Residuos leñosos de gran tamaño en un torrente de la Cordillera de Los Andes, Chile: su funcionalidad e importancia

The study analyzed amounts, characteristics and morphological impact of Large Woody Debris (LWD) in Tres Arroyos torrent of the Chilean Southern Andes draining an old-growth forested basin. All woody debris pieces greater than 10 cm in diameter and 1 m in length were surveyed along a 1.5 km-long stream section presenting a general step-pool/cascade morphology. The total amount of large woody debris within the fluvial corridor was on average 1,500 m3/ha, very high value comparable only to data from old-growth forested basins in the Pacific Northwest of North America. Around two thirds of LWD volume were found in accumulations while half of the LWD elements were located on the active floodplain. As much as 83% of the LWD pieces showed signs of in-stream transport, 13% were directly associated to natural tree falls, and the remaining to landslides and bank erosion. Different types of log-jams were observed, some heavily altering channel morphology (log-steps and valley jams), others just lining the channel edges (bankfull bench jams). The percentage of log-steps over the total number of steps is around 22%, whereas the elevation loss due to LWD (log-steps and valley jams) was 27% the total potential energy. Finally, 2,000 m3 of sediments were estimated to be stored in the main channel behind LWD structures, corresponding to approximately 150% of the annual basin sediment yield

A stochastic model of throughfall for extreme events

International audienceAlthough it is well known that forest canopies reduce the amount and intensity of precipitation at the ground surface, little is known about how canopy interception modifies extreme events. The effects of forest cover on intensity-duration-frequency relationships were investigated, using a stochastic model to extrapolate measured rainfall and throughfall to throughfall expected during extreme events. The model coupled a stochastic model of rainfall with stochastic representations of evaporation and precipitation transfer through canopies. Stochastic evaporation was governed by probability distributions sensitive to storm size, and transfer through canopies was governed by a black-box linear system. The modelled reduction of extreme-event intensities by canopies was 5?30%, depending on duration and return interval. The reduction was 15?20% in low return interval events (2 y) at all durations. In contrast, intensities of high return interval events (90 y) were proportionally more reduced at short durations (~30% reduced) than at long durations (~5% reduced). The model suggested that evaporative losses reduced intensity in the frequent events (2 y return interval), but water transfer through the canopy was more important for the reduction in intensity in the rarest extreme events. High return intervals of long duration were least affected by canopies because evaporative losses were the least proportion of rainfall. Extreme events larger than 10- or 20-y return interval probability threshold occurred only 31?69% as often in throughfall as in rainfall. Keywords: canopy interception, throughfall, stochastic rainfall modelling, rainfall intensity, linear systems, landslide

De la agresión a las palabras. Apuntes para una historia de la violencia política en Chile

La violencia política en nuestra sociedad chilena no es más que la prolongación delpapel de la venganza en la justicia antigua, la figura del juez desaparece en beneficio del poder político, el soberano se convierte en el verdugo y las víctimas que caen bajo su yugo, se transforman en insignificantes individuos absorbidos por la enormidad del poderoso. El vehículo para pensar nuestra violencia, es la historia, a partir de ella analizaremos cómo ha ocurrido antes y en otros lugares, y a su vez, cómo es abandonada y superada por las palabras, lenguaje que importa el respetode la comunidad humana, a lo razonable y verdadero

Movilidad y reclutamiento de material leñoso de gran tamaño en dos cauces de la Cordillera de la Costa de Chile

In-stream large wood mobility was studied in segments of the main channels of Vuelta de Zorra and Pichún catchments, located in the Coastal Mountain Range, Chile. Statistical significant relationships were found only between bankfull channel width and dependent variables associated to some large wood characteristics measured in the channel reaches. This would indicate that large wood length, and not diameter, would regulate the mobility of the wood pieces during high flows. After the winter season of 2009, the 12 and 6.5 % of the wood elements were mobilized in Vuelta de Zorra and Pichún, respectively. The lower percentage in Pichún would be associated to the lower bankfull width/mean piece length ratio. A statistical non-significant trend was found between mobilized distance and piece length/mean bankfull width; indicating that distance decreased with increased dimensionless piece length. Large wood mobility in the two channels was associated to normal peak flows and could be considered as a minimum mobility rate at annual level. The recruited wood volume was higher than the exported wood volume in both segments, therefore the study period can be considered as an in-stream large wood stocking-period

Modelling the Effects of Changes in Forest Cover and Climate on Hydrology of Headwater Catchments in South-Central Chile

This study analyses the changes in the runoff of forested experimental catchments in south-central Chile, to determine to what extent observed trends can be attributed to effects of intensive forestry and/or climate change. For this, we applied the distributed TETIS® model to eight catchments (7.1−413.6 ha) representative of the land uses and forestry activities in this geographical area. Rainfall and runoff data collected between 2008 and 2015 were used for modelling calibration and validation. Simulation of three land uses (current cover, partial harvest and native forest) and 25 combinations of climatic scenarios (percentage increases or decreases of up to 20% of rainfall and evapotranspiration relative to the no-change scenario applied to input series) were used in each calibration. We found that changes in land use and climate had contrasting effects on runoff. Smaller catchments affected by the driest climatic scenarios experienced higher runoff when the forest cover was lower than under full forest cover (plantations or native forests). In contrast, larger catchments under all climatic scenarios yielded higher runoff below the full forest cover than under partial harvest and native forest. This suggests that runoff can be influenced, to a great extent, by rainfall decrease and evapotranspiration increase, with the model predicting up to a 60% decrease in runoff yield for the dry’s climatic scenario. This study proves to be relevant to inform ongoing discussions related to forest management in Chile, and is intended to minimize the impact of forest cover on runoff yield under uncertain climatic scenarios.The authors acknowledge the support from the Economy and Knowledge Department of the Catalan Government through the Consolidated Research Group ‘Fluvial Dynamics Research Group’—RIUS (2017 SGR 459)

Reach scale ecologic influence of in-stream large wood in a Coastal Mountain range channel, Southern Chile

The ecologic influence of in-stream large wood (LW) was evaluated at reach scale in a third-order channel located in the Coastal Mountain Range, southern Chile. The tested hypotheses were that Coarse Particle Organic Matter (CPOM) retention is higher and benthic macro-invertebrates are more diverse in channel reaches storing higher volumes of LW. Three LW-low-volume reaches (mean LW volume of 8 m³ 100m-1 of reach channel length; plane bed morphology) and three LW-high-volume reaches (mean volume 262 m³ 100m-1; forced pool riffle bed morphology) were selected within a 1557 m-long segment of the study channel. CPOM retention was inferred from the difference between the number of leaves collected in the downstream end of each reach and the leaves introduced in the upstream end. Mean CPOM retention was 72 and 90% among LW-low and high volume reaches, and showed a positive correlation (R = 0.61) with LW volume. Macro-invertebrates were more diverse and abundant in LW-high-volume reaches and the richness was twice than in LW-low-volume reaches. Bray-Curtis similarity index was low (40%) between LW-high and low volume reaches, while it was high (> 60%) among LW-low-volume reaches. Results confirm the hypothesis that OM retention and benthic macro-invertebrate diversity and abundance were higher in high LW volume reaches. The higher diversity and abundance of macro-invertebrates associated with LW accumulations confirm that logjams constitute a biotope that offers shelter and food. These results can guide the assessment of environmental impacts of stream intervention and restoration projects. The study was undertaken within the framework of Project Fondecyt 1110609. La influencia ecológica de la madera en el cauce (LW) fue evaluada a nivel de tramo en un canal de tercer orden localizado en la Cordillera de La Costa, sur de Chile. Las hipótesis planteadas fueron que la retención de Materia Orgánica Particulada Gruesa (CPOM) es mayor y que la diversidad de macro-invertebrados bentónicos es también mayor en tramos que almacenan una mayor cantidad de LW. Tres tramos con bajo volumen de LW (volumen medio de 8 m³ 100m-1 de largo de tramo; morfología de lecho plano) y tres tramos con alto volumen de LW (volumen medio de 262 m³ 100m-1; morfología de lecho de rápidos y pozas) se seleccionaron dentro de un segmento de 1557 m de largo en el canal estudiado. La retención de CPOM fue inferida de la diferencia entre el número de hojas recogidas al final de cada tramo y las hojas introducidas al canal en el extremo de aguas arriba. La retención promedio de CPOM fue 72 y 90% en tramos con bajo y alto volumen de LW, y mostró una correlación positiva (R = 0,61) con el volumen de LW. La diversidad y abundancia de macro- invertebrados fuero mayor en tramos con mayor volumen de LW y la riqueza fue el doble que en tramos con bajo volumen de LW. El índice de similaridad de Bray-Curtis fue bajo (40%) entre tramos con alto y bajo volumen de LW, mientras que fue alto (> 60%) entre tramos con bajo volumen de LW. Los resultados confirman la hipótesis que la retención de CPOM y la diversidad y abundancia de macro-invertebrados bentónicos fueron mayores en tramos con mayor volumen de LW. La mayor diversidad y abundancia de macro-invertebrados asociadas con acumulaciones de LW confirman que los diques de madera constituyen un biotopo que ofrece protección y alimento. Estos resultados pueden guiar la evaluación de los impactos ambientales de proyectos de intervención o restauración de cauces. Este estudio fue desarrollado en el marco del Proyecto Fondecyt 1110609

Material leñoso de gran tamaño en dos cuencas de la Cordillera de la Costa de Chile con diferente historia de uso del suelo

Previous researches indicated that in-stream large wood (LW) depended on the characteristics of the riparian vegetation. This is the first study about LW abundance, longitudinal distribution and sediment trapped by large wood structures comparing two stream segments in catchments with contrasting land use, located in the Coastal Mountain Range, Southern Chile. One such catchment (Vuelta de Zorra) is covered mainly by native forests and the other (Pichún) runs under productive plantations. In a stream segment of each catchment the riparian vegetation was characterized, each LW piece was measured and geo-referenced and the channel morphology was defined. Vegetation along the Vuelta de Zorra channel corresponded to an adult second growth forest ca. 200 years old with a basal area of 69 m² ha-1; while in Pichún, it was a much degraded remnant of old native forests with a basal area of 24 m² ha-1. Considering channel bankfull area as reference, LW volume at Vuelta de Zorra and Pichún was 109 and 56 m³ ha-1, respectively. Trapped sediment values in the stream segments were 82 and 30 m3 km-1 for Vuelta de Zorra and Pichún, respectively; in both channels the accumulations of LW played a key role in the amount of trapped sediment. LW volume was associated with the characteristics of the riparian vegetation which is a consequence of the history of forest interventions and land uses in each catchment

Is the reputation of Eucalyptus plantations for using more water than Pinus plantations justified?

The effect of Eucalyptus plantations on water balance is thought to be more severe than for commercial alternatives such as Pinus species. Although this perception is firmly entrenched, even in the scientific community, only four direct comparisons of the effect on the water balance of a Eucalyptus species and a commercial alternative have been published. One of these, from South Africa, showed that Eucalyptus grandis caused a larger and more rapid reduction in streamflow than Pinus patula. The other three, one in South Australia and two in Chile, did not find any significant difference between the annual evapotranspiration of E. globulus and P. radiata after canopy closure. While direct comparisons are few, there are at least 57 published estimates of annual evapotranspiration of either the Eucalyptus or Pinus species. This paper presents a meta-analysis of these published data. Zhang et al. (2004) fitted a relationship between the vegetation evaporation efficiency and the climate wetness index to published data from catchment studies and proposed this approach for comparing land uses. We fitted this model to the published data for Eucalyptus and Pinus and found that the single parameter of this model did not differ significantly between the two genera (p = 0.48). This was also the case for all parameters of an exponential relationship between evapotranspiration and rainfall (p = 0.589) and a linear relationship between the vegetation evaporation index and rainfall (p = 0.155). These results provide strong evidence that, for a given climate wetness index, the two genera have similar annual water use. The residuals compared to the model of Zhang et al. (2004) were significantly correlated with soil depth for Eucalyptus, but this was not the case for Pinus. For Eucalyptus, the model overestimates the vegetation evaporation efficiency on deep soils and underestimates the vegetation evaporation efficiency on shallow soils