Global Remote Sensing of Water Use Efficiency: Initial Test and Application of a Synergistic Approach

Water use efficiency (WUE) provides a critical link between carbon and water cycles in terrestrial ecosystems. At leaf and canopy scales, WUE relates to the ratio of carbon fixed via photosynthesis to water lost through transpiration. WUE is mechanistically related to Ci/Ca, or the ratio of leaf internal to atmospheric CO2 concentrations. In turn, Ci/Ca ratios are related to discrimination against stable carbon isotopes during photosynthesis, an important constraint on the global carbon cycle. Methods to assess canopy WUE at canopy to global scales are therefore relevant to assessing plant Ci/Ca ratios and carbon isotope discrimination. We describe and test a synergistic method for remote sensing of water use efficiency that builds on rapidly developing methods for remote sensing of photosynthesis and transpiration. In this approach, photosynthesis (A) and transpiration (T) are assessed independently then combined to estimate WUE=A/T. We first describe methods for remote sensing of A and T based on visible and near-infrared reflectance, solar radiation, temperature and humidity, and methods of calculating Ci/Ca from WUE. We then validate these methods using eddy covariance and stable isotope measurements in a Mediterranean oak-grass savanna that exhibits dynamic seasonal changes in WUE and Ci/Ca. Finally, we apply the method at the global scale using satellite observations from the Advanced Very High Resolution Radiometer (AVHRR). In addition to highlighting spatial patterns and temporal trends in WUE, we demonstrate that knowledge of C3 or C4 photosynthetic pathway is not necessary for estimating transpiration when leaf area index (LAI) and net radiation (Rn) are used as inputs, but it is required for estimating photosynthesis. Knowledge of C4 plant distributions is therefore critical to remote sensing of regional to global scale WUE.