Ecohydrological Classification: Flow Regimes

Ecohydrological Classification: Flow Regimes

Flow Regime classes in north Australia

Class 1 – Stable baseflow

Class 1 streams are perennial with high baseflow contribution and comparatively high constancy of monthly mean flows (mean BFI= 0.35, mean Colwell’s C = 0.37). This high baseflow constancy is further indicated by a generally flat slope of the flow duration curve throughout the range of percentile flow values. These streams are highly predictable due to this baseflow constancy but have a relatively weak seasonal signal (low M/P) because discharge magnitude is relatively uniform throughout the year. Streamflow tends to be very stable within and among years (low variability in daily and annual flow), with low skewness and low rates of rise and fall. High flow events (e.g. > 1st percentile) are comparatively small, frequent and of short duration. The variability in timing of maximum flows (i.e. CV of Julian date) is moderately low (0.27). These streams also have high discharge per unit catchment area (i.e. mean annual runoff). These streams are uncommon and limited to spring fed creeks and the Jardine River.

Class 3 – Stable summer baseflow

Streams in class 3 are perennial with a high baseflow contribution derived from groundwater. High flow events (e.g. > 1st percentile exceedence flows) tend to be of slightly higher magnitude and longer duration, but less frequent than non-perennial river flow regimes elsewhere in northern Australia (e,g. classes 10 and 11) and this results in comparatively lower constancy of monthly mean flows (Colwell’s C) and comparatively lower predictability of mean, minimum and maximum flows. None-the-less, moderate Colwell’s P values are recorded for daily, minimum and maximum flows for streams within class 3 streams. A strong seasonal signal of discharge (M/P) occurs for this class with the majority of runoff occurring in summer. Streamflow tends to be very stable within and among years (low variability in daily and annual flow), with low skewness and low rates of rise and fall. Moderately high levels of flow reversal are recorded for this regime class. . This class is characterised by high values of mean annual runoff. Class 3 streams are characterised by floods of comparatively high magnitude, high frequency and long duration. Class 3 streams are characterised by high 25th percentile exceedence flows but of low frequency of occurrence and long duration. The 90 day mean flow also is high in class 3 streams.

Class 7 – Unpredictable intermittent

Class 7 streams are characterised by low predictability in minimum and especially maximum monthly flows and exhibit weak seasonality. Although still summer dominated, the higher CV of Julian date of maximum flow indicate that high flows occur at any time during the summer period. Rise and fall rates are comparatively rapid and the frequency of flow reversals is low. Class 7 streams have high flow variability, skewness and rates of rise and fall.

Class 10 – Predictable summer highly intermittent

Streams in classes 10 are highly intermittent (usually 100-200 zero flow days per year) which lead to high flow constancy. When they do flow, class 10 streams are dominated by summer runoff. The strong seasonality of mean and minimum flows contributes to high predictability of flows in this stream class. High flow events (e.g. > 1st percentile) tend to be of large magnitude, occur infrequently but are of relatively long duration. Class 10 streams have high rates of rise and fall of the flood hydrograph and few flow reversals.

Class 11 – Unpredictable summer highly intermittent.

Class 11 streams have similar high flow characteristics to class 10 streams but differ from other highly intermittent stream classes in that minimum and especially maximum monthly flows are less predictable and exhibit weaker seasonality, and although still summer dominated, the higher CV of Julian date of maximum flow indicates that high flows can occur at any time during the summer period. Rise and fall rates are comparatively rapid and the frequency of flow reversals is low. Class 11 streams have high flow variability, skewness and rates of rise and fall.

Class 12 - Variable summer extremely intermittent

Class 12 streams are extremely intermittent (> 250 zero flow days per year) resulting in high flow constancy and hence high predictability. Although summer dominated, seasonality of flows is very weak. These streams are dominated by infrequent large floods which, while of similar magnitude from year to year (resulting in high predictability of maximum flows), can occur at any time of year (e.g. high variability in Julian date of maximum flows). These streams are also characterised by very high flow variability, skewness and rates of rise and fall.

 

Hydrographs

Class 1 – Stable baseflow (catchment area=0.27% of all north Australian catchments)

Class 1: Stable Baseflow

Class 3 – Stable summer baseflow

Class 3 – Stable summer baseflow

Class 7 – Unpredictable intermittent

Class 7 – Unpredictable intermittent

Class 10 – Predictable summer highly intermittent

Class 10 – Predictable summer highly intermittent

Class 11 – Unpredictable summer highly intermittent

Class 11 – Unpredictable summer highly intermittent

Class 12 - Variable summer extremely intermittent

Class 12 - Variable summer extremely intermittent