## 3 Emerging Patterns from Existing Studies
### Spatial resolution requirements
While the NDVI signal associated with irrigation allows identification of irrigated lands, particularly in semi-arid areas characterized by a single irrigation period and simple land cover types, in many areas of the world there are multiple irrigation periods and several crop types with different schedules. In these locations, the NDVI signal associated with irrigation might be more subtle. Frequent observations of the NDVI from sensors like MODIS may help to map these lands and thus determine acquisition times for high spatial resolution imagery. However, difficult cases for distinguishing irrigated from non-irrigated crops occur in locations where the same crop type is grown with and without irrigation during the same growing season. Temporal NDVI profiles of both irrigated and non-irrigated crops in these locations may exhibit an identical pattern. While irrigated fields often exhibit greater greenness due to the constant availability of moisture, the NDVI difference between irrigated and non-irrigated fields is small and probably useless in distinguishing them. Thus, a more sensitive index may be required to make this distinction. A large body of research of spectral remote sensing of vegetation canopies indicates that moisture stress in vegetation is strongly manifested in spectral indices related to chlorophyll content [78,79]. One such index, suggested by [PERSON] et al. [80] to be used with the MODIS sensor, is the Green Index (GI) defined as:  
\[GI=\rho_{{}_{int}}/\rho_{{}_{green}} \tag{1}\]  
where \(\rho_{{}_{green}}\) is the reflectance in the green spectral region. The theoretical foundations of the GI are given in [PERSON] _et al._[78]. Briefly, it is based on evidence that in the green spectrum (centered around 510 nm) the specific absorption coefficient of chlorophylls is very high while green leaves are absorbing more than 80 percent (e.g., [81]). In contrast, depth of light penetration into leaves in the blue and red spectral ranges is one-fourth to one-sixth as much (e.g., [82]). Therefore, in the green spectrum, absorption of light is great enough to provide high sensitivity of the GI to chlorophyll content but much lower than in the blue and red to avoid light saturation [78]. The theoretical foundation for using the GI for irrigation assessment is that irrigated crops with very little or no soil moisture stress will exhibit higher chlorophyll content than non-irrigated crops that potentially experience moisture stress. To test this hypothesis and the sensitivity of the GI to the presence of irrigation (moisture), [PERSON] and [PERSON] [20] compared four vegetation indices in the midwestern US: NDVI, Enhanced Vegetation Index (EVI) [81], Wide Dynamic Range Vegetation Index (WDRVI) [82], and the GI, using the following Relative Sensitivity Index (RSI):  
\[RSI=\frac{I_{irr}-I_{non}}{I_{non}(\max)-I_{non}(\min)} \tag{2}\]  
In Equation (2), \(I_{irr}\) and \(I_{non}\) are the irrigated and non-irrigated values for each index, I, at each time period, normalized by the seasonal amplitude (maximum-minimum) of non-irrigated values of each index. RSI represents the difference between irrigated and non-irrigated index value compared to the seasonal maximum change non-irrigated value of the same index. Comparison of relative sensitivity of all four indices reveals that the GI shows the greatest sensitivity to irrigation presence during peak crop growth (Figure 3). Similar results were obtained by [PERSON] _et al._[83] in the same location. Recognizing the increased sensitivity to irrigation, we suggest that future mapping studies include the GI in the classification process. As noted, irrigated areas are very dynamic in time and space and thus their spectral signatures quickly change across the growing seasons.