Any gauging station loaded within Qube with natural or naturalised annual flow duration curve data can be nominated as a local data gauge. Gauged Flow Naturalisation is described here.

The User can select whether local data are to be incorporated within the estimation procedure. The algorithms for explicit use of local data within Qube also offer the potential for over-riding the natural estimates by alternative sources of data (e.g. groundwater models). This is not explored further within this method statement but is achieved by setting up a dummy gauging station and loading up with FDC data.

Potential Configurations of Local Gauging Stations

An outflow site of an ungauged catchment may be positioned upstream or downstream of sites on the same river system, at which flow data have been recorded. Where a gauging station is positioned downstream of the ungauged site there may be additional gauges on tributaries which also contribute to flows at the downstream gauging station which may be considered to be adjacent to the ungauged site. The upstream gauges are nested within the ungauged catchment and the ungauged catchment is nested within the catchment of the downstream gauge. The adjacent catchments are nested within the downstream gauge but not the ungauged catchment. Within the local data algorithm is assumed that a water balance is closed by a gauging station (with no significant gauge bypass) and that travel times are insignificant over the scales considered.

An ungauged site may have an arbitrary number of upstream or downstream gauges positioned in direct sequence and which are hence nested with one another. However, in the current algorithm, only flow data from the first upstream gauge(s) and/or downstream gauge are used to constrain estimates of flow statistics for the ungauged catchment.

The algorithm is based on the following assumptions:

data from all gauges defined as local data gauges is of equal quality and of zero hydrometric error.
the gauged flows are natural (i.e. the gauged catchment has few influences or the flows have been derived/modelled, for example by flow naturalisation, to produce a record that can be considered natural).
an ungauged catchment may have multiple non-overlapping proximal upstream gauges, which are entirely contained within the boundary of the ungauged catchment.
an ungauged catchment may have a single proximal downstream gauge within the boundary of which the ungauged catchment is entirely contained.
all of the flows recorded at an upstream gauge, pass directly through all points further downstream, including the ungauged catchment outflow.
all of the outflow from an ungauged catchment flows through a downstream gauge.
the catchment of a downstream gauging station may contain multiple adjacent upstream gauges, which do not overlap either with themselves or the ungauged catchment.

Figure 1 shows examples of the configurations in which local data may be used to constrain estimates of flow statistics at ungauged sites. In each example, the shaded areas denote the catchments for which local gauged data are available.

Figure 1 - Definition of a) upstream local gauges b) downstream and adjacent local gauges

Use of Upstream Gauged Data

Where gauged data are available from one or more upstream gauges (Figure 1.a), the flow at the ungauged catchment may be considered to be the total of all the flows recorded at the upstream gauges, with additional runoff from the incremental area, which is the area within the ungauged catchment which is not included in any of the gauged sub-catchments.

Qube estimates an ROI FDC in Ireland for the catchment of interest (this will provide flow in m³/s for the annual FDC 101 points). The annual FDC 101 points are then adjusted for the upstream LDG(s) as follows:

$FINALQ_{ug} = \bigg( {ROIQ_{ug} \times {IncArea_{ug} \over TotalArea_{ug}} \bigg) + \sum_{n=1}^i Q_{gi}}$

where; $FINALQ_{ug}$ = the final flow for a given annual percentile for the ungauged catchment (m³/s) $ROIQ_{ug}$ = the ROI estimated flow for a given annual percentile for the ungauged catchment (m³/s) $IncArea_{ug}$ = the incremental catchment area for the ungauged site (total area minus sum of all upstream non-nested LDG catchment areas) (km²) $TotalArea_{ug}$ = the catchment area for the ungauged site (km²) $Q_{gi}$ = the recorded flow for a given annual percentile for the $i$ th upstream LDG (m³/s)

The monthly FDC is then calculated and fit to the adjusted annual FDC (as completed for ROI estimates that are not adjusted for local data).

Finally, the annual and monthly mean flow are then calculated as the area under the annual and monthly FDCs (as completed for ROI estimates that are not adjusted for local data).

Downstream Local Data Gauges

Flow estimates in Ireland are not be adjusted for Downstream Local Data Gauges.

If your catchment of interest is just upstream of a ROI LDG with a similar catchment area, it will have similar fractional extents and thus have a high weighting in the ROI pools. As the FDC in Ireland is estimated as a flow per unit area, the gauged flows will be explicitly and fully incorporated into the estimate of the FDC and hence mean flow (as mean flow is estimated as the average of the volume under the FDC).

In the UK, in contrast, the FDC is an ROI estimate of the FDC normalised by mean flow scaled by a natural estimate of mean flow. The normalised FDC makes maximum use of the normalised FDC from the downstream gauge. However the estimate of mean flow is calculated based upon a 1km runoff grid, therefore the resultant FDC does not fully utilise the downstream gauging station. That is, the Ireland ROI at the location of a downstream gauging station will yield the gauged FDC whereas in the UK at the downstream gauge the estimate will be the normalised FDC for the gauge multiplied by a estimate of mean flow.