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Abstract Dissolved solids (DS) and suspended sediment (SS) loads are considered relevant environmental problems. They are related to a wide range of on-site and off-site impacts, such as soil erosion or salinization of water bodies. In this study, the dynamics of DS and SS concentrations and loads were assessed in five small watersheds covering representative agricultural land uses in Navarre (Spain). To this end, discharge, DS and SS concentration data were collected during ten hydrological years at each watershed outlet, and loads were computed from discharge and concentration values. DS concentration followed a seasonal pattern imposed by the availability of water, with higher concentrations recorded in low-flow periods and lower concentration in the high-flow period. SS concentration was extremely variable, with a range of 2–4 orders of magnitude in concentration for any specific discharge. Temporal variations (both intra- and inter-annual) in DS loads were explained by differences in runoff, whereas those of SS were not, being the SS loads associated mainly with specific high flow events. These temporal patterns were observed for both agricultural (this study) and non-agricultural (literature) watersheds. From the data in the Navarrese watersheds and those available in the literature, we inferred that agricultural land use, in general, tends to increase the concentration of both DS and SS. Regarding DS and SS yields, the effects of agricultural land use on DS yields are controlled by the changes in runoff rather than the (small) changes in DS concentration. In this sense, land uses changes expected to increase runoff (i.e., a shift from forested to arable or from rainfed to irrigated agriculture) would increase DS yields. On the other hand, agricultural land use tends to increase SS yields, although the effect is highly variable depending on site-specific factors, both natural (e.g., watershed shape) and anthropogenic (e.g., degree of soil conservation practices). In the Navarrese watersheds, DS yields ranged from 1.1 to 2.2 Mg ha−1 year−1 whereas SS yields ranged from 0.3 to 4.3 Mg ha−1 year−1. DS yields seem to dominate under non-agricultural conditions and in most agricultural land uses at the small watershed scale. On the other hand, SS yields dominate in watersheds with increased soil erosion as a consequence of arable land use over erosion-prone watersheds. Highlights • Dissolved solids concentration follows a seasonal pattern. • Suspended sediment concentration was extremely variable. • Dissolved loads behave conservatively with water. • The dissolved load dominated the exportation in most of the cases. • Agricultural land use increases suspended sediment concentration and loads. [ABSTRACT FROM AUTHOR]

Summary: In this study the water draining from a mixed land use catchment of 16km2 in NW Spain was monitored over a 5-year period (2004/2005–2008/2009) to determine the temporal variability in phosphorus (P) transport, to examine the P fluxes and controlling factors, and to analyse the relationships between discharge and P (PP: particulate phosphorus and DP: dissolved phosphorus) at event scale. A total of 132 runoff events was analysed, providing an extensive dataset on P values under different hydrological conditions. Mean annual P export from the catchment amounted to 184kg equivalent to 11.5kgkm−2, a value relatively lower than those obtained in other mixed land use catchment. The PP accounted for 74% of the total phosphorus (TP). The intra- and inter-annual variability in P transport was high due to the fact that most of P was discharged during a short-time period: 35% of PP load and 19% of DP load occurred in only 2% of the time. Runoff events contributed to 68% of P exported during the period of study. A high variability in P transport during different runoff events was observed. Phosphorus export increased with the magnitude of runoff events and rainfall amount, suggesting that these could be the key factors controlling P yield. Erosion was also an important driving force behind P export in the catchment. The relationships between PP and DP and discharge resulted in different hysteresis patterns. The relationship PP-discharge was mainly characterised by clockwise behaviour, linked to sediment in the surface runoff, while DP-discharge was predominately anticlockwise due to transport in subsurface flow, showing different pathways of transport between PP and DP. [Copyright &y& Elsevier]

Abstract: The hydrological and geomorphic effects of land use/land cover changes, particularly those associated with vegetation regrowth after farmland abandonment were investigated in the Central Spanish Pyrenees. The main focus was to assess the interactions among slope, catchment, basin, and fluvial channel processes over a range of spatial scales. In recent centuries most Mediterranean mountain areas have been subjected to significant human pressure through deforestation, cultivation of steep slopes, fires, and overgrazing. Depopulation commencing at the beginning of the 20th century, and particularly since the 1960s, has resulted in farmland abandonment and a reduction in livestock numbers, and this has led to an expansion of shrubs and forests. Studies in the Central Spanish Pyrenees, based on experimental plots and catchments, in large basins and fluvial channels, have confirmed that these land use changes have had hydrological and geomorphic consequences regardless of the spatial scale considered, and that processes occurring at any particular scale can be explained by such processes acting on other scales. Studies using experimental plots have demonstrated that during the period of greatest human pressure (mainly the 18th and 19th centuries), cultivation of steep slopes caused high runoff rates and extreme soil loss. Large parts of the small catchments behaved as runoff and sediment source areas, whereas the fluvial channels of large basins showed signs of high torrentiality (braided morphology, bare sedimentary bars, instability, and prevalence of bedload transport). Depopulation has concentrated most human pressure on the valley bottoms and specific locations such as resorts, whereas the remainder of the area has been affected by an almost generalized abandonment. Subsequent plant recolonization has resulted in a reduction of overland flow and declining soil erosion. At a catchment scale this has caused a reduction in sediment sources, and channel incision in the secondary streams. At the regional scale, the most important consequences include a reduction in the frequency of floods, reduced sediment yields, increasing stabilization of fluvial channels (colonization of sedimentary bars by riparian vegetation and a reduction in the braiding index), and stabilization of alluvial fans. These results demonstrate the complexity and multiscalar nature of the interactions among land use and runoff generation, soil erosion, sediment transport, and fluvial channel dynamics, and highlight the need to adopt a multiscale approach in other mountain areas of the world. [Copyright &y& Elsevier]