Stefan Wirtz Vom Fachbereich VI (Geographie/Geowissenschaften ...

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Experimentelle Rinnenerosionsforschung vs. Modellkonzepte – Quantifizierung der hydraulischen und erosiven Wirksamkeit von Rinnen Soil erosion on abandoned land in Andalusia – a comparison of interrill- and rill erosion rates <strong>Wirtz</strong>, S. (1); Iserloh, T. (1); Rock, G. (2); Hansen, R. (1); Marzen, M. (1); Seeger, M. (1); Betz, S. (1); Remke, A. (1); Wengel, R. (1); Butzen, V. (1); Ries, J.B. (1) (1): Trier University, Dep. of Physical Geography, Behringstraße, 54286 Trier, Germany (2): Trier University, Remote Sensing Department, Behringstraße, 54286 Trier, Germany Abstract The present paper is based on several field investigations (monitoring soil and rill erosion by aerial photography, rainfall simulations with portable rainfall simulators and manmade rill flooding) in southern Spain. Experiments lead now to a closer understanding of the dynamics and power of different soil erosion processes in a gully catchment area. The test site Freila (Andalusia, SE Spain) covers an area of 10.01 ha with a rill density of 169 m ha -1 , corresponding to a total rill length of 1694 m. Assuming an average rill width of 0.15 m, the total rill surface can be calculated at 250 m² (0.025 ha). This given, the surface covered by rills makes up only 0.25 % of the total test site. Since the rill network drains 1.98 ha, 20 % of the total runoff comes from rills. The rills’ sediment erosion was measured and the total soil loss was then calculated for detachment rates between 1685 g m -2 and 3018 g m -2 . The interrill areas (99.75 % of the test site) show values between 29-143 g m -2 . This suggests an important role of rill erosion concerning runoff and soil detachment. Keywords Soil erosion, water erosion, rill erosion, interrill erosion, rainfall simulation 1 Introduction Soil erosion by water involves different physical processes at variable spatial and temporal scales. The two main processes are interrill and rill erosion by runoff water, the mechanisms of these two processes are completely different. The soil detachment in interrill erosion is induced and enhanced by splash and shallow overland flow [1]. In addition, it is influenced by the intrinsic characteristics of the soils [2, 3, 4] and rainfall intensity [5, 6]. In contrast, rill erosion is caused by a concentrated overland flow [6, 7, 8]. This is considered to be the most important process of sediment erosion and soil loss [9, 10]. As a result, the new rills can 39
Experimentelle Rinnenerosionsforschung vs. Modellkonzepte – Quantifizierung der hydraulischen und erosiven Wirksamkeit von Rinnen become persistent and form gullies, potentially constraining further land use [11, 12, 13, 14]. Especially on fallow and shrub land, rills can develop without being disturbed by land management measures like ploughing. Since huge areas in the Mediterranean are covered by fallow and shrub land [15], rill erosion can be assumed to be a major process of soil erosion in the Mediterranean. The outstanding importance of linear erosion for all intents and purposes (geomorphology, hydrology and economy) can be explained by the amount of kinetic energy of water, running as a concentrated runoff in channels. When concentrated, water reaches its maximum impact concerning erosion and transport [16]. The percentage of interrill-, rill- and gully erosion on the total soil loss in a catchment area is difficult to determine, and results of many research groups differ considerably in comparing the effectiveness of the different soil erosion processes. Results concerning the proportion of gully erosion on total soil erosion vary between 10 % and more than 90 % [17]. Gullies need high-intensity rainfall events to be activated. Most thunderstorms do not activate the gully in a catchment, but are able to generate or reactivate smaller forms, the rills. Faust & Schmidt [18] considered the geomorphologic importance of gullies as quite low, corresponding to the rare activity of the gullies. They state an activity frequency of one single event in 20 years, compared with an assumed activity of the rills of about four times per year. On that account, the rills deliver smaller sediment quantities, but this can occur several times within one year. Meyer et al. [19] suggested a triplication of erosion rates due to rill development. Cornfields in Bedfordshire (Great Britain) lost as much as 9 to 21 times more sediment by rill erosion, compared with erosion only affected by interrill erosion [20]. Cerdan et al. [9] reported the soil loss by rill erosion in Normandy during only a few heavy rainfall-events to be up to 90 % of the total soil loss. During the last decades, several approaches have been developed to describe and predict soil detachment and sediment transport in rills in a reliable way [7, 21, 22]. In contrast, the factors influencing the development and the behaviour of rills have not been comprehensively assessed yet. The relationships between factors influencing these processes remain unclear as well. Knapen et al. [8] attribute this to the lack of comparable data that could be used for a meta-analysis on rill erosion. Furthermore, there is still little known about the function of rills in specific environments. In this paper, the results of a set of experimental methods are used to draw conclusions from the differences between interrill- and rill erosion dynamics on a gully catchment in Andalusia (Spain). Three main questions were to be answered: 40
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Dipl. Geogr. Stefan Wirtz; Physisch
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