Identification of the best spectral indices to remotely trace the diurnal course of water use efficiency of Tamarix ramosissima in the Gurbantunggut Desert, China

Water availability is one of the most important factors limiting photosynthetic assimilation of carbon dioxide and growth of individual plants in terrestrial ecosystems. Water use efficiency (WUE) of plants has been widely assessed using ecological methods in field measurements; however, approaches for remotely sensing WUE are still lacking, particularly in arid ecosystems. In this study, a comprehensive analysis of diurnal WUE via spectral indices in arid ecosystems was assessed. Analyses were conducted on a native dominant desert shrub, Tamarix ramosissima, in its original habitat on the southern periphery of the Gurbantunggut Desert, China. Based on diurnal measurements of spectral reflectance, photosynthesis, and micrometeorological variables, simple and useful spectral indices for estimating diurnal WUE at the assimilative organ scale were explored. From six types of spectral indices, ranging from simple to sophisticated, the best wavelength domains for a given type of index were determined by screening all combinations using correlation analysis. The coefficient of determination (R 2), ranging from 0.19 to 0.60, for WUE was calculated for all indices derived from spectra taken from the assimilative organs. With only two wavelengths and a significant correlation coefficient (R 2 = 0.60, P < 0.001), the simple ratio (SR) type index was the most sensitive to WUE among all of the indices. Furthermore, SR is a useful indicator to determine the dynamic and diurnal processes of photosynthesis and transpiration of T. ramosissima. Although it has a few weaknesses, SR serves as a simple and robust indicator of WUE in arid ecosystems.