Mapping the maximum inundation extent of lowland intermittent riverine wetland depressions using LiDAR

Accurate high-resolution maps of maximum wetland inundation extents are valuable inventorial resources, but mapping such boundaries can be difficult, time consuming and involve a level of subjectivity from the surveyor. A novel, objective and efficient method is presented for delineating the maximum inundation extents of lowland intermittent riverine wetland depressions using a high resolution LiDAR-derived digital terrain model (DTM). The method was specifically developed for wetland depressions associated with the lower reaches of the Murrumbidgee River in the southern Murray Darling Basin, Australia, but is suitable for any similar intermittent wetland depressions where the natural inundation extent is otherwise unclear. The method is based on the premise that immediately after rising water reaches a threshold level, much larger volumes of water are required to inundate not just the discrete wetland depression but also its surrounding local area; that threshold level can be considered the wetland fill level. Using a series of water levels and a corresponding series of the volume between each water level and a DTM of a wetland area, the maximum rate of increasing acceleration in volume with respect to water level, i.e. the wetland fill level, can be identified. The fill levels of eight wetland areas, ranging in size from 44 to 384 ha, were produced by this method and subsequently used to map their maximum inundation extents. Corresponding maps of the inundation frequency gradient were independently determined from a time series of Landsat derived inundation maps. The level of separation in inundation frequency of map cells inside the maximum inundation extents compared to those outside, determined by the Kolmogorov Smirnov statistic, validated the technique, while identifying that a minimum level of variation in the elevation of the wetland area may be necessary for it to be successfully applied. The technique has potential to be applied more widely as an objective and relatively inexpensive procedure to identify the potential maximum inundation extent of intermittent floodplain wetlands when a good quality high resolution DTM, acquired during a dry phase, is available.