Chapter 17 Runoff and Erosion Processes Within a Dune System G.J. Kidron and A. Yair(* ü ) 17.1 Introduction The occurrence of surface runoff processes is often regarded by geomorphologists or hydrologists as irrelevant within sandy areas and dune systems, due to the very high infiltration rates of sand. The presumed lack of runoff processes has been mentioned several times in the case of the northwestern Negev sand field (Hillel and Tadmor 1962; Tsoar and Zohar 1985; Tsoar and Møller 1986). The high infil- tration rates of over 100 mm h -1 for loose sand, far above the common range of rain intensities in the Negev, may explain the assumed unlikelihood of runoff genera- tion within the Negev dune systems. However, field observations have drawn atten- tion to the existence of widespread biological topsoil crusts (Danin 1978; Tsoar 1990; Yair 1990), rich in fine-grained particles that are expected to reduce infiltra- tion rate (Chaps. 10 and 18, this volume). In order to obtain an initial insight into the issue of runoff generation in the Nizzana sandy area, a sprinkling experiment, with an intensity of 18.4 mm h -1 , was conducted over a small plot covering 1.5 m 2 (Fig. 17.1). The experiment was per- formed in the winter time, under wet surface conditions. Runoff developed within 3 minutes, after only 1 mm of rain. Final infiltration rate was reached quickly, and amounted at ~12 mm h -1 (Yair 1990). Shortly after this run, a second sprinkling experiment was performed over the same plot after the topsoil crust had been removed. Rain intensity in the second run was increased to 53 mm h -1 . Despite the antecedent wet conditions, and the extremely high rain intensity applied, no runoff was observed for 42 minutes, by which time the accumulated rain amount reached the value of 37 mm. The results of the two experiments clearly demonstrated the importance of the biological crust in limiting infiltration and enhancing runoff generation. It was therefore postulated that the microbiotic crust may affect the hydrological behavior of the surface under natural rain conditions, and the spatial distribution of water resources in the study area. The results of the sprinkling experiments cannot adequately represent processes under natural rainfall conditions, for the following reasons: S.-W. Breckle et al. (eds.), Arid Dune Ecosystems. Ecological Studies 200. 239 © Springer-Verlag Berlin Heidelberg 2008 Breckle_Ch17.indd 239 Breckle_Ch17.indd 239 11/29/2007 9:56:21 AM 11/29/2007 9:56:21 AM