Francesc Gallart

Rainfall interception by a Pinus sylvestris forest patch overgrown in a Mediterranean mountainous abandoned area I. Monitoring design and results down to the event scale

Monitoring (in 5 min steps) of precipitation, throughfall, stemflow and bulk canopy wetness, and also weather conditions and soil moisture, was carried out from July 1993 to December 1995, in a Pinus sylvestris forest patch located in a Mediterranean mountainous former agricultural basin subject to spontaneous change from pasture to forest. Throughfall collectors were designed to obtain hydrologically representative data and they consist of nine troughs with a total catchment area of 9 m2. The bulk interception rate measured after 30 months of monitoring was about 24%. Relative interception was irregular and decreased with the magnitude of the event; it was at least 15% for events of more than 20 mm. Multivariate analysis of the events demonstrates that their characteristics can be simplified in two main factors which respectively represent the duration of the event and its magnitude. The magnitude of the event biases the characterization because of the non-linearity of the rainfall-interception relationship. Long events do not produce higher interception rates than shorter ones because of the occurrence of low vapour pressure deficits during the former. In atmospheric dry conditions the rainfall intensity provides the main control on interception rates.

A simplified method for forest water storage capacity measurement

A simplification of direct methods to measure canopy storage capacity is presented. It is based on measurement of water retained by vegetal entities (needles, stems and branches) and the up-scaling of these measurements using the determination of the surface of canopy elements from common vertical photographs taken from the ground. The specific water retention capacities of Pinus sylvestris pine needles ranged between 0.104 and 0.043 mm, depending on the simulation of still air or windy conditions. These values are low when compared with the specific water retention capacity of branches and stems, 0.62 mm. The water retained in branches and stems, therefore, plays a key role in rainfall interception. The canopy storage results obtained are consistent with the spatial distribution of throughfall measured in five experimental plots located in a heterogeneous 40-year-old Pinus sylvestris stand in a Mediterranean mountain area of the South Eastern Pyrenees (Catalonia, Spain), and are 30% higher than the values estimated through indirect methods.

Studying the role of old agricultural terraces on runoff generation in a small Mediterranean mountainous basin

Abstract The small drainage basin of Cal Parisa was instrumented in early 1989 to study the hydrological and sediment routing behaviour of Mediterranean mountain areas formerly used for agriculture but now abandoned. Environmental changes produced by agricultural land use included the construction of terraces on the major part of the basin and subsequent artificial channelling of surface waters. Field observations and hydrological data suggest that storm runoff is generated by the contributing role of saturated areas, most of them originating from the terrace system. Modelling of the natural conditions before terracing, with the help of TOPMODEL, supports the hypothesis that terracing promotes the premature formation of saturated areas, increasing saturation overland flow at the expense of lower baseflow and actual evapotranspiration. Now, after land abandonment, the more relevant environmental hazard is linked to the spontaneous reorganization of the unmaintained artificial drainage network.

Regolith behaviour and physical weathering of clayey mudrock as dependent on seasonal weather conditions in a badland area at Vallcebre, Eastern Pyrenees

Abstract Studies on badland areas in the Vallcebre basin (Pyrenees) suggest that erosion rates are controlled by the weathering rate of mudrocks. To obtain the temporal pattern of physical weathering and its control on the erosive processes, monitoring of regolith temperatures at different aspect, depth and lithology, and periodical determination of regolith moisture and bulk density were carried out. Changes in surface regolith have also been monitored by means of photographic techniques, using an especially designed tripod. Finally, the hydrological and erosive response of the regolith trough the seasons were studied using rainfall simulations. The results obtained support and confirm previous knowledge on the role of frost action in mudrock weathering and the role of subsequent regolith development and wetness status on infiltration and runoff generation. These seasonal changes of behaviour are so important that they raise serious questions about the validity of observations or experiments carried out in only one season. This work confirms that erosion processes are controlled by regolith formation during winter and regolith depletion by storm rainfall erosion during summer, spring and early autumn. It also raises important points about annual trends of infiltration and erodibility, that should be used as guidelines for modelling runoff and sediment production.

Catchment Management under Environmental Change: Impact of Land Cover Change on Water Resources

Abstract Since the second third of the 20th century, it has been clearly established in the hydrological science that an increase of the forest cover in a catchment determines a decrease in water resources from this catchment. Nevertheless, the application of this paradigm for catchment management has been delayed. In Spain, extensive mountain areas that were farmed or grazed in the past have been abandoned during the last 50 years and undergone spontaneous or induced afforestation, mainly by pine trees. The historical flow records of several Spanish rivers during the last 50 years show significant decreases that average an annual reduction of about 0.4 percent of mean annual flow, whereas in the Ebro River the annual decrease was as high as 0.63 percent of mean annual flow. About one-third of these decreases were not explained by increases in water consumption by irrigation or by climate variability, but should be attributed to an increase in evaporation from the headwaters. These results confirm that the assessment of future water resources must take into account the past and expected changes in land cover in the catchment headwaters and that land use and cover planning should be integrated in the management of catchment water resources, especially in scarce water environments.

Recent gully erosion in the El Cautivo badlands (Tabernas, SE Spain).

Abstract Detailed geomorphic observations in El Cautivo badland area, in the semiarid SE of Spain, show a recent renewal of linear channel erosion following a period of aggradation. Prior to this renewal, the landscape was characterized by smooth colluvial pediments on the north-east facing aspects and relatively steep crusted microhillslopes on south-west facing aspects. First-order valleys within the badland landscape were smooth-floored and filled with silty deposits connected with pediment colluvial deposits. The conditions under which valley bottom deposition was replaced by channel incision were analyzed by a study of flow hydraulics (magnitude–frequency of eroding events vs. resistance of valley floor to water flow) and the pollen sequence in the recently cut deposits. Changes in the vegetation-carpeting the valley bottom seem to be responsible for the deposition–erosion alternation rather than changes in the magnitude–frequency of major runoff events, given the high sensitivity of flow hydraulics to changes in roughness. Present rainfall rates would not, on their own, be able to erode a well-vegetated valley bottom. Yet, the pollen sequence in the valley fills demonstrates a progressive deterioration in climatic conditions with the eventual disappearance of groups that require a constant level of soil moisture (Alnus, Corylus, Cyperaceae). An increase in yearly rainfall or a change in temporal patterns would increase infiltration leading to permanent denser vegetation on footslopes and valley floors, helped by the natural water harvesting system. This vegetation cover would reduce the flow velocity and induce the deposition of the colluvium and valley fills, whereas erosion on headslopes would have been active enough to supply the sediments that were deposited downslope or on the valley bottoms. The reactivation of linear erosion would seem, therefore, to be a consequence of the decay of this permanent vegetation, induced by an increase in the duration or severity of drought periods, which meant that the water storage capacity of the valley fillings was insufficient to support it.

Geomorphic agents versus vegetation spreading as causes of badland occurrence in a Mediterranean subhumid mountainous area.

Abstract Previous evidence on the chief role of physical weathering on badland development in the Vallcebre area (South Eastern Pyrenees) led to the study of the amounts of energy available for the main geomorphic agents, and to analyze the results in the light of the patterns of vegetation cover and species distribution, in order to explain both the occurrence of these badland forms and their preference for north-facing (shady) hillslopes. Results showed that the weathering potential provided by freezing is about two orders of magnitude greater than rainfall kinetic energy, and that the former strongly depends on hillslope aspect. The study of vegetation demonstrated that vegetation is poorer in cover and species composition on shady hillslopes than on sunny ones, spontaneous revegetation of these surfaces being restrained by cold thermal conditions rather than by water availability. These results show that the occurrence of these badland areas depends on montane climatic attributes that control geomorphic and biological processes, and that are different from those that promote the occurrence of badlands in dry Mediterranean areas. The preference for northern aspects provides evidence for these differences.

Hydrological functioning of mediterranean mountain basins in Vallcebre, Catalonia: Some challenges for hydrological modelling

The Vallcebre research catchments are located in the south-eastern Pyrenees, in an area of diverse land use and varying levels of degradation, including forested hillslopes, abandoned agricultural terraces and badland areas. Outside the badlands, the hydrological response is controlled by saturation mechanisms. Between September and June the spatial patterns of saturated areas and soil moisture are determined by subsurface flow, modified by the premature saturation of the inner parts of agricultural terraces, and the negative soil moisture anomalies induced by forest patches overgrown in grassland areas. During summer, this behaviour ceases because of soil moisture depletion and badland surfaces are the only hydrologically active areas, producing excess runoff in response to the intense rainstorms. During the beginning and the ending of the wet season, the saturation of active areas shows a hysteretic behaviour that breaks the linearity between the mean water reserve of the basin and the relative saturated area.

Fluvial geomorphology and hydrology in the dispersal and fate of pyrite mud particles released by the Aznalcóllar mine tailings spill

The Agrio-Guadiamar River reach affected by the Aznalcollar mine tailings spill can be divided into three main sectors taking account of geomorphic characteristics and human-induced changes. Along the first 15 km, the valley presents the characteristics of a middle fluvial reach of moderate sinuosity and gradient; the floodplain declines gently towards the main channel and is built up mainly by lateral accretion deposits, which are mined by open pits. The second sector is located between 15 and 30 km downstream of the mine, and is characterised by higher channel sinuosity and a lower gradient; the floodplain is built on overbank deposits, separated from the main channel by natural levees, showing some low sinuosity flood channels. Finally, the lowest reach, with a similar low gradient, can be described as the proximal area of the Donana marshlands, whose geomorphology has been modified by agricultural practices for the last 40 years. The spread of mining tailings deposited a mat of pyrite mud (fine milled heavy metal-rich ore) over the main channel and floodplain with a thickness of up to 50 cm in the first sector and then, progressively decreasing downstream. The pyrite mud was deposited over the natural sediments without significant mobilisation of the latter and without relevant geomorphic changes. The subsequent cleaning up of the pyrite mud with the help of heavy machinery resulted in the removal of most of the bushy and herbaceous vegetation as well as the reworking of bed sediments and the destruction of their natural armouring. Relatively small amounts of submerged pyrite mud that could not be removed from the main channel and that mixed with the shallow natural sediments during the cleaning works provides a long-term pollution element in the system. Pyrite mud remnants on floodplains are not being directly removed by fluvial activity in the mid-term because these sedimentation areas are susceptible to erosion only by the wandering of the main channel. However, these materials can be easily oxidised, therefore, plant uptake and underground water pollution should be monitored. On the other hand, the channel bed and banks are source areas of sediments and pyrite materials that can be easily eroded due to the destruction of the natural protection features (vegetation and armouring). Once eroded, the pyrite particles will be transported over longer distances than the coarser host natural alluvium, and deposited on floodplains or areas with low flow velocity.

Studying solute and particulate sediment transfer in a small Mediterranean mountainous catchment subject to land abandonment

The sediment budget of the small research catchment of Cal Parisa (Vallcebre, Eastern Pyrenees) was studied by hydrological monitoring and assessment of the erosion rates in the major sediment sources. This area is characterized by clayey mudrock prone to landsliding and badland erosion, but the catchment was selected in an area free of major badland features, as a representative of middle mountain regions where a system of terraces and drainage ditches had been built for agricultural use but is now abandoned. Streamwater chemistry is dominated by Ca 2+ and HCO 3 - at concentrations close to calcite saturation. Total dissolved solids show dilution during runoff peaks and positive hysteresis loops that support a slow contribution of subsurface water. Relative dissolved ion concentrations are different for each event analysed. Particulate sediment yield is very low and represents only about 1 per cent of gross erosion in the catchment. Mineralogical analysis of suspended sediments shows an enrichment in calcite because of precipitation. Chemical analysis of suspended sediments, using common one-litre water samples, shows higher contents of Ca, P and Mn in transported sediment than in sediment source areas, attributed to the precipitation of calcite, and enrichment in organic particulate matter during events respectively for the two first elements, whereas enrichment in Mn remains uncertain. Solid matter yield is therefore clearly dominated by dissolved transport as a result of both high calcium bicarbonate concentrations in runoff waters and strong suspended sediment conveyance discontinuities. Land conservation structures are very effective because they are in good condition whereas the soil is covered by dense permanent vegetation. Nevertheless, this state is unstable because the network of drainage ditches needs maintenance; its spontaneous breakdown after abandonment may result in the rearrangement of the elementary stream network and gullying of old fields in hollows.