**3.1.1 Plant covering**

The plant covering on cultivated areas can be estimated by the foliar surface of plants. In French regions with temperate climate, it is very difficult to get reliable data about monthly rates of plant covering and their intra-annual evolution. The data that we show (Fig. 8) have been sent by the Technical Institute for Plants specialized in the agronomic research (ARVALIS – Institut Technique du Végétal), a Calvados-based and nationally recognized organism. These data concern the principal plants cultivated annually or in intercrop periods, in Normandy.

Fig. 8 enables to show on a monthly scale three types of evolution for the plant covering by plants in crop areas. A first type gathers the plants characterized by a poor rate of plant covering at the end of the first month of growth and then by a very quick and totally covering rate at the end of the second month. This type concerns maize silage and ray grass. The following type refers to rapeseed and mustard. The rate of plant covering is very

SCALES: An Original Model to Diagnose Soil Erosion Hazard

initial data that showed an intra-annual variability.

abundant annual rainfalls, around 950 and 1000 mm.

and 36% of the cultivated areas.

**3.2.2 Acquisition of input data** 

agricultural parcels.

(mustard) and wheat stubble.

**3.2.3 Results** 

**3.2 Application to the Branche catchment** 

Normandie, on a catchment scale (Fig. 1A).

**3.2.1 Study area** 

and Assess the Impact of Climate Change on Its Evolution 241

classes presented previously are reused to estimate the seasonal levels of pressure for the

The monthly and seasonal approach of the SCALES model has been tested in Basse-

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

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%

To calculate slopes inclination, we used Digital elevation Model of Manche with a grid resolution of 50 X 50 m. The climate data start from the Torteval-Quesnay station, based 15 km East from our site. As for the general model, we collected the daily data for the 1991- 2004 period. These data were used for mapping the soil erosion hazard on monthly, seasonal and annual scales within the framework of an average climatic year. The data related to the agricultural practices result from a survey carried out among farmers who exploited catchment lands between 2005 and 2008. Those were used to map the annual hazard. For the other temporal scales, the hazard was estimated from agricultural data from the 2007-august 2008 period. Finally, the soil data (structural stability, available water content) were obtained from soil boreholes according to a density of 1 hole for 10 ha about agricultural area and from laboratory analysis concerning the structural instability of the superficial horizons. The spatial units of integration of the SCALES data correspond to the

The monthly maps obtained for the September 2007-august 2008 period primarily reveal a

One note a first sequence, between September and October, characterized by quite a low hazard on the majority of the cultivated parcels (approximately 300 hectares). The surfaces with medium level represent 2% of the catchment area. During this period, the erosivity is low because of nonexistent or very low hydrological surplus and insofar as the plant covering is relatively important because of the presence of fast-growing temporary crop

significant intra-annual variability of the soil erosion hazard (Fig. 9).

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. Cereals correspond to this type.

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

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