.png){kind=small}
Hydro Resource Evaluation Tool
physical characteristics of a hydro site
Tier 1. Transposing measured data to estimate mean flow
Selection of an analogue catchment
Due to the large spatial variations in rainfall and hydrogeology across the UK, transposition techniques all attempt to adjust the flow record for the analogue for differences in hydrological scale (differences in rainfall and catchment area) between the analogue and ungauged catchments. An analogue catchment is usually a catchment that is:
- Geographically close to the ungauged catchment, hence has the same climatic regime; a general rule of thumb is within 50km. You can use the NRFA website interactive map scale to estimate the distance between your proposed scheme and gauging station, or determine the exact distance using the grid references.
- Similar rainfall. You can use your estimate of SAAR for your catchment, and SAAR is provided on the NRFA websitefor gauging stations.
- Hydrogeologically similar. The geology of the catchments may be found by consulting the relevant Catchment Abstraction Management Strategy (CAMS) documents which should provide descriptions of the hydrogeology within the area of interest (see the EA website). A description of the geology is provided by the NRFA website for gauging stations. Using these sources of information you can use your own judgement as to whether the two catchments are hydrogeologically similar.
- Similar in size; a general rule of thumb is the catchment size should be within a factor of two. You can use your calculated catchment area and catchment area is provided by the NRFA website for gauging stations.
- Either a natural catchment (no major artificial influences), or a catchment for which there is a naturalised flow record. You can use your assessment of artificial influences in your catchment and a description of influences is provided by the NRFA website for gauging stations.
- Ideally, the analogue catchment should lie upstream or downstream of the ungauged catchment (termed connected).
In the case of a connected analogue, there is a strong serial correlation between the flow measured at the analogue gauge and the flows at the ungauged site as the water that flows past both points has a common component. It is quite common that there is no connected analogue available, thus a catchment from an adjacent system or tributary for the same system is selected as the analogue for the ungauged catchment. For this case the analogue would be termed unconnected.
These are relatively stringent requirements and in many cases it will be necessary to relax these criteria with the consequence that the choice of analogue catchment might not be as good. The technical report discusses this further.
You should now have a list of potential analogue gauging stations for your proposed site. If there is more than one potential gauging station, use your judgement to decide which is most similar to your catchment in terms of meteorological conditions, gauging stations directly upstream or downstream would be ideal.
