Land use influences on suspended sediment yields and event sediment dynamics within two headwater catchments, Waikato, New Zealand

Abstract

Information on the effect of human-induced disturbances on sediment yields and sediment sources from New Zealand catchments is limited. This study analyses the sediment yields and event sediment dynamics for a 12-year period from two paired headwater catchments within the Waikato region. One catchment (Whakakai) was entirely vegetated in native forest, while the other (Mangaotama) was of mixed land use. Near the beginning of the study period, an integrated catchment management plan was implemented with the Mangaotama catchment. This plan included riparian planting, planting of degraded land in Pinus radiata and changes in stock management. Long-term in-stream turbidity data was calibrated with suspended sediment concentration (SSC) measurements and used to determine event and annual sediment yields. Hysteresis of the discharge (Q)–SSC relationship during events was used to infer differences in sediment sources. Event sediment yields within the Mangaotama catchment were similar to Whakakai catchment during small events (c. 1.01-year return period). For larger and less frequent events (>5-year return period), the event yields from Mangaotama exceeded Whakakai by about a factor of three, probably due to mass movement initiation. The specific sediment yield (±1σ) from Mangaotama (97±39 t/km2/year) was around 60% higher than Whakakai (60±22 t/km2/year). Clockwise hysteresis in the Q–SSC relationship dominated at Mangaotama, indicating the importance of near-channel sources. The persistence of near-channel source dominance throughout the study period (despite major land use changes) was attributed to switching of erosive mechanisms (i.e. from cattle-induced mechanical disturbance to subaerial processes). The prevalence of anti-clockwise hysteresis in the Q–SSC relationship of events at Whakakai suggests that hillslope sources are likely to be a principal source of eroded sediment. Such information provides valuable information that may help guide future decisions on the application of catchment rehabilitation measures.