a climatic region having an annual precipitation, usually between 25.4 and 50.8 centimeters (10-20 inches) per year, this being greater than a truly arid climate; typically, the vegetation is composed of grasses with or without woody plant layers.
Semi-arid generally describes climatic regions that receive low annual rainfall (250-500 mm or 10-20 in) and have predominantly scrub or short-grass vegetation. A more precise definition is given by the Köppen climate classification that treats steppe climates (BS) as intermediates between the true deserts (BW) and humid climates in ecological characteristics and agricultural potential. The Köppen climate classification allows adjustments for temperature and for seasonality of precipitation, effectively excluding forested regions (such as those in Canada and Siberia).