Modelling river and floodplain interactions for ecological response

The study of "environmental flows"-selecting regulated flow regions that su pport 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 dame, weirs, and levee banks which have changed the natu re of instream and floodplain habitat by altering the time-varying depth of river channels and the frequency and duration of floodplain inundation. Th e ability to model water movement between the instream and floodplain is fu ndamental for assessing the quality of aquatic habitat for riverine biota, particularly floodplain vegetation and waterbirds. Precise hydraulic modell ing of these relationships is difficult because of the lack of appropriatel y scaled information to describe floodplain topography and surface roughnes s, both of which vary in space and time. This paper describes a framework t o allow simple modelling of average water depth and flood duration in flood plain environments using a partial water balance. Water bodies are defined as a one-dimensional storage representing the quantity of water in the stor age 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 an d a position along the storage vertical axis which determines the threshold when water is released along the pipe and in what quantity. Each storage h as an exponential decay (loss) term and a maximum capacity. Water that is p assed to a storage which has reached its maximum capacity is discarded, whi ch means that there is no attempt to balance the water over the whole syste m. This simplification allows the framework to be easily set up yet still m odel properties of interest. Pipes are connected between storages to move w ater around the landscape. A storage may have many input and output pipes. For example, losses to groundwater and evapotranspiration may be represente d using an output pipe that is not connected to another storage, or may be associated with the decay term. This approach allows mixed scale representa tions and incremental improvements to modelled water behaviour without chan ging the underlying structure of the system. This framework is one componen t of an environmental flows decision support system being developed by CSIR O Land and Water, Environment Canada, and the Murray-Darling Basin Commissi on. The modelling of vegetation habitat is used to illustrate the approach. (C) 2001 Elsevier Science Ltd. All rights reserved.