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Hydro Resource Evaluation Tool
physical characteristics of a hydro site
Tier 3. Short term continuous measurement
Introduction
For larger schemes it is recommended that developer’s measure flows continuously at the site of interest for as long as possible; no less than 6 months and preferably for a year or more. The longer the period of measurement the more certain the estimate of resource availability.
It is important that the regulator is contacted prior to continuous monitoring to ensure that their requirements are met as monitoring can become both expensive and time consuming.
Flow duration statistics may be calculated directly from the short record. Whilst these data have undoubted value being direct measurements of flows at the design site, the flow duration statistics compiled from these data are very sensitive to sampling errors when used to represent the long term resource availability.
The aim of this section is to provide guidance on the methodologies that may be used, but these should only be used by a qualified professional.
Typically the standard error of estimates for MF and Q95 statistics drawn from 1 year of data is between 20% and 50% respectively within the type of catchment used for hydropower development. Use of short records must therefore be analysed in the context of both the seasonal and annual variability of the climatology at the site.
Methodologies
If a suitable analogue catchment exists, which has a long flow record overlapping the short flow record at the site of interest, then it is possible to address the limitations associated with the short record by synthesising a flow record at the site of interest from the long record. The following three methods may be used to extend your short record:
- Record extension using FDC ratios;
- Record extension using regression; and
- Record extension using matching pairs.
Method 2 is very dependent on the synchronicity of the flows within the two catchments and hence is only applicable to the relatively close analogues, whereas Methods 1 and 3 are generally dependent upon the climatology being generally similar over both catchments.
The uncertainty associated with these methodologies is highly dependent upon the length of record and where possible the length of record should be maximised. As a rule of thumb, the absolute minimum length of record used with these methodologies is one complete year for method 1 and 6 months for methods 2 and 3. These short records should also be representative of an average year and if the period of measurement is particularly dry or wet then measurement should be extended to a longer period of time.
If you are interested in low flows, a fourth method may be used to estimate the Q95 from BFI. For this methodology the same principles of record length apply with a minimum of 1 year and preferably 2 years continuous measurement. See Technical Report for further details.
