Estimating the Water Requirements for Plants of Floodplain Wetlands

egin to swell, closing off the cracks. Once the cracks are filled, verylittle water enters the soil profile. The volume of water that infiltrates isdetermined by the volume of the cracks.Figure 19. Cracks after winter rainsGermination in shallow cracks on cracking soil on the Hay Plains, July1990. On some floodplain wetlands, notably riverine lakes in semiaridareas, deep holes and cracks are important as habitat for native fauna(Briggs and Jenkis 1997).Estimating crack volumeThe volume represented by the cracks and holes depends on soil typeand on how long the soil has been drying out. For most cracking clays,the soil moisture content at saturation is about 40% by volume, but whenair-dry, it is about 9% by volume. This means approximately 31% (and asmuch as 35%) of the total soil volume may be available to absorb water.If the soil is dry, and assuming a typical cracking depth of around 1 m,then this is equivalent to an infiltration depth of 300 mm, whichcorresponds to a volume of 3 ML/ha. In very dry conditions, cracks mayextend to 2 m, which is equivalent to an infiltration depth of 600 mm or6 ML/ha.If the soil is not fully dry, then crack volume can be estimated as afunction of the drying period. For this, crack volume, expressed as apercentage of the fully dry volume, is a linear function of the square rootof the drying period expressed as a proportion of the time required todry fully (Figure 20). For example, for a floodplain where completedrying takes 400 days and leads to a crack volume equivalent to 600 mminfiltration depth, then the crack volume after 200 days would beequivalent to around 425 mm.Spatial variationsUsing just the surface water component of the wetland storage volumeallows calculation of only a spatially averaged depth value. Forfloodplain wetlands that are topographically complex, water depth willvary considerably from place to place across the floodplain.Section 6: Using Water Regime Data 67