Effects of Land Use and Extreme Precipitation on Hillslope Erosion and Suspended Sediment Yields in the Manawatu River, New Zealand

Abstract:

Landscapes disturbed by intensive land uses are susceptible to sediment erosion that may degrade river water quality. In February 2004, the southern portion of New Zealand’s North Island, which is covered by livestock grazing, experienced an extreme precipitation event which resulted in extensive landsliding in the Oroua and Pohangina subcatchments of the Manawatu River catchment. In this study, we (1) assess whether land use influences landslide occurrence; (2) identify landslides that are active sediment sources to river channels; and (3) determine if land use can switch subcatchments from supply-limited (river sediment loads are limited by available sediment in the landscape) to transport-limited (river sediment loads are limited by flow capacity). Results revealed that landslides caused by this extreme precipitation event occurred disproportionately in areas identified as pasture, suggesting that these areas are more susceptible to landsliding than forested areas. The observed sediment load response to flood events for the period of 1999-2014 in the Pohangina subcatchment was variable in relation to flood magnitude, indicating a supply-limited landscape, while in the Oroua subcatchment the observed sediment response was relatively proportional to flood magnitude, indicating a transport-limited landscape. Further analysis of sediment storage and delivery processes within the two subcatchments is necessary to explain the observed sediment load responses. Channel connectivity analysis revealed that approximately 65% of landslide scars were unconnected to river channels, so there will likely be a response lag in sediment loads. As livestock grazing intensity increases in New Zealand, an understanding of the short-term and long-term impacts of land use on soil erosion and water quality is necessary to develop effective management practices.