## 1 Introduction  
The quantitative description of water, carbon and energy fluxes between the land surface and the atmosphere plays an important role in current hydrological and climate research studies, especially at larger spatial scales [1]. An effective tool to derive relevant land surface characteristics over the last decades has been the development and use of satellite remote sensing techniques in the visible (VIS: 350-700 nm) and near infrared (NIR: 700-1,300 nm), short wave infrared (SWIR: 1,300-2,500 nm), microwave and thermal infrared spectral region of the electromagnetic spectrum (see e.g., [2] for an extensive review of methods in the field of eco-hydrology). In particular, the monitoring of narrow band spectral reflectance from plant surfaces using multi- and hyper-spectral sensors has proven very promising in gathering a large variety of eco-physiological information including photosynthetic activity (absorbed photosynthetically active radiation, APAR) [3], vegetation structure (leaf area index, LAI) [3,4], biomass and ecosystem productivity (net ecosystem productivity/gross primary productivity, NEP/GPP) [5], plant/leaf water content [6] or vegetation light use efficiency (LUE) [7], which is the ratio of GPP to APAR and controlled by the phenological stage, solar radiation conditions and the availability of soil moisture and nutrients [8]. The relationship between spectral reflectance and ecophysiological characteristics is often modeled via the use of spectral indices (SI's) that are calculated from spectral reflectance of multiple bands. There have been a large number of investigations over the last years and a summary of developed and applied SI's is given in Table 1. Spectral indices are here grouped into three major classes:  
1. The first group, including the normalized differential vegetation index (NDVI) as the most prominent representative, shows response to the general greenness, biomass and structure of the vegetation. The NDVI and its variants have successfully been related to properties such as the leaf area index (LAI), the fraction of absorbed photosynthetically active radiation (FPAR) or the biomass of many different ecosystems and environments [9,10]. 2. The second group of indices is related to the leaf pigment activity (such as xanthophyll or carotenoid) that shows sensitivity to plant physiological processes and in particular to the photosynthetic radiation use efficiency, a major component in many current eco-climatic models and analysis. Specifically, the photochemical reflectance index (PRI, the normalized difference of the 531 and 570 nm bands) has received large attention over the last years allowing relating spectral response to carbon fluxes and GPP [7,11-13]. As the PRI is sensitive to the plant xanthophyll cycle, active when dissipating excess energy under intensive radiation, and also dependent on water and nutrient availability, it will be a very useful indicator of the soil moisture and nutrient regime at a particular location [6,14]. 3. The third group of indices mainly uses water absorption bands in the near- and mid-infrared region. They are sensitive to the leaf/plant water concentration that is controlled by the soil water availability and climatic conditions. A detailed discussion about pros and cons of individual spectral bands/indices can be found in [6,15,16]. In general, the indices of this group are applied in areas such as drought assessment, irrigation practice or wild fire risk [17].