Linking Land Cover Changes in the Sub‐Alpine and Montane Belts to Changes in a Torrential River

Abstract

Channel changes are the consequence of changes in sediment yield from the slopes and in the connectivity between slopes and channels because of distinct land use and climate impacts. In this study, we investigated the characteristics and evolution of a short reach in the headwater of the Ijnez River, central-southern Pyrenees. Assessment of a series of sedimentary and geomorphic structures confirmed major changes to the valley bottom, mainly related to changes in the intensity of human activity. The oldest sedimentary structure is a terrace level located 3 to 4 m above the current alluvia] plain. General deforestation, overgrazing and recurring fires in the montane belt (1100-1600 m a.s.1.) have led to increased soil erosion and connectivity, and to the triggering of debris flows that have been deposited on the fluvial terrace. Woody fragments from within the debris flows were dated using accelerator mass spectrometry "C radiocarbon techniques (AMS), yielding ages between 100 and 115cal years BP, which coincides with the period of maximum deforestation and human density in the Pyrenees. Depopulation and farmland abandonment since the beginning of the 20th century has resulted in generalized natural and artificial reforestation, a shrinkage of the eroded areas and a decline in connectivity between slopes and the channel. The most important consequence has been channel incision and narrowing, and the development of a sediment armour layer. Active sediment transport is continuing, although there has been a decrease in sediment yield from the slopes.

Los efectos geoecológicos del cambio global en el Pirineo Central español: una revisión a distintas escalas espaciales y temporales

“…Ese es el caso de las realizadas en el Prepirineo, dentro de la cuenca de drenaje del embalse de La Peña (río Gállego) o las del valle del río Ijuez, una cuenca extremadamente degradada a mediados del siglo xx y que fue casi completamente reforestada. En ambos casos se ha detectado una notable disminución de la producción de sedimento (García-Ruiz & Ortigosa, 1988;Sanjuán et al, 2014) y de su tamaño, dando lugar además al restablecimiento de la vegetación en los cauces y orillas . Sin embargo, Ortigosa (1991) demostró en el Sistema Ibérico riojano que la utilización de técnicas agresivas de repoblación (construcción de terrazas de bulldozer) no había contribuido en muchos casos a un descenso de la erosión del suelo.…”

“…Pero, en general, las áreas erosionadas han visto reducida su extensión en los últimos 50 años (ver, por ejemplo, el caso de la cuenca del río Ijuez: Sanjuán et al, 2014) y se limitan cada vez más a los sectores que sufrieron una degradación extrema por la agricultura nómada (slash-and-burn o shifting agriculture), o por el paso frecuente del ganado. Además, algunas de las principales fuentes de sedimento conectan difícilmente con la red fluvial.…”

Catchment based hydrology under post farmland abandonment scenarios

“…Similarly, a decrease in connectivity in the central part of the Ebro basin was found to be directly related to a 71% decrease in agricultural land between 1957 and 2010 (Lizaga et al, 2016). Revegetation of former grazing areas in the subalpine belt of the Central Pyrenees was found to lead to morphological changes in the river channel (incision and development of a terrace level) and to a subsequent reduction in connectivity between the slopes and the channel (Sanjuán et al, 2016).…”

“…Hydrologically, the growth of vegetation directly affects interception, infiltration, evapotranspiration and soil storage of water, usually reducing runoff production and peak flows (López-Moreno et al, 2008;Lana-Renault et al, 2011). Vegetation expansion also causes a shrinkage of sediment sources (Bakker et al, 2008;Lana-Renault and Regüés, 2009) and a decrease in sediment supply from hillslopes to channels (Piégay et al, 2004;Quiñonero-Rubio et al, 2016), reducing the sedimentation levels of reservoirs, rivers and coastal areas (Keesstra, 2007;López-Moreno et al, 2011) and altering stream morphology, including the narrowing and incision of alluvial plains (Beguería et al, 2006;Sanjuán et al, 2016). The mechanisms by which changes in a landscape (e.g., due to farmland abandonment) affect the transfer of water and sediment through a drainage basin are a "hot topic" of hydrological and geomorphological research and have led to the increased use of the concept of hydrological connectivity.…”

Land use and land cover change based on historical space–time model

“…Single dynamic degree of land use and cover refers to the number of some kind of land use and cover change in a certain time in a study area (Sanjuán et al, 2016), and its ex-…”