Simple and Innovative Methods to Estimate Gross Primary Production and Transpiration of Crops: A Review

Sustainable agricultural production to support food security is one of the main targets of the 2030 agenda for global sustainable development goals (SDGs). New technology advancements and sources of information play a critical role in supporting agriculture to achieve the SDGs goals and increase production capabilities to meet rising food demands. Gross primary production (GPP) and transpiration (T) of crops are the largest carbon and water fluxes in agroecosystems. GPP data products are used to estimate crop yield, an important metric for agricultural resiliency and food security. Over land surfaces, T is the largest component of evapotranspiration and represents more than 60% of the precipitated water returned to the atmosphere. T is used to represent water use and improve irrigation in croplands, thereby helping to reduce production costs and support sustainable crop production. In this chapter, we review the state-of-the-science approaches for estimating GPP and T, including in situ and remote sensing methods, while focusing on the biophysical foundation behind the major available techniques. Furthermore, given the linkage between GPP and T through the behavior of stomates, it is possible to simplify the calculation of both variables by estimating common biophysical parameters (i.e., minimum stomatal conductance) that influence water loss and vegetation carbon uptake. We highlight innovative approaches that enhance the calculation of both variables based on this relationship and review the extent and limitations of the algorithms used in some of the most popular satellite products like Moderate Resolution Imaging Spectroradiometer (MODIS) global evapotranspiration product (MOD16) and EcoStress. Furthermore, this chapter explores and describes the potential for using solar-induced fluorescence (SIF) data to calculate agricultural GPP and T. Finally, given the importance of certain variables in agricultural and forest meteorology, this chapter also includes a section highlighting the main conversion factors useful in calculating energy, carbon, and water fluxes.