Modelling river and floodplain interactions for ecological response

Abstract

The study of “environmental flows”—selecting regulated flow regions that support aspects of the biotic and abiotic environment—has received increasing attention in the Murray-Darling Basin during the 1990s. The Basin contains a large number of dams, weirs, and levee banks which have changed the nature of instream and floodplain habitat by altering the time-varying depth of river channels and the frequency and duration of floodplain inundation. The ability to model water movement between the instream and floodplain is fundamental for assessing the quality of aquatic habitat for riverine biota, particularly floodplain vegetation and waterbirds. Precise hydraulic modelling of these relationships is difficult because of the lack of appropriately scaled information to describe floodplain topography and surface roughness, both of which vary in space and time. This paper describes a framework to allow simple modelling of average water depth and flood duration in floodplain environments using a partial water balance. Water bodies are defined as a one-dimensional storage representing the quantity of water in the storage per unit time. The storages are filled and drained by conceptual pipes that have a given discharge per unit time. Pipes have a limited capacity and a position along the storage vertical axis which determines the threshold when water is released along the pipe and in what quantity. Each storage has an exponential decay (loss) term and a maximum capacity. Water that is passed to a storage which has reached its maximum capacity is discarded, which means that there is no attempt to balance the water over the whole system. This simplification allows the framework to be easily set up yet still model properties of interest. Pipes are connected between storages to move water around the landscape. A storage may have many input and output pipes. For example, losses to groundwater and evapotranspiration may be represented using an output pipe that is not connected to another storage, or may be associated with the decay term. This approach allows mixed scale representations and incremental improvements to modelled water behaviour without changing the underlying structure of the system. This framework is one component of an environmental flows decision support system being developed by CSIRO Land and Water, Environment Canada, and the Murray-Darling Basin Commission. The modelling of vegetation habitat is used to illustrate the approach.