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Aridity increased, with the Ghaggar-Hakra River retracting its reach towards the foothills of the Himalayas, [64] [67] [68] leading to erratic and less-extensive floods, which made inundation agriculture less sustainable. Aridification reduced the water supply enough to cause the civilisation's demise, and to scatter its population eastward.
Desertification is a gradual process of increased soil aridity.Desertification has been defined in the text of the United Nations Convention to Combat Desertification (UNCCD) as "land degradation in arid, semi-arid and dry sub-humid regions resulting from various factors, including climatic variations and human activities."
An aridity index (AI) is a numerical indicator of the degree of dryness of the climate at a given location. The American Meteorological Society defined it in meteorology and climatology, as "the degree to which a climate lacks effective, life-promoting moisture".
Aridification is the process of a region becoming increasingly arid, or dry.It refers to long term change, [1] rather than seasonal variation. It is often measured as the reduction of average soil moisture content.
For example, temperature increase by 1.5–2.1 percent across the Nile Basin over the next 30–40 years could change the region from semi-arid to arid, significantly reducing the land usable for agriculture. In addition, changes in land use can increase demands on soil water and thereby increase aridity. [6]
For example, Phoenix, Arizona, receives less than 250 mm (9.8 in) of precipitation per year, and is immediately recognized as being located in a desert because of its aridity-adapted plants. The North Slope of Alaska's Brooks Range also receives less than 250 mm (9.8 in) of precipitation per year and is often classified as a cold desert. [24]
Title page of J.W. Powell's 1879 Report on the Lands of the Arid Region of the United States.. Report on the Lands of the Arid Region of the United States is a scientific report and policy recommendation written by American explorer, geologist, and anthropologist John Wesley Powell, and first published in 1878.
Thornthwaite developed four indices: the Moisture Index (Im), the aridity and humidity indexes (Ia/Ih), the Thermal Efficiency Index (TE) and the Summer Concentration of Thermal Efficiency (SCTE). Each of the four climatic types can be described by an English alphabet letter and are arranged exactly by the order shown previously. [5]