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3.3 Second phase: Descriptive approach

The implementation of the descriptive approach directly relates to the legend presented in the Chapter 2.2, and in particular in Box 1, point B (site-descriptive mapping).

The descriptive approach consists of the application of the legend to the polygons of the erosion status map resulting from the first phase (predictive approach).

Two elements/procedures are used for such application: photo-interpretation, and field observation. Both are necessary and complementary.

Some of the aspects or categories considered in the legend are rather conspicuous in aerial photographs. Easily identifiable by photo-interpretation are: wastelands (rock outcrops, cliffs, stony or sandy areas), some types of afforestation, physical structures (e.g. terraces), dominant gullies, some cases of mass movements and sediment deposits.

Assuming that overall geographical environments are to be subdivided in two broad categories, i.e. Stable non-erosion-affected areas, and Unstable erosion-affected areas, continued field observation and erosion process identification procedures consist of several very distinct steps:

• Step 1: For stable/stabilized environments defining the grade of erosion risks/potentials;

• Step 2: For unstable environments defining the type of dominant erosion process, its relative intensity and evolutive trend;

• Step 3: Identifying and assessing specific local features such as main erosion-prone areas or special causative agents.

The above-mentioned procedures are mainly qualitative assessments and are to be considered as complementary to the predictive phase.

Step 1: Defining the grade of erosion risk/potential

• The procedure only applies to stable, non-erosion-affected areas which are defined as showing very few or no evident signs of erosion, with well developed topsoil and good soil structure; these areas are usually unused or very lightly/suitably used by man: either the present vegetation cover is adequate and/or topographic and soil conditions prevent erosion.

• Different types of stable and/or rehabilitated areas are identified according to their use, management and grade of erosion risk: the erosion risk ranges from 0 (no risk = highest grade of stability) to 3 (stability threshold = highest grade of risk).

• In most cases, the main erosion risk causative agents are easily identifiable; they might be indicated by extra codes as described in Box 1-Legend.

Step 2: Identifying and defining dominant erosion processes

• All unstable areas are affected by one or several erosion processes ranging from slight to moderate and severe degradation which for each specific process can be assessed in relative terms of intensity (depth of gullies, volume of soil or sediments removed), or extension of space affected (localized, dominant or generalized): adapted codes are given in the legend.

Some practical field observation indicators can be easily identified:

• Slight erosion usually shows some localized surface wash, as a consequence of sheet erosion with a soil loss rate slightly greater than the soil formation rate;

• Moderate erosion clearly shows localized losses of topsoil mainly due to a combination of sheet and rill erosion, particularly on cultivated areas or those covered by scarce vegetation;

• Severe erosion generally removes most of topsoil by sheet and/or rill erosion; gullies have begun to form, fertility as well as the potential land use for many purposes have been seriously affected;

• In severely gullied areas almost all topsoil is removed and more than 50% of the land is gullied and much or all of the affected land may lack any vegetation; these areas are the so-called "badlands" hardly useful for any purpose thus classified as non-recoverable areas in the legend (LX, CX);

• Complementary indicators on erosion dynamics can be provided through the identification and evaluation of erosion evolutive trends expressed by sub-digits as indicated in the legend: either stabilization or expansion and/or intensification of the referred erosion process can be indicated by a sub-digit as described in the legend (see Box 1- Legend).

Step 3: Identifying specific local features or particularities, such as causative agents of erosion risks for stable environments. multiple and/or associated erosion processes and their main evolutive trends (see Box 1 - Legend).

3.4 Third phase: Integration

The final map is a descriptive/prescriptive erosion qualitative map containing rather complete information on a number of relevant aspects of the erosion phenomena.

• The final consolidated Erosion Map is the result of associating and integrating predictive and descriptive data in an iterative way using the potential erosion map as a reference cartographic layer which provides a basic grid of homogeneous units where various different erosion processes may occur. The description and mapping of active erosion as well as more specific erosion risks (better identified through their main causative agents) are complementary to the predictive cartographic data which implies that final integrated erosion units be expressed by one single comprehensive symbol.

• In stable areas grade of stability and erosion risks are identified by the means of the related descriptive symbols without referring to erosion status.

• Example: 032g = Stable managed area with high erosion risk mainly due to geologic factors

• In unstable environments erosion affected units are identified by consolidated symbols integrating both erosion status (expressed by the level number figured in brackets) and erosion process (figured by the related descriptive symbol expressing the nature, intensity, extension and evolutive trend of the process); no causative agents are to be identified for actually unstable areas.

• Example:(2) L21 = unstable unit with level (2) erosion status affected by dominant sheet erosion with a trend to local expansion or intensification.

As a general recommendation, the developed methodology and mapping legend should not be considered as a rigid scheme, but rather as a set of practical guidelines in which cartographic concepts, units and symbols should be flexible and easily adapted to specific geographical country or regional features.

In the descriptive part of the legend the adaptation is possible by opening new categories of processes or features as proposed in the original legend under paragraph 'Multiple Processes" where symbols P. 3 are available for newly identified erosion forms or trends.

4. Implementation aspects

4.1 Technical management components

4.1 Technical management components

The application of the represented mapping methodology requires multidisciplinary technical teams. The composition could be as follows:

• 1 specialist in remote sensing (images and processing of data, photo-interpretation) and mapping activities to be fulfilled during the surveying and prospective phases of the programme.

• 1 geomorphologist for the analysis, identification and classification of landscape units and their related morphogenic features and surface water dynamics.

• 1 biogeographer and/or soil specialist for the identification and assessment of various types of both land use and vegetation covers and canopies.

• 1 team leader who should be an expert in soil erosion and be particularly familiar with integrated mapping techniques. He would be responsible for the elaboration of a locally adapted legend and the final integrated predictive/descriptive soil erosion map.

Training at team level should be initiated step-by-step, following the methodological sequences described in the present Guidelines. A second training phase could consist of practical mapping of a small area, preparing the complete sequence of maps and providing a final assessment of both results and problems.

Basic technical data and materials usually consist of the following:

• Complete aero-photographic coverage and/or enlarged satellite images of the study area at scales preferably medium to large (i.e. 1:50,000 to 1: 10,000).

• Topographic/hypsometric (contour) maps at scales as near as possible to the scale of aerial photographs and/or enlarged satellite images.

• Geologic/lithologic existing or derived interpreted maps covering the survey area at scales which can be smaller than the basic topographic mapping grid.

• Photo-interpretation material and tools for both office and field work activities (table and pocket stereoscopes).

• Field visits in different periods of the year might be considered in order to take into account variations in vegetation cover and land use as well as extreme rainfall events.

An example of practical application of the proposed methodology is provided by the Adra river basin (726 km²) case study, where the programme of activities was as follows:

1. Basic data and material preparation
2. Preparation of slope map and derived erodibility map.

1 Geomorphologist

3. Preparation of land use and vegetation cover map

Steps 2 and 3 can be performed simultaneously by different members of the team.

4. Production of the Erosion Status Map by overlapping Erodibility and Soil Protection Maps 2 m/weeks
5. Production of the final Descriptive and Predictive Erosion Map

Use of colours

When presenting the maps in colours, the following should be applied:

• stable / non affected areas to be presented in blue or green;

• unstable areas to be presented from yellow Þ orange Þ light red Þ dark red, with progressive growing density of the colour selected (from dots to full colour).

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