Unit Hydrograph Methods and Applications
The unit hydrograph method of runoff estimation has become a powerful tool of hydrology since it was first proposed by Sherman in 1932. The approach has been applied widely for design of flood alleviation schemes, for water resource studies, for operational control and for pollutant load assessment. The attraction of the approach is the basic simplicity of the definition of the unit hydrograph: that is the direct runoff resulting from unit depth of excess rainfall produced by storm of uniform intensity and specified duration. Whilst simple in concept, the application of unit hydrograph theory is not always so straightforward. Catchment behaviour, and hence rainfall-runoff response, is highly non-linear with vegetation and physical controls on evapotranspiration, complex flow paths and water routing, channel hydrodynamic processes and spatial variability of rainfall across catchments. Thus the linear theory of unit hydrographs has had to be adapted and modified to take into account these non-linearities.
In this special issue new techniques for the estimation of unit hydrographs are considered and issues of non-linearity investigated. The first paper by Chapman investigates the derivation of unit hydrograph and input rainfall excess directly from streamflow data, making use of numerical procedures for estimation purposes.
The question of baseflow separation is addressed and the problem of baseflow separation is a common theme throughout the special issue with a novel approach proposed using the model IHACRES. The nonlinearity of the rainfall loss process is also considered by Chapman and four alternative modelling approaches are compared.
With the increasing importance of water quality and pollutant transport in catchments, the debate over 'old' and 'new' water is particularly relevant. Barnes and Bone11 consider the linkages between unit hydrograph techniques, residence time distributions and solute transport, and propose modifications to the basic model to allow for solute transport and predictions. The model modification is related to hillslope processes and the geomorphological controls of hydrograph response are analysed in depth in papers by Rinaldo and Rodriquez-Iturbe, Maidment et al. and Post and Jakeman. In each of these papers different approaches to incorporating catchment characteristics into unit hydrograph generation and streamflow response are described with the first using a travel time distribution approach, the second utilising velocity fields and a GIS approach and the third relating IHACRES model parameters to catchment characteristics. The paper by Robinson and Sivapalan generalises the unit hydrograph theory to create non-linear instantaneous response functions.
Finally the scaling up problem of moving from small river basins to large catchments is investigated by Schreider while Dietrich considers the problems of numerical identification of unit hydrograph response.
This special issue has brought together new approaches and modelling procedures to extend the utility of unit hydrograph theory, and linked such approaches with the need for techniques for solute prediction.