Albert Solé

Differential responses of hillslope and channel elements to rainfall events in a semi-arid area

The degree of hydrological connectivity of hillslope elements in a semi-arid climate was studied at the season and event timescales. Field data were obtained in Rambla Honda, a Medalus project field site situated in SE Spain, on micaschist bedrock and with 300 mm annual rainfall. The season timescale was assessed using correlation analysis between soil moisture and topographic indices. The event timescale was studied by a quasi-continuous monitoring of rainfall, soil moisture, runoff and piezometric levels. Results show that widespread transfers of water along the hillslope are unusual because potential conditions for producing overland flow or throughflow are spatially discontinuous and extremely short-lived. During extreme events, runoff coefficients may be locally high (ca. 40% on slope lengths of 10 m), but decrease dramatically at the hillslope scale (<10% on slope lengths of 50 m). Two mechanisms of overland flow generation have been identified: infiltration excess, and local subsurface saturation from upper layers. The former occurs during the initial stages of the event while the latter, which is quantitatively more important, takes place later and requires a certain time structure of rainfall intensities that allow saturation of the topsoil and the subsequent production of runoff. Hillslopes and alluvial fans function as runoff sources and sinks respectively. Permanent aquifers are lacking in Rambla Honda. Variable proportions of hillslope areas may contribute to flash floods in the main channel, but their contribution to the formation of saturated layers within the sediment fill is very limited.

How mudrock and soil physical properties influence badland formation at Vallcebre (Pre-Pyrenees, NE Spain)

Abstract Soils, regoliths and their parent materials were investigated in relation to the development of badlands in the Pre-Pyrenees, in NE Spain, on Late Cretaceous mudrocks under a mountain Mediterranean climate. Whilst all the soils in the area which are developed on mudrocks show a good structure and support a dense plant cover, the bare parent materials slake very rapidly giving rise to badlands characterized by high erosion rates. Several physical, chemical, mineralogical and micromorphological properties of soils, regoliths and related parent materials were analyzed in order to explain their different susceptibility to erosion and to determine which of all these properties are better related to such erosive behaviour. Only micromorphological and porosity properties were able to explain the erosion susceptibility, whilst mineralogical, chemical and physico-chemical (specific surface area) properties were not. Among the aggregate stability tests performed to estimate the relationships of soil components with the susceptibility to erosion, the Emerson test for macroaggregate evaluation, was particularly well suited. Although no good correlation was found between the Emerson test classes and organic matter content, we observed that the degree of slaking in organic-rich horizons, was always lower than in subsurface horizons and regoliths. Consequently, as far as organic rich aggregates are present in the soil surface, the influence of water in producing slaking, swelling and then erosion, will be minimum. However, when regoliths and rocks are uncovered, the influence of water in these particular materials, through freeze-drying and swell-shrinking, will give rise to mudrock desintegration leading to accelerated erosion.

The Effects of Burning and Clearcutting in Forest Floor Layers of Holm Oak Forest

A clearcutting and an experimental fire were carried out in a Ouercus ilex L. forest. The evolution of amounts of forest floor, litter decomposition and respiration was studied. The clearcut plot showed an increase of organic matter because of the input of leaves and branches of slash. The burned plot showed an important loss of OC and N during the first and second years after the fire.