Low Flows 2000 – the estimation of river flows

River flow estimation – the requirement

Consents to discharge, planning approvals, environmental impact statements, abstraction licences applications, reservoir planning and the assessment of hydropower potential are all examples of projects that require information on river flows. However, there is little or no measurement of river flow for the majority of river reaches within the UK.

In lieu of site specific observed data hydrologists are faced with the problem of estimating flows. Developed by staff now forming part of the WHS team, Low Flows 2000 meets this challenge by integrating within a GIS framework the latest regionalised models for predicting for the estimation of natural and artificially influenced river-flows, as represented by flow duration statistics, within ungauged catchments.

Low Flows 2000 has been adopted by the Environment Agency (EA) and the Scottish Environment Protection Agency (SEPA) as a best practice tool for estimating low flows in ungauged catchments. It is widely used by the EA within the Resource Assessment Methodology (RAM) process and the associated development of Catchment Management Abstraction Strategies (CAMS).

Development and distribution

WHS is the sole appointed developer and distributor of the CEH Low Flows 2000 software system. The software is available as two versions:

• Low Flows 2000 Enterprise
• Low Flows 2000 Lite

Please contact us by e-mail or phone for further information and software prices. We also offer a cost effective retrievals service for the low volume user, or the user without experience in the estimation of river flows within ungauged catchments for the delivery of flow estimates from Low Flows 2000.

Hydrological models within Low Flows 2000

The main hydrological models integrated within the Low Flows 2000 software were originally developed by CEH in collaboration with the Environment Agency for England and Wales and subsequently with the Scottish Environment Protection Agency for Scotland.

The principle flow statistics that can be estimated using Low Flows 2000 are:

• Mean flow – This is estimated within the software using the outputs from a deterministic model based on observed flow data from over 500 gauged of catchments in the United Kingdom. The model has been published within the scientific literature by Holmes et al (2002a).
• Annual flow duration curves (FDCs) – The annual FDC model incorporated within Low Flows 2000 makes use a 'region of influence (ROI)' approach to hydrological regionalisation. Dimensionless, observed flow duration statistics from catchments 'similar' to the user-defined catchment are combined to estimate statistics for that catchment. Similarity is defined using a Euclidean metric based on catchment characteristics. The derived dimensionless FDC is then rescaled by mean flow. The model was published as Holmes et al (2002b).

The process associated with deriving an annual flow duration curve is illustrated as:

Low Flows 2000 methodology for deriving an annual flow duration curve
within England and Wales

In addition to the estimation of annual statistics the software also contains models for estimate the equivalent monthly flow statistics:

• Monthly mean flow – A similar ROI approach is applied to estimate the proportion of annual runoff distributed over each month. Full details can be found in Holmes et al (2002c).
• Monthly flow duration curves – These are estimated from dimensionless observed monthly FDCs using an ROI model identical to the annual FDC model. The estimates of monthly mean flows are then used to rescale these dimensionless curves to flow units. Full details can be found in Holmes et al (2002c).

Low Flows 2000 also facilitates the estimation of the Base Flow Index for the ungauged site using the model development as part of the development of the Hydrology of Soil Types (HOST) hydrological classification of soil associations within the UK.

Low Flows 2000 Bibliography

Holmes MGR, Young AR, Gustard AG and Grew R (2002a)
A new approach to estimating Mean Flow in the United Kingdom. Hydrology and Earth System Sciences 6 (4) 709-720.

Holmes MGR, Young AR, Gustard AG and Grew R (2002b)
A Region of Influence approach to predicting Flow Duration Curves within ungauged catchments. Hydrology and Earth System Sciences 6 (4) 721-731.

Holmes MGR and Young AR (2002c)
Estimating seasonal low flow statistics in ungauged catchments BHS 8th National Symposium - Birmingham, 2002

Holmes MGR, Young AR, Goodwin TH and Grew R (2005)
A catchment-based water resource decision-support tool for the United Kingdom
Environmental Modelling & Software Vol.20 Issue 2:197-202.

Young AR, Grew R & Holmes MGR (2003)
Low Flows 2000: a national water resources assessment and decision support tool.
Water Science & Technology Vol.48 No.10 pp 119-126