## 1 Introduction
Thus, from the remote sensing perspective, studies that attempt to map irrigated areas have been rare and scientific consensus on mapping methodologies is fragmented and evolving. This review seeks to synthesize current studies on identification and mapping of irrigated areas by remote sensing. Our goal is two-fold. First, we will provide a reference guide to the spatial, spectral, and temporal information requirements for monitoring irrigated areas, derived from case studies that have successfully mapped irrigated lands. Second, and more important, we will establish the state-of-the-art in this field by providing a comprehensive assessment and a taxonomic synthesis of studies to date. This information can provide a foundation for future studies to expand on these methods and fill data gaps. The approaches that have been adopted to tackle irrigation are diverse; they vary in scale, extent, data inputs and processing requirements. It is also clear that a consensus within the scientific community as to the 'best practices' for mapping irrigation are still evolving, although certain methods appear to be common among different studies. Moreover, reviews such as this portray information needs for timely and accurate monitoring of irrigation. This is necessary in order to form the basis for development of sustainable water management practices within the context of what is perhaps the greatest human intervention in the hydrological cycle. _Advantages and disadvantages of remote sensing_  
We will begin with a brief discussion of the benefits and drawbacks of remote sensing for mapping crop location, productivity, and change in irrigated settings. Remote sensing has been an effective tool to monitor irrigated lands in many locations around the world under a variety of environmental conditions [19-21,28,33,35,50,60,65]. It provides synoptic coverage of irrigated fields in several spectral regions and with temporal frequencies sufficient to assess vegetation growth, maturity, and harvest. Archived data that span many years allow comparison of images, thus revealing change. The digital nature of satellite data also makes it relatively easy to integrate into a Geographic Information System (GIS) for synthesis or comparison with other data sources. Remotely sensed data are also less costly and time-consuming than traditional statistical surveys that may require aerial photography over large areas. This makes remote sensing particularly valuable for inventories of irrigated land and for monitoring in developing countries, where funds are limited and little objective information is available. Moreover, remote sensing delivers useful spatial information on the exact locations of irrigated lands rather than mere totals within arbitrary political units. This is important for prioritizing water delivery, assessing irrigation performance, providing irrigation intensities (e.g., single crop _vs._ double crop), quantifying environmental impact, objectively assessing irrigation water use and understanding changes where irrigation occurs. Finally, remote sensing can provide information on timing, both in the number of irrigation-related vegetation peaks and in the length of time irrigation is utilized over the course of a year. However, satellite imagery also has limitations. Because of the spatial resolution of most operational imagery (15-60 m), it is difficult to identify small irrigated areas which, taken together, may cover significant parts of the earth. It is also difficult to separate irrigated fields from non-irrigated plots in humid areas because of substantial overlap in their spectral signatures. For example, the signatures of flooded irrigated fields at certain growth stages may overlap with those of natural wetlands, thus limiting accuracy in mapping. Researchers have overcome these limitations by using temporal information on crop planting, maturity, and harvest in conjunction with spectral information [21].