A remote sensing-based two-leaf canopy conductance model: Global optimization and applications in modeling gross primary productivity and evapotranspiration of crops

摘要

The temporal dynamics of optimum stomatal conductance (g(smax)), as well differences between C-3 and C-4 crops, have rarely been considered in previous remote sensing (RS)-based Jarvis-type canopy conductance (G(c)) models. To address this issue, a RS-based two-leaf Jarvis-type G(c) model, RST-G(c), was optimized and validated for C-3 and C-4 crops using 19 crop flux sites across Europe, North America, and China. RST-G(c) included restrictive functions for air temperature, vapor pressure deficit, and soil water deficit, and it used satellite-retrieved NDVI to formulate the temporal variation of g(smax) defined at a photosynthetic photon flux density (PPFD) of 2000 mu mol m(-2) s(-1) (g(sm, 2000)). Results showed that the parameters of RST-G(c) differed between C-3 and C-4 crops. RST-G(c) successfully simulated variations in Penman-Monteith (PM)-derived daytime G(c) with R-2 = 0.57 for both C-3 and C-4 crops. RST-G(c) was incorporated into a revised evapotranspiration (ET) model and a new gross primary productivity (GPP) model. The two models were validated at 19 crop flux sites. Daily mean inputs were generally incorporated into a PM approach to model daily transpiration. This is inappropriate because available energy and stomatal conductance vary significantly on a diurnal basis, with both non-linearly regulating transpiration rate. The PM approach with daily mean inputs produced unreasonable transpiration rate estimates. Efforts were made in the revised ET model (denoted as RS-WBPM2), which was modified from the water balance based RS-PM (RS-WBPM) model of Bai et al. (2017), to address this issue by calculating transpiration using daytime inputs. The photosynthesis-based stomatal conductance model, developed by Ball et al. (1987a) and improved by Leuning (1995) (BBL model), was inverted to calculate GPP using canopy conductance; the inverted model was denoted as IBBL. Cross validation showed good agreement between flux tower measurements and modeled ET (R-2 = 0.79