Estimating the Water Requirements for Plants of Floodplain Wetlands

Seven points over the flow range is barely enough to define the shapeof an incremental inundation map, and ten points is suggested as atarget. Nine points for the Chowilla floodplain (Figure A1 – 1) wereenough to define the linear part of the inundation-area curve (Figure20).The type of floodplain, whether confined or unconfined, can influencehow inundated area is presented, and what hydrologic variable to use.On confined floodplains, such as Chowilla, where the total floodplainarea can be defined, then inundated area can be expressed as apercentage of the floodplain, which can be a more effectivecommunication tool than absolute area. The data shown (Figure A1 – 1)range from 5.2% at 33,110 ML/ day to 77.2% at 101,100 ML/day.Figure A1 – 1. Inundation-area curveInundation-area curve for a confined floodplain wetland, the Chowillafloodplain, South Australia. The nine images cover a range of flows fromwithin a relatively narrow time frame, from 1984 to 1990, for a risinglimb (from Noyce and Nicolson 1993).15000Inundated area (ha)10000500000 25 50 75 100 125River Murray flow (ML day) ¥ 10 –3Inundation area can be related to hydrologic variables using linear andnon-linear regression analysis. If the regression is statistically significant,it can be used predictively, but should not be applied beyond the rangeof the data used in the defining the linear part of this curve. Numericalanalysis is relatively new and has been tried in relation to the GreatCumbung Swamp by Sims (1996) and by Brady et al. (1998). Multipleregression is one way to accommodate antecedent conditions, and wastried on the Lachlan River. The mixed success of this first attempt (TableA1 – 3) does not invalidate this approach, which could have great valueif predictive ability could be improved. In this case, the combinedrainfall/evaporation factor contributed most to the discrepancy.Standardisation Standardisation is as important as other factors inlimiting interpretation, and in achieving cost-efficiency. Nearly allinvestigation on floodplain wetlands in the Murray–Darling Basin havefound that, for a number of reasons, the number of images used inanalysis was considerably less than the number purchased.This is a clear message that image selection needs to consider twoantecedent conditions: soil moisture, whether wet or dry (time sincelast flood, time since major rainfall); and timing of image relevant toflood hydrograph (rising limb, flood peak, falling limb). These can be104 Estimating the Water Requirements for Plants of Floodplain Wetlands