Importance of Flood Flows to the Productivity of Dryland Rivers and Their Floodplains

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Final Report
Prof P.M. Davies, Prof S. E. Bunn and Ms F. Balcombe
Environment Australia, 2003

Predictive Model Development

Tracing the structure of aquatic food webs using stable isotope analysis showed the overriding importance of benthic algal material. This was shown during both the 'wet' and 'dry' (Bunn, Davies and Winning submit. Consequently, the processes that regulate and control algal production are critical for model development.

In the 'dry', elevated Gross Primary Production (GPP) was restricted to a 'bath-tub ring' in the littoral edges of pools (Bunn, Davies & Winning, 2003). Approximately 7% of the pools were characterised by this bath-tub ring; a function of turbidity. Rates of GPP within the channel are very small and, as such, would not overly influence the magnitude of processes within the model. During flooding, both the newly-inundated benthos and water column had elevated rates of carbon respiration (R24) with GPP increasing with the period of inundation. When floodwaters started to recede, rates of GPP were over 1300mgC-2day-1 and the system had switched to be a net producer of carbon (GPP/R24>1). A two dimensional model showed the amount of aquatic carbon produced by a day of floodplain inundation was equivalent to 82 years of aquatic production during the dry.

Rapid water abstraction during the dry could desiccate the littoral zone and subsequently remove this production from the aquatic food web. Quantitative models predicting the ecological consequences of water abstraction during the dry would depend on pool morphology and the rates of drawdown (as a consequence of pumping). This needs to be assessed on a waterhole by waterhole basis.

During the 'wet', rates of GPP on the floodplain are initially dominated by processes in the water column, after about a week the benthic processes predominate. At a catchment scale, GPP is largely determined by the area of inundation, time since inundation and depth of inundation, where total GPP = benthic + open water rates. Benthic GPP is a function (time, depth, area) and open water GPP as a function (time, area). The elementary component of the model is the area of inundation. This needs to be determined at a resolution depth at the level of centimeters (as turbidity controls benthic GPP).

To be able to quantitatively assess varying levels of water abstraction on catchment scale GPP (during the wet) a digital elevation model may have adequate resolution. A simple GIS assessment of the area of inundation would not be able to resolve the depth component, as shallow depths and turbidity are not distinguishable.

The productivity parameters and rates have been communicated to ARIDFLO (through Julian Reid) for incorporation into the ARIDFLO model.

Measuring benthic metabolism on the inundated floodplain.

Measuring benthic metabolism on the inundated floodplain.