ON THE REPRESENTATIVE ELEMENTARY AREA (REA) CONCEPT AND ITS UTILITY FOR DISTRIBUTED RAINFALL-RUNOFF MODELING

Since the paper of Wood et al. (1988), the idea of a representative el ementary area (REA) has captured the imagination of catchment modeller s. It promises a spatial scale over which the process representations can remain simple and at which distributed catchment behaviour can be represented without the apparently undefinable complexity of local het erogeneity. This paper further investigates the REA concept and reasse sses its utility for distributed parameter rainfall-runoff modelling. The analysis follows Wood et al. (1988) in using the same topography a nd the same method of generating parameter values. However, a dynamic model of catchment response is used, allowing the effects of flow rout ing to be investigated. Also, a 'nested catchments approach' is adopte d which better enables the detection of a minimum in variability betwe en large- and small-scale processes. This is a prerequisite of the exi stence of an REA. Results indicate that, for an impervious catchment a nd spatially invariant precipitation, the size of the REA depends on s torm duration. A 'characteristic velocity' is defined as the ratio of a characteristic length scale (the size of the REA) to a characteristi c time-scale (storm duration). This 'characteristic velocity' appears to remain relatively constant for different storm durations. Spatially variable precipitation is shown to dominate when compared with the ef fects of infiltration and flow routing. In this instance, the size of the REA is strongly controlled by the correlation length of precipitat ion. For large correlation lengths of precipitation, a separation of s cales in runoff is evident due to small-scale soil and topographic var iability and large-scale precipitation patterns. In general, both the existence and the size of an REA will be specific to a particular catc hment and a particular application. However, it is suggested that a se paration of scales (and therefore the existence of an REA), while bein g an advantage, is not a prerequisite for obtaining simple representat ions of local heterogeneity.