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.

Land-use changes affecting Mediterranean mountains represent the intensification of use in valley bottoms, accompanied by land-use conflicts, and a generalized abandonment of the hillslopes, which in the past were perfectly integrated in the system of land management. Farmland abandonment, reforestation, diminution of the livestock pressure and substitution of cereal crops by meadows are the most outstanding features of the recent land-use changes. The question is whether the new spatial organisation is in accordance with a longterm policy of sustainable development in mountain areas. The results obtained confirm that farmland abandonment on steep slopesand the resulting colonization of old fields by a dense shrub coverand afforestation contribute to control both soil erosion and surface runoff. As a result some of the most important rivers and alluvial fans have recently stabilized their sedimentary structures.

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This paper studies the hydrological response to rainstorm events of a small experimental catchment in the Central Spanish Pyrenees. The Arnás catchment was cultivated until 40 years ago, and then abandoned and affected by plant recolonisation, especially shrubs. A rainfall of a few mm is enough to produce a sudden increase in discharge, due most probably to the steep gradients and the small size of the catchment and the extensive areas with low vegetation density and thin soils. The intensity of the response shows a very high variability, depending on the intensity of precipitation and soil humidity conditions before the flood. This paper identifies two types of floods according to the relationships between precipitation and discharge, and confirms that antecedent soil moisture explains much of the response. The shape of the hydrograph, very similar to the hyetograph, suggests that the Arnás catchment is dominated by overland flow processes. However, more intense rainstorms do not generate higher peak flows, thus demonstrating the existence of different runoff generating areas.

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On the evening of the 7 August 1996 an intense storm occurred over the Ara s catchment near Biescas in the central Pyrenees. Eighty-seven people were killed as a result of the subsequent¯ood, which hit a campsite located on the alluvial fan at the outlet of the 18 . 8 km 2 catchment. This paper presents the main results of a hydromorphological study of the event. The Bete s subcatchment received the most intense rainfall, estimated at somewhat in excess of 250 mm, which resulted in a peak¯ow from this tributary of 300 m 3 s À1 . Just downstream from the Bete s river junction,¯ow in the main channel reached 400 m 3 s À1 , increasing to around 500 m 3 s À1 further downstream. Rainfall in the larger Aso tributary was less intense, and in the head reaches¯ow remained within-bank, representing a one in two-year return period event.Flow from this tributary did not exceed 100 m 3 s À1 , indicating that the Bete s subcatchment supplied some 75% of thē ow from just 28 . 7% of the catchment area. The extreme¯ows caused the collapse of a series of sediment trap dams in the Ara s channel downstream of the Bete s junction. This resulted in the addition of 68 000 m 3 of sediment to an already disastrous event. Data from other rain gauges in the area showed both the extremely local nature of the event, and the problems of return period analysis for such storms, whose peaks are rarely observed at gauges. Together with the high geomorphological risks of the zone, this leads to the conclusion that a new method of spatial and temporal risk analysis is required for infrastructure planning.

In light of the significance that ecosystem service research is likely to play in linking conservation activities and human welfare, systematic approaches to measuring, modeling and mapping ecosystem services (and their value to society) are sorely needed. In this paper we outline one such approach, which we developed in order to understand the links between the functioning of the ecosystems of Tanzania’s Eastern Arc Mountains and their impact on human welfare at local, regional and global scales. The essence of our approach is the creation of a series of maps created using field-based or remotely sourced data, data-driven models, and socio-economic scenarios coupled with rule-based assumptions. Here we describe the construction of this spatial information and how it can help to shed light on the complex relationships between ecological and social systems. There are obvious difficulties in operationalizing this approach, but by highlighting those which we have encountered in our own case-study work, we have also been able to suggest some routes to overcoming these impediments.

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