Cell-based IQQM wetland modelling for Yanga National Park, a forested lowland floodplain in southern New South Wales

Developing wetland hydrological models was an essential component of the NSW Rivers Environmental Restoration Program (RERP). The RERP aimed to arrest the decline of nationally important wetlands in the Murray-Darling Basin, and Decision Support Systems (DSS) have been developed for several wetland complexes to evaluate and compare the ecological impact of different river management policies, strategies and programmes. Each DSS is underpinned by a long-term hydrological model that provides inundation extents, volumes and durations for each wetland feature. This paper details development of the Yanga National Park hydrological model that provides hydrology inputs to the Lowbidgee DSS. Model development followed a novel approach, combining the advantages of hydrodynamic and hydrological modelling. Hydrodynamic models such as MIKE21 are routinely used to simulate flood inundation for a range of environments and spatial scales. However, the hydrodynamic model computation time required to simulate an area as large as Yanga National Park over many years and for repeated scenarios makes it impractical to use such a model to provide direct inputs into a DSS. By comparison hydrological models such as IQQM generally take minutes to simulate long periods and allow wetlands to be integrated into broader valley-based water resource model systems. Such hydrological models are normally developed and calibrated directly from observed hydrometric data. However, in the case of Yanga National Park very little hydrometric data was available, and a different approach was required. As an interim step in hydrological model development, a detailed hydrodynamic model was developed to provide insight into the floodplain hydraulics of Yanga National Park. Hydrodynamic model results were used to schematise and construct the hydrological model. Hydrological model schematisation was established from the location of key wetland features identified on the ground, but also from the flowpaths and hydraulic controls identified by the hydrodynamic model simulations. This resulted in a total of 34 "cells" in the Yanga National Park hydrological model. For each of these cells simplified cell volume / inter-cell flow relationships were derived from the hydrodynamic model results. The hydrological model was then validated against inundation mapping based on satellite images between 1988 and 2006. The validated IQQM model provides a range of inputs to the DSS, including inundation surface area, volume and water depth. It has been used to estimate actual historical wetland inundation patterns between 1975 and 2010, and to model wetland inundation patterns and durations for different Murrumbidgee River management scenarios for periods greater than 100 years. Further work is required to address uncertainty in modelling inundation extents due to infiltration and vegetation effects, and to validate the model further against recent flood inundation between December 2010 and February 2011.