of a decade in order to characterize sediment regimes of the Oroua and Pohangina. An aggregated dataset of sediment-bearing events for the period of record was analyzed with a local polynomial regression (LOESS) to examine the suspended sediment response of individual storms relative to runoff magnitudes. The findings of this study demonstrate that large storms have the ability to generate enough sediment via landsliding to temporarily convert these catchments from a supply-limited state to a transport-limited state. Land use in the form of livestock grazing on steep hillslopes was an important control on the location of landslides and thus sediment supply. The timing and intensity of storms were also important influences on these state transitions. The suspended sediment response to the February 2004 storm (relative to the mean condition) was smaller in the Pohangina than in the Oroua. The occurrence of another large storm in the Pohangina that closely preceded the February 2004 storm likely depleted available sediment in the landscape. In both subcatchments, suspended sediment loads were elevated for a period of approximately four years following the landslide-generating February 2004 storm. The spatial and temporal interactions between land cover/land use and climate have important implications for land management strategies to reduce erosion and improve river water quality in landscapes dominated by storm-induced erosion.