Wednesday, December 6, 2023

Mapping surface erosion on a national scale

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The first national-scale map of surface erosion in New Zealand has been developed, which is also the first in the world to include the impact of livestock grazing.
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AgResearch landscape scientist Mitchell Donovan says the map will help gauge where land is susceptible to soil loss.

The first national-scale map of surface erosion in New Zealand has been developed, which is also the first in the world to include the impact of livestock grazing. Colin Williscroft reports.

Topsoil erosion threatens food production, land stability, water quality and functioning ecosystems around the world, with around 192 million tonnes of soil lost to erosion in New Zealand alone.

Globally, soil erosion models play a critical role in understanding where and how to conserve soil but until recently, the impact of livestock grazing has not been accounted for.

For NZ, where 40% of land is in pasture, that means there is an incomplete picture of surface erosion susceptibility.

A new high-resolution map, developed through Our Land and Water’s Land Use Sustainability programme, incorporates grazing to provide a more accurate picture of land’s susceptibility to soil loss from surface erosion at farm, catchment and national scales.

To provide a more detailed picture of surface erosion, researchers, led by AgResearch landscape scientist Mitchell Donovan, produced the national map by developing a novel grazing model, which they plugged into a globally recognised model for calculating susceptibility to surface erosion, the RUSLE (Revised Universal Soil Loss Equation) model.

Donovan says initial results show that integrating the grazing model into the RUSLE model vastly improves our ability to understand where and how susceptible land is to surface erosion, soil degradation and estimates of soil loss from pastoral land, with modelled surface erosion aligning well with observed measurements. 

In doing so, Donovan suggests we can proactively mitigate soil losses by minimising the overlap between susceptible lands and moderate- to high-intensity practices.

The model used calculates soil erosion susceptibility using seasonal rainfall, slope length and steepness, soil erodibility and groundcover and management factors.

Donovan says when combined with the sub-model, it can incorporate the impacts of grazing and stock treading on soil’s physical properties and groundcover and therefore on surface erosion.

He says the sub-model uses observed relationships between grazing/treading intensity (stock hoof pressure, grazing density, duration and history) and damage to soil’s physical properties (permeability and structure).

It further accounts for susceptibility due to clay content and seasonal soil moisture, which alter soil’s susceptibility to pugging and compaction and incorporates natural variability in seasonal groundcover and the erosion potential of monthly rainfall.

Donovan says the maps can provide two primary tangible benefits to land health and farm economic standing if the results are used to inform on-farm decisions.

“In the long run, I imagine these benefits will stem from less degradation to and loss of topsoil, which will increase the storage and retention of nutrients (N, P, C).

“Increased retention of soil and nutrients will mean a long-term increase in productivity with fewer fertilisers or inputs,” Donovan said.

The first is that it provides a spatial representation of where farmland is most and/or least susceptible to surface erosion based on the inherent/natural properties, which are unmanageable. These are soil type, slope and rainfall intensity.

He says some farmers will have an idea of this already, but having high-resolution evidence supporting farmer’s knowledge can improve that understanding and have the potential to inform farm environmental planning and soil conservation goals.

“I see the maps as a second approach or method for figuring out where the land is susceptible to soil loss, or as a first glance for farmers who are working a new set of lands,” he said.

“The second benefit that I see is through collaboration between myself and farmers.

“Working together, we can use the farm management scenarios to estimate the change in soil losses when each paddock is grazed at a specific density by a particular stock type. This goes beyond understanding the land and towards an understanding that includes the interaction between animal grazing intensity, physics and the land’s susceptibility.”

He says because the maps work up to 15 m2 resolution, they can distinguish susceptible areas within paddocks that could benefit from being fenced off, rather than just providing a general estimate for the entire paddock.

The maps also have the potential to inform tools like freshwater farm plans.

Because the farm maps can be nested within the broader catchment erosion model, Donovan says this approach would be the best available option for providing a catchment context of each farm within a catchment, as well as risk impact and assessment, which are two key aspects of freshwater farm plans.

A third aspect – actions to reduce risk – would not be a direct output of the maps, but could be possible if a farmer used knowledge of the land’s susceptibility in order to avoid grazing specific paddocks or areas that are particularly susceptible.

“We often come at the framework of ‘patching up’ our damages and overlook the idea that proactive planned farm management is an action in itself. For example, farmers who use a soil susceptibility map to plan grazing strategies is a proactive action, though it’s not often listed in remediation strategies or ‘actions’ in many environmental guidance documents/websites,” he said.

Donovan says it will probably be a few months before the national maps are released publicly because it will take time to find the appropriate online platform and storage. 

He says the national maps are not likely to be immediately useful to regional councils, catchment groups, or individual farmers who want to understand their farm, but in those cases, he recommends getting in touch to explore the appropriate options to develop a regional, catchment or individual farm map that accounts for specific grazing densities at paddock scales.

“I’m working hard to get these in the hands of those that can use them, which will be most effective if I am supported by those who are working with the land,” he said.

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