LiDAR reveals drainage risks to wetlands have been under-estimated

Drainage is a recognised cause of wetland loss worldwide, and New Zealand is no exception. In the last 200 years drainage has reduced the natural extent of wetlands in New Zealand by c. 90%. Avoiding further loss is a national priority. Despite recent reform to restrict new drains within 100 m of existing wetlands in New Zealand, little is known about the extent and effect of existing drains in and near wetlands. Using a national layer of wetland extent (Freshwater Ecosystems of New Zealand) we calculated the area of wetlands currently within a zone of potential drain effects in the North and South Islands, by buffering an existing national drain layer by 100 m and 50 m and stratifying these results by peat/non-peat, and wetland type. We show that 7476 ha (c. 3%) of New Zealand wetlands identified in the national FENZ dataset are potentially affected by drainage when intersected with the national drains layer buffered by 100 m. Of these wetlands near drains, 4387 ha were wetlands with high organic matter (peat) that are vulnerable to drainage-induced subsidence and release of greenhouse gases. We then conducted a case study within the Waituna catchment (Southland, New Zealand) to assess if the national drain data is under-estimating the extent of wetland drainage by comparing the area affected by drainage detected using the national drains data with an algorithm to identify drains from LiDAR. Our catchment case study revealed that our LiDAR method more than tripled the area of wetland near drains suggesting that the existing national drains layer is underestimating wetland drainage extent. We highlight that further work should be undertaken to develop an accurate stocktake of drains near wetlands, given the increasing availability of LiDAR and the ongoing efforts to improve wetland mapping by territorial authorities.