Dryland is defined as an area where the aridity index (AI), which represents the degree of dryness (AI = P (annual precipitation)/PET (annual potential evapotranspiration)), is 0.65 or less. These areas are divided into four categories based on the value of AI: hyper-arid areas <0.05 ≤ arid areas <0.20 ≤ semi-arid areas <0.50 ≤ dry sub-humid areas <0.65. Hyper-arid areas with an AI value of less than 0.05 are mostly deserts where plants cannot grow and are therefore excluded from the scope of “desertification” as defined in UNCCD (the United Nations Convention to Combat Desertification).