**2.2.2 Parameters and input data**

The choice of input data (Fig. 2) in the view of characterizing factors of evaluation of erosion hazard result from well-established scientific concepts in the literature, expert opinions and by conclusions originating from numerous personal observations for two years in the Department of Calvados (Le Gouée et al., 2008).

In order to estimate the soil erodibility, we selected and considered the structual instability as input data. This characteristic corresponds to the soils sensibility to degradation of its superficial structure by rainfalls. The degradation by water can be explained by the different physical and physico-chemical mechanisms among which we can cite: bursting, mechanical disaggregation, disaggregation by differential blow, and the chemical spread. Mechanical disaggregation due to the impact of rain drops constitute, in the temperate regions influenced by the ocean, the main mechanism acting on the soil crusting.

SCALES: An Original Model to Diagnose Soil Erosion Hazard

erosive pressure levels due to agricultural practices.

possibilities increase rapidly as soon as the slopes are strong.

rainfalls. This led us to propose two types of input data.

management" input data.

stations of Météo France network.

**2.2.3 Steps of the modeling** 

refers to a very high pressure level.

Topography

Rainfall erosivity

and Assess the Impact of Climate Change on Its Evolution 231

For the latter, another type of data aims to precise the profile of crop rotation. It allows us to gather information about the duration of the crop rotations and of the duration of the intercrop periods. This intercrop period data is essential for the application of SCALES because it specifies the amount of time during which the soil is directly subjected to the erosive action by the rainfalls. When the intercrop period is persisting, the risk of soil erosion increases. The mean duration of intercrop period has been chosen to estimate

For cultivated lands, a third type of input data is used. It is related to intercrop management. This management is indeed leading to very different soil erosion responses whether keeping a bare soil or establishing temporary plant cover such as temporary crops or ray-grass. The duration and terms of intercrop management are linked to the types of crop rotations. The diversity of observed situations during the rotation cycle is integrated in the form of erosive pressure levels calculated for "intercrop period" and "intercrop

The role of topography in the assessment of erosion hazard is expressed through the selection of only one type of input data which is slope inclination. Possibilities of runoff of the non-infiltrated water are depending on the slope at every point of space. These

Rain is the main factor of soil erosion by water. Its capacity to damage the soils depends on rainfall intensity, on volume of precipitations and on the hydrological response of the soil to

The first type defines the rainfalls ability to erode the soil based on its intensity. Among the indicators usually employed, we chose to considerate the yearly number of days with a Daily Intensity is above 10 mm (Fig. 2). Data comes from records of local meteorological

The second type of data results from the response of the soils to the rainfalls. It refers to the concept of yearly positive hydrological balance and is given by applying the methods of hydrous budget. The positive hydrological balance is regarded as available water either for drainage or runoff. Data are resulting from the combination of potential evapotranspiration, rainfalls and available water content. If it is difficult to estimate the part of drainage and runoff, we can recognize that the risk of runoff increases with an increase of positive hydrological balance. These data are calculated at monthly scale and then cumulated to obtain results at yearly scale. The data used are means of climatic period. The main problem encountered to get this type of data comes from the methods implemented to gather accurate and reliable data about available water content. This problem is solved since it has

The first step aims to convert all input data into erosive pressure levels (Fig. 3). 6 levels of pressure had been specified, from 0 to 5. Level 0 indicates absence of pressure and level 5

Some types of input data have only 5 levels. In this case, there is no level 0. This applies to "intercrop period", "daily intensity of rainfalls", "intercrop management" and, "yearly

been decided to start a wide program of soil mapping over the Calvados.

Fig. 2. Types of parameters and input data and levels of pressure concerning soil erosion

#### Soil erodibility

However, one must be aware that the sensibility of the soil to erosion is not always correlated with the soil crusting notion and a fortiori to the soil characteristics which lead to this phenomenon. In case of heavy rainfalls combined with water-saturated soils we will observe the establishment of a runoff on soils with stable structure able to transport heavy aggregates or stony load. This case will be integrated to the model when we will show input data relative to rainfall erosivity factors.

#### Agricultural practices

In temperate regions, soil erosion is linked to agricultural land. We can observe this erosion when soils are not protected by a permanent or well-developed plant cover. At this stage, a first distinction must be established between areas entirely and durably covered by agricultural vegetation i.e., permanent grassland and orchard, and the ones dedicated to crops. In the first case, water erosion remains absente or very anecdotic. In the second case, the risk to soil erosion is closely linked to the agricultural practices and their temporality.

The choice between crops or grassland and the technical management of cultivated parcels is integrated within the framework of logics of management of the farms fixed over several years. The assessment of the pressure generated by the agriculture on erosion hazard fulfills these logics. This is the reason why we prefer talking about agricultural practices more than land cover: the first term refers to the impact of agriculture at a multiannual scale when the second refers to plant covers (grassland, cereal crops, fodder crops) at a given moment. As we explained about rainfall erosivity, the role of agricultural practices on erosion is finally estimated by a mean pressure level characterizing the global erosive impact of technical practices during agricultural cycles. Therefore, the soil erosion hazard is an indicator of the soil degradation at multiannual scale.

The impact of agricultural practices in the modeling is established out of 3 types of input data. The first type of input data is related with the presence or absence of plant cover. A distinction is made between the areas characterized by permanent plant cover (grassland or orchards) and cultivated areas.

For the latter, another type of data aims to precise the profile of crop rotation. It allows us to gather information about the duration of the crop rotations and of the duration of the intercrop periods. This intercrop period data is essential for the application of SCALES because it specifies the amount of time during which the soil is directly subjected to the erosive action by the rainfalls. When the intercrop period is persisting, the risk of soil erosion increases. The mean duration of intercrop period has been chosen to estimate erosive pressure levels due to agricultural practices.

For cultivated lands, a third type of input data is used. It is related to intercrop management. This management is indeed leading to very different soil erosion responses whether keeping a bare soil or establishing temporary plant cover such as temporary crops or ray-grass. The duration and terms of intercrop management are linked to the types of crop rotations. The diversity of observed situations during the rotation cycle is integrated in the form of erosive pressure levels calculated for "intercrop period" and "intercrop management" input data.

#### Topography

230 Soil Erosion Studies

Fig. 2. Types of parameters and input data and levels of pressure concerning soil erosion

However, one must be aware that the sensibility of the soil to erosion is not always correlated with the soil crusting notion and a fortiori to the soil characteristics which lead to this phenomenon. In case of heavy rainfalls combined with water-saturated soils we will observe the establishment of a runoff on soils with stable structure able to transport heavy aggregates or stony load. This case will be integrated to the model when we will show input

In temperate regions, soil erosion is linked to agricultural land. We can observe this erosion when soils are not protected by a permanent or well-developed plant cover. At this stage, a first distinction must be established between areas entirely and durably covered by agricultural vegetation i.e., permanent grassland and orchard, and the ones dedicated to crops. In the first case, water erosion remains absente or very anecdotic. In the second case, the risk to soil erosion is closely linked to the agricultural practices and their temporality. The choice between crops or grassland and the technical management of cultivated parcels is integrated within the framework of logics of management of the farms fixed over several years. The assessment of the pressure generated by the agriculture on erosion hazard fulfills these logics. This is the reason why we prefer talking about agricultural practices more than land cover: the first term refers to the impact of agriculture at a multiannual scale when the second refers to plant covers (grassland, cereal crops, fodder crops) at a given moment. As we explained about rainfall erosivity, the role of agricultural practices on erosion is finally estimated by a mean pressure level characterizing the global erosive impact of technical practices during agricultural cycles. Therefore, the soil erosion hazard is an indicator of the

The impact of agricultural practices in the modeling is established out of 3 types of input data. The first type of input data is related with the presence or absence of plant cover. A distinction is made between the areas characterized by permanent plant cover (grassland or

Soil erodibility

Agricultural practices

data relative to rainfall erosivity factors.

soil degradation at multiannual scale.

orchards) and cultivated areas.

The role of topography in the assessment of erosion hazard is expressed through the selection of only one type of input data which is slope inclination. Possibilities of runoff of the non-infiltrated water are depending on the slope at every point of space. These possibilities increase rapidly as soon as the slopes are strong.

#### Rainfall erosivity

Rain is the main factor of soil erosion by water. Its capacity to damage the soils depends on rainfall intensity, on volume of precipitations and on the hydrological response of the soil to rainfalls. This led us to propose two types of input data.

The first type defines the rainfalls ability to erode the soil based on its intensity. Among the indicators usually employed, we chose to considerate the yearly number of days with a Daily Intensity is above 10 mm (Fig. 2). Data comes from records of local meteorological stations of Météo France network.

The second type of data results from the response of the soils to the rainfalls. It refers to the concept of yearly positive hydrological balance and is given by applying the methods of hydrous budget. The positive hydrological balance is regarded as available water either for drainage or runoff. Data are resulting from the combination of potential evapotranspiration, rainfalls and available water content. If it is difficult to estimate the part of drainage and runoff, we can recognize that the risk of runoff increases with an increase of positive hydrological balance. These data are calculated at monthly scale and then cumulated to obtain results at yearly scale. The data used are means of climatic period. The main problem encountered to get this type of data comes from the methods implemented to gather accurate and reliable data about available water content. This problem is solved since it has been decided to start a wide program of soil mapping over the Calvados.
