Hydrological processes and their seasonal controls in a small Mediterranean mountain catchment in the Pyrenees

The Vallcebre catchments are located in a middle mountain area of the Pyrenean ranges, built up by sedimentary rocks and loamy soils. The vegetation cover is pastures and forests of Pinus sylvestris, mostly occupying former agricultural terraces. Some relatively small, heavily eroded landscapes (badlands) occur in the catchments, playing a relevant hydrological and geomorphic role. Annual precipitation is 924 mm and potential (reference) evapotranspiration is about 700 mm. Rainfall interception in forests represents about 24% of precipitation; interception rates were similar throughout the seasons because of a compensation between rainfall intensities and atmospheric conditions. Soil moisture showed a temporal pattern characterised by the occurrence of marked deficit periods in summer and also, but less pronounced, in winter. During most of the year, subsurface flows on hillslopes drove the spatial organisation of soil moisture and the occurrence of saturated areas. Nevertheless, this spatial organisation was also controlled by the patterns of vegetation cover. During dry periods, subsurface flow ceased, saturated areas disappeared and the spatial patterns of soil moisture changed. Stream flow from these catchments was dominated by storm flow, and the runoff generating mechanisms showed a clear seasonal pattern, controlled mainly by the soil moisture and the extent of saturated areas. During the dry periods, runoff was produced only on impervious areas and badlands. At the end of the dry periods, some large rainfall events generated significant runoff because of the perched saturation of the shallow soil horizons. Thereafter, runoff generation was dominated by the role of saturated areas. Stream waters in catchments with badlands had very high suspended sediment concentrations. The seasonal pattern of erosion processes in badlands was characterised by physical weathering during winter, regolith breakdown and vigorous hillslope erosion during spring and summer, and efficient transport of sediments in autumn.This research was supported by the projects VAHMPIRE (ENV4-CT95-0134) funded by the European Commission and EPROHIDRO (HID98-1056-C02-01) funded by the Programa Nacional de Recursos Hídricos, Spanish Government. The Vallcebre catchments also operate with support from the RESEL network, through an agreement between CSIC and DGCONA.Peer Reviewe

Streamflow response and water-table dynamics in a sub-Mediterranean research catchment (Central Pyrenees)

11 páginas, 6 figuras, 1 tabla.Rainfall–runoff relationships at the event scale were studied in a small sub-Mediterranean research catchment in the central Spanish Pyrenees (Arnás catchment, 2.84 km2 in area). The strong non-linearity between these two variables evidenced the high variability in the hydrological response of the catchment, linked to seasonal dynamics of the water reserves. Both the magnitude of rainfall events and the baseflow at the start of each event had a pronounced influence on the hydrological response, while the rainfall intensity played a major role in terms of discharge increment. Water-table data suggested a strong topographic control on saturation patterns and showed high seasonal dynamics, with a dry period during summer and a wet period during winter. The transition between dry and wet periods was characterised by high spatial variability in the water-table pattern. An analysis of streamflow and water-table response during a series of four events revealed the complexity of the hydrological processes operating within the catchment. During dry conditions, infiltration excess over localised areas is likely to be the main runoff process, whereas during wet conditions both saturated excess runoff and subsurface flow are most likely to be the dominant runoff processes. During the wetting-up period, the hydrological response was variable, depending mainly on the depth of the water table before the event and rainfall depth and intensity. In these conditions, the interpretation of runoff processes is not straightforward and requires further research.This research was financially supported by the following projects: ‘‘Modelling hydrological processes in gauged catchments for predictions in non-gauged catchments’’ (CANOA, CGL2004-04919-C02-01/HID) and ‘‘Processes and sediment balances at different spatial scales in Mediterranean environments: Effects of climate fluctuations and land use changes’’ (PROBASE, CGL2006-11619/HID), both funded by the Spanish Ministry of Education and Science, and ‘‘Validation of a coupled model for the simulation of hydrological and hydraulic processes using data from experimental catchments in a Mediterranean mountain’’ (PM088/2006) funded by the DGA. Monitoring of the catchment was supported by an agreement between the CSIC and the Spanish Ministry of Environment (RESEL). N. Lana-Renault is grateful to the CSIC for financial support within the framework of an I3P PhD grant. J. Latron has benefited from a research contract (Juan de La Cierva programme), funded by the Spanish Ministry of Education and Science.Peer reviewe

Effects of sample and grid size on the accuracy and stability of regression-based snow interpolation methods

15 páginas, 11 figuras, 1 tabla.This work analyses the responses of four regression-based interpolation methods for predicting snowpack distribution to changes in the number of data points (sample size) and resolution of the employed digital elevation model (DEM). For this purpose, we used data obtained from intensive and random sampling of snow depth (991 measurements) in a small catchment (6 km2) in the Pyrenees, Spain. Linear regression, classification trees, generalized additive models (GAMs), and a recent method based on a correction made by applying tree classification to GAM residuals were used to calculate snow-depth distribution based on terrain characteristics under different combinations of sample size and DEM spatial resolution (grid size). The application of a tree classification to GAM residuals yielded the highest accuracy scores and the most stable models. The other tested methods yielded scores with slightly lower accuracy and varying levels of robustness under different conditions of grid and sample size. The accuracy of the model predictions declined with decreasing resolution of DEMs and sample size; however, the sensitivities of the models to the number of data points showed threshold values, which has implications (when planning fieldwork) for optimizing the relation between the effort expended in gathering data and the quality of the results.This work has been supported by the research projects CGL2008-01189/BTE, CGL2006-11619/HID, and CGL2008-1083/CLI financed by the Spanish Commission of Science and Technology and FEDER, EUROGEOSS (FP7-ENV-2008-1-226487), and ACQWA (FP7- ENV-2007-1- 212250) financed by the VII Framework Programme of the European Commission, ‘Las sequías climáticas en la cuenca del Ebro y su respuesta hidrológica’ and ‘La nieve en el Pirineo aragonés: Distribución espacial y su respuesta a las condiciones climática’ Financed by ‘Obra Social La Caixa’ and the Aragón Government and ‘Programa de grupos de investigación consolidados’ financed by the Aragón Government.Peer reviewe

Effects of sample and grid size on the accuracy and stability of regression-based snow interpolation methods

15 páginas, 11 figuras, 1 tabla.This work analyses the responses of four regression-based interpolation methods for predicting snowpack distribution to changes in the number of data points (sample size) and resolution of the employed digital elevation model (DEM). For this purpose, we used data obtained from intensive and random sampling of snow depth (991 measurements) in a small catchment (6 km2) in the Pyrenees, Spain. Linear regression, classification trees, generalized additive models (GAMs), and a recent method based on a correction made by applying tree classification to GAM residuals were used to calculate snow-depth distribution based on terrain characteristics under different combinations of sample size and DEM spatial resolution (grid size). The application of a tree classification to GAM residuals yielded the highest accuracy scores and the most stable models. The other tested methods yielded scores with slightly lower accuracy and varying levels of robustness under different conditions of grid and sample size. The accuracy of the model predictions declined with decreasing resolution of DEMs and sample size; however, the sensitivities of the models to the number of data points showed threshold values, which has implications (when planning fieldwork) for optimizing the relation between the effort expended in gathering data and the quality of the results.This work has been supported by the research projects CGL2008-01189/BTE, CGL2006-11619/HID, and CGL2008-1083/CLI financed by the Spanish Commission of Science and Technology and FEDER, EUROGEOSS (FP7-ENV-2008-1-226487), and ACQWA (FP7- ENV-2007-1- 212250) financed by the VII Framework Programme of the European Commission, ‘Las sequías climáticas en la cuenca del Ebro y su respuesta hidrológica’ and ‘La nieve en el Pirineo aragonés: Distribución espacial y su respuesta a las condiciones climática’ Financed by ‘Obra Social La Caixa’ and the Aragón Government and ‘Programa de grupos de investigación consolidados’ financed by the Aragón Government.Peer reviewe

Internal evaluation of a physically-based distributed model using data from a Mediterranean mountain catchment

An evaluation of the performance of a physically-based distributed model of a small Mediterennean mountain catchment is presented. This was carried out using hydrological response data, including measurements of runoff, soil moisture, phreactic surface level and actual evapotranspiration. A-priori model parameterisation was based as far as possible on property data measured in the catchment. Limited model calibration was required to identify an appropriate value for terms controlling water loss to a deeper regional aquifer. The model provided good results for an initial calibration period, when judge in terms of catchment discharge. However, model performance for runoff declined substantially when evaluated againts a consecutive, rather drier, period of data. Evaluation against other catchment responses allowed identification of the problems responsible for the observed lack of model robustness in flow simulation. In particular, it was shown that an incorrect parameterisation of the soil water was preventing adequate representation of drainage from soils during hydrogeraph recessions. This excess moisture was then being removed via an overestimation of evapotranspiration. It also appeared that the model underestimated canopy interception. The results presented here suggest that model evaluation against catchment scale variables summarising its water balance can be of great use in identifying problems with model parameterisation, even for distributed models. Evaluation using spatially distributed data yielded less useful information on model performance, owing to the relative sparseness of data points, and problems of mismatch of scale between the measurement and the model grid.This work was carried out as part of project VAHMPIRE (Validating Hydrological Models using Process Studies and Internal Data from Research Basins: tools for assessing the hydrological impacts of environmental change), which was funded by the European Commission Framework IV Environment and Climate Program (Contract No. ENV4- CT95-0134). Simulations were carried out on a UNIX workstation funded jointly by UK Nirex Ltd. and NERC grant GR3/ E0009.Peer Reviewe

El papel del bosque en los procesos hidrológicos: ejemplos en las cuencas de investigación de Vallcebre (Prepirineo catalán)

The Vallcebre research catchments are representative of Mediterranean mountain areas with spontaneous afforestation by Scots pine (Pinus sylvestris L.) as a consequence of the abandonment of agricultural practices since the middle of the XX century. In this context, hydrological processes have been modified as a consequence of the emergence of a mosaic of different land covers, formed by patches of the original oak forest (Quercus pubescens Willd.) or of Scots pine forest scattered in the old abandoned terraces mainly covered by mesophilous meadows. Water fluxes and stores have been studied at the Vallcebre research catchments with a twofold objective, the understanding of the processes describing the water use by different vegetation covers in areas with larger water deficits and the extrapolation of this knowledge at catchment scale by means of hydrological modelling.Las cuencas de Vallcebre son representativas de áreas de montaña Mediterránea con forestación espontánea por pino albar (Pinus sylvestris L.) consecuencia del abandono de las labores agrícolas desde mediados del siglo XX. En este contexto, los procesos hidrológicos se han visto modificados como consecuencia de la aparición de un mosaico de cubiertas vegetales, formado por manchas del bosque original de robles (Quercus pubescens Willd.) o de pino albar dispersas en las terrazas de cultivo abandonadas, estas últimas, cubiertas mayoritariamente por prados mesófilos. Los flujos hídricos y compartimentos a escala de cada una de estas cubiertas vegetales están siendo estudiados en las cuencas de investigación de Vallcebre con la doble finalidad, comprender el funcionamiento de los procesos descriptores del uso de agua por diferentes cubiertas vegetales en áreas con importantes déficits hídricos y extrapolar estos conocimientos a escala de cuenca a partir de la modelización hidrológica

El papel del bosque en los procesos hidrológicos: ejemplos en las cuencas de investigación de Vallcebre (Prepirineo catalán)

The Vallcebre research catchments are representative of Mediterranean mountain areas with spontaneous afforestation by Scots pine (Pinus sylvestris L.) as a consequence of the abandonment of agricultural practices since the middle of the XX century. In this context, hydrological processes have been modified as a consequence of the emergence of a mosaic of different land covers, formed by patches of the original oak forest (Quercus pubescens Willd.) or of Scots pine forest scattered in the old abandoned terraces mainly covered by mesophilous meadows. Water fluxes and stores have been studied at the Vallcebre research catchments with a twofold objective, the understanding of the processes describing the water use by different vegetation covers in areas with larger water deficits and the extrapolation of this knowledge at catchment scale by means of hydrological modelling.Las cuencas de Vallcebre son representativas de áreas de montaña Mediterránea con forestación espontánea por pino albar (Pinus sylvestris L.) consecuencia del abandono de las labores agrícolas desde mediados del siglo XX. En este contexto, los procesos hidrológicos se han visto modificados como consecuencia de la aparición de un mosaico de cubiertas vegetales, formado por manchas del bosque original de robles (Quercus pubescens Willd.) o de pino albar dispersas en las terrazas de cultivo abandonadas, estas últimas, cubiertas mayoritariamente por prados mesófilos. Los flujos hídricos y compartimentos a escala de cada una de estas cubiertas vegetales están siendo estudiados en las cuencas de investigación de Vallcebre con la doble finalidad, comprender el funcionamiento de los procesos descriptores del uso de agua por diferentes cubiertas vegetales en áreas con importantes déficits hídricos y extrapolar estos conocimientos a escala de cuenca a partir de la modelización hidrológica

Cross-site Comparison of Variability of DOC and Nitrate c–q Hysteresis during the Autumn–winter Period in Three Mediterranean Headwater Streams: A Synthetic Approach

23 páginas, 8 figuras, 2 tablas.The forms, rotational patterns and trends of hysteretic loops of dissolved organic carbon (DOC) and nitrate (NO3) were investigated in three headwater Mediterranean streams during one autumn–winter period using two biogeochemical descriptors summarizing the changes in solute concentrations (ΔC) and the overall dynamics of each hysteretic loop (ΔR). The study had two aims: (1) to examine whether the variability of solute hysteretic loops monitored in different streams during cold seasons followed a consistent and recurring pattern, (2) to identify hydrological parameters which could potentially influence features of the DOC and NO3 hysteresis. Relationships between hysteresis features and hydrological parameters in the studied streams were explored using multivariate redundancy analysis (RDA). Both DOC and NO3 typically increased in concentration during storm events, although hysteretic loops did not obey any consistent pattern across the three streams. The rotational patterns of DOC and NO3 hysteresis ranged widely, from clockwise to counterclockwise. Storm hydrographs and also the magnitude of antecedent storm events were explanatory of the DOC and NO3 concentration changes across the study sites. However, the detailed hydrological information did not offer a satisfactory explanation of the entire DOC and NO3 concentration dynamics during the storm events.This study was supported the Comisión Interministerial de Ciencia y Tecnología (CICYT, C). Work by AB and JL was financed by European funds post doctoretl contract (I3P-2002/2 and I3P-2003/4 respectively). AB, EV and FS are members of the Limnology Group (CEAB (CSIC)-UB).Peer reviewe

Cross-site Comparison of Variability of DOC and Nitrate c–q Hysteresis during the Autumn–winter Period in Three Mediterranean Headwater Streams: A Synthetic Approach

23 páginas, 8 figuras, 2 tablas.The forms, rotational patterns and trends of hysteretic loops of dissolved organic carbon (DOC) and nitrate (NO3) were investigated in three headwater Mediterranean streams during one autumn–winter period using two biogeochemical descriptors summarizing the changes in solute concentrations (ΔC) and the overall dynamics of each hysteretic loop (ΔR). The study had two aims: (1) to examine whether the variability of solute hysteretic loops monitored in different streams during cold seasons followed a consistent and recurring pattern, (2) to identify hydrological parameters which could potentially influence features of the DOC and NO3 hysteresis. Relationships between hysteresis features and hydrological parameters in the studied streams were explored using multivariate redundancy analysis (RDA). Both DOC and NO3 typically increased in concentration during storm events, although hysteretic loops did not obey any consistent pattern across the three streams. The rotational patterns of DOC and NO3 hysteresis ranged widely, from clockwise to counterclockwise. Storm hydrographs and also the magnitude of antecedent storm events were explanatory of the DOC and NO3 concentration changes across the study sites. However, the detailed hydrological information did not offer a satisfactory explanation of the entire DOC and NO3 concentration dynamics during the storm events.This study was supported the Comisión Interministerial de Ciencia y Tecnología (CICYT, C). Work by AB and JL was financed by European funds post doctoretl contract (I3P-2002/2 and I3P-2003/4 respectively). AB, EV and FS are members of the Limnology Group (CEAB (CSIC)-UB).Peer reviewe

Instalación y uso de un colchón de nieve para la monitorización del manto de nieve. Cuenca Experimental de Izas (Pirineo Central)

13 páginas, 3 figuras, 2 fotos..[EN]: Snowpack density is one of the most difficult hydroclimatic variables to be monitored in mountain environments. Snow pillows are a commonly used tool for this purpose. Despite they have been widely used in North America, Scandinavia, and several mountains areas, they had not been installed yet in any Spanish mountain system. In 2005, a snow pillow was installed in Izas Experimental basin (Central Pyrenees). Since then, it has provided information on the evolution of density and snow water equivalent (SWE) for a three years period. This paper reviews the history of the use of snow pillows, specifics of its utilization and considerations to have into account when they are installed. Moreover, results obtained during the three monitored years are presented. Several uncertainty sources are detected, and the usefulness for understanding the hydrological behaviour of high mountain environments is highlighted.[ES]: La densidad del manto de nieve resulta una de las variables hidroclimáticas de montaña más difíciles de medir. Los colchones de nieve (snow pillows) representan la herramienta más utilizada para este fin. A pesar de que su uso es generalizado en zonas de Norte América y Escandinavia, y se cuentan con ejemplos de su uso en otras zonas de montaña, no había sido utilizado hasta el momento en la montaña española. En el año 2005 se instaló un colchón de nieve en la Cuenca Experimental de Izas (Pirineo Central) y desde entonces ha proporcionado información de la evolución de la densidad de nieve y el equivalente del manto de nieve en agua (SWE) durante tres años. Este trabajo revisa la historia del uso de este instrumento, las peculiaridades de su uso y los requisitos a considerar durante su instalación. Además, se presentan los resultados obtenidos durante los tres años disponibles detectando algunas fuentes de incertidumbres y destacando su utilidad para la comprensión del comportamiento hidrológico de zonas de alta montaña.Este trabajo ha sido financiado con los proyectos CGL2008-01189/BTE, CGL2006-11619/HID y CGL2008-1083/CLI, financiados por la Comisión Española de Ciencia y Tecnología y FEDER, y "Las sequías climáticas en la cuenca del Ebro y su respuesta hidrológica" y "Lanieve en el Pirineo aragonés: Distribución espacial y su respuesta a las condiciones climáticas", financiados por la Obra Social de La Caixa y el Gobierno de Aragón, y el Programa de grupos de investigación consolidados financiados por el Gobierno de Aragón.Peer reviewe