C

I | No erosion I 1 Moderate sheet wash Strong sheet wash | Rill erosion I Accumulation

No change Q Sheet wash stopped IBB Accumulation stopped Q Sheet wash diminished IBS Sheet wash intensified I

FIGURE 16-5 Test site Maria de Huerva 1. (A) Vegetation cover in April 1997. (B) Vegetation cover change, April 1997-April 1998. (C) Geomorpho-dynamics in April 1997. (D) Change of geomorphodynamics, April 1997-April 1998. Adapted from Marzolff (2003, color plates 1—4).

analysis, this would appear to be a typical rather homogeneous fallow field.

As expected, statistical analysis (predominantly by cross-tabulations and correspondence analyses, see Mar-zolff, 1999) showed a generally negative correlation of geomorphological process activity with vegetation cover density for all study sites. However, relationships are much more complex than anticipated and vary considerably between climatic regions and within those for different plant life forms. An influence of microrelief on geomorphological processes exists mainly for linear erosion forms. The influence of topography as an erosioncontrolling factor outweighs the role of vegetation density only when vegetation cover is sparse; in this case, nano-relief (e.g. tillage pattern on set-aside field) considerably influences pattern and intensity of geomorphological processes, also.

As a most important result, it was concluded that on the observed test sites erosion processes are occurring in patterns of high spatial frequency at far higher percentages of vegetation cover than tends to be assumed by most investigations into land degradation (Marzolff, 1999). Generally, a vegetation cover of 30-40% is taken as a threshold beyond which runoff and soil erosion rates reach negligible amounts. In contrast, results of this study showed sheet erosion on fallow land with up to 70% overall vegetation cover. Looking at small-sized patterns, moderate sheet erosion can even be observed at up to 90% vegetation density, and process dynamics may during several observation periods intensify even with increasing vegetation cover.

In the semi-arid region in particular, where vegetation succession is limited by water stress, at least 60% vegetation cover is required in order to bring prevailing sheet erosion processes to a halt. A possible reason for the disparity between this threshold and the 30-40% value identified by numerous authors may be the improved assessment of percentage vegetation cover on the basis of SFAP. Terrestrial observations (field mapping) and conventional airphoto or satellite analysis tend to overestimate vegetation cover owing to the shadowing between plants and leaves.

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