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Irrigation in the Sahara Desert.

Irrigation is the artificial application of water to the soil usually for assisting in growing crops. In crop production it is mainly used to replace missing rainfall in periods of drought, but also to protect plants against frost. Additionally irrigation helps to suppress weed growing in rice fields. In contrast, agriculture that relies only on direct rainfall is sometimes referred to as dryland farming or as rain fed farming. Environmental problems such depletion of underground aquifers, ground subsidence, buildup of toxic salts on soil surface in areas of high evaporation, overirrigation and the possibility of water pollution.