Australia: The Land Where Time Began

A biography of the Australian continent 

Murray-Darling Basin - Hydrology

According to The authors¹ the flow regimes of the inland rivers of Australia have been greatly affected by climatic swings in the past. The river characteristics of the present are maintained by the current climate and the hydrology of their catchments, though they have been greatly affected by water resource developments, the regulation of rivers and water extraction to irrigate crops, etc. In the Murray-Darling Basin the rivers of the lowland-dryland regions have hydrological regimes that are either perennial, seasonal, intermittent or ephemeral, and their flows tend to vary from year to year, and even on time scales of decades or centuries (Finlayson & McMahon, 1988). It can be seen from monthly maximum hydrological data that for some of these rivers there are very high flow rates for short periods. these periods being interspersed with periods in which the flow rate was moderate to low, of even no flow.

In some of the rivers there is also a general decline of river discharge and the slope of valleys downstream resulting in energy conditions that are lower and a tendency for reduced stream capability and efficiency. Compared to upstream reaches of such rivers, in the downstream reaches there is a decline in discharge and stream power that tends to lead to higher levels of sedimentation, as less sediment is carried by lower energy flows. As a result of this, during floods along the lower reaches of the lowland-dryland rivers a greater proportion of overbank flows during floods is promoted compared to the what occurs in the upper reaches, as well as a greater degree of interconnection occurring between the main river channels and the channels of the surrounding floodplain wetlands.

The authors say there are in general 5 groups of flow rate variables that are critical for floodplain-wetland ecosystems, as well as the plants and animals that depend on the water from lowland-dryland rivers of the Murray-Darling Basin.(Young, 1999).

Flow magnitude, the 1st of these variables describes the total or maximum discharge volumes and the water levels associated with them (area inundated) and in a river or wetland, the duration of flooding over periods of 1 day, 1 month or 1 year

Flow variability is the 2nd variable, describing the frequency and periodicity of certain volumes of floods and the water levels occurring during a certain time period.

Magnitude and frequency of extreme events is the 3rd variable, describing the volume or size and time length between floods or droughts that are prolonged.

Flow rate changes are the 4^th variable, the speed at which rivers rise and fall.

Flow seasonality, the 5^th variable, is the timing of flows for a series of months or for a particular season in a year.

Patterns of inundation and flooding duration in floodplain wetlands, to which the plants and animals typically respond, disturb normal flow regimes, strongly influencing the structure and function of wetland ecosystems.

Sources & Further reading

1. Ralph, Timothy & Rogers, Kerrylee, 2011, Floodplain Wetland Biota in the Murray-Darling Basin, Water and Habitat Requirements, CSIRO Publishing.