By analysing changes in the subindicators in the context of local assessments of climate, soil, land use and any other factors influencing land conditions, national authorities can determine which land units are to be classified as degraded, sum the total, and report on the indicator. A conceptual framework, endorsed by the UNCCD’s governing body in September 2017,^{[30]} underpins a universal methodology for deriving the indicator. The methodology helps countries to select the most appropriate datasets for the subindicators and determine national methods for estimating the indicator. In order to assist countries with monitoring and reporting, Good Practice Guidance for SDG Indicator 15.3.1^{[31]} has been developed by the UNCCD and its partners.
The indicator is derived from a binary classification of land condition (i.e., degraded or not degraded) based primarily, and to the largest extent possible, on comparable and standardized national official data sources. However, due to the nature of the indicator, Earth observation and geospatial information from regional and global data sources can play an important role in its derivation, subject to validation by national authorities.
Quantifying the indicator is based on the evaluation of changes in the subindicators in order to determine the extent of land that is degraded over total land area. The subindicators are few in number, complementary and nonadditive components of landbased natural capital and sensitive to different degradation factors. As a result, the 1OAO principle is applied in the method of computation where changes in the subindicators are depicted as (i) positive or improving, (ii) negative or declining, or (iii) stable or unchanging. If one of the subindicators is negative (or stable when degraded in the baseline or previous monitoring year) for a particular land unit, then normally it would be considered as degraded subject to validation by national authorities.
The baseline year for the indicator is 2015 and its value (t_{0}) is derived from an initial quantification and assessment of time series data for the subindicators for each land unit during the period 20002015. Subsequent values for the indicator during each monitoring period (t_{1n}) are derived from the quantification and assessment of changes in the subindicators as to whether there has been positive, negative or no change for each land unit relative to the baseline value. Although the indicator will be reported as a single figure quantifying the area of land that is degraded as a proportion of land area, it can be spatially disaggregated by land cover class or other policy‐relevant units.
As detailed in the Good Practice Guidance for SDG indicator 15.3.1, deriving the indicator for the baseline and subsequent monitoring years is done by summing all those areas where any changes in the subindicators are considered negative (or stable when degraded in the baseline or previous monitoring year) by national authorities. This involves the:
(1) assessment and evaluation of land cover and land cover changes; (2) analysis of land productivity status and trends based on net primary production; and (3) determination of carbon stock values and changes, with an initial assessment of soil organic carbon as the proxy.
It is good practice to assess change for interim and final reporting years in relation to the baseline year for each subindicator and then the indicator. This facilitates the spatial aggregation of the results from the subindicators for each land unit to determine the proportion of land that is degraded for the baseline and each monitoring year. Furthermore, it ensures that land classified as degraded will retain that status unless it has improved relative to the baseline or previous monitoring year.
Land degradation (or improvement) as compared to the baseline may be identified with reference to parameters describing the slope and confidence limits around the trends in the subindicators, or to the level or distribution of conditions in space and/or time as shown during the baseline period. The evaluation of changes in the subindicators may be determined using statistical significance tests or by interpretation of results in the context of local indicators, data and information. The method of computation for SDG indicator 15.3.1 is illustrated in Figure 2.
Figure 2: Steps to derive the indicator from the subindicators, where ND is not degraded and D is degraded.
The area degraded in the monitoring period t_{n} within land cover class i is estimated by summing all the area units within the land cover class determined to be degraded plus all area units that had previously been defined as degraded and that remain degraded, minus area units that have improved from a degraded to a nondegraded state:
${A\left(Degraded\right)}_{i,\mathrm{n}}={A\left(recent\right)}_{1,n}+{A\left(persistent\right)}_{i,n}\mathrm{\ }{A\left(improved\right)}_{1,n}$ (1)
Where:
${A\left(Degraded\right)}_{i,n}$ is the total area degraded in the land cover class i in the year of monitoring n (ha);
${A\left(recent\right)}_{i,n}$ is the area defined as degraded in the current monitoring year following 1OAO assessment of the subindicators (ha);
${A\left(persistent\right)}_{i,n}$ is the area previously defined as degraded which remains degraded in the monitoring year following the 1OAO assessment of the subindicators (ha);
${A\left(improved\right)}_{i,n}$is the area that has improved from a degraded to a nondegraded state following the 1OAO assessment of the subindicators (ha).
The proportion of land cover type i that is degraded is then given by:
${P}_{i,n}=\frac{{A\left(Degraded\right)}_{i,\mathrm{n}}}{{A\left(total\right)}_{i,0}}$ (2)
Where
${P}_{i,\mathrm{n}}\mathrm{\ }$is the proportion of degraded land in that land cover type i in the monitoring period n;
${A\left(Degraded\right)}_{i,\mathrm{n}}$ is the total area degraded in the land cover type i in the year of monitoring n (ha);
${A\left(total\right)}_{i,0}$ is the total area of land cover type i within the national boundary (ha).
The total area of land that is degraded over total land area is the accumulation across all land cover classes within the monitoring period n is given by:
${A\left(Degraded\right)}_{\mathrm{n}}=\sum _{i}^{}{A\left(Degraded\right)}_{i,\mathrm{n}}$ (3)
Where
${A\left(Degraded\right)}_{\mathrm{n}}$ is the total area degraded in the year of monitoring n (ha);
${A\left(Degraded\right)}_{i,\mathrm{n}}$ is the total area degraded in the land cover type i in the year of monitoring n.
The total proportion of land that is degraded over total land area is given by:
${P}_{\mathrm{n}}=\frac{{A\left(Degraded\right)}_{\mathrm{n}}}{\mathrm{A}\left(\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}\right)}$ (4)
Where
${P}_{\mathrm{n}}$ is the proportion of land that is degraded over total land area;
${A\left(Degraded\right)}_{\mathrm{n}}$ is the total area degraded in the year of monitoring n (ha);
$\mathrm{A}\left(\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}\right)$ is the total area within the national boundary (ha).
The proportion is converted to a percentage value by multiplying by 100.
