Estimating the Water Requirements for Plants of Floodplain Wetlands

section, using storage volume and inundation area, including how touse topographic data to determine the spatial variations in depth.Techniques available for determining inundation areas are presented,followed by a description of the hydrological techniques fordetermining the different components of a water balance.Using storage volume and inundation areaAn average water depth (in metres) for a floodplain wetland can becalculated by dividing storage volume (in cubic metres) by inundatedarea (in square metres). For this, inundation area and storage volume fora large flood are needed.This average water depth ignores the proportion of the storage volumeheld in the soil. An improved estimate can be obtained by determiningthe soil storage volume. The spatial variability in water depth can beestimated using descriptions of the surface topography of thefloodplain.Soil storage volumeStorage volume includes both surface water and water held in the soil,also known as the soil moisture store, and this can account for asignificant fraction of the total storage volume. Subtracting the soilmoisture store from the total storage volume will leave surface water,giving a more accurate estimate of average water depth.The soil moisture store, as a volume, is the depth of water thatinfiltrates the soil profile, integrated across the inundated area.Infiltration is mainly a function of soil hydraulic conductivity, the rateat which water moves through the soil; soil cracking and flood durationalso contribute. For example, in soils with low hydraulic conductivity,the depth to which flood water infiltrates is defined by the length oftime the surface is inundated. Surface drainage and evapotranspirationmay deplete the storage volume before the soil is fully saturated.Estimating the soil moisture store for a floodplain wetland is not simple,partly because the process of infiltration varies greatly between soiltypes. For soils other than cracking clays, infiltration occurs by thedownward advance of water through the soil profile. For these soils,the moisture store, when saturated, can be estimated as a function ofsoil depth and soil porosity (the proportion of the soil volumerepresented by pore spaces). When the soil moisture store is not full,for example, if infiltration has been limited by flood duration,infiltration rates and flooding duration need to be estimated. Hydraulicconductivities have been established for many soils, but the applicationof these values in field situations is usually constrained by a lack of datadescribing the spatial variability in soil types. Furthermore, soilcracking and macro-pores can increase initial infiltration rates to ordersof magnitude higher than laboratory derived values.In the Murray–Darling Basin, the lowland river floodplains on theMurray and Darling Riverine Plains are characterised by cracking clays.These soils behave very differently from lighter soils, and the infiltrationprocess is very different. As these soils dry, deep cracks form in the soil,sometimes forming discrete columns of soil (Figure 19). Infiltrationoccurs as a rapid filling of these cracks, and water subsequently moveslaterally through the soil away from the cracks. As the soils wet up, they66 Estimating the Water Requirements for Plants of Floodplain Wetlands