Impact of DEM Resolution on Topographic Indices and Hydrological Modelling Results J. Vaze 1, 2 and J. Teng 1, 2 1 Department of Water and Energy, NSW, Australia 2 eWater Cooperative Research Centre, Australia Email: Jai.Vaze@dnr.nsw.gov.au Keywords: DEM, LiDAR, spatial indices, spatial modelling, hydrological modelling EXTENDED ABSTRACT Topography is an important land-surface characteristic that affects most aspects of the water balance in a catchment, including the generation of surface and sub-surface runoff; the flow paths followed by water as it moves down and through hillslopes and the rate of water movement. All of the spatially explicit fully distributed hydraulic and hydrological models use topography (represented by the DEM of the area modelled) to derive bathymetry. DEM is also used to derive some other key information critical in fully distributed hydraulic and hydrological models. In most of the distributed modelling implementations, relatively large DEM grid cells make up the model domain in order to reduce the computation time. This is to allow quick model calibrations and model sensitivity analysis but also, in operational mode, it allows model simulations in real time. A major disadvantage of the use of low resolution input data is the loss of important small-scale features that can seriously affect the modelling results. If the input DEM is at a higher grid resolution, during the transformation or re-sampling of the original DEM data to a lower model resolution, important topographic details are lost mainly as a result of averaging. If the input DEM is already at a low resolution, it does not represent the actual on-ground topographic features which might significantly affect the accuracy and reliability of the results from the modelling exercise. There are numerous studies reported in literature which compare spatial indices derived from different coarse resolution DEM’s (eg. 100m and 1000m grid cell resolution) and researchers have also investigated the effect of using coarse resolution DEM on the results from hydrological and hydraulic modelling. Most or almost all of the reported studies focus on coarse resolution DEM (100m or above). With higher resolution DEM’s such as LiDAR (Light Detection and Ranging) becoming more readily available and also with the advancements in computing facilities which can handle these large datasets, there is a need to quantify the impact of using these different resolution DEM’s (eg. 1m against 10m or 25m) on the modelling results and the loss of accuracy and reliability of the results as we move from high resolution to coarser resolution. This paper presents the results from an investigation where we re-sampled the 1m LiDAR DEM in steps (2m, 5m, 10m, and 25m) and compared the different spatial indices derived from these different resolution DEMs against the ones derived from the base data (1m LiDAR DEM). By re-sampling to coarser grid cell size, averaging across increasingly larger domains is realised and has resulted in an increased loss of detailed topographic properties that affect the spatial indices derived from the DEM. We also compared these outputs against the widely available and most commonly used 25m DEM across NSW, which is derived from contour maps. The results indicate that the quality of DEM-derived hydrological features is sensitive to both DEM accuracy and resolution. The contour derived 25m DEM across NSW has substantial differences when compared to the 1m LiDAR DEM and also with the 25m coarsened LiDAR DEM. The results also indicate that the loss of details by re-sampling the higher resolution DEM to lower resolution are much less compared to the details captured in the widely available low resolution DEM derived from contour maps. As such, where available, the higher resolution DEM should be used instead of the coarse resolution one. 706