How will climate change affect erosion and suspended sediment loads across New Zealand?

Understanding the potential impact of climate change on erosion and suspended sediment loads is critical for developing catchment management plans that achieve land productivity and water quality objectives under future climate. The response of erosion and sediment loads to climate change can be expected to vary across catchments and regions due to variations in the magnitude and direction of climate change and the occurrence of different erosion processes. Policymakers and catchment managers therefore need to plan for variation in the impact of climate change on erosion and sediment loads. However, there has been no published quantitative assessment of national-scale changes in erosion or suspended sediment loads anticipated under future climate for New Zealand.

To address this gap, Manaaki Whenua – Landcare Research, in collaboration with AgResearch and NIWA, produced New Zealand’s first national assessment of the potential impact of climate change on erosion and suspended sediment loads. Future erosion and sediment loads were estimated for mid-century (2040) and late century (2090) based on projected changes in hydro-climatic drivers. Changes in sediment loads were modelled under four potential CO₂ concentration pathways. The pathways, referred to as representative concentration pathways, or RCPs, represent radiative forcing based on greenhouse gas trajectories under a mitigation scenario (RCP 2.6), two stabilisation pathways (RCP 4.5 and RCP 6.0), and a continual increase scenario (RCP 8.5).

The results of this modelling highlight clear regional differences in the effects of projected climate change on erosion and sediment loads across New Zealand, and emphasise the impact of higher CO₂ concentrations on erosion rates. In the North Island, soft-rock hill country shows the largest erosion response to climate change (Figure 1), which increases under higher CO₂ concentrations. In the South Island, the most noticeable differences are seen in the eastern regions (Figure 1), where drier conditions under a low CO₂ concentration scenario produce decreasing erosion, whereas wetter conditions under higher CO₂ concentration increase erosion.

Up to 28% of the North Island and 8% of the South Island are estimated to experience a two-fold or greater increase in average annual sediment yield (sediment supplied to the channel network) by late century (Figure 1). Regional sediment loads delivered to coastal receiving environments are estimated to increase between 1% and 233% (Table 1). These responses are driven by different directions and magnitudes of change in erosion and the cumulative impact on suspended sediment loads.

This work has been published, open access, in the journal Geomorphology, and can be accessed here for more information.

Figure 1. Percentage change in average annual sediment yield relative to the baseline (1995) under low (left) and high (right) CO2 concentration scenarios by late century (2090). Purple represents an increase in sediment yield, and green represents a decrease. (Source: Neverman et al. 2023).

For regional applications, erosion processes are best represented by the SedNetNZ model using region-specific data, such as spatial information on erosion mitigation works by councils. SedNetNZ has been used to assess the impacts of climate change on erosion and sediment loads in the Wairoa catchment (Hawke’s Bay) and the Manawatū-Whanganui region (Smith et al. 2022; Vale et al. 2022), with results helping to inform implementation of the NPS-FM 2020 (Vale et al. 2022).

Table 1. Baseline total suspended sediment loads supplied to the coast of each region, and the proportional change in total suspended sediment loads under future climate scenarios. The RCPs represent a CO2 mitigation scenario (RCP 2.6), two stabilisation pathways (RCP 4.5 and RCP 6.0), and a continual increase scenario (RCP 8.5). (Source: Neverman et al. 2023)

		2040 change in load (%)				2090 change in load (%)
Region	Baseline load (Mt/yr)	RCP 2.6	RCP 4.5	RCP 6.0	RCP 8.5	RCP 2.6	RCP 4.5	RCP 6.0	RCP 8.5
Auckland	0.47	18	24	25	36	14	30	45	75
Bay of Plenty	6.2	16	21	21	29	13	25	37	56
Canterbury	9.4	6	7	11	10	2	8	16	26
Gisborne	28	23	31	30	40	20	36	53	81
Hawke's Bay	10	24	29	30	41	19	38	54	84
Manawatū-Whanganui	12	33	45	44	59	29	55	83	127
Marlborough	1.2	7	6	7	4	1	5	10	18
Nelson	0.036	9	13	20	6	2	11	23	39
Northland	14	59	86	88	113	49	105	153	233
Otago	1.6	13	16	16	21	8	21	37	55
Southland	4.1	12	12	17	15	9	14	31	47
Taranaki	2.8	21	32	30	42	20	39	60	95
Tasman	2.4	13	15	21	21	10	20	31	46
Waikato	5.9	28	42	40	57	25	53	75	121
Wellington	5.2	36	47	47	61	33	58	84	127
West Coast	66	12	15	22	23	7	18	31	52

References

Neverman AJ, Donovan M, Smith HG, Ausseil A-G, Zammit C 2023. Climate change impacts on erosion and suspended sediment loads in New Zealand. Geomorphology 427: 108607. https://doi.org/10.1016/j.geomorph.2023.108607

Smith HG, Vale S, Neverman AJ, Robson-Williams M, Harris L 2022. Climate change impacts on sediment loads in the Wairoa catchment, Hawke’s Bay. Manaaki Whenua ‒ Landcare Research contract report LC4121 prepared for Our Land and Water National Science Challenge.

Vale S, Smith HG, Neverman AJ, Herzig A 2022. Application of SedNetNZ with SLUI erosion mitigation and climate change scenarios in the Horizons region to support NPS-FM 2020 implementation. Manaaki Whenua ‒ Landcare Research contract report LC5033 prepared for Horizons Regional Council.