1.   Purpose and objectives

     A storage routing method developed specifically for short-term flow forecasting. Inflow-Storage-Outflow (ISO) function models can be used to forecast flows from rainfall (either measured or forecast) or from tributary flows.

2.   Description

     ISO-function models offer a simple, but flexible, approach to hydrological modelling and are capable of solving a variety of forecasting problems. The method assumes that the outflow rate from a catchment (or reservoir, or river reach) is uniquely related to the quantity of water stored, and ensures that the water balance equation is satisfied at all times. These two conditions lead to a first order differential equation expressing the outflow rate in terms of inflows (eg. rainfall) and the characteristics of the storage.

     Several types of ISO-model are possible, depending on the functional form used for the storage/outflow relationship. For certain ISO-functions, analytical solutions are available which provide explicit formulae for forecasting both rate of flow and volume of runoff. Equations are also available to evaluate the sensitivity of the flow forecast to errors in the input data or model parameters. For more complicated ISO-functions, a numerical or graphical solution is used.

     ISO-function models are designed to make maximum use of telemetered river flows to correct forecasts in real-time. The method allows immediate state-updating of the model - and hence its forecasts - whenever a telemetered flow is received.

3.   Input

     In calibration: rainfall hyetographs, runoff hydrographs. In real time: rainfall measurements and/or forecasts, telemetered river flows.

4.   Output

     Flow forecasts, volume of runoff forecasts, and accuracy of forecasts subject to estimated errors in inputs.

5.   Operational requirements and restrictions

     For simple applications, the model can be expressed in graphical form or as a formula for hand calculation. Thus it is suitable for use by technicians with calculators, using real-time data obtained manually by telephone or radio transmission. In more complex applications, the method is used to model individual subcatchments (or river reaches) as a component of a basinwide computerized flow forecasting system.

     The ISO-function model makes only an implicit allowance for rainfall losses; this is because the method was designed specifically for operational flood forecasting on saturated or near-saturated catchments. However, the simplicity of the model is seen as an advantage rather than a disadvantage: the model is able to make effective use of telemetered river flows and is readily initialized at the start of a flood event or following periods of data loss. Once the important parameter of translation lag has
been derived from analysis of hydrometric records, the model discriminates the relative importance of measured and forecast rainfalls to short-period flow forecasts.

6.   Form of presentation

     A report in English describing the concept, theory and application of ISO-function models, and drawing attention to elements of similarity between ISO-models and Storage Function/Transfer Function/Unit Hydrograph methods.

7.   Operational experience

     ISO-models have been used successfully by several water authorities in the UK, both in simple graphical form and as a component of computerized flood forecasting systems such as the Dee River scheme installed in 1976.

8.   Originator and technical support

     Originator: Mr. A.O. Lambert, Welsh Water Authority

     Technical support: Centre for Ecology and Hydrology, Wallingford, formerly the Institute of Hydrology.

9.   Availability

     From the HOMS National Reference Centre for UK.

10.  Conditions on use

     None.