Juan Puigdefábregas

The effect of land use on runoff and soil erosion rates under Mediterranean conditions

Abstract The effect of land use and precipitation on annual runoff and sediment loss was investigated in eight different sites along the northern Mediterranean region and the Atlantic coastline located in Portugal, Spain, France, Italy and Greece. These sites represent a variety of landscapes and are under a number of land-uses representative of the Mediterranean region, such as agricultural land with rainfed cereals, vines, olives, eucalyptus plantation or natural vegetation (shrubland). It was found that land use can greatly affect runoff and soil erosion. The greatest rates of runoff and sediment loss were measured in hilly areas under vines (average sediment loss 142.8 t km −2 yr −1 ). Areas cultivated with wheat are sensitive to erosion, especially during winter, generating intermediate amounts of runoff and sediment loss (17.6 t km −2 yr −1 ) especially under rainfalls higher than 280 mm per year. Olives grown under semi-natural conditions, as for example with an understory of vegetation of annual plants greatly restrict soil loss to nil values (0.8 t km −2 yr −1 ). Erosion in shrublands increased with decreasing annual rainfall to values in the range 280–300 mm and then it decreased with decreasing rainfall (average sediment loss 6.7 t km −2 yr −1 ).

Facilitation and succession under the canopy of a leguminous shrub, Retama sphaerocarpa, in a semi-arid environment in south-east Spain

frequently with a growth of herbs in its understorey which contrasts with the surrounding areas by its higher biomass and diversity. We examined the relationships between the shrub and the herbs underneath along a gradient of shrub age. A total of fifty individuals were selected to fit five age classes and shrub characteristics, soil properties and flora under the canopy were examined along the estimated chronosequence. All shrub size variables increased with time, as did the amount of nutrients stored by the shrub, but differences were often significant only between the three oldest classes. Concentration of nitrogen and phosphorus in photosynthetic stems remained constant, but nutrient pools in stem biomass increased with time. The shrubs changed the soil environment under their canopies with age by ameliorating soil texture, nutrient content and capacitance of water. The improvement was most pronounced between the two oldest classes (IV and V), and was probably due to the high biomass of perennial species in the understorey which increased the production of litter, and the interception of wind-blown dust. Plant diversity in the understorey increased with shrub age, likely due to a greater heterogeneity under larger canopies. Drought-resistant species, typical of the open areas between shrubs, were displaced from the centre of the understorey by taller, more mesic species over the age gradient. Retama sphaerocarpa shrubs benefited from the increase of resources in the understorey and showed a higher reproductive output with shrub age, but decreased at the beginning of senescence. The indirect interactions between R. sphaerocarpa and its understorey herbs could be considered as a two-way facilitation in which both partners benefit from their association.

Hydrological and erosion response of a badlands system in semiarid SE Spain

A long term monitoring program is being conducted to study runoff and erosion processes in a semiarid badlands environment (Tabernas Desert, SE Spain). The first six years of data from an instrumented experimental area with nested microcatchments are presented. The overall area is composed of a complex mosaic of soil surfaces with contrast hydrological and erosion behaviour. At microcatchment scale, runoff and erosion are controlled by the types of soil surfaces: small bare microcatchments had the highest runoff coefficients and the highest erosion rates, while those completely covered by vegetation had the lowest. Rainfall intensity significantly affected water and sediment budgets. The effect of antecedent soil moisture could only be observed when soil was near saturation and a few millimetres of additional rainfall were sufficient to produce Horton-type runoff, but it was very difficult to separate this from the effect of surface crusts formed in the first minutes of rainfall. Most of the rainfall events were below the threshold for producing runoff although they were important for sediment preparation through weathering. Small magnitude, low-intensity rainfall events along with protective plant cover over half of the total surface, are the main factors explaining low overall erosion rates at microcatchment scale.

Interactions of plant growth and sediment movement on slopes in a semi-arid environment

Abstract The morphology, and architecture of the tussocks of alpha grass (Stipa tenacissima) as well as its associated micro topography have been described in several sites on the south versant of the Filabres Range, in southern Spain. The annual rainfall of this area is between 225 and 275 mm, and plant cover ranges from 20 to 45%. The results show the formation of micro topographic mounds upslope of the tussocks which are interpreted as deposition bodies originating from the interception of downslope sediment movement by the vegetated patches. The tussock architecture shows an outer growing zone, a middle mature zone and a central senescent zone. The outward expansion of the tussock is performed by tillers in the growing zone which bend at the nodes and root when they come into contact with the soil surface. The mat of dead leaves inside the tussock may prevent the stems from bending and this factor plays a significant role in controlling tussock growth. A similar effect on slopes is produced by the mound of sediments which accumulate upslope of the tussock and press upon the growing stems. A simple simulation model has been built from the above field observations. The model is able to reproduce the tussock shapes found in nature and predicts that at low plant rates, the tussocks become unstable and collapse, while at higher levels of plant vigour and sediment movement rates, sedimentation occurs upslope of the tussock and erosion downslope. These foci of sedimentation and erosion are crescent shaped and their relative importance depends on the splash factor.

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.

Measurement and modelling of rainfall interception by three semi-arid canopies

The main aims of this study were, firstly, to adapt the rainfall interception model of Rutter et al. (Agric. Meterology, 1971, 9, 367‐384) to individual plants of two semiarid shrubs (Anthyllis cytisoides L. and Retama sphaerocarpa (L.) Boiss.) and a tussock grass (Stipa tenacissima L.) and secondly, to understand how the different canopy structures influence rainfall partitioning by individual plants. The selected species represent contrasting canopy types typical of vegetation of semiarid areas. Free throughfall coefficients were estimated from field measurements of low volume rainfall events and vertical photographs taken beneath the plant canopy. Canopy drainage curves were measured by continuous weighing of wetted plants. Canopy boundary layer conductances were calculated by measuring the evaporation of water from wet canopies. Field measurements of gross rainfall, throughfall and stemflow were taken for each rainfall event for A. cytisoides and R. sphaerocarpa. The Rutter type model of rainfall interception was adapted for individual shrubs and tested with measured rainfall events showing a good agreement between observed and predicted values for R. sphaerocarpa and for A. cytisoides. The interception model was then run to simulate interception loss during actual rainfall events, using atmospheric conditions measured every 5 s. The results from this simulation showed significant differences in interception loss between species, which can be explained by differences in canopy drainage and boundary layer conductance, and are caused primarily by the structural differences in their canopies. R. sphaerocarpa gave lower interception than the other two species, S. tenacissima gave higher interception, while A. cytisoides had an intermediate value. The low interception loss by R. sphaerocarpa can be explained by its low total area index, thus, high free throughfall and high canopy drainage rate per unit projected canopy area. On the other hand, S. tenacissima and A. cytisoides, show a low free throughfall and drainage rate per unit projected canopy area because of their higher aerial biomass density. The ecological implications of these adaptations are discussed. # 1998 Elsevier Science B.V. All rights reserved.

An investigation of rooting depth of the semiarid shrub Retama sphaerocarpa (L.) Boiss. by labelling of ground water with a chemical tracer

1. (1) Water containing lithium chloride as a chemical tracer was applied to the bottom of two survey wells 16 and 28 m deep and the concentration of Li+ in young cladodes of Retama sphaerocarpa shrubs growing around the wells was monitored for 9 days after the subterranean application. 2. (2) The mean concentration of Li+ in the young cladodes of the shrubs increased from a natural background of 1.8 ± 0.8 μg Li+ g−1 dry mass to 91 ± 18 μg g−1 within 24 h of application. 3. (3) The mean concentration of Li+ was significantly higher in shrubs around the shallower well where a maximum of 184 ± 52 μg g−1 was measured 5 days after application. 4. (4) At the deeper well, mean concentration of Li+ averaged over the 9 day period of observation was positively correlated with shrub size.

Effects of soil and vegetation on runoff along a catena in semi-arid Spain

Runoff and infiltration were investigated on abandoned fields of patchy vegetation in semi-arid Spain during 15 months of natural rainfall and by rainfall simulations. The aim was to ascertain sources and sinks of runoff and the effects of soils and plant cover. Soils of the catena developed from mica schists of the upper hillslopes, fan deposits of the lower hillslopes, and an alluvial terrace at the bottom. Runoff from natural events were from three sets of three pairs each of 10 × 2 m runoff plots. The pairs of each set had different densities of plant cover; the sets were vegetated with tussock grass, Stipa tenacissima, a shrub, Anthyllis cytisoides, and a bush, Retama sphaerocarpa. Nineteen natural rainfall events of intensities up to 18 mm/h produced 400 mm of rain during the study period. Because the rainfall threshold for runoff production was about 20 mm, only eight events produced runoff. The rainfall simulations used a sprinkler that produced 50 mm/h of rain for 30 minutes; runoff was recorded each minute in 0.24 m2 bounded plots. The depth and structure of the soil mantle provide the main controls on runoff rates, which are lowest on the lower fan deposits and highest on the thin upslope soils. The river-bank terrace, with a surface covered by crusts and mosses, also yields relatively high runoff. In general, vegetation density varies inversely with runoff. Nevertheless, shrub and bush litter favor runoff, as does a particular spatial distribution of individual plants on the hillslope. Settling of the upper few centimeters of soils of the alluvial fan following cessation of cultivation 15 to 40 years ago has produced a near-surface compacted layer favoring shallow subsurface runoff. Apparently contradictory results between runoff plots and rainfall simulations are the result of differing processes.

Response of high mountain landscape to topographic variables: Central pyrenees

An objective method for inductively modelling the distribution of mountain land units using GIS managed topographic variables is presented. The landscape of a small high mountain catchment in the Spanish Pyrenees, covered with grassland, was classified in ten land units by hierarchical agglomerative clustering, using a sample of 194 random plots, in which classes of vegetation, soils and landforms were defined. Additionally, seven layers of topographic variables (altitude, slope angle, aspect, solar radiation, topographic wetness index, specific catchment area, and regolith thickness) were created from a Digital Elevation Model. The affinity of each land unit to the topographic variables was calculated using Binary Discriminant Analysis (BDA), after dichotomising the latter around their mean values. Then, the distribution of each land unit was predicted by boolean operations combining step by step distributions for the seven topographic variables ordered, for each unit, after the absolute values of the Haberman’s residuals in BDA. The predicted distributions were tested (χ2) against that of the observed sampling plots. From the original ten land units, the distributions of eight of them were successfully predicted (four are related to the slope sequence, two reflect the water accumulation in the soil, and two respond to geomorphic processes) while the remaining two had to be rejected. Part of the catchment (39%) was not assigned to any land unit, probably because more distributed variables accounting for snow distribution are necessary.

Evaluating the long-term water balance of arid zone stream bed vegetation using evapotranspiration modelling and hillslope runoff measurements

The difference between long-term actual evapotranspiration (AET) and precipitation (P) provides a useful indication of the extent to which a site retains or loses water resources, and therefore of the likely occurrence of specific land degradation processes. Sink areas OAET q PU receive lateral water inputs from other parts of the catchment. In arid and semi-arid environments these areas are frequently found in, or next to, the stream beds of ephemeral rivers and are often characterised by intensive land use or high conservation values. For both types of land use it is important to know if, and how much, AET exceeds P, and where the lateral water inputs come from. Thick sedimentary fills in the stream bed, variable climate conditions and ephemeral flow conditions pose specific difficulties to the evaluation of the water balance of these sites. The objective of this study was to develop an approach to explore the relative importance of lateral water inputs to shrub stands growing in thick sedimentary fills of semi-arid ephemeral rivers. The approach is based on (i) estimating long-term AET2 P balances in the channel sediments and (ii) assessing whether these inflows originate mainly from surrounding hillslopes or from the upstream part of the catchment. A physically based evapotranspiration model for sparse vegetation was used to estimate the long-term AET rates. The relative importance of hillslope runoff and channel flow was evaluated in a semi-quantitative fashion from a combination of surface area estimates and mostly published values of soil hydrological parameters. The approach was developed and tested in a selected stand of Retama sphaerocarpashrubs in a stream bed at the Rambla Honda field site (Tabernas,