Characterisation of the hydrology of an estuarine wetland

The intertidal zone of estuarine wetlands is characterised by a transition from a saline marine environment to a freshwater environment with increasin g distance from tidal streams. An experimental site has been established in an area of mangrove and salt marsh wetland in the Hunter River estuary, Au stralia, to characterise and provide data for a model of intertidal zone hy drology. The experimental site is designed to monitor water fluxes at a sma ll scale (36 m). A weather station and groundwater monitoring wells have be en installed and hydraulic head and tidal levels are monitored over a 10-we ek period along a short one-dimensional transect covering the transition be tween the tidal and freshwater systems. Soil properties have been determine d in the laboratory and the field. A two-dimensional finite element model o f the site was developed using SEEP/W to analyse saturated and unsaturated pore water movement. Modification of the water retention function to model crab hole macropores was found necessary to reproduce the observed aquifer response. Groundwater response to tidal fluctuations was observed to be alm ost uniform beyond the intertidal zone, due to the presence of highly perme able subsurface sediments below the less permeable surface sediments. Over the 36 m transect, tidal forcing was found to generate incoming fluxes in t he order of 0.22 m(3)/day per metre width of creek bank during dry periods, partially balanced by evaporative fluxes of about 0.13 m(3)/day per metre width. During heavy rainfall periods, rainfall fluxes were about 0.61 m(3)/ day per metre width, dominating the water balance. Evapotranspiration rates were greater for the salt marsh dominated intertidal zone than the non-tid al zone. Hypersalinity and salt encrustation observed show that evapotransp iration fluxes are very important during non-rainfall periods and are belie ved to significantly influence salt concentration both in the surface soil matrix and the underlying aquifer. (C) 1998 Elsevier Science B.V. Ail right s reserved.