Modelling floodplain inundation for environmental flows: Gwydir wetlands, Australia

Abstract

Floodplain wetlands rely on catchment flows to maintain the flooding cycles critical to their ecological integrity. The development of water resources has significantly altered the flow patterns in many river systems. Recent research into water requirements for wetland systems shows that duration, frequency, depth, timing and extent of flooding are the most important influences on ecological communities. Modelling these systems is hampered by a lack of data and inappropriate model structures. Remote sensing using AVHRR satellite data were shown to be an effective option for assessing flood dynamics. Vegetation communities and flood patterns of the Gwydir floodplain were analysed to provide a model structure based on channel, flowpath and core wetland components. The temporal and spatial behaviour of each component was described using water balance principles. The model was calibrated and tested under a range of climatic conditions and the results demonstrate that the model can simulate ecologically significant flood dynamics of the floodplain wetland. This study demonstrates that a conceptually based, semi-distributed water balance approach can provide the basis for an effective decision support system for water management.

Introduction

Floodplain wetlands rely on inflows from catchments to maintain the flooding and drying cycles critical to their ecological integrity. In inland Australia, these wetland systems can be permanent, semi-permanent or ephemeral and may link to downstream river systems or become terminal inland deltas. Many of these wetlands have national and international significance to a range of vegetation and bird communities (Allan and Lovett, 1997, Environment Australia, 2001). The development of water resources has significantly altered the flow patterns in many river systems (Chauhan and Gopal, 2005, Kingsford, 2000, Sanchez-Carrillo et al., 2004). Management is essential because of the impact of river regulatory structures that control most of the flow. Community and government pressure has seen a change in water management practices towards more equitable and sustainable sharing of water resources between the environment and water users. In order for this change to be effective the water requirements of the environment need to be understood. This paper proposes a parsimonious semi-distributed water balance model of ecologically significant flood dynamics. The key criteria required as outputs of the model over identified ecological communities include the depth and duration of flooding, the area inundated by a flood event, and the soil moisture balance for floodplains and wetlands to assess drying and parching.

Current approaches to water resource planning include the use of decision support systems, with an emphasis on testing water management scenarios (Letcher et al., 2004, Podger and Hameed, 2000, Simons et al., 1996, Young et al., 2003). These decision support systems can provide an estimate of daily flow into a floodplain or wetland system from a range of scenarios. To subsequently test these scenarios against a range of ecological benefit or response over large scale floodplain wetland systems is more difficult. Recent research into water requirements for floodplain vegetation (Brock and Casanova, 1997, Casanova and Brock, 2000, Mawhinney, 2003, Roberts and Marston, 2000, Roberts et al., 2000) has shown that the duration and frequency of flooding, as well as pre-existing community structure, are the most important influences on plant community composition. Inflow volumes, water depth, timing of flows, the extent of flooding and the flooding cycles in other wetlands have been shown to influence the number of colonial waterbirds nesting (Kingsford and Auld, 2005, Kingsford and Johnson, 1998). A model that simulates the impact of inflows on the frequency and duration of flooding as well as the depth and area inundated is required.

Complex hydraulic and one-, two- and three-dimensional models, based on the differential equations of hydrodynamics, are routinely used to model flood flows over floodplains for floodplain development and asset protection (Department of Infrastructure Planning and Natural Resources, 2004, Thompson et al., 2004, Whigham and Young, 2001). These models require large amounts of data and are generally applied to assess floodplain development impacts at a design flood level. Another common approach to modelling floodplain water requirements is the use of water balance equations (Bennett and McCosker, 1994, Ferrati and Canziani, 2005, Keyte, 1994, Young et al., 2003). Simple daily mass-balance water budget models including inflows, outflows, precipitation, evapotranspiration and groundwater seepage have also been used to model constructed through-flow wetlands (Zhang and Mitsch, 2005) and integrated into grid-based models in low-gradient dryland rivers (Costelloe et al., 2003). In the Murray River, Australia, the Murray Flow Assessment Tool (MFAT) (Young et al., 2003) and the Flood Inundation Model (Overton, 2005) have been developed with more spatial and temporal representation of flood dynamics and have linked the flood dynamics to ecological response in the form of habitat preference curves. These often take the form of an empirical relationship between water depth, length of inundation and plant response. The application of the models and estimates in their current forms are limited in terminal floodplain wetland systems. This is mainly due to the lack of soil moisture accounting within the model structure, and the paucity of traditional calibration data as there are no downstream outlet points to calibrate against.

Spatial modelling using remote sensing to map and classify wetlands, and to estimate flood extent (Costelloe et al., 2003, Faulkenmire, 2004, Kingsford and Thomas, 2002, McCarthy et al., 2003) is also used in floodplain wetland assessment and is of value in determining flood patterns and dynamics. Roberts et al. (2000) suggest building volume–area relationships to describe hydrologic patterns in floodplain wetlands using data from remote sensing. This type of spatial approach must still be coupled with temporal modelling to enable testing of flow management scenarios against ecologically significant flood dynamics.

The papers outlined above have focussed on the assessment of free surface water (flooding) at a range of temporal and spatial scales. In inland Australia antecedent conditions impact on flood duration and extent and the inclusion of soil moisture status into a floodplain model is critical to assess the benefit to a range of vegetation types. The aim of this study was to assess the use of remote sensing to provide spatial and temporal analysis of flood dynamics and soil moisture status and to apply the results to the development of a semi-distributed daily water balance model relevant to ecological water management in a terminal wetland system.