## 2 Review of Existing Studies
### Local Scale Studies  
To date, most research on remote sensing-based irrigation mapping occurs at local scales for several reasons. First, local areas provide familiar environments where investigators have extensive knowledge of their study sites. Second, the purpose of a remote sensing study may be to demonstrate the utility of a method rather than an operational assessment encompassing all irrigation. In these cases a small area suffices. Third, irrigation practices take many forms and this variation increases as the \begin{table}
\begin{tabular}{l l l l l l l} \hline
**Satellite** & **Technical** & **Access to** & **Continuous** & **Pre-** & **Image** & **Technical** & **Frequency of** \\
**observation** & **observation** & **information**/ & **observation** & **processed** & **data** & **difficulty** & **use in** \\
**system/prog** & **challenges** & **data** & **program with** & **datasets** & **cost** & **required to** & **irrigation** \\
**ram** & **solved** & **worldwide** & **global** & **accessible** & & **produce** & **studies** \\  & & & & **coverage** & & **maps** & \\ \hline RapidEYE & yes & no & yes & yes & high & medium & low \\ Landsat & yes & yes & yes & yes & low & medium & high \\ SPOT & yes & yes & yes & yes & high & medium & medium \\ AWiFS & yes & no & no & yes & high & medium & low \\ LISS & yes & no & no & yes & high & medium & low \\ ASTER & yes & yes & no & yes & low & medium & low \\ CBERS & no & no & no & no & low & medium & low \\ THEOS & yes & no & no & yes & medium & medium & low \\ MODIS & yes & yes & yes & yes & low & high & medium \\ MERIS & yes & yes & yes & yes & low & high & low \\ AVHRR & yes & yes & yes & yes & low & high & medium \\ SPOT VEG. & yes & no/yes & yes & maybe & low & medium & low \\ \hline \end{tabular}
\end{table} 
Table 1: Current operational optical sensors and their technical/logistical capacities for irrigation mapping. Some of the table headings are adapted from the United Nations Collaborative Programme on Reducing Emissions from Deforestation (UN-REDD) sourcebook and Forest Degradation in Developing Countries [97]. \begin{table}
\begin{tabular}{l l l} \hline
**Spatial scale** & **Sensors used** & **Method of mapping** & **References** \\ \hline local & Landsat TM/ETM+, SPOT, & Photo interpretation, Image arithmetic, & [26-29,31-33, \\  & LISS, ASTER, AWiFS, & Image classification, segmentation, & 42-44,98] \\  & CBERS, THEOS & image fusion & \\ regional & Landsat TM/ETM+,MODIS, & Times-series analysis, & [21,47-50,53] \\  & MERIS, AVHRR, SPOT VGT & Supervised/unsupervised classification, & \\  & masking & & \\ continental & Landsat TM/ETM+, MODIS, & Times-series analysis with other & [7,19-21,49, \\  & MERIS, AVHRR, SPOT VGT & ancillary data, data fusion & 52,98] \\ global & MODIS, MERIS, AVHRR, & Unsupervised clustering, machine & [22,54-57] \\  & SPOT VGT & learning algorithms applied to time- & \\  & & series data, also employ other ancillary & \\  & & data(statistic, ground truth data...) & \\ \hline \end{tabular}
\end{table} 
Table 2: Summary of spatial scales, sensors, methods, and example applications in the literature. study area becomes larger. Therefore methods developed in one place and time may not be appropriate for other locations and durations. Two forms of mapping methods emerge as most common in local area studies. These methods are visual interpretation and digital image classification. Early work concentrated on determining the utility of remotely sensed imagery using various visual interpretation techniques adapted from the airphoto sector to map and update irrigated acreage, mainly in the U.S. and India [26-32]. More recent studies have developed digital image classification methods specifically adapted to mapping irrigated lands automatically [33-37]. Within each category, several methods have been tested. _Visual interpretation of satellite imagery_  
Early work with satellite imagery at the local scale relied on visual interpretation of large hard-copy satellite image prints to identify and map irrigated lands [26-28].