Classification of arid lands, including soil degradation and irrigated areas, based on vegetation and aridity indices

Abstract

It is preferable to prepare internally consistent maps of arid regions on a global scale in order to understand the present conditions of arid regions, especially deserts and soil degradation areas. We attempted to delimit arid regions at a global scale by combining climate data, i.e. aridity index (AI), and vegetation data, i.e. vegetation index. The annual AI was estimated by the ratio of mean annual precipitation to mean annual potential evapotranspiration, using the Thornthwaite method. The long-term mean of yearly maximum normalized difference vegetation index (NDVI ymx ) was used as an indicator of the vegetation condition. Arid regions of the world were classified into four categories, namely A, severe deserts, where both aridity and vegetation indices are very small; G, semi-arid regions, where the vegetation index is proportionally related to the AI; I, irrigated areas and oases, where the vegetation is relatively abundant despite severe dryness; and S, soil degradation areas, where the vegetation is poor despite relatively humid conditions. The Sahel from Niger to Chad, the Sahel in Darfur, and the Ordos Plateau in China are within Category S. The standard deviation of NDVI ymx is very small/large in severe deserts/semi-arid areas, respectively. Thus, the Sahara desert was clearly distinguished from the Sahel; the latter belongs to Category G and drought occurs frequently here. In Category S zones, the standard deviation of NDVI ymx is relatively small compared with that within the Category G zone because the return rainfall does not seem to promptly restore productivity. Category S was divided into three subdivisions according to the degree of degradation, expressed by the ratio of the AI to vegetation index. Category G was also divided into four classes, according to degree of vegetation (or aridity). The distribution of Category S is comparable to the soil degradation areas mapped by Global Assessment of Human-Induced Soil Degradation (GLASOD) data. True deserts, where the standard deviation of NDVI ymx is very small, were selected from the 'severe desert' group. Desert areas were classified as true deserts, severe deserts, grassland deserts (Category G), and soil degradation deserts (Category S).

Using Remote Sensing to Map and Monitor Water Resources in Arid and Semiarid Regions

“…Gamo et al [11] developed a method for classifying arid lands. The objective of that project was to prepare internally consistent maps of arid regions on a global scale in an effort to understand the conditions of existing arid regions, especially deserts and soil degradation areas.…”

“…Desert areas were further classified into severe deserts (Category A), grassland deserts (Category G), and soil degradation deserts (Category S). As a result, a map was produced showing the global distribution of arid regions using these unified criteria with both physical and biological meaning [11].…”

Future Hydroclimatic Impacts on Africa: Beyond the Paris Agreement

“…Because of the diverse-and inconsistent-approaches used to delineate the boundaries between aridity classes (Gamo et al, 2013;Maliva & Missimer, 2012;Thornthwaite, 1948), we here used a simple method to split our group of basins in two major aridity groups, those tending to arid conditions and those to humid ones. We first classified the basins in two main aridity classes using the average value of the PET/P in the period 1960-1989 as measure of aridity.…”

Evaporation From Deep Aquifers in Arid Regions: Analytical Model for Combined Liquid and Vapor Water Fluxes

“…Population in these areas heavily relies on groundwater, so any loss of groundwater by evaporation could be critically important for sustaining their water supply. Evaporation in these environments is also important at the global scale, as hyperarid deserts represent 10% or more of the terrestrial landscape (e.g., Gamo et al, , classified 11.1% as “severe deserts”); therefore, the water balance in these areas, and especially the evaporation component, may play an important role in the global hydrologic cycle. Hence, it is important to improve our ability to quantify evaporation under hyperarid conditions for understanding the local and global impacts of evaporation on the hydrologic cycle.…”