Abstract:Multivariate statistical analysis was used to explore relationships between catchment topography and spatial variability in snow accumulation and melt processes in a small headwater catchment in the Spanish Pyrenees. Manual surveys of snow depth and density provided information on the spatial distribution of snow water equivalent (SWE) and its depletion over the course of the 1997 and 1998 melt seasons. A number of indices expressing the topographic control on snow processes were extracted from a detailed digital elevation model of the catchment. Bivariate screening was used to assess the relative importance of these topographic indices in controlling snow accumulation at the start of the melt season, average melt rates and the timing of snow disappearance. This suggested that topographic controls on the redistribution of snow by wind are the most important influence on snow distribution at the start of the melt season. Furthermore, it appeared that spatial patterns of snow disappearance were largely determined by the distribution of snow water equivalent (SWE) at the start of the melt season, rather than by spatial variability in melt rates during the melt season. Binary regression tree models relating snow depth and disappearance date to terrain indices were then constructed. These explained 70-80% of the variance in the observed data. As well as providing insights into the influence of topography on snow processes, it is suggested that the techniques presented herein could be used in the parameterization of distributed snowmelt models, or in the design of efficient stratified snow surveys.
19The hydrological and geomorphic effects of land use/land cover changes, 20 particularly those associated with vegetation regrowth after farmland abandonment were 21 investigated in the Central Spanish Pyrenees. The main focus was to assess the 22 interactions among slope, catchment, basin, and fluvial channel processes over a range 23 of spatial scales. In recent centuries most Mediterranean mountain areas have been 24 subjected to significant human pressure through deforestation, cultivation of steep 25 slopes, fires, and overgrazing. Depopulation commencing at the beginning of the 20th 26 century, and particularly since the 1960s, has resulted in farmland abandonment and a 27 reduction in livestock numbers, and this has led to an expansion of shrubs and forests.
The present paper analyses and quantifies the sensitivity of snowpack to climate change, and assesses implications for snow processes in the central Pyrenees under the climatic conditions projected by a set of 9 regional climate models (RCMs) for the end of the 21st century, under the IPCC emission scenario SRES A2. The methodology was based on comparison of the snow series obtained by simulating the energy balance of a snow surface driven by climatic conditions recorded during the period from 1996 to 2006, with snow series obtained by simulations for the same period that included various changes in magnitude for each of the climatic drivers (sensitivity analysis); and the changes projected by the RCMs to the end of the 21st century. Results showed a marked sensitivity of snowpack duration and thickness to shifts in temperature, precipitation and solar radiation. RCMs suggest that the most significant changes expected in the study area are related to temperature, which is the main parameter responsible for the predicted changes in future snow processes. A large coherence was found among the simulations made using the projections of the 9 regional climate models. Comparison with respect to current conditions indicated a decrease of 50 to 60% in maximum snow water equivalent, the occurrence at least 1 mo earlier of the maximum snow water equivalent, and a reduction in the duration of the snowpack by around 2 mo. In addition, the 3 snow parameters will be subject to a marked increase in inter-annual variability compared to the observed conditions. KEY WORDS: Snow · Snow energy balance · SEB · Climate change · Regional climate models · RCMs · Pyrenees Resale or republication not permitted without written consent of the publisherClim Res 36: 203-217, 2008 from December to April in areas >1500 m above mean sea level (a.s.l.), with a longer duration of snow cover at higher altitudes and in shaded areas (García-Ruiz et al. 1986, López-Moreno & Nogués-Bravo 2005. In this region, snow controls many ecological processes and also has important socio-economic implications, mainly due to (1) the large contribution of snow melt to the amount and seasonal distribution of runoff in Pyrenean river basins, which plays a major role in water management in the semiarid and highly populated Ebro valley (López-Moreno & García-Ruiz 2004) and (2) the significant development of winter tourism in recent decades, which represents one of the main sources of revenue for the region. Thus, this area could be particularly sensitive to the impact of expected climatic change in the 21st century (López-Moreno et al. 2008).Within this context, the aim of the present study was to assess the impact of predicted climate change on snow processes in the Pyrenees. For this purpose, the snow energy balance (SEB) in the area was simulated for climatic conditions in the recent past (1996 to 2006) and for future conditions driven by projections of 9 different regional climate models (RCMs) for the end of the 21st century. The use of a multi-mod...
The hydrological and geomorphological effects of an exceptional 14 rainstorm event that occurred in the central Spanish Pyrenees during 19-21 October 15 2012 were studied in five experimental catchments under various land covers: (i) 16 subhumid badlands; (ii) dense forest; (iii) an abandoned farmland area recolonized by 17 shrubs and forest patches; and (iv) subalpine grasslands. Hydrographs and sedigraphs 18 demonstrated that vegetation cover is a major factor affecting the control of floods even 19 during exceptional rainstorms, at least at the spatial scale at which the phenomenon was 20 studied (catchment sizes: 0.3-2.8 km 2 ) and under dry catchment conditions. The 21 combined precipitation over the two days (ca. 250 mm) was the greatest for any two-22 day event recorded since 1950 in the central-western Pyrenees for all but one of the 23 stations in the study. Five pulses of most intense rainfall were recorded. The forested 24 catchment did not react to the two most intense rainfall pulses, because of the very low 25 antecedent level of the water table. The main peak flow occurred only when at least a 26 part of the catchment was saturated. The abandoned farmland catchment had two small 27 peak discharges at the beginning of the event, which were produced by infiltration 28 excess overland flow from eroded areas close to the main stream. During the third most 29 intense rainfall period a large part of this catchment contributed to runoff and a 30 relatively high peak discharge was produced. The badland catchment reacted 31immediately from the beginning of the rainstorm, yielding very high discharges 32 accompanied by high suspended sediment concentrations. The subalpine catchment 33 showed a hydrograph mirroring the hyetograph, with brief but intense hydrological 34 responses to increased precipitation, because of the marked gradients and the presence 35 of bare rock in the headwaters. A high volume of bedload was carried during the peak 36 discharge. Mediterranean region, particularly in coastal areas where rainfall of more than 200 mm 44 in 24 hr is commonly recorded, generally in autumn (López Bermúdez and Romero 45 Díaz, 1993; Poesen and Hooke, 1997; González Hidalgo et al., 2003). These rainstorms 46 are usually caused by warming of the Mediterranean Sea at the end of summer and the 47 occurrence of cold advection at high atmospheric levels, which results in 48 thermodynamically unstable cut-off systems (Nieto et al., 2005) having a duration of 49 two or three days. As a consequence, the most extreme flash floods within Europe tend 50 to occur in the Mediterranean region (Gaume et al., 2009), with some studies reporting 51 a change in flow from zero to hundreds of cubic meters per second in minutes 52 (Camarasa Belmonte and Segura Beltrán, 2001). 53Very large and intense rainstorms also occur in the interior of the Iberian 54Peninsula. These are typically localized and affect relatively small areas. This was the 55 case for the Rivillas ravine in the Guadiana River basin, where a flash ...
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