Estimating the Water Requirements for Plants of Floodplain Wetlands

water. This requires representing the ground surface slopes (floodplaintopography) and the surface roughnesses. Surface roughness canchange dramatically with vegetation type and condition, or withchanging soil surface condition.Topographic and roughness data can give a two-dimensionalrepresentation of the movement of water across the floodplain, basedon complicated hydraulic calculations. The data needs and complexityof the calculations depend on the level of spatial representation that isused. The level of detail should be determined by the level of spatialresolution required to provide information to enable the requiredpredictions of vegetation response. Hydraulic modelling of floodplains isa major undertaking, usually based on complex computer modellingusing large data sets. Accurate calibration or even validation of suchmodels is often constrained by the lack of appropriate data to describefloodplain water movement.Australian examplesFive Australian examples of wetland or floodplain hydrology modellingare described below, all from the Murray–Darling Basin. The first is asimple water balance approach. The second is an empirical model basedon relating river levels to inundation imagery. The third and fourthexamples are of hydraulic floodplain modelling: one of the lowerMacintyre River floodplain by Connell Wagner (Qld) Pty Ltd, the otherof Condamine–Balonne River floodplain by the Snowy MountainsEngineering Corporation (SMEC) (Marr 1999). The last exampledescribes the range of different wetland and floodplain modellingapproaches used in the IQQM river hydrology model developed by theNSW Department Land and Water Conservation.EFDSS and the Murray–Darling BasinOne of the simplest approaches to representing water movementthrough a wetland complex was taken in the development of afloodplain water balance model by Whigham and Young (1999), for usein an environmental flows decision support system (EFDSS)(Young et al. 1999).The EFDSS imports daily river flow data from an external river hydrologymodel, and uses this to run a simple water balance model for linkedstorages or elements (Whigham and Young 1999). The water balancemodel is conceptualised as a series of connected pipes and storages.Pipes have a minimum level at which flow begins, and a maximumcapacity. Storages have a constant area, a maximum capacity and anexponential decay on storage volumes. These parameters can be used to‘calibrate’ the behaviour of the floodplain model to the observed patternof storage volumes or water levels. It is not intended that the model beused to represent groundwater inflows or direct rainfall inputs, althoughthese could be represented using pipes. Pipes are used to representsurface inflows: while outflows are represented using pipes and/or thedecay function on storage volumes. Typically, surface outflows arerepresented using pipes, and the decay function is used to represent thecumulative effects of evapotranspiration and groundwater outflows.The two main limitations of the model in its present form are thatstorages have a constant area (rather than a volume–area relationship)Section 6: Using Water Regime Data 81