**3.2.1 Study area**

240 Soil Erosion Studies

important at the end of the first month of growth and reaches 100% at the end of the second month. Finally, the third type is slightly different from the two others as it needs 4 months of growth to get a rate of plant covering of 50% and then 6 months for a complete rate.

Fig. 8. rate of covering according to the vegetative period of various cultures (Arvalis)

<=10%. Those are respectively corresponding to the levels of pressure 0, 1, 2, 3, 4, 5.

For YPHB: <30 mm; [30-60 mm[; [60-90 mm[; [90-120 mm[; >=120 mm Those are respectively comparable with the levels of pressure 1, 2, 3, 4, 5.

A first adaptation of the SCALES model consisted then in replacing the initial data "intercrop period" and "intercrops management" by the data "rate of plant covering". The monthly classes selected are as follows: >90%; [90-70%[; [70-50%[; [50-30%[; [30-10%[;

The data of the initial model that characterized the rainfall erosivity have been conserved. Concerning the number of days when the rainfall intensity is above 10 mm, we made an average monthly counting for the 1991-2004 period starting from the network of meteorological stations presented previously. For the average monthly positive hydrological balance, we used the monthly data of hydrous budget reports used in the initial version of

In order to estimate the soil erosion hazard at seasonal scale, we calculated, for the initial data rate of plant covering, DI10 and YPHB, the average of values of the months that characterized each season. Thus, the value relative to the spring results from the average of values obtained for March, April and May. The values of June, July and August provide the summer value ; the ones from September, October and November give the autumnal value and the ones from December, January and February are the winter value. Thus, the monthly

Cereals correspond to this type.

**3.1.2 Climate** 

the SCALES model.

Monthly classes are the following. For DI10: =1; =2;=3;=4; >=5

**3.1.3 From monthly data to seasonal data** 

Localized in the French department of the Manche, the Branche catchment covers 1100 hectares and is a part of the Vire catchment, a larger hydrological area of 1270 km². The uphill of the test zone shows an undulating relief resulting of many small valleys. The slopes are between 1% and 5%. The downhill is marked by deeper steeply sided settings of the rivers. The slopes are more abrupt, between 5 and 15%. The study area is situated in the Armorican block, formed by Precambrian schist and sandstones. Umbric leptosols and cambisols cover the major parts of the slopes and gleysols can be found in the valleys bottoms. Their thickness varies from 40 cm to 120 cm. The local climate is characterized by abundant annual rainfalls, around 950 and 1000 mm.

The average size of the farms is 100 ha. The local agriculture shows a system of intensive pastures with a high proportion of ploughings dominated by spring crops. The grass surfaces represent 60% of the agricultural land. Crop and wheat respectively occupy 53% and 36% of the cultivated areas.
