Flow variability index

Find further information on the Conservation of Freshwater Ecosystem Values (CFEV) Program and its data at www.dpipwe.tas.gov.au/cfev.

Attribute data

TitleFlow variability index

CustodianWater and Marine Resources Division, Department of Primary Industries, Parks, Water and Environment

CreatorGIS Unit, Information and Land Services Division, Department of Primary Industries and Water

DescriptionThe degree of change in flow regime variability as a result of human flow manipulation at major regulatory structures.

Input data

1. CFEV Mean Annual Run-off (MAR)
2. CFEV Rivers spatial data
3. CFEV Waterbodies spatial data
4. Hydro infrastructure and discharge data (volume, location, operational regime (e.g. Hydro peaking, etc.), Hydro Tasmania

Lineage

An index of change in flow variability can only be estimated based on the scales of major change in flow regime known to be associated with large storages, especially Hydro dams and power stations (e.g. (Davies et al. 1999)).

The flow variability index was calculated using the CFEV river spatial data layer, current and natural MAR, the CFEV waterbodies spatial data layer and the wetlands spatial data layer.

Waterbodies such as Hydro storages, irrigation/town water supplies, etc. (listed in Appendix 14 of the CFEV Project Technical Report) were rated according to the following categories using expert knowledge (Peter Davies, Freshwater Systems and Mick Howland, Hydro Tasmania):

0No change to flow variability (i.e. no dam/structure present, unmodified waterbodies)

0.3Low level of variability at seasonal to annual scales (e.g. irrigation storages, weirs, etc.)

0.6Moderate to high variability at daily to seasonal scales (e.g. headwater storages, some run of river storages)

1Very high variability at monthly to hourly scales (e.g. hydro peaking power stations).

These scores (listed for waterbodies in Appendix 14 of the CFEV Project Technical Report) are designed to reflect the nature of variability in flow releases from the artificial waterbodies (by contrast, the abstraction and regulation indices reflect changes in the quantity and degree of regulation of flow in the drainage). Waterbodies not listed in Appendix 14 were considered natural and rated as 0 for flow variability.

The flow variability rating only describes the influence of major infrastructure on variability, and many river sections will have varying degrees of flow abstraction and regulation, i.e. may have reduced (or enhanced) base/flood flows (this is reflected in the abstraction and regulation indices) without corresponding flow variability scores. For reservoirs used for irrigation/water supply, the rating is 0.3, with some larger storages rated 0.6. Hydro storages have been rated 0.6 or 1, based on knowledge of their release strategies. Different ratings have been assigned for relevant Hydro storages to the river section receiving power station discharge and to that immediately downstream of the storage dam(s).

The calculation of the flow variability index for each river section (RS_FLOVI) was based on the presence and initial rating of upstream storages. The indices were altered (reduced in severity) with each river section downstream. The specific rules for assigning the index to river sections are given below.

Data limitations

The flow variability index inherits all the data limitations of the derivation processes and input data.

Date createdOctober 2004

Scale and coverage1:25 000; Statewide

References

Davies, P.E., Cook, L.S.J. and McKenny, C.E.A. (1999). The influence of changes in flow regime on aquatic biota and habitat downstream of the Hydro-electric dams and power station in Tasmania. Hydro Technical Report Project No. 95/034. Hobart, 128 pp.

Column headingRS_FLOVI

Type of dataContinuous but also exists in a categorical format (see Table 1).

Number of classes4

Assigning values to ecosystem spatial units

Each river section was assigned a flow variability index score (RS_FLOVI) according to the following rules:

1. Is the river section immediately downstream of an artificial waterbody (WB_ARTIF = 0) or associated downstream canal/pipe (listed in Appendix 14 of the CFEV Project Technical Report) (RS_PIPE =1)?

Yes: Assign score to that river section. Note that there may be up to two different downstream paths (natural and normal) as per drainage flow regimes, and hence there can be two flow variability scores associated with each storage. If there is more than one value given, then the maximum value is taken.

Scores were separately applied to river sections immediately downstream of dam structures as well as river sections immediately downstream of any canal/pipe which serves to discharge water from that waterbody (and the canal/pipe’s associated infrastructure e.g. power station).No: Assign score of 0.
2. Using the river sections that were assigned values in Step 1 as starting points, dilute the flow variability scores downstream (using a MAR-weighted average) as per the equation below. Typically, as MAR increases downstream, the score decreases until a minimum value close to 0 is reached. Note, that the downstream accumulation stops at any waterbody, and re-starts downstream of that waterbody with the flow variability rating relevant to that waterbody.

Where:

The final value of the flow variability index was assigned to the river sections as the inverse of the above score, i.e. 1 = high condition, with no change in natural flow variability, and 0 = low condition with maximum change in flow variability.

The river spatial data layer had the continuous flow variability data categorised according to Table 1. The categorical data was used for reporting and mapping purposes.

Table 1. Flow variability categories for rivers.

CFEV assessment framework input

1. Rivers> Condition assessment>Naturalness score (RS_NSCORE)>Geomorphic condition (RS_GEOM)>Flow change (RS_FLOW)
2. Rivers> Condition assessment>Naturalness score (RS_NSCORE)>Biological condition (RS_BIOL)>Macroinvertebrate condition (RS_BUGCO)