centuries, the Moors developed irrigation and associated agricultural ... in the lower part of the Rambla Honda, 45 km north of Almeria, Spain, (UTM 30S WG.
Hydrological Interactions Between Atmosphere, SoU and Vegetation (Proceedings of the Vienna Symposium, August 1991). IAHS Publ. no. 204, 1991.
Plant Physiological Behaviour in Relation to Water in a Semiarid Environment in Southeast Spain
F. DOMINGO & J. PUIGDEFABREGAS CSIC, Estacion Experimental de Zonas Aridas, 04001 Almeria, Spain S. C. CLARK & L. D. INCOLL Department of Pure and Applied Biology, University of Leeds, Leeds LS2 9JT, UK J. G. LOCKWOOD Department of Geography, University of Leeds, Leeds LS2 9JT, UK ABSTRACT An investigation of the effects on the hydrological balance of type of vegetation, changing land practises and increasing demands for water was started in 1989 in the Rambla Honda in southeast Spain. From May 1989 to October 1990 meteorological data were recorded continuously. Water relations of plants of Stipa tenacissima, dominant on denuded valley sides, and Anthyllis cytisoides, dominant on alluvial fans, were measured at approximately 14 day intervals, as were the water relations of the soils on which they grew. There were significant differences in plant water potential between species and in soil water content. In addition there were significant differences between plants within stands. Soil water content in relation to transpiration was predicted, using the meteorological data, in a sparse vegetation model. Precipitation events and saturation deficits could explain the measured seasonal differences in plant and soil water relations. Predictions from the model were in broad agreement with direct observations. INTRODUCTION Almeria province is the driest in Spain, precipitation can be below 250 mm and the climate is semi-desert. The effects of the low rainfall are exacerbated by the impervious nature of the thin soil in many parts of the region. This increases run-off and reduces infiltration, thereby decreasing the water available for storage and plant growth. The area has a long history of human influence. During the eighth and ninth centuries, the Moors developed irrigation and associated agricultural techniques, that used a high proportion of the rainfall effectively to maintain a stable ecosystem. They cultivated crops that either could tolerate the high temperatures and associated summer drought e.g. olive and apricot, or grew in the cooler and wetter conditions of winter e.g. winter wheat or barley. Their technology maintained a stable agriculture in some areas for as long as seven or eight centuries. The decline in these practises was associated with the "Reconquest" and the Moors' system was abandoned and replaced by pastoral ones where overgrazing with its associated degradation of the vegetation commenced. This system of grazing by sheep and goats still operates to-day. Because of the decline in vegetation cover brought about by grazing, there has been an increase in the ratio of run-off to infiltration which has therefore reduced the yield of water available for storage in the soil and for plant growth. The seriousness of the problems associated with this long term decrease in effective rainfall have become more acute during the last decade, due to increased demands for water on the coast, 335
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from new and extending resorts and from the growers of high water-demanding salad crops. In 1989, a project was started by the Estacion Experimental de Zonas Aridas, Almeria, and the Universities of Leeds and Cambridge, UK, to carry out a detailed investigation of the relationship between vegetation, land form, soil stability and hydrological balance, in a valley 45 km north of Almeria. The project has now become part of the MEDALUS project within the EPOCH programme of the EC which is investigating the water relations, carbon acquisition and phenologies of the predominant species on the valley floor, alluvial fans and valley sides, in relation to seasonal changes in temperature, humidity, precipitation and soil water. This paper is a preliminary report about seasonal and diurnal changes in soil and plant water relations in Stipa tenacissima and Anthyllis cytisoides, the dominant species of the denuded valley sides and alluvial fans respectively. Differences in the measured attributes between plants within stands are also reported, as are the results of using the data in a sparse vegetation model to predict moisture content of the soil at two depths.
DESCRIPTION OF SITE The field site lies in the Betic Cordillera, on the southern slopes of the Filabres Range in the lower part of the Rambla Honda, 45 km north of Almeria, Spain, (UTM 30S WG 555880, 4109600). Records from the weather station at Tabernas, 10 km away, show a mean annual temperature of 17.8°C, and an annual rainfall of 225 mm. Precipitation is concentrated mainly in winter, with approximately 25 mm falling each month from October to April. Severe drought prevails for at least three months in summer. The area forms part of the Nevado-Filabride complex, and includes metamorphic rocks, graphytic mica-schists and some quartzite beds. All are intensively tectonized, with isoclinal folding the dominant structure (Harvey, 1984, 1987). The slope morphology is controlled by the structure of the bedrock. This supports a thin transit regolith sheet, in which plate-shaped pebbles are abundant. This regolith is fed by the rapidly weathering mica-schists. At the base of the slopes, there are extensive alluvial fan systems and colluvia that have developed since the late Pleistocene. The field site is located in the marginal zone between the slopes and fans. The slopes are covered with extensive Stipa tenacissima communities containing variable amounts of Anthyllis cytisoides. Small remnants of the original matoral vegetation also occur infrequently, with Rhamnus lvcioides and Pistacea lentiscus. Abandoned fields that have been uncultivated for widely differing periods, occur on the colluvial formations of the slopes and on the fans. Stands of Rétama sphaerocarpa grow in the abandoned fields on the fans, Artemisia spp and Anthyllis cytisoides form communities in the most recently and the longest abandoned fields respectively. Cereals are cultivated in the lower parts of the fans in wet years and on the lower fluvial terraces near the bed of the rambla, where flood waters increase yields. There is extensive gallery vegetation in the valley bottom where Rétama sphaerocarpa dominates the shrub layer.
METHODS Gravimetric soil water content, total water potential, and leaf relative water content were generally measured every 14 days.
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Particle size distribution of soils Particle size distribution of soil samples from the 0-60 cm horizon from the slopes and alluvial fans was determined by sieving air-dry samples into 16 mm fractions. Gravimetric water content of soils (9) Soils were sampled at randomly chosen co-ordinates in two 20 m x 20 m plots, one for each species. Five cores 10 cm in diameter and 10 cm high, were extracted from the 10 to 20 cm deep layer in each plot. Hourly climatological record An automatic weather station (DIDCOT Instruments, Abingdon) measured total solar and net radiation, wet- and dry-bulb temperatures, wind speed, wind direction and rainfall. Signals were processed by a data logger to produce hourly and daily means. Relative water content (RWC) Relative water content of leaves was measured after Barrs & Weatherley (1962). Total water potential (V) Total water potential of Anthyllis shoots and Stipa leaves was measured at dawn using a pressure chamber apparatus (Forestry and Natural Resources Dept., Edinburgh University). For both RWC and \\f, ten replicate bushes (Anthyllis) or tussocks (Stipa) were used on each occasion. The same plants were used throughout the period. Numerical model of vegetation water balance Soil moisture was estimated by a version of the model for predicting evaporative loss for sparse crops described by Shuttleworth & Wallace (1985). The original model was modified to include a simple description of stomatal conductance which varied with solar radiation, atmospheric water vapour pressure deficit and soil moisture and is described in Lockwood et al. (1989). In this model a bulk canopy resistance is calculated by dividing the inverse of the stomatal conductance by the leaf area index. Interception loss from the vegetation is described by a 'Rutter'-type drainage / evaporation expression with the assumption that drainage occurs when the storage of rainfall on the vegetation exceeds 0.2 mm / leaf area index. A soil surface resistance is calculated which is analogous to the bulk canopy resistance for vegetation. This resistance is estimated using a simple two-layer parameterization of the hydrology of the surface litter / soil system.
RESULTS There were no marked differences in particle size distribution between the soils under the two species. Both soils were stony. There was evidence for heterogeneity in these
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soils in that the 20-60 cm horizon in the soils under Stipa had a higher proportion of the
