Sediment yields from a forested and a pasture catchment, coastal Hawke's Bay, North Island, New Zealand

This study compares the suspended sediment load from two catchments, one in pasture (7.95 km²) and the other in a mature exotic plantation forest (3.45 km²) for a 29-month period (January 1995-May 1997) before harvesting. Both catchments are underlain by gently dipping Tertiary sediments capped with gravels, volcanic ash, and loess. The mean annual rainfall is 1300 mm, but the annual variability is large. Streamflow is measured with broad-crested weirs, and suspended sediment concentrations are monitored with automatic water samplers. The sediment yield response to nine storms was monitored in both catchments and when the calculated suspended sediment yields were summed, the catchment in pasture generated almost 2.5 times the amount of sediment per unit area compared with the one in forest. Initially, sediment rating curves based on log-transformed concentration and instantaneous flow data were used in conjunction with flow duration curves to estimate annual suspended sediment yields, but the scatter in the plots was unacceptably high. Instead, the relationship between peak discharge and storm suspended sediment load for both catchments was used to estimate the total yields. The total suspended sediment yield for the forested catchment (January 1995-May 1997) was estimated at 32.7 (± 3.0) t /km², and for the catchment in pasture at 104.4 (± 8.3) t/km², which is roughly three times the former. Between a quarter and a third of the total suspended sediment was contributed by one storm. Bedload made up less than 1% of the total sediment yield.