*Correspondence to: Raymond Torres, Department of Geological Sciences, University of South Carolina, Columbia, SC 29208, USA. E-mail: torres@geol.sc.edu The unsaturated flow equation is a robust tool for predicting water movement through fully characterized soils with well-defined sets of initial and boundary conditions. On the other hand, Beven (1989) and Germann (1990) argued that the governing equation rarely applies to field conditions, primarily due to macropore flow (e.g. Mosley, 1979, 1982; Beven and Germann, 1982; Tsuboyama et al., 1994). Other preferential flow processes that violate bulkaveraged soil-water flow assumptions are finger flow (e.g. Hill and Parlange, 1972; Hendrickx et al., 1993; Glass and Nichol, 1996) and ‘mesopore’ flow (e.g. Wilson and Luxmoore, 1988; Hornberger et al., 1991; Jackson, 1992; Luxmoore and Ferrand, 1993; Taha et al., 1997). These preferential flow processes, and others, can give the appearance of a ‘rapid’ pressure head or water content response deep in an unsaturated soil profile. Field and laboratory data indicate that a ‘pressure perturbation’, ‘pressure wave’ or ‘water content wave’ may also give rise to a rapid unsaturated zone response, even when Darcy assumptions seem not to be violated (e.g. Zimmermann et al., 1966; Andersen and Sevel, 1974; Gilham, 1984; Kayane and Kaihotsu, 1988; Novakowski and Gilham, 1988; Marui et al., 1993; Jayatilaka and Gilham, 1998; Reid et al., 1998; Rasmussen et al., 2000). For example, Torres et al. (1998) conducted field irrigation experiments to characterize unsaturated zone influences on runoff generation and slope stability. They drove the soil profile to near-zero pressure head with corresponding soil-water contents at ∼0·30 (porosity ∼0·60). A natural storm augmented the applied rain with a ten fold spike increase in rain intensity. By comparing pressure wave velocity with approximate wetting front velocity (after Warrick et al. (1971)) they showed that the 1·75 h peak rainfall to peak discharge resulted from a rapid unsaturated zone response. Tracer data during a later irrigation experiment, one without a spike increase, showed that preferential flow did not occur (Anderson et al., 1997). Therefore, rapid pressurewave-like responses in the unsaturated zone may be related to spike increases in rainfall. Reid et al. (1998) conducted sprinkler irrigation experiments on a ‘soil’ prism and observed a pressure-wave-type response. Reid et al. (1998) applied water to an inclined soil prism at 100 mm h−1 for 50 min, stopped irrigating for 20 min, and then rained at 200 mm h−1 until the prism collapsed at 11 min. During the initial irrigation, the pressure heads increased to near-zero, and soil-water contents increased to ∼0·35 (porosity ∼0·50). These values changed