Validating the Carnegie-Ames-Stanford Approach for remote sensing of perennial grass net primary production

Under optimal growth conditions, net primary productivity ( NPP ) is a product of intercepted photosynthetic active radiation ( Ipar ) and maximum radiation use efficiency ( RUE max ; conversion of Ipar to biomass). The objective of this study was to improve and validate the RUE max -based Carnegie-Ames-Stanford Approach ( CASA ) for the determination of grassland NPP by canopy multispectral reflectance collected at field (handheld sensor) and airborne ( UAV ) scale considering environmental constraints. The analysis was based on multi-year field experiments on sandy loam soil in Denmark, measured shoot and estimated root biomass to calculate NPP , long-term meteorological data, and daily NPP estimated from CO 2 flux chamber measurements for deriving environmental constraints. The results derived from CO 2 flux data showed that NPP and plant respiration were higher in the middle of the season before the second harvest when temperature was also high. The daily maximum air temperature optimal for grass biomass production was 16.5 °C. The improved CASA model built in this study was accurate for modeling NPP at both daily ( nRMSE decrease of 9 %) and seasonal ( nRMSE decrease of 8–34 %) scales when considering the best environmental constraints such as maximum air temperature, vapor pressure deficit, cloudiness, and water stress, compared to no constraints. Maximum air temperature and water stress were the most important environmental constraints to the grass RUE max . Seasonal RUE max for modeling NPP after considering best environmental constraints was 1.9–2.7 g C MJ −1 for ryegrass and 1.9–2.2 g C MJ −1 for grass-legume mixture using the two remote sensors for measuring spectral reflectance. Over the whole growing season, tall fescue (3.1 g C MJ −1 ) and festulolium (2.9 g C MJ −1 ) obtained higher RUE max than perennial ryegrass (2.3 g C MJ −1 ). This study highlights the practical implications of using the CASA model improved by maximum temperature and water stress functions for real-time, remote sensing-based assessments of grassland productivity. • Improved CASA model with high accuracy by considering environmental constraints. • NPP was predicted well both at field- and airborne-scale, daily and seasonal scale. • Maximum air temperature and water stress were major environmental constraints. • Actual evapotranspiration estimation by remotely sensed energy balance method. • Tall fescue displayed higher RUE max than festulolium and perennial ryegrass.