Estimating the Water Requirements for Plants of Floodplain Wetlands

Section 6: UsingWater Regime DataThe hydrologic variable of most relevance to plants is waterdepth. However, water depth data are rarely available for largewetlands. Depth can be obtained directly, or indirectly by waterbalance calculations from existing data. This section focuses onthe indirect ways for obtaining depth, but recognises that waterregime can be defined in other ways. Options for estimating thedifferent components of a floodplain wetland water balance aredescribed, with emphasis on spatial and temporal variationsacross the floodplain wetland. The application of water regimedata for hydrological modelling is described, with currentAustralian examples, although these are rarely based on depth.Estimating water depthWater depth is the hydrological variable that most clearly summarisesplant water regime, if expressed as a spatially and temporally variablemeasurement (Section 2: Focusing on depth). Water depth can bemeasured directly, but good sets of direct measurements are rarelyavailable for characterising the spatial variability of depth in largefloodplain wetlands, and are hard to obtain. This is in contrast to smallindividual wetlands where the spatial variability of depth is low, and sochanges in depth can be recorded at a single location. On largefloodplain wetlands, water depth can be estimated indirectly usingwater balance calculations (Section 1). Changes in water inputs andoutputs can be used to estimate changes in the volume of water in thefloodplain wetland, and volume can be expressed as depth usinginundation area, as this is a function of surface topography.Although depth is an effective way of summarising plant water regime,it has not been central to Australian studies of floodplain wetlands,which have tended to focus on flood frequency. The aim of this sectionis to present the ideal hydrological variable, ie. water depth, and at thesame time present other aspects of plant water regime that have beenthe backbone of floodplain research.Direct estimates. Changes in depth can be measured directly usingwater level recorders of various types. In large floodplain wetlands, thespatial variations in depth will vary through a flood and will be differentfrom one flood to the next. Many water level recorders are needed tocapture this spatial variability of depth. Knowing how many recordersare needed and where to site them is process of trial and error.Maximum flood height recorders, as used in the Macquarie Marshes,record maximum depth at a location, but must be modified if they are torecord time.Indirect estimates. Indirect estimates can be made usingmeasurements of the inundation area, the changes in storage volume,and descriptions of the floodplain topography. These are outlined in thisSection 6: Using Water Regime Data 65