NZeem was used in particular for its ability to model suspended sediment load within a system. Fine suspended sediment in particular had been outlined within the Waikato region as causing serious impact to water clarity in most river systems. As such this models ability to provide insight into the matter made it a perfect choice.
The Waikato Regional Council (WRC) Healthy Rivers: Plan for Change/Wai Ora: He Rautaki Whakapaipai aims to reduce inputs of sediment, bacteria and nutrients (primarily nitrogen and phosphorus) to water bodies (including groundwater) in the Waikato and Waipa River catchments.
Therefore a key objective of this case study was to review NZeem generated sediment load estimates for the Waikato and Waipa Rivers as a part of a broader study of sediment transport through these systems for the healthy rivers plan change.
Landcare research used this model to estimate sediment loads by sub-catchment in the Waikato catchment. The reliability of NZeem estimates was assessed by comparing them with sediment load estimates from related literature (Hoyle et al., 2011). Statistically, as discussed in the study, modelled results vary within acceptable confidence intervals when compared to literature and as such the conclusion can be drawn that that the NZeem model provides a reasonable estimate of catchment suspended sediment load.
Further comparison to literature indicated that when specific portions of the catchment are considered NZeem values are similar to previous estimates which were based on historical data from 1990-2010. The issue is raised however that the NZeem model does not account for sediment derived from bank erosion (or losses to flood plain deposition), and when coupled with other software packages (SedNet model) figures produced for bank erosion are created with a large margin of error, and also are of a magnitude greater than the total catchment sediment yield as determined by NZeem; something that is not realistic.
Model developers argue that that there is a balance between flood plain deposition and bank erosion, something that may be a true statement long term, however on the timescales that the model will likely be run the assumption is probably less applicable.
NZeem ultimately appears to produce reasonable estimates of suspended sediment loads for the Waikato and Waipa catchments. Uncertainties arise when bank erosion and floodplain deposition are considered given the dynamic nature of these inputs and stores with suspended load, bed load transport and storage; something that is not modelled well in NZeem.
This study highlighted the fact that contrary to previous studies river banks have the potential to be significant and important contributions to a rivers’ over all sediment budget, with the example of the Waipa river being highlighted. This may be one assumption that could be revisited or considered in the future use of the NZeem model.
NZeem® partitions the New Zealand landscape according to the factors controlling erosion: rock type, land form (especially slope angle), rainfall and land cover.
Andrew Hughes (2015) Waikato River suspended sediment: loads, sources, and sinks Information to inform economic modelling for the Healthy Rivers Wai Ora Project. NIWA Client Report No: HAM2015-059
Hoyle, J., Hicks, M., Roulston, H. (2011) Sampled suspended sediment yields from the Waikato region, Waikato Regional Council, Technical Report 2012/01: 53.