Abstract
In the semi-arid Central Ebro basin experimental rainfall simulations with a small mobile jet rainfall simulator according to Calvo et al. (1988), Lasanta et al. (1994 and 2000) were carried out on abandoned fields near María de Huerva to investigate the conditions of runoff generation and local erodibility. On the silty soils, susceptible to soil sealing and crusting very divergent rates of runoff and erosion rates caused by water are found. On the crusted young fallow land runoff coefficients range between 20 and 95%, erosion rates range between 9 and 460 gm-2 in the experiments. The start of runoff was recorded between the second and fifth minute. Soil surface sealing leads to a decrease in infiltration rates and a consequent increase in runoff in comparison to freshly tilled agricultural fields. On the old fallow land, values range from 0 to 76% and 0 to 78 gm-2 depending on other parameters such as slope and the activity of the edaphon. The start of runoff was recorded between the third and the fifteenth minute. A remarkable fact is that material delivery does not significantly decrease with increasing vegetation cover. Only with a vegetation cover of over 60% does material output decrease obviously. The runoff rates show the same order of magnitude than those recorded by Lasanta et al. (2000) in abandoned fields near Peñaflor, but the erosion rates in María de Huerva are significantly lower.
Highlights
In the research project EPRODESERT – the acronym stands for Evaluation of Processes Leading to Land Degradation and Desertification under Extensified FarmingSystems in North-East Spain - the interaction of vegetation succession and geomorphodynamics in different test areas is investigated along a transect from the High Pyrenees to the Central Ebro Basin
Set-aside programmes of arable land have recently been supported by subsidies of the EU with the result that the number of abandoned fields has increased in the Ebro Basin since the beginning of the nineties (Figure 2). This young fallow land shows very complex fluvial-geomorphologic processes: When the former arable land is no longer ploughed, harrowed and rolled regularly, as it was usual in the traditional dry farming system, soil crusts develop and reduce the hydraulic conductivity in the upper millimeters, what causes heavily reduced local infiltration capacity
The vegetation succession and the soil erosion are investigated on this young fallow land and are compared to the situation on older abandoned fields which are not used as agricultural fields since about 60 years (Ries et al, 2000)
Summary
This transect follows a strong topographic and climatic gradient from perhumid climate in a High Mountain Region to a semi-arid climate in the lowlands in front of them (Ries, Seeger and Marzolff , 1997). This paper presents the results from rainfall simulations for determining the sitespecific erodibility of soil and runoff generation on fallow land of different ages. It explains the wide range of results by the vegetation distribution, the zoological influences of soil fauna and the role of soil crusts for runoff coefficients and soil erosion rates. It is very close to the paper from Tilmann Sauer, showing how the development of soil water content can be calculated for different soils in all five test areas for daily time steps
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