Small Mediterranean Catchment Research Articles

Modelling the hydrological response of mediterranean catchments, Prades, Catalonia. The use of distributed models as aids to hypothesis formulation

This paper attempts to extend the physical arguments underlying the distributed TOPMODEL concepts in an application to the strongly seasonal contributing area responses in two adjacent small mediterranean catchments in the Prades region of Catalonia, Spain. A perceptual model of hydrological response in these catchments is used to suggest possible modifications of the model in a hypothesis testing framework, including an attempt to modify the topographic index approach to reflect the expansion of the effective area of subsurface flow during the wetting-up sequence. It is found that slight improvements in modelling efficiency are possible but that different model parameter distributions are appropriate for different parts of the record. The model was much more successful for the catchment producing the higher runoff volumes. © 1997 John Wiley & Sons, Ltd.

Geochemistry and water dynamics: application to short time-scale flood phenomena in a small Mediterranean catchment: I. Alkalis, alkali-earths and Sr isotopes

We report major, trace elements and Sr isotope data for water samples taken regularly during a four-day-long September flood of a Mediterranean river, the Vène (Hérault, S. France). The objective is to combine all these data into a dynamic model that describes the origin(s) and movements of waters and their loads. This river drains the runoff from a small, mainly carbonate, partly karstified watershed with Miocene and Jurassic lithologies. The watershed is also impacted by both agricultural and urban activities. Both the dissolved and the particulate loads were analyzed. Concentrations of the dissolved components show major remobilization of almost all elements during the first few hours of the flood (water treatment plants and aerosol scavenging), followed by a sharp concentration decrease. Some major species return to their previous summer values (Ca, HCO 3) while others reach low `background' levels (Na, K, Cl, S0 4). Some trace elements (Rb, Sr, Cs) show similar behaviour but (Ba) appears somewhat unaffected. Trace element concentrations and ratios define two main periods (three in the suspended particulate matter). Ratios do not allow distinguishing between the three main sources for the dissolved load in the first period (Miocene, Jurassic, water treatment plants), but clearly show the Jurassic karst influence later on. The 87Sr /86Sr of the suspended particulate matter is more variable and more radiogenic than in the dissolved phase. Variations in concentration ratios and Sr isotope composition in particulates indicate the large and variable contribution of Miocene silicates with some carbonate. However, there is a need for another component with [Rb]/[Sr] higher than bedrocks, internal or external to the watershed, possibly due to differential erosion. Dissolved Ca and Mg fluxes during the flood were calculated at 0.26 ton and 0.029 ton/km 2, respectively. Even though the carbonate nature of the watershed restricts variability in Sr isotope composition in the dissolved load, we distinguish several endmembers: seawater (≈marine rain), Miocene marls, Jurassic limestones, water treatment plants (and possibly another attributable to fertilizers). Combined with major and trace element variations, Sr isotope fluctuations indicate time-varying proportions of different water endmembers at the outflow and suggest a general dynamic model. Based on PCA (principal component analysis), a 3D representation allows to visualize the geochemical evolution of the Vène waters. In particular, Sr isotopes clearly indicate that the inflow of karstic waters during the flood was not continuous but occurred as a series of marked oscillations between flowing waters with chemical signature of Miocene lithologies and increasing flushes of deeper waters that interacted with Jurassic lithologies.

Validation of catchment models for predicting land-use and climate change impacts. 2. Case study for a Mediterranean catchment

Validation methods commonly used to test catchment models are not capable of demonstrating a model's fitness for making predictions for catchments where the catchment response is not known (including hypothetical catchments, and future conditions of existing catchments which are subject to land-use or climate change). This paper describes the first use of a new method of validation (Ewen and Parkin, 1996. J. Hydrol., 175: 583–594) designed to address these types of application; the method involves making ‘blind’ predictions of selected hydrological responses which are considered important for a particular application. SHETRAN (a physically based, distributed catchment modelling system) is tested on a small Mediterranean catchment. The test involves quantification of the uncertainty in four predicted features of the catchment response (continuous hydrograph, peak discharge rates, monthly runoff, and total runoff), and comparison of observations with the predicted ranges for these features. The results of this test are considered encouraging.

The relative influence of riparian habitat structure and fish availability on otter Lutra lutra L. sprainting activity in a small Mediterranean catchment

The influence of riparian habitat structure and fish availability on otter Lutra lutra marking activity was studied at the head of the Guadalete river (southern Spain). Otter habitat in the study area could be ordered along two gradients in structural properties (altitude, stream order, channel form, water current, bank vegetation), water quality and bankside plant species composition. Generally, higher altitude sites had more dense bankside vegetation cover and better water quality. The dominant upstream bankside plant assemblage consisted of species such as Rubus sp., Salix spp., and Arundo donax, which provided secure shelter for otters. In contrast, at downstream sites the typical plant assemblage was Tamarix africana, Nerium oleander and helophytes, overall bankside cover was lower but wider and the water was more eutrophic. Otter sprainting activity was correlated with this gradient, being higher in downstream sites, especially in areas dominated by short vegetation and scarce human presence. Similarly, fish biomass was greater downstream. A strong positive correlation was observed between otter sprainting activity and larger fish size. Thus, the variability in otter marking intensity along the upper Guadalete seemed to be more dependent on fish availability, and secondly on human disturbance, than on particular riparian habitat features, including the species composition of vegetation growing on the river banks. Implications for conservation management of otters in Mediterranean areas are discussed.