Water Environment and Water Services (Scotland) Act 2003 implementation: environmental standards - consultation

Consultation seeking views on our proposals for new or updated environmental standards for Scotland’s water environment, and updates to some of the assessment methods used for deriving such standards.

9. River flows

We are proposing to introduce revised standards for two aspects of river flows. SEPA uses river flow standards to assess the risk to the ecological quality of rivers posed by new abstractions and to identify the scale of improvements likely to be needed to achieve our objectives for rivers that are already under pressure from water abstraction.

The new proposed standards that redefine the existing standards for High status are set out in Table 9.1. The proposed revision of existing flow standards that allow for a short term exceedance from the thresholds set for classification of Good status, provided that a number of test are met, are shown in Table 9.2. Further technical information about the proposed standards and their derivation is available from UKTAG[7].

9.1 High hydrological status and artificially increased flows

Evidence reviewed by UKTAG suggests that persistent artificially increased flows have a negative impact on river macroinvertebrates. Under WFD, High ecological status is defined as near naturalness associated with no or very low human pressure. Consequently, we consider High hydrological status thresholds should take account of persistent artificially elevated flows in addition to the current limits on flow from abstraction pressures.

We propose that the existing flow standards for High hydrological status are amended to include an upper threshold of 5% deviation above natural at flow less than or equal to Qn95 and 10% deviation above natural where flows are greater than Qn95. This mirrors the thresholds for flow reduction for High status. Details are shown in Table 9.1.

Table 9.1 Recommended revisions to the "High" standards for river flows

Permitted maximum as a proportion of natural flow

At daily flows (Qn) less than Q95

At daily flows (Qn) greater than Q95

River Type

Existing standards

Proposed revision

Existing standards

Proposed revision



+/- 5%


+/- 10%

No changes are proposed to the existing standards for daily flows for Good status

The above proposal to redefine High status does not apply to Heavily Modified Waterbodies (HMWBs).

Implications of the proposal

There are approximately 2000 non-HMWB river waterbodies in Scotland. Of these, there are 84 which meet the current high hydrology environmental standard, but would breach the revised standard. However, of these, only two would drop in overall ecological status from high to good with the other 82 already being at less than high status due to other parameters. The two waterbodies which would be downgraded to good are those with elevated flows due to catchment transfers as part of large storage hydropower schemes. However, there would be no regulatory impact on the two hydropower schemes as the waterbodies will be at good status.

9.2 Good hydrological status and short-term abstractions

The current flow standards are in the form of a series of limits of allowable reduction from natural flows at a range of flow conditions. However, this takes no account of the duration of an abstraction, nor how frequently it occurs. This means that an abstraction that breaches a standard for a few days once a year is treated the same as one causing a continuous breach; the same limits apply to both.

River animals and plants have evolved to live under a highly variable flow regime. This includes short-term periods of naturally low flow, which animals and plants are expected to be better adapted to than longer term events. Evidence reviewed by UKTAG found that for flow reductions lasting less than one month, impacts on aquatic life forms were low, provided some flowing water remains in the channel. The exception to this is where the low flows result in a loss of connectivity, with the appearance of isolated pools. Under these conditions significant ecological impacts can arise quickly.

We propose that a temporal element is applied to the flow standards, such that, depending upon frequency and duration, short-term exceedances might not result in a deterioration in class. The magnitude of allowable exceedance would depend on both the duration of, and typical interval between, exceedances. This accounts for the resilience of aquatic ecology to short low flow events but also the need for a recovery period.

Table 9.2 shows a matrix of allowable flow standard exceedances for short-term flow reductions. An allowable exceedance means High or Good hydrology status can be confirmed even if the threshold for High or Good status is exceeded (within the given time constraints) if the waterbody is at Good or High hydrology status prior to the low flow event. If hydrology is at less than Good status pre low flow event, it will inform deterioration risk to a lower class. Exceedances are not permitted, i.e. current standards continue to apply, where:

  • the standard for Poor is exceeded, or
  • an exceedance lasts more than twenty days, or
  • exceedances typically occur more frequently than once every two months, or
  • the natural daily mean flow is below Qn98.

The revision allows an increasing degree of exceedance of the current standards as flow reduction events become shorter and less frequent. For example, an event of a magnitude that breaches the existing Moderate standard (i.e. Poor class) which occurs typically between one and six times per year (interval 2 months to ≤1 year) and lasts up to five days would still meet the Moderate short-term standard.

The allowable exceedances would mean that, where the frequencies and durations of abstraction events are small, a higher class than permitted by the current standards may be assigned. This would apply to the waterbody (not an individual abstraction) and would need to take account of any cumulative effects from multiple abstractions, as well as effects on flow on any downstream waterbodies. Normal classification spatial rules would apply.

Table 9.2 Revised classification accounting for short-term flow deviations
Table 9.2

Implications of the proposal

These changes would potentially change the way ecological evidence of a failure of Good status is interpreted where there are short-term abstractions i.e. whether the ecological class of less than Good status is due to a water resource pressure. The impact of the proposed change will be on waterbodies currently assigned a class of Moderate or Good for hydrology due to the influence of intermittent abstractions, typically for irrigation. It is estimated that up to 20 waterbodies impacted by short-term abstractions could change class from Moderate to Good status as a result of these changes. These improvements would be seen in areas that have high numbers of agricultural abstractions such as Angus, Fife, Tay and Tweed. However, there is still significant pressure from irrigation on many of these waterbodies and it will not open up significantly more capacity for abstraction in most cases.


Email: eqce@gov.scot

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