ESTIMATION OF SUBGRID SCALE KINEMATIC WAVE PARAMETERS FOR HILLSLOPES

The kinematic wave approximation is commonly used in the new generatio n of so-called physically based, distributed rainfall-runoff models. H owever, although the kinematic wave approximation is commonly accepted for channel and experimental flows, its applicability to actual hills lopes remains unvalidated. Because it is not possible to measure, nor model, all of the details of the flow on any realistic surface, we use subgrid approximations to provide an effective parameterization of th e processes that occur on scales smaller than those that can be modell ed. This paper explores different effective parameterizations, the dat a required to identify the correct parameterization, and the implicati ons of not being able to identify all of the parameters on the scale d ependence of flood hydrology. Data from small-scale plot experiments ( 100 m(2)) and large-scale catchments (1 km(2)) are used to explore the se issues. It has been found that infiltration parameters can be adequ ately calibrated from small-scale plots. However, it is more difficult to calibrate the kinematic wave parameters using small-scale data alo ne. The conveyance properties of the hillslope cross-sections are para meterized by two kinematic wave parameters, c(r) and e(m), to yield th e discharge Q = c(r)A(cs)(em)S(0.5) with S being the slope and A(cs) t he cross-sectional area. It is shown that these two parameters are hig hly correlated, particularly when inferred from small-scale data. The surface roughness, amount of rilling and undulations of the surface al l influence the kinematic wave parameters. The runoff response at larg e scales is very sensitive to changes in c(r) and e(m), yet is not rea dily apparent in small-scale data. Unfortunately, using small-scale da ta c(r) and e(m) cannot be estimated with acceptable precision to reli ably extrapolate to larger scales. The significance of this behaviour is demonstrated and some possible solution strategies are discussed.