Circular features in the desert, which almost always indicate irrigated agriculture, appear in this near-vertical photograph of the Al Khufrah Oasis in southeastern Libya. This center-pivot irrigation system uses ground water that is pumped under pressure into a gantry or tubular arm from a central source. Anchored by a central pivot, the gantry slowly rotates over the area to be irrigated, thereby producing the circular patterns. Although the field diameters vary, these fields are approximately 0.6 mile (1 kilometer) in diameter. Darker colors indicate fields where such crops as wheat and alfalfa are grown; lighter colors show fields that have been harvested recently; fields that are lying fallow, fields that have just been planted, or fields that have been taken out of production. Because hot, dry winds have a devastating effect on plants and livestock in this desert environment, adequate water and suitable soils are not always sufficient for agriculture and livestock production. Visible are a long, north-south-aligned airport runway in the center of the photograph and hexagonal features southwest of the airport runway that are perhaps either more desert agriculture (without center-pivot irrigation) or military sites.