The Chillán River in Central Chile plays a fundamental role in local society, as a source of irrigation and drinking water, and as a sink for urban wastewater. In order to characterize the spatial and temporal variability of surface water quality in the watershed, a Water Quality Index (WQI) was calculated from nine physicochemical parameters, periodically measured at 18 sampling sites (January-November 2000). The results indicated a good water quality in the upper and middle parts of the watershed. Downstream of the City of Chillán, water quality conditions were critical during the dry season, mainly due to the effects of the urban wastewater discharge. On the basis of the results from a Principal Component Analysis (PCA), modifications were introduced into the original WQI to reduce the costs associated with its implementation. WQI DIR2 and WQI DIR , which are both based on a laboratory analysis (Chemical Oxygen Demand) and three (pH, temperature and conductivity), respectively, four field measurements (pH, temperature, conductivity and Dissolved Oxygen), adequately reproduce the most important spatial and temporal variations observed with the original index. They are proposed as useful tools for monitoring global water quality trends in this and other, similar agricultural watersheds in the Chilean Central Valley. Possibilities and limitations for the application of the used methodology to watersheds in other parts of the world are discussed.

The Biobío River basin is of high strategic importance for Chilean development, at both the regional and the national levels. For this reason, advances in the general understanding of, and in the capacity to describe and predict, in a spatially explicit way, the impact of climate and anthropogenic forcing on the hydrology of the Biobío basin are urgently needed. The work presented here attempts to set the basis for future modelling applications within the Biobío basin by analysing the applicability of a readily available modelling tool, the SWAT model, to one of its sub-basins. Modelling results show that the model performs well in most parts of the study basin. The SWAT model application for the Vergara basin confirms that SWAT is a useful tool and can already be used to make a preliminary assessments of the potential impacts of land-use and climate changes on basin hydrology.Key words hydrological modelling; SWAT; calibration; Chile; Biobío Modélisation hydrologique avec SWAT en conditions de données peu disponibles: évaluation des résultats d'une application au ChiliRésumé Les eaux du bassin du Biobío sont d'une importance stratégique pour le développement du Chili, au niveau régional comme au niveau national. Pour cette raison, il est urgent d'avancer dans la compréhension générale de l'hydrologie du bassin, afin de pouvoir décrire et prévoir, d'une manière spatialement explicite, l'impact du forçage climatique et anthropique. Le travail présenté ici prétend établir la base pour les futurs efforts de modélisation du bassin du Biobío, en analysant l'applicabilité d'un outil de modélisation existant, le modèle SWAT, à un de ses sous-bassins. Les résultats indiquent que le modèle a une bonne performance dans la presque totalité du bassin étudié. L'application du modèle SWAT au bassin de Vergara confirme qu'il est un outil utile, qui peut d'ores et déjà être utilisé pour faire des évaluations préliminaires des impacts potentiels des changements d'occupation du sol et de climat sur l'hydrologie du bassin.

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RESUMENEn enero de 2006 se estudiaron los patrones espaciales de distribución de comunidades de macroinvertebrados bentónicos de la cuenca hidrográfica del río Baker (45º50' O y 47º55' S) y los principales factores controladores, intentando cubrir la mayor variedad de ecosistemas lóticos. Para llevar a cabo el estudio se seleccionaron 27 estaciones de muestreo ubicadas en las diferentes subcuencas del río. En cada estación se realizó una caracterización fisicoquímica del agua (conductividad, oxígeno disuelto, pH, temperatura y turbidez), y se documentaron las características del tramo de río (e.g. ancho del cauce y tipo de sedimento) e información cartográfica utilizando un sistema de información geográfica (SIG). Se identificaron un total de 51 taxa que correspondieron en su mayoría a larvas de insectos (80 %). Los grupos con mayor riqueza fueron los órdenes Ephemeroptera (15 taxa), Plecoptera (8 taxa) y Trichoptera (8 taxa). Los análisis de clasificación y ordenación realizados con los datos de abundancia, permitieron reconocer siete grupos de estaciones diferentes (A-F) que fueron estadísticamente significativos (P < 0.05). El análisis de regresión múltiple, muestra que la variable ambiental que mejor explica los patrones biológicos observados, fue el porcentaje de cobertura de bolones, explicando el 44 % de la varianza de los datos biológicos (r 2 = 0.44 P < 0.05).Palabras clave: cuenca del río Baker, hábitat fluvial, macroinvertebrados bentónicos, número de Froude, ríos patagónicos. ABSTRACTIn January of 2006 we studied the distributional patterns of benthic macroinvertebrate communities of the Baker river basin (45º50' O and 47º55' S) and their main controlling factors trying to cover the greater variety of the lotic ecosystems. To carry out the study, 27 sampling stations were located in the different sub basins of the river. In each station, physical-chemical parameters of the column of water were quantified (conductivity, dissolved oxygen, pH, temperature and turbidity), and documented characteristics of the segment (e.g. wide of the channel and sediment type) and cartographic information using a geographic information system (GIS) and complemented with cartographic information using GIS. Identified a total of 51 taxa, are mostly insect larvae (80 %). The groups most richness were orders Ephemeroptera (15 taxa), Plecoptera (8 taxa) and Trichoptera (8 taxa). The classification and ordination analyses carried out with the data of abundance, allowed to recognize seven groups stations (A-F) statistically significant (P < 0.05). The multiple regression analysis, showed that the environmental variable that accounts for the observed biological patterns, was coverage cobble, explaining 45.1 % the variance of the biological data (r 2 = 0.44 P < 0.05).