The Role and Significance of Surface and Subsurface Hydrology on Gully Head Growth in South East Spain

Abstract

This thesis seeks to extend the work of Thornes and Scoging on hillslope processes in south east Spain by paying particular attention to the relative roles of surface and sub-surface water movement in gully head change on two contrasting soil types.Gully growth is a major agent of erosion in semi-arid environments, yet studies to date have assumed the dominance of surface wash, and only speculated on the role of subsurface water movement.

The sampling design was based on the hillslope hydrological cycle and incorporates measures of precipitation, infiltration, and runoff and subsurface flow on catchment areas above gully heads. Additional data on vegetation and some soil properties were collected. The field work was undertaken on three occasions in the summer, autumn and spring of 1982/3 to examine seasonal variations.

The analysis of surface flow was hindered by the drought which meant there were only 10 rain days in 1982, and only one storm occurred during the field sessions on 26 November 1982. Despite this several observations can be made. Wash volumes were twice as high on the marl on 26 November. Both lithologies are susceptible to relatively high erosion rates by surface wash although rates tend to be higher on the marl, and there is considerable variation on both lithologies. However for neither lithology is the amount of sediment transported sufficient to fill in the gullies in the medium term. The analysis of the subsurface hydrology shows that saturated conditions were not monitored anywhere, and maximum soil moisture values reached between 50--60% saturation on the marl and conglomerate soils. There are marked seasonal variations in soil moisture and most of the variation occurs in the upper horizons. Flux rates are negligble on the marl and dominantly in the vertical plane. On the conglomerate rates are much faster, and throughflow may well occur on occasions, and at rates exceeding e'apotranspiration. This will contribute to wetter conditions around and in gullies on the conglomerate.