Effects of Surface Runoff and Subsurface Flow on the Spatial Variability of Water Resources in Longitudinal Dunes

The relative textural homogeneity of sand particles in desert dunes, coupled with their high porosity and low water-holding capacity, have led to the widespread idea that water movement in unsaturated sand is fast and predominantly vertical. Under such conditions, one would expect a quite uniform depth of water penetration, of soil moisture and vegetation cover in dune areas. However, several studies have cast doubt on the vertical water movement in dunes. Miles et al. (1988) showed that slight differences in the compaction of texturally homogeneous sand layers affected the hydraulic conductivity and controlled water movement under unsaturated conditions. Yeh and Harvey (1990) drew attention to heterogeneities between layers as factors controlling the direction of flow. Zaslavsky and Sinai (1981) suggest that lateral flow can be expected over a sloping surface, even in the absence of an impeding layer at a shallow depth. Water concentrates especially in slope concavities. Dunes are characterized by a layered cross-bedded structure and lateral changes in slope angles. In view of the arguments presented above, a lateral flow component would be expected for steep dunes, especially if slight differences in compaction and texture exist between adjoining layers. Such a phenomenon should lead to a water redistribution process of infiltrated waters, a process by which water infiltrated in the upper part of a dune ridge moves laterally towards the base of the dune slope at a shallow depth. Local slope concavities would be expected to increase the effect of local water concentration. The actual existence of a lateral flow component in dune areas under field conditions was demonstrated in several studies conducted in the desert of New Mexico (USA), where average annual rainfall is 220 mm (Stephens and Knowlton 1986; McCord and Stephens 1987; Stephens 1994). Similar results have been reported in a study conducted in the Nizzana sand field in Israel (Yair et al. 1997). Lateral water movement at a shallow depth is supported by field observations. Vascular plant cover in the Nizzana area increases from the upper to the lower part of the dune slope, with high concentrations in slope concavities, especially at the base of steep dune ridges (Fig. 18.1).