A better knowledge of hydrological processes is essentials for water resources management in terms of water quantity (floods and droughts) as well as water quality (pollution). 
The hydrological functioning of Mediterranean systems is still largely unknown despite several studies have been carried out during the last twenty years. Progresses in the identification and modelling of hydrological processes are almost entirely due to research in temperate-humid climate (Bonell y Balek, 1993; Buttle, 1994). According to Bonell (1993), the lack of knowledge forces to ‘the transfer of results’ in spite of the clear need to develop different approaches particularly concerning catchment modelling (Pilgrim et al. 1988).  
Concerning the hydrological modelling, the available studies (Durand et al., 1992; Parkin et al., 1996; Piñol et al., 1997 among others) show serious models difficulties in reproducing the first autumnal discharge events, just after the dry summer period. Moreover, generally it seems difficult to model the hydrological behaviour of Mediterranean systems with just one set of parameters (Piñol et al., 1997, Bernal et al., 2004).
Mediterranean climate is characterized by a marked seasonality of rainfall and evapotranspiration processes, which produces alternating wet and dry periods throughout the year. This strongly modifies catchment moisture conditions, which leads to a complex and non linear hydrological behaviour (Piñol et al. 1999).
The need to understand the hydrological functioning of a system responds to two important issues: one is the most appropriate procedure to provide useful elements for the integrated management of water resources and at the same time it is essential for modelling the behaviour of nutrients such as nitrate, due to its high solubility. 
During the last decades, nitrate export has become a major concern in river systems because of increases in both atmospheric deposition of nitrogen and diffuse transport from agricultural land uses (Vitousek et al., 1979). Quantifying inorganic nitrogen loads and the mechanisms that govern its dynamic at catchment scale is essential to predict the effect that would occur in water quality due to changes in land use or climate (Payraudeau et al., 2001). 
Water quality modelling, which is generally a complex issue, is even more difficult when it concerns Mediterranean systems, characterized by alternating wet and dry conditions that lead to highly non-linear hydrological and biological behaviours (Bernal et al., 2004; Medici et al., 2008). In fact, the variation in the availability of some resource can significantly alter the ecosystem functioning, especially with respect to bacterial population dynamics and organic matter and nutrients cycle. To this end, arid and semiarid environments represent systems in which the availability of resources such as water, is intermittent and where such availability is represented by ‘pulses’ within long dry periods (Schwinning et al., 2004a). 
The task of developing parsimonious and robust models with which to understand and predict the movement of inorganic nitrogen in Mediterranean-type catchments is difficult but extremely necessary (Neal et al., 2002, Liu et al., 2005). Dynamic, process-based models of pollutant sources and catchment dynamics are necessarily complex because they attempt to describe all factors and processes so that the relative importance of these may be understood and investigated in response to environmental change (Dean et al., 2009). However, models will always necessarily be simplification of reality. These simplifying assumptions are a source of uncertainty in a model, and the robustness of any model application will be dependent upon the validity of the assumptions made. Sensitivity analysis provides model users with information regarding the effect of model parameters on the resultant model prediction. 

The study case of this PhD thesis is the Fuirosos catchment that is located in the northern slopes of Catalan Littoral Range, near Barcelona (Spain). The drainage area at the Fuirosos flowgauge station is approximately 13 km2 and it drains an intermittent stream. 
The first part of this research has been focused on the catchment hydrological modelling. A progressive perceptual understanding approach was used in order to identify a model structure able to represent the non-linear behaviour of the hydrological cycle in a small intermittent Mediterranean stream. 
The initial lumped model structure consisting in a series of three connected water tanks (LU3) progressed to a model with four tanks (LU4), and finally to a semidistributed model structure (SD4) in which spatial variability of the evapotranspiration according to the vegetation cover and to the local aspect was considered. In the final model structure, which gave the best fit (Nash-Sutcliffe efficiency index = 0.78), an additional tank representing the riparian zone was included (SD4-R). Results showed that the abrupt changes of the riparian water table during summer and the formation of a perched water table during the transition from dry to wet conditions were the main mechanisms leading to the non-linear hydrological behaviour. The transpiration process from the saturated zone and the spatial variability of evapotranspiration resulted key factors to successfully represent the annual water balance. The spatial and temporal validations carried out for each of the four model structures considered in this study supported the hypothesis adopted during the calibration process. 
The aim of the second part of this work was to couple a nitrogen (N) sub-model to already existent hydrological lumped (LU4-N) and semi-distributed (LU4-R-N and SD4-R-N) conceptual models, to improve our understanding of the factors and processes controlling nitrogen cycling and losses in Mediterranean catchments. The N model adopted provides a simplified conceptualization of the soil nitrogen cycle considering mineralization, nitrification, immobilization, denitrification, plant uptake, and ammonium adsorption/desorption. It also includes nitrification and denitrification in the shallow perched aquifer. We included a soil moisture threshold for all the considered soil processes. The results suggested that all the nitrogen processes were highly influenced by the rain episodes and that soil microbial processes occurred in pulses stimulated by soil moisture increasing after rain. The riparian zone was a key element to simulate the catchment nitrate behaviour and our simulation highlighted it as a possible source or sink of nitrate depending on the period of the year and the soil moisture conditions. 
In the last part of the work the developed models (LU4-N, LU4-R-N y SD4-R-N) have been examined according to an extensive general sensitivity analysis based on 100,000 Monte Carlo simulations (GSA, Hornberger and Spear, 1980 and GLUE, Beven and Binley, 1992). The aim of this part of the study was to determine if additional model complexity actually gives a better capability to model the hydrology and nitrogen dynamics of the Fuirosos catchment. The results obtained highlighted the most complex structure (SD4-R-N) as the most appropriate one representing the non-linear behaviour of this small Mediterranean catchment.