From 1996 to 1999, an European project called WELSONS was led to better understand the impacts of changes in climate and land-use on soil degradation by wind erosion for agricultural soils in a semiarid region of north-east Spain (Central Aragon). This paper presents the main results obtained from the measurement activities of the WELSONS project aimed to generate an original data base to study wind erosion processes. The first objective was to evaluate the effects of climate and anthropogenic processes on surface characteristics of agricultural soils. During the whole WELSONS experiment, a physical dynamics characterization of the soil surface and of the atmosphere near the ground were determined for each field plot studied. Results from the characterisation of soil surface shows that reduced tillage (RT) lower soil erodibility by wind compared to conventional tillage (CT).Soil crusting, which is a common feature of soils in the area, can also be considered as a protecting element against wind erosion. The second objective was to study the mobilisation processes of soil-particles into the saltation layer. Strong and erosive Cierzo winds occurred frequently in the area of the experimental field, but because a soil surface crust was present, only a limited supply of material was available for wind erosion. However, a measured saltation transport flux could be obtained for each sampled period from the measurement of the mass of saltating particles transported during each erosion event. The third objective was to assess the atmospheric emission of fine dust into the suspension layer. Significant vertical dust flux of PM20 was observed only in the CT plot because, as the erosion events are supply-limited, a lower amount of material lying on the crust was transported by the wind and sandblasted to provide suspended dust. Finally, the sandblasting efficiency calculated in the present experiment could be interpreted as the result of a higher binding energy for silt loam soil particles compared to sandy loam, loamy sand and sandy textured soils.
Abstract. This paper deals with the characteristics of turbulent flow over two agricultural plots with various tillage treatments in a fallow, semiarid area (Central Aragon, Spain). The main dynamic characteristics of the Atmospheric Surface Layer (ASL) measured over the experimental site (friction velocity, roughness length, etc.), and energy budget, have been presented previously (Frangi and Richard, 2000). The current study is based on experimental measurements performed with cup anemometers located in the vicinity of the ground at 5 different levels (from 0.25 to 4 m) and sampled at 1 Hz. It reveals that the horizontal wind variance, the Eulerian integral scales, the frequency range of turbulence and the turbulent kinetic energy dissipation rate are affected by the surface roughness. In the vicinity of the ground surface, the horizontal wind variance logarithmically increases with height, directly in relation to the friction velocity and the roughness length scale. It was found that the time integral scale (and subsequently the length integral scale) increased with the surface roughness and decreased with the anemometer height. These variations imply some shifts in the meteorological spectral gap and some variations of the spectral peak length scale. The turbulent energy dissipation rate, affected by the soil roughness, shows a z-less stratification behaviour under stable conditions. In addition to the characterization of the studied ASL, this paper intends to show which turbulence characteristics, and under what conditions, are accessible through the cup anemometer.
Abstract. This study presents the preliminary results of the local energy budget and dynamic characteristics of the surface atmospheric boundary-layer (SBL) during the WELSONS (wind erosion and losses of soil nutrients in semiarid Spain) experiment. Some Mediterranean regions suer land degradation by wind erosion as a consequence of their particular soil and climate conditions and inappropriate agricultural practice. In Spain, where land degradation by water erosion is well known, the lack of ®eld studies to quantify soils losses by wind erosion resulted in the European Community organizing a scienti®c program for this speci®c issue. The European programme known as WELSONS was devoted to study the wind erosion process in central Aragon (NE Spain). This multidisciplinary experiment, which began in 1996 and ®nished in 1998, was carried out over an agricultural soil which was left fallow. Within the experimental ®eld, two plots were delimited where two tillage treatments were applied, a mould-board ploughing (or conventional tillage denoted CT) and chisel ploughing (reduced tillage denoted RT). This was to study on bare soil the in¯uence of tillage method on surface conditions, saltation¯ux, vertical dust¯ux, erosion rates, dynamics characteristics such as friction velocity, roughness length, etc., and energy budget. The partitioning of the available energy, resulting from the dynamics of the SBL, are quite dierent over the two plots because of their own peculiar soil and surface properties. The ®rst results show that the RT treatment seems to provide a wind erosion protection. Because of the long data recording time and particular phenomena (formation of a crust at the soil surface, very dry conditions, high wind speed for instance), these microclimatological data acquired during the WEL-SONS programmes may be helpful to test atmospheric boundary-layer models coupled with soil models.
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