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Abstract

Hicks, D.M.; Shankar, U.; McKerchar, A.I.; Basher, L.; Jessen, M.; Lynn, I.; Page, M. Suspended sediment yields from New Zealand rivers, #50, 2011

Suspended sediment yields from New Zealand rivers
D. Murray Hicks,1 Ude Shankar,1 Alistair I. McKerchar,1 Les Basher,2 Ian Lynn,3 Mike Page4 and Murray Jessen*
1 NIWA, PO Box 8602, Christchurch, New Zealand. Corresponding author: m.hicks@niwa.co.nz
2 Landcare Research, Private Bag 6, Nelson Mail Centre, Nelson 7042, New Zealand,
3 Landcare Research, PO Box 40, Lincoln 7640, New Zealand.
4 GNS Science, PO Box 30-368, Lower Hutt, 5040, New Zealand.
* Deceased, previously Landcare Research
Abstract
River suspended sediment yield estimates from 233 New Zealand catchments are presented and used to calibrate an empirical, raster-type GIS model for predicting suspended sediment yield from any river in New Zealand. The calibration dataset is mostly based on suspended sediment gaugings and flow records, but includes data from lake and fiord bed sedimentation studies. The model relates sediment yield to the spatial integration of the product of a ‘driving’ factor and a ‘supply’ factor. The driving factor is P 1.7, where P is the local mean annual precipitation extracted from a precipitation grid. The supply factor depends on an erosion terrain classification that spreads erosion potential by slope and lithology, and to some extent by erosion process. With 74 unique terrains defined for New Zealand, coefficients for each terrain were determined by a range of approaches based on the availability of calibration data. Comparison of measured yields with those predicted by a preliminary calibration procedure highlighted several regions with systematic over- or under-prediction. These regions have differences in land cover, glacial history, tectonic regime, and/or climate that are not incorporated into the erosion terrain classification or the driving factor based on mean annual rainfall. Separate coefficients were determined for the dominant erosion terrains in each special region. With these adjustments, the model explained 97% and 96% of the variance in the measured South and North Island (log-transformed) yields, respectively, while the standard errors of the predictions equated to factors of 1.55 and 1.8 for the South and North Islands catchments, respectively. The factorial error in predicted yield decreased as catchment area increased. When totalled over all measured catchments, the predicted yield differed from the measured yield by only 2.3% in the South Island and 6.5% in the North Island. Summing measured yields and model-predicted yields from ungauged catchments, the South Island yield is 91 Mt/y and the North Island yield is 118 Mt/y. Natural lakes intercept 14% of the potential sediment yield of the South Island but only ~ 1% of that of the North Island. Hydro-lakes intercept ~ 3% of the South Island yield and ~ 0.3% of the North Island yield. Overall, floodplains and estuaries intercept only a small percentage of the total sediment delivery to the coast (although this may not be the case for particular floodplains or estuaries). The ~ 209 Mt/y total suspended sediment yield from New Zealand amounts to ~ 1.7% of the global sediment delivery to the oceans, making New Zealand a significant contributor on a unit area basis by virtue of its steep terrain, high rainfall, and tectonic activity. The sediment yield model is available in an easily used form on the Internet (http://wrenz.niwa.co.nz/webmodel), but in its present form it is not suited to assessing the effects of changes in land cover or land use on sediment delivery because this was not explicitly included as a controlling factor.

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