Susanne Schnabel

Soil hydrological response under simulated rainfall in the Dehesa land system (Extremadura, SW Spain) under drought conditions

Soil hydrology was investigated in the Guadelperalon experimental watershed in order to determine the influence of land use and vegetation cover on runoff and infiltration within the Dehesa land system. Five soil–vegetation units were selected: (1) tree cover, (2) sheep trials, (3) shrub cover, (4) hillslope grass and (5) bottom grass. The results of the simulated rainfall experiments performed at an intensity of 56·6 mm h−1 during one hour on plots of 0·25 m2, and the water drop penetration time test indicate the importance of water repellency in the Dehesa land system under drought conditions. Low infiltration rates (c. 9–44 mm h−1) were found everywhere except at shrub sites and in areas with low grazing pressure. Soil water repellency greatly reduced infiltration, especially beneath Quercus ilex canopies, where fast ponding and greater runoff rates were observed. The low vegetation cover as a consequence of a prolonged drought and grazing pressure, in conjunction with the soil water repellency, induces high runoff rates (15–70 per cent). In spite of this, macropore fluxes were found in different locations, beneath trees, on shrub-covered surfaces, as well as at sites with a dominance of herbaceous cover. Discontinuity of the runoff fluxes due to variations in hydrophobicity causes preferential flows and as a consequence deeper infiltration, especially where macropores are developed.

Hydrological behaviour of a small catchment in the dehesa landuse system (Extremadura, SW Spain)

Investigations of the hydrological processes operating in a small experimental catchment representative of the dehesa ecosystem were carried out. The dehesa constitutes a system of agro-silvo-pastoral landuse, which is characterized by a Mediterranean, semi-arid climate. The study includes an analysis of the relationships between rainfall, soil water content and discharge, as well as the establishment of the annual water budget. The results demonstrate a complex hydrological response. The relationships between the factors involved and the operating processes are difficult to explain because of the decisive role played by the valley bottoms. These areas typically possess a sediment fill, and contrast with the shallow soils developed on the hillslopes. Genesis and quantity of runoff (Hortonian or saturation) measured at the outlet depend on the antecedent moisture conditions of the valley bottoms because of their water-retention capacity. Annual runoff coefficients are similar to those reported from other semi-arid areas. The analysis of the annual water budget shows that rainfall is positively related with both actual evapotranspiration and discharge.

Soil and Water Dynamics

Soil properties and water dynamics play a crucial role in the function of oak woodland ranches and dehesas. They are largely controlled by climate conditions, terrain morphology and parent material, but also by land use and management. We review results obtained from research carried out in California and Spain on topics related to soil quality, soil degradation, and water dynamics. Of particular interest is gaining understanding of the influence of land-use and management practices. The distribution of vegetation produces spatial and temporal variation in soil properties that are described in detail. The influence of trees on soil water content is discussed and the dynamics of catchment hydrology is presented, for both California and Spanish cases. An important characteristic is high variability in precipitation, with the occurrence of prolonged dry periods (droughts) that affect water availability for plants. On ranches the effects are two-fold, influencing pasture productivity and water resources for livestock rearing. Soils in the Spanish dehesas have been subject to degradational processes as a consequence of centuries of agricultural use. Water erosion resulting in the reduction of organic matter and physical degradation is the most important phenomena. For California, with a much shorter history of plowing and livestock grazing, we present results from studies on water quality and the effects of vegetation conversion on water yield, soil stability and erosion.

Evaluating the influence of physical, economic and managerial factors on sheet erosion in rangelands of SW Spain by performing a sensitivity analysis on an integrated dynamic model.

An integrated dynamic model was used to evaluate the influence of climatic, soil, pastoral, economic and managerial factors on sheet erosion in rangelands of SW Spain (dehesas). This was achieved by means of a variance-based sensitivity analysis. Topsoil erodibility, climate change and a combined factor related to soil water storage capacity and the pasture production function were the factors which influenced water erosion the most. Of them, climate change is the main source of uncertainty, though in this study it caused a reduction in the mean and the variance of long-term erosion rates. The economic and managerial factors showed scant influence on soil erosion, meaning that it is unlikely to find such influence in the study area for the time being. This is because the low profitability of the livestock business maintains stocking rates at low levels. However, the potential impact of livestock, through which economic and managerial factors affect soil erosion, proved to be greater in absolute value than the impact of climate change. Therefore, if changes in some economic or managerial factors led to higher stocking rates in the future, significant increases in erosion rates would be expected.

Comparison of two methodologies used to estimate erosion rates in Mediterranean ecosystems: 137Cs and exposed tree roots

The 137Cs deposited in soil and exposed tree roots have been widely applied to estimate medium-term soil erosion rates. However, comparative studies between these methods are scarce. For this purpose, three hillsides in two Mediterranean dehesas (rangeland with disperse tree cover) were selected. Regarding the 137Cs technique, a reference site close to the study areas and with similar altitude and rainfall was selected. In order to reduce uncertainties related to the use of point soil profiles, all those collected in an area were combined to form a representative composite profile. The total inventory was 2790±50Bq/m2, and the relaxation coefficient indicated it was an undisturbed soil. The radiocaesium inventory in the study areas was 14-23% lower than in the reference area. The erosion rates for 137Cs were in the range 20.9-38.1tha-1y-1. The exposed root technique was applied to holm oak trees (age about 90years), and the erosion rates were in the range 22-34tha-1y-1. The ratio between exposed root and 137Cs techniques was 1.02±0.11 (S.D.) within the range 0.89-1.2. Both methods produced very similar results equally with respect to the mean erosion rate as well as the relative difference between the hillslope sections, i.e. displaying the same spatial variation in the study areas. As the accounting time for these two techniques is different, 50 and 90y for 137Cs and exposed roots respectively, results suggest that no change in mid-term erosion rates was implied for these areas for almost a century. The use of 137Cs and exposed roots methodology for the determination of mean erosion rates can be reproduced in other ecosystems, but a careful selection of the reference site for 137Cs is essential.

Temporal variability of gully erosion in valley bottoms with dehesa land use system

This paper analyzes gully erosion in valley bottoms of rangelands with a disperse tree cover in SW Spain (dehesas) and its relation with hydrology and land use. The study was carried out in a small catchment (99.5 ha). Two dif- ferent approaches were used; i) first, gullying was determined by means of a repeated survey of topographic cross sections and discharge and rainfall were