Modelling reveals reductions in erosion due to soil conservation

Region-wide sediment loads to water are estimated to have decreased by 4 per cent regionwide between 2002 to 2017 in response to soil conservation efforts, Waikato Regional Council’s Integrated Catchment Management Committee heard last week.

Tim Norris, one of the council’s soil and land scientists, gave the committee a presentation about the work done with Manaaki Whenua Landcare Research to assess the effectiveness of catchment management and river restoration through monitoring and sediment modelling.

SedNetNZ modelling was used to estimate baseline erosion and evaluate the effectiveness of historic riparian restoration efforts (specifically fencing and stock exclusion) and hill-country erosion control measures (such as space planting) for reducing erosion, and it was found that between 2002 and 2017, modelled erosion decreased across most of the council’s eight management zones, with reductions ranging from 3.5 per cent (Lake Taupō zone) to 8.5 per cent (Waipā zone).

Only the upper Waikato zone showed a modelled increase of 9.4 per cent, which Norris attributed to land use changes from forestry to farming.

“The modelling implemented a series of backward-looking scenarios at four points of time: 2002, 2007, 2012 and 2017,” said Norris.

“Given the council’s large investment in soil conservation programmes, and the significant investment by landowners, we want to know what impact these programmes are having on reducing erosion and sediment delivery to receiving environments.” 

The council’s Integrated Catchment Management (ICM) directorate incentivises landowners to undertake soil conservation mitigations in priority catchments and reaches, providing co-funding support particularly from Waikato River Authority and the Ministry of Primary Industries (Hill Country Erosion Fund).

It assists landowners with funding to enable them to take an integrated approach to keeping soil on land to reduce the impacts of sediment in waterways, and to maintain channel capacity.

Priority catchments for funding are identified through science, such as the results from the council’s catchment environmental monitoring programme and the recent erosion and sediment modelling undertaken using the SedNetNZ model.

Norris said the council undertook a regional survey every five years to assess the overall state of riparian margins across the region, looking at fencing, vegetation and buffer width. 

“Over time, we have seen a consistent increase in the proportion of bank length effectively fenced across the region,” said Norris.

“For dairy farms, the 2017 survey found that 87 per cent of streambanks were effectively fenced.

More than 1 million hectares in the Waikato region is affected by erosion to varying degrees.

The West Coast zone has the highest average  erosion (190 tonnes per square kilometre per year) due to steep hill-country, soft rock geology and high rainfall in some areas, followed by Waipā and Upper Waikato (105 tonnes per square kilometre per year and 70 tonnes per square kilometre per year respectively), while Central Waikato (the smallest zone by area) has the lowest (39 tonnes per square kilometre per year). Across the region, approximately 1700 kilotonnes per year of sediment is delivered to the coast.

The council also modelled potential future erosion mitigations for scenarios of assumed soil conservation works on high-risk pastoral land (LUC class 6e, 7e and 8e) of 20 per cent, 40 per cent, 50 per cent and 100 per cent treatment through space planting of poles or revegetation, and assuming climate conditions remain unchanged.

“This produced decreases of up to 41 per cent under the most ambitious scenario [100 per cent], with13 per cent reductions for 20 per cent of the at-risk areas treated.”

Modelling showed climate change was also expected to exacerbate erosion and increase sediment loads, with a modelled increase of 12-24 per cent by 2040 under the most conservative scenario of RCP2.6.

Norris said the council’s ICM and Science Policy and Information directorates were working together on the use of the findings to inform more targeted catchment management priorities and programmes and upcoming freshwater management policy.

The report Assessing the Effectiveness of Catchment Management and River Restoration through Monitoring and Sediment Modelling is in the 7 November agenda to the Integrated Catchment Management Committee meeting, which can be viewed here.