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The combined effects of areal heterogeneity of the soil hydraulic properties and the surface seal formation dominate the hydrological response of arid and semiarid water catchments. Here these two phenomena were simulated to study their mutual role in runoff generation in small bare catchments. Seal formation during rainfall was simulated applying the dynamic model of Assouline and Mualem (1997). Areal heterogeneity of the soil was represented by a lognormal distribution of the saturated hydraulic conductivity of the initially undisturbed soil, K-s, and by related distributions of the other soil parameters. The runoff hydrograph, at the outlet of a hypothetical bare catchment of 0.5 km(2), was calculated using the cell model of Diskin et al. (1984). Two water catchments types, homogeneous and heterogeneous, with two different soil surface states, unsealed (mulched) and ongoing dynamic sealing, were considered. Four spatial organizations of the 10 cells in the heterogeneous catchment, all representing the same discrete distribution of Ks, were studied. Rainfall events of two rainfall intensities, 20 and 40 mm h(-1), of different durations ranging from 10 to 120 min, were applied uniformly over the catchments to study the effect of rainfall intensity and duration on runoff characteristics under the different soils and catchments conditions. The state of the soil surface seal was found to be a dominant factor with regard to runoff generation. Relative to the runoff produced in the homogeneous unsealed catchment under a (40 mm h(-1), 45 min) rainfall, the runoff was augmented by a factor of 10 during soil surface sealing and still more, by a factor of 20, when the soil surface was already sealed. On a relative basis the impact of soil sealing on runoff is much more important than that of soil heterogeneity. The effect of areal heterogeneity on runoff seems to depend on both the soil surface condition and the rainfall intensity and duration. When the soil is unsealed, the total runoff and the discharge peak are higher for the heterogeneous catchment. When the soil surface undergoes a sealing process, the total runoff and the discharge peak are higher for the heterogeneous catchment for the lower rainfall intensity of 20 mm h(-1). For the 40 mm h(-1) rainfall the catchment relative response was found to depend on the rainfall duration. A higher peak was obtained in the homogeneous catchment for rainfall durations above 60 min, and more runoff was produced for rainfall durations above 90 min. The spatial pattern of the cell organization in the heterogeneous catchment is an additional factor affecting the hydrological response. The hydrographs corresponding to each of four patterns representing the same areal heterogeneity displayed differences regarding concentration time (which varied between 7 and 28 min), timing of the peak runoff (varying between 77 and 146 min), and the peak discharge (varying between 0.41 and 0.57 m 3 s(-1)).