Vegetation indices (VIs) have been shown to be a proxy of green leaf area index (gLAI); however, it has not been verified whether the relationships VI vs. gLAI are the same, as well as VIs retaining their accuracy, for various crop types for estimating gLAI. The goal of this study was to (1) determine if the best VIs used in previous studies for gLAI estimation in maize and soybean may be applicable for potato and wheat and vice versa, and (2) determining the cause of a hysteresis between green up and reproductive stages for the VI vs. gLAI relationship. Spectral measurements of wheat and potato were obtained in Israel and of maize and soybean in the USA. In Israel, remote estimates of gLAI were compared with in-situ canopy transmittance measurements of irrigated potato and wheat under various nitrogen treatments from 2004-2007 for a total of 15 field-years. In eastern Nebraska, USA, remote estimates of maize and soybean gLAI data were compared with destructive gLAI determination in two irrigated/rainfed maize/soybean rotation sites and in one irrigated site under continuous maize. These data were collected during eight years (2001-2008) for a total of 24 field-years. For all four crops, the ten VIs examined showed similarities in relationships between VIs and gLAI with the exception of Red-edge Inflection Point (REIP) and the MERIS Terrestrial Chlorophyll Index (MTCI). REIP and MTCI have very different relationships with maize and soybean gLAI in green up and reproductive stages, thus, they require re-parameterization during the season. This study outlines the two major factors that influence the VI vs. gLAI relationship in the green up and reproductive stages. While the results suggest that relationships VI vs. gLAI are quite close for all four crops, different methodologies in determining the ground-truth measurements of gLAI prevent us to confirm whether algorithms calibrated for one crop can be used with no re-parameterization for other crops. These concerns aside, we found that normalized difference VIs (NDVI, Green NDVI, etc.) were capable of estimating accurately gLAI below 2 and ratio VIs (Simple Ratio, CIgreen, etc.) were best for gLAI above 2. For all four crops, green and red-edge chlorophyll indices appear to be the most accurate for gLAI estimation.
|Original language||English GB|
|State||Published - 3 Dec 2012|
|Event|| American Geophysical Union, (AGU) Fall Meeting 2012 - San Francisco, San Francisco, United States|
Duration: 3 Dec 2012 → 7 Dec 2012
|Conference||American Geophysical Union, (AGU) Fall Meeting 2012|
|Period||3/12/12 → 7/12/12|