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RSEHN >> Publicaciones >> Bol. R. Soc. Esp. Hist. Nat. 99 (1-4), 2004 >> Trabajos presentados en la VI Reunión de la IPA España

Bol. R. Soc. Esp. Hist. Nat. 99(1-4), 2004


Trabajos presentados en la VI Reunión de la IPA España

Efectos de la nieve y la temperatura del suelo en la actividad geomorfológica: primeros resultados de su monitorización en la Sierra de Guadarrama (España)


Snow cover and ground temperature effects on geomorphic activity: initial results of its monitoring in Sierra de Guadarrama (Spain)

F. Javier de Marcos García-Blanco y David Palacios Estremera

Bol. R. Soc. Esp. Hist. Nat. 99 (1-4): 25-36, 2004

Resumen

Para comprobar la influencia de la permanencia de la nieve y el régimen térmico del suelo, se ha realizado un experimento en el sector culminante de un pequeño nicho de nivación, ubicado en la vertiente oriental del macizo de Peñalara (40º51’N y 3º57’W), Sierra de Guadarrama, entre las cumbres de Hermana Menor (2.269 m) y Hermana Mayor (2.284 m). El experimento ha consistido en el establecimiento de una estación geomorfológica donde se ha monitorizado la extensión y profundidad de la cubierta nival, la temperatura del aire y del suelo, en tres niveles de profundidad, y la movilidad del suelo, desde octubre de 2001 a mayo del 2002. Los resultados obtenidos muestran como la cubierta nival, a pesar del escaso espesor alcanzado durante el periodo monitorizado, impidió un mayor número de ciclos hielo-deshielo, al actuar como aislante térmico de la superficie del suelo respecto a la temperatura del aire. Pero, por otro lado, en los sectores del suelo con abundancia de finos, cooperó a incrementar el levantamiento. En estos casos, suministró humedad al estrato helado, que gracias a este aporte, se fue incrementando a lo largo del prolongado periodo en que el suelo permaneció con temperaturas negativas. El desplazamiento de los clavos en sentido de la pendiente fue efectivo siempre que ésta superara los 15º, alcanzando un desplazamiento máximo de 50 mm en áreas con predominio de finos. Este lento y masivo desplazamiento se pudo relacionar con pequeñas coladas de gelifluxión, alimentadas por el aporte de agua de fusión al subsuelo y su posterior congelación y deshielo. Donde predominaba la fracción más gruesa tan sólo se apreciaron desplazamientos aislados de algunos cantos y gravas, relacionados con la formación de agujas de hielo.

Palabras clave: Geomorfología nival, Nieve, Temperatura del suelo, Guadarrama, España.

Abstract

Recent studies link prolonged snow cover to the effects of geomorphic activity in above timberline areas of Sierra de Guadarrama. To verify this relationship we conducted a study in a small snow hollow located at the summit between two peaks, Hermana Menor (2,269 m) and Hermana Mayor (2,284 m), on the eastern slope of Peñalara massif in Sierra de Guadarrama. We established an experimental plot at 2,235 m on the eastern side of the Hermanas peaks, at the headwall of the snow hollow. The extension and depth of the snow cover, air and ground temperatures and surface movement were monitored from late October 2001 through May 2002. Digital photographs were taken of predetermined georeference points and snow depth was measured with a standardized measuring stick each week. Ground temperature was recorded at 1-hour intervals using three automatic temperature sensors (UTL model) located at three depths: -5 cm, -20 cm and -80 cm. These temperatures were compared to air temperature readings for the same time interval at 160 cm above the ground. Surface movement was determined by measuring variations in the initial position of rows of steel stakes (stake size 5.5 mm × 140.0 mm), bands of vertically and horizontally placed boulders and painted lines during the observation period. The results can be summarized as follows: a) Snow cover: The observation period that began on January 1, 2002, was characterized by a severe winter drought with only 120 days of prolonged snow cover and a snow depth of < 30 cm. b) Ground temperature: The sensor at -5 cm recorded freezing temperatures for a period of almost 160 continuous days from November to May 14. The sensor at -20 cm showed temperatures below 0º C for 149 days from November 27 to April 24, while the sensor at -80 cm recorded freezing temperatures for 131 days from December 14 to April 23. The most significant finding was the reduced number of freeze-thaw cycles: 15 at -5 cm, and only one at -20 and -80 cm. This contrasts with the average of 10 days/month of positive and negative thermal changes recorded by the air temperature sensor for the period November 2001 to May 2002. c) Surface movement: The degree of displacement of the buried stakes varied from no movement in sectors consisting mainly of coarse material (boulders and gravel) to > 50 mm in areas composed primarily of fines (sands and clays). The displacement of the stakes in the direction of the slope frequently occurred if the slope gradient was >15º and the greatest displacement (> 50 mm) took place in areas with fines. In sectors with coarse material and a gradient of < 5º, there were isolated examples of displacement in areas with a few boulders and gravel. The findings revealed that there is significant upward movement of the ground (> 50 mm/yr) caused by frost heave despite the reduced number of freeze-thaw cycles. Although snow cover may be thick, it insulates the ground surface from air temperature fluctuations, which reduces the number of cycles. Snow cover does have a positive effect, however, in that it provides ground moisture which together with many days of below freezing temperatures, facilitates the formation of a layer of subsurface ice. It also promotes ground movement in a direction horizontal to the slope. Likewise, as meltwater is released from the snow cover, surrounding areas may be affected by needle ice which superficially reworks the gravel and sand. In other instances, increased subsurface water and subsequent freezing and thawing can form small gelifluction lobes where fine matrix is abundant.

Keywords: Nival geomorphology, Snow, Ground temperature, Guadarrama, Spain.





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(c) Real Sociedad Española de Historia Natural. Facultades de Biología y Geología. Universidad Complutense de Madrid. 28040-Madrid - e-mail: rsehno@bio.ucm.es