María Mardones

La zonificación y evaluación de los riesgos naturales de tipo geomorfológico: un instrumento para la planificación urbana en la ciudad de Concepción

Concepcion city, situated at the central south of Chile, is an urban center that in the course of his history has been affected frequently by naturals risks. In this article, we have separated and evaluated the areas exposed at risks: seismic; of overflowing; of fluvial inundation and removal in mass. For that objective we have adapted the methodology of aggregation of charts of risks factors: danger, vulnerability and exposition, proposed by the NU. As a result we obtained a natural risks letter and we suggest it is application in the preparation of plans and programs prevention and natural risks correction and also in the programs organization of emergencies administration.

The effect of crop-pasture rotations on the C, N and S contents of soil aggregates and structural stability in a volcanic soil of south-central Chile

Abstract The effects of six crop-pasture rotations were evaluated on properties of soil aggregates in a volcanic soil (Humic Haploxerand) of south-central Chile. Rotations that included intensive cropping without pastures, and crops with short- or long-term pastures were maintained for 12 years after which soil samples were taken at 0–5 and 5–10 cm depths for analysis of the C, N and S contents in the different grades of water-stable aggregates. The mean weight diameter of the aggregates was also determined as an indicator of structural stability. The results showed that the rotations which included long-term pastures of alfalfa (Medicago sativa L.) or white clover (Trifolium repens L.) had higher contents of C, N and S in soil and there was a higher concentration of these elements in macro-aggregates (>0.5 mm) than in micro-aggregates (<0.5 mm). Additionally, the rotations with pastures also produced greater structural stability of the soil aggregates. Consequently, crop rotations that included pastures, particularly those of longer duration, improved the soil and were therefore a more sustainable use of the soil resource compared to the more intensive rotations. Additionally, there was evidence of hierarchical organization of the soil structure, which is a previously non-described feature of volcanic soils.

Contrasting mountain and piedmont dynamics of sediment discharge associated with groundwater storage variation in the Biobío River

Supply and transport of sediment in catchments involve processes with fundamental consequences for river management, land use, and the prediction of climate-driven sediment fluxes. In the present study we addressed spatial variability in the water routes through the surface and subsurface of a catchment and the suspended sediment discharge (Qs) over a mountain-piedmont system. We analyzed daily suspended sediment concentration (Cs) and water discharge (Q) measurements at stations located in different topographic settings (mountain and piedmont) in the Biobio River basin (southern central Andes, 37–39°S). In steep catchments, the Q versus Qs relationship has a marked seasonal hysteresis. In the piedmont, Qs is proportional to Q, with no seasonal hysteresis. The contrast in the hysteresis pattern between catchments with different topographies is explained by differences in the routing of rainfall-derived water. In the piedmont, most of the rainfall is converted into surface runoff because the water table is near the surface. In the mountains, groundwater storage results in large seasonal variations in the proportion of Q that flows at the surface and transports sediment from the hillslopes, producing hysteresis. By separating the total Q into two components (direct discharge, Qd and base flow, Qb), we observed the response of Qs to the fraction of water that quickly leaves the catchment after a rainfall event (Qd). Similar results between the mountain and piedmont and the absence of hysteresis simplify the behavior of Qs into a linear relationship with Qd over the entire catchment and lead us to propose that sediment mobilization to the river along the Biobio catchment is primarily controlled by overland flow. Our findings highlight the importance of an adequate hydrological model for understanding the erosion and transport processes of a catchment, and which can be applied to other natural and modeled mountain-piedmont systems.

El patrón hidrográfico de la cuenca del río Blanco: control tectónico y geomorfológico

La geometria y dinamica de la red hidrografica presentan gran sensibilidad en el corto periodo geologico, frente a variaciones tectonicas, climaticas y geomorfologicas. Se analiza y explica el patron hidrografico de la cuenca del rio Blanco, particularmente las capturas y reorganizacion Postglaciar de la red. Para tal finalidad, se desarrollo un analisis morfometrico de tipo hortoniano en Imagenes de Radar de la Mision SRTM de la NASA (2000), con el Programa River Tools y ENVI y en la carta topografica del I.G.M. a escala 1:50.000. El levantamiento geomorfologico se efectuo mediante fotointerpretacion y reconocimiento en terreno. Dos dataciones radiometricas 14C fueron obtenidas de sedimentos morrenicos y lacustres. Se concluye que el patron hidrografico actual del rio Blanco y particularmente las capturas son de origen Holoceno y estarian relacionadas principalmente con factores tectonicos y con los procesos de deglaciacion de la zona austral de Chile. Se consigna una edad aproximada de 2.000 anos para la red hidrografica actual del rio Blanco

Una revisión de inundaciones fluviales en Chile, período 1574-2012: causas, recurrencia y efectos geográficos

RESUMEN Una revision bibliografi ca de inundaciones fl uviales historicas en el periodo 15742012 a traves de diferentes publicaciones, permite analizar, explicar y sistematizar causas o factores detonantes, periodos de recurrencia, distribucion y efectos geografi cos de las inundaciones acaecidas en Chile continental. Segun el factor detonante se identifi can 5 tipos principales de inundaciones asociadas a: procesos volcanicos, procesos nivoglaciares, deslizamientos, intervenciones antropicas y precipitaciones; estas ultimas causan el 71% de las inundaciones en el periodo analizado. A nivel nacional, se observa un patron zonal de los tipos de eventos dividido en 5 areas homogeneas, cuya mayor recurrencia se concentra en el dominio climatico mediterraneo, consistente con montos pluviometricos intensos y cambios ambientales ocurridos durante las ultimas tres decadas. Se constata un incremento de inundaciones catastrofi cas, relacionadas principalmente con procesos nivoglaciares e intervenciones antropicas, desde la segunda mitad del siglo XX a la fecha. Palabras clave: inundacion fl uvial, inundaciones historicas, peligro natural, tipos de inundaciones, Chile.

Hidrogeoquímica y comportamiento del agua subterránea en la cuenca del río Claro, Región del Biobío, Chile

espanolSe presenta un estudio hidrogeoquimico de las aguas subterraneas y superficiales, asociadas a la cuenca hidrografica del rio Claro en el cuadrangulo Yumbel, Region del Biobio, su clasificacion y comportamiento. Para tal efecto, se realizaron los analisis quimicos a 24 muestras de aguas subterraneas, 3 muestras del sistema fluvial del rio Claro y un estudio de la composicion granulometrica de 4 muestras de suelo. De acuerdo a los resultados obtenidos, es posible senalar que las aguas analizadas corresponden al tipo genetico bicarbonatadas calcicas y bicarbonatadas magnesicas. Los tipos de agua calcica y magnesica tienen relacion directa con el terreno que atraviesan (terrenos graniticos meteorizados y arenas negras basalticas, respectivamente). Las aguas mixtas calcico-magnesica y magnesico-calcica, con contenido mas alto de sales totales disueltas, se ubican a lo largo de los limites roca-sedimentos indicando la direccion del flujo subterraneo. El estudio de la composicion granulometrica del suelo permite deducir que el area investigada esta constituida por material residual, producto de la meteorizacion de rocas graniticas y por material acumulado de arenas basalticas proveniente del volcan Antuco, debido a lo cual se presentan fuertes variaciones de porosidad eficaz y de permeabilidad, que son caracteristicas especificas para la infiltracion, movimiento y formacion de acuiferos controlados por la litologia. Del estudio de los analisis quimicos de las aguas subterraneas se concluye que son aptas para el consumo y, tambien, pueden utilizarse en faenas de regadio, sin riesgos de alcalinizacion y salinizacion del suelo. English A hydrogeochemical study of the groundwater and surface water of the Rio Claro hydrographic basin of the Yumbel quadrangle (VIII Region, Biobio), including their classification and behaviour is presented. 24 groundwater samples and 3 surface water samples from the rio Claro system were chemically analysed. The grain size distribution of 4 soil samples was determined. Based on these results, the waters belong to the calcium bicarbonate and magnesium bicarbonate genetic types. The calcium and magnesium water types are in direct relation to the areas crossed, weathered granite and black basaltic sands, respectively. Mixed calcium-magnesium and magnesium-calcium waters, with high contents of total dissolved solids, were found at the igneous rock-sediment boundary, thus indicating the direction of subterraneous flow. Soil granulometry indicates that the area consists of residual material, product of the weathering of granitic rocks and accumulations of basaltic sands from the Antuco Volcano. This results in strong variations of effective porosity and permeability, which are specific characteristics for infiltration, movement and formation of aquifers controlled by lithology. Chemical analyses of subterranean waters demonstrate them to be apt for consumption, and irrigation without risk of soil alkalinisation and salinification.

Hydrogeochemistry and behavior of groundwater in the Río Claro basin, Biobío Region.

Se presenta un estudio hidrogeoquimico de las aguas subterraneas y superficiales, asociadas a la cuenca hidrografica del rio Claro en el cuadrangulo Yumbel, Region del Biobio, su clasificacion y comportamiento. Para tal efecto, se realizaron los analisis quimicos a 24 muestras de aguas subterraneas, 3 muestras del sistema fluvial del rio Claro y un estudio de la composicion granulometrica de 4 muestras de suelo. De acuerdo a los resultados obtenidos, es posible senalar que las aguas analizadas corresponden al tipo genetico bicarbonatadas calcicas y bicarbonatadas magnesicas. Los tipos de agua calcica y magnesica tienen relacion directa con el terreno que atraviesan (terrenos graniticos meteorizados y arenas negras basalticas, respectivamente). Las aguas mixtas calcico-magnesica y magnesico-calcica, con contenido mas alto de sales totales disueltas, se ubican a lo largo de los limites roca-sedimentos indicando la direccion del flujo subterraneo. El estudio de la composicion granulometrica del suelo permite deducir que el area investigada esta constituida por material residual, producto de la meteorizacion de rocas graniticas y por material acumulado de arenas basalticas proveniente del volcan Antuco, debido a lo cual se presentan fuertes variaciones de porosidad eficaz y de permeabilidad, que son caracteristicas especificas para la infiltracion, movimiento y formacion de acuiferos controlados por la litologia. Del estudio de los analisis quimicos de las aguas subterraneas se concluye que son aptas para el consumo y, tambien, pueden utilizarse en faenas de regadio, sin riesgos de alcalinizacion y salinizacion del suelo.