National mapping of winter forage cropping and intensive grazing on hill country land.
The Ministry for the Environment (MfE), as part of the Essential Freshwater policy development process, engaged Manaaki Whenua – Landcare Research (MWLR) to assess the spatial extent of hill country forage cropping and intensive grazing, as well as the severity and timing of de-vegetation that results from these practices.
The work, as part of the Advanced Remote Sensing Aotearoa (ARSA) programme, used recently developed methods for automated cloud clearing to temporally analyse Sentinel 2 imagery for the period autumn to spring 2018, and to identify all paddocks with a slope of 7 degrees or greater where winter forage paddocks became bare due to cattle grazing.
The analysis found that over 9,000 paddocks on land over 7 degrees were identified to have gone bare during the 2018 winter and assessed to be at risk of high soil erosion. These paddocks collectively covered an area of 42,082 ha, which is 0.76% of the New Zealand hill country agricultural land with a slope greater than 7 degrees. The regions with the most winter forage cropping, as a percentage of the hill country agricultural land, were Southland (2.21%), Otago (1.61%), and Canterbury (1.20%). The data have also enabled soil erosion scientists to quantify for the first time the national total of soil lost from winter forage cropping on hilly land (>7 degrees) at 689,000 tonnes. Compared with total sediment loads in rivers this is small at ~1%, but it has a high impact on water quality in small streams near winter forage paddocks.
This work has built on MWLR’s existing expertise and processing capability with time-series satellite imagery, with this national analysis of high-risk agricultural practices in hill country requiring significant processing advances, particularly code and workflow development on the New Zealand eScience Infrastructure (NeSI) high-performance computing facility. As a result of these advances, a national map of winter bare ground could be produced each year as a standard product for the Land Cover Database.
Figure 1. A typical example of the temporal pattern that indicates a winter forage crop.
The large paddock in the upper centre of this subscene is fully vegetated (green) in early June and then, from mid-June, the vegetation cover is gradually reduced by strip-grazing until by mid-September the ground is completely bare (pink/brown). To the left of the winter forage paddock (on the other side of the river) is a paddock that is bare in early June and then greens up, remaining green right through to September. This could be an example of a summer crop that has been replaced by an intercrop pasture. Source: 2018 imagery of an area in northern Hawke’s Bay.
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