### Land degradation risk

#### Indicators:

- Water-erosion sensitivity index
- Rainfall erosivity index
- Land-use pressure index

Food security may be threatened by loss of soil productivity as a result of human-induced land degradation. The qualitative modelling approach for land degradation in IMAGE 2.4 is restricted to the assessment of the land's sensitivity to water erosion, which is the most prominent cause of land degradation in the world (see land degradation model). Other forms of land degradation risks such as salinization, alkalinization and wind erosion are not yet simulated.

Sensitivity to water erosion is used to express the chance of short-term occurrence of water erosion under current conditions of terrain erodibility, rainfall erosivity, and land cover and land use. The sensitivity is assumed to be lowest under natural vegetation, which provides maximum protection. Under agricultural systems the actual vulnerability depends on the type of crop grown. The land's sensitivity may change, along with changes in land use and climate.

| #### Water-erosion sensitivity index
| unit: none (index, no dimension) dimension: region (in graph), index category

The water-erosion sensitivity index expresses the sensitivity of the land under a specific form of land use. The sensitivity is composed of the land's so-called susceptibility and the type of land use (i.e., the land-use pressure index). The susceptibility is defined as the sensitivity to water erosion of the bare soil (i.e., terrain erodibility), based on the specific soil and terrain conditions (soil properties, slope, slope length) and climate (i.e., rainfall erosivity).

In the map and graph the index is presented for all classes or categories from 0 (no water erosion risk in case of natural vegetation) to 1.0 (highest risk) in steps of 0.05. Based on validation of the model results against the GLASOD water erosion assessment, actual water erosion risks occur at index values greater than 0.15. High water erosion risks are associated with index values of 0.30 and beyond.

The classification in the table below resulted in maximum correspondence with the GLASOD degradation status maps and can be used as a guide when analyzing the water-erosion sensitivity indicator.

Classification of water-erosion sensitivity index
Water erosion-sensitivity index | Erosion risk
< 0.15 | no/low
0.15-0.30 | moderate
0.30-0.45 | high
> 0.45 | very high

| #### Rainfall erosivity index
| unit: none (index, no dimension) dimension: rainfall erosivity index

The rainfall erosivity index describes the effect of climate on water erosion. As soil loss generally occurs during showers of high intensity, the month with maximum rainfall intensity is considered to be indicative for rainfall erosivity. An indication of rainfall intensity was inferred from the monthly precipitation values and the number of rain-days.

For each grid cell, maximum mean-monthly rainfall intensities of below 2 mm per day result in an index value of 0, while those exceeding 20 mm per day are assigned a value of 1.0. Between these two extremes a linear relationship is assumed.

| #### Land-use pressure index
| unit: none (index, no dimension) dimension: land-use pressure index

The land-use pressure index represents the degree of protection of the soil surface provided by land use and land cover. The IMAGE land-cover maps containing the geographic distribution of different land-cover types, forms the basis for estimating current and future pressure due to land use and cover. The effect of soil and crop management and soil conservation practices is not included in the land-use pressure index.

Natural vegetation such as forests provide a high degree of protection against rainfall erosion and which results in a low land-use pressure index value. Agriculture generally leads to higher sensitivity of the land surface to the destructive action of intensive rainfall and thus goes with higher values of the land use pressure index. A grid cell (partly) covered by agricultural crops is assigned a composite index value, which is based on the distribution of agricultural crops within the specific world region to which the grid cell belongs.