## 1 Introduction  
The intensification of agricultural practices--under the auspices of the \"Green Revolution\" that includes better seeds, extensive fertilizer use, and irrigation--has dramatically altered the relationship between humans and environmental systems across the world. Today many agricultural lands are being used much more intensively as opportunities for expansion are being exhausted elsewhere. In the last 40 years, global agricultural production has more than doubled--although cropland has increased by only 12%--in part through increased reliance on irrigation [1,2]. Currently, irrigated agriculture is the principal consumer of fresh water resources; it accounts for more than 70 percent of water withdrawn from lakes, rivers, and groundwater aquifers [3]. As the earth's population continues to increase and the demand for food, fuel, and fiber rises, continued agricultural intensification will require at least a 50 percent increase in water resources, especially in arid and semi-arid regions [4]. While these modern agricultural practices have successfully increased food production, they have also caused significant environmental change in many regions. Accurate information on the extent of irrigation is thus fundamental to many aspects of Earth System Science, and global change research in general. These aspects include modeling of water exchange between the land surface and atmosphere [5-8], analysis of the impact of climate change and variability on irrigation water requirements and supply [9-13], management of water resources that affect global food security [14], and climatic feedbacks, including the effect that results from evaporative cooling in intensely irrigated arid areas [15,16]. Despite their significance for food security and the water and energy cycles, the extent and distribution of irrigated areas worldwide still remain uncertain [17]. Existing maps, especially those covering large areas, have been derived primarily from country-level statistics. The politically charged nature of irrigation often sets the stage for under-reporting of water use; this is especially true in countries that share resources across borders with their neighbors [18]. Country-level estimates also mask the considerable spatial variability in irrigation practices, and simply cannot reflect the location or extent of irrigation across large areas [19]. Even in countries such as the U.S., where the extent of irrigated areas is known, irrigation-related information exists in disparate sources and cannot be easily synthesized into a single continental scale database [20]. Also, information on irrigated areas in many countries is reported only from officially recognized management units (or command areas) servicedby large scale irrigation projects. As a result, subsistence-scale irrigation is not reported, although these areas could collectively account for a substantial land area and significant amount of water use. Satellite remote sensing offers tremendous potential for routine monitoring of irrigation due to the synoptic nature of the data and readily available archives of imagery. Yet studies that have used remote sensing to map irrigated lands remain relatively rare. This is a direct result of the complexity associated with trying to map _land use_ as opposed to _land cover_. While it may be straightforward to detect the high near-infrared signal of mature crops given appropriate spatial, spectral and temporal resolution data (_i.e._, land _cover_), detecting irrigation requires knowledge of land management, or some understanding of where and when humans have provided water or supplemented rain-fed crops (_i.e._, land _use_). Because of the difficulty in isolating these practices with satellite observations, a literature search reveals only 65 peer-reviewed papers that use remote sensing to map irrigation, compared to thousands that report agricultural or land-cover mapping activities.