The brown and red discs scattered throughout this coloured SPOT-1 satellite image are irrigated fields in the desert of Saudi Arabia. The fields, typically 600 meters across, are irrigated using a center-pivot system, where water drawn from a well in the center of the field is applied to the crops by a sprinkler system rotating about the well. Such irrigation schemes have enabled Saudi Arabia to become a net exporter of many crops, particularly wheat, although there are worries about the long-term viability of the aquifers that feed the wells.