Estimating the Water Requirements for Plants of Floodplain Wetlands

used as exclusions; or can be quantified, and the values incorporated asa co-variate in a numerical analysis.The falling limb may be useful in defining areas of greater flood durationbut their value for defining flood extent is likely to be compromised: bythe tail of a flood, vegetation will have had time to respond, obscuringthe inundation area. The problem of vegetation overhang or growingthrough water means the rising limb is preferred for inundationmapping, by satellite imagery.Table A1 – 3. Measured and predicted inundated areaComparison of measured and predicted inundation area for eight dates for the Great Cumbung Swamp. Observedinundated area was based on satellite imagery. Predictions were made using an empirically-derived multiple linearregression based on previous inundation area and changes to wetland storage volume, over the same time-frame.Changes in storage volume were based on measured in-flows from three gauging stations, adjusted for direct inputsand losses with a rainfall and evaporation factor (from Brady et al. 1998).M-area (ha) 3,560 6,320 4,240 13,100 4,400 2,800 3,790 4,160P-area (ha) 3,950 4,030 3,810 12,990 4,540 3,710 4,970 4,390% difference 10.8 –36.3 –10.1 –0.9 3.2 32.2 31.2 5.4In addition, if it is known that changes have occurred on the floodplainwhich might affect the passage or flood waters, such as vegetationclearance, erosion of channels, concrete structures, channel blockagesor clearing, then it would be advisable to select images representingjust only the most recent situation.Commissioning new imagery. Current practice in terms ofestablishing a volume or flood inundation area relationship is to usehistorical data. The use of historical data can be restrictive, because ofthe difficulty of standardisations, as described above, and because acomplete range may not be available. Commissioning a special set ofimages, using hyperspectral imagery or radar, has not beenimplemented yet in Australia. With falling costs, and greater need forprecision, this is likely to become a real option in the future.An in-house cost–benefit analysis is suggested before making a decisionwhether to proceed with historical satellite or specially-commissionedimagery. This should consider not just technical issues such as dataquality, but additional benefits.InterpretationIn inundation mapping, the initial step is to interpret the images anddetermine areas that are water, and which are dryland. This can be doneby visual interpretation, or by computerised analysis. A visualrepresentation of this for a river–billabong–floodplain is shown inFigure A1 – 2.Visual interpretation. Visual interpretation is open to criticisms ofsubjectivity, observer bias, and problems of inter-observer consistency:these can be important if a time series is being prepared, or if maps arebeing overlaid. Visual interpretation requires hard copy, either of airphotos or satellite imagery, or a single-band (grey scale) satellite imageand the user visually discriminates and delineates inundated areas. Itrelies on the user to visually discriminate and delineate inundated areas.Despite the obvious subjectivity of this, some skilled practitioners find106 Estimating the Water Requirements for Plants of Floodplain Wetlands