Thermal-based modeling of coupled carbon, water, and energy fluxes using nominal light use efficiencies constrained by leaf chlorophyll observations

摘要

Recent studies have shown that estimates of leaf chlorophyll content (Chl),defined as the combined mass of chlorophyll a and chlorophyll b per unit leafarea, can be useful for constraining estimates of canopy light use efficiency(LUE). Canopy LUE describes the amount of carbon assimilated by a vegetativecanopy for a given amount of absorbed photosynthetically active radiation(APAR) and is a key parameter for modeling land-surface carbon fluxes. Acarbon-enabled version of the remote-sensing-based two-source energy balance(TSEB) model simulates coupled canopy transpiration and carbon assimilationusing an analytical sub-model of canopy resistance constrained by inputs ofnominal LUE (β_n), which is modulated within the model in response tovarying conditions in light, humidity, ambient CO₂ concentration, andtemperature. Soil moisture constraints on water and carbon exchange areconveyed to the TSEB-LUE indirectly through thermal infrared measurements ofland-surface temperature. We investigate the capability of using Chlestimates for capturing seasonal trends in the canopy β_n from in situmeasurements of Chl acquired in irrigated and rain-fed fields of soybean andmaize near Mead, Nebraska. The results show that field-measured Chl isnonlinearly related to β_n, with variability primarily related tophenological changes during early growth and senescence. Utilizing seasonallyvarying β_n inputs based on an empirical relationship with in situmeasured Chl resulted in improvements in carbon flux estimates from the TSEBmodel, while adjusting the partitioning of total water loss between planttranspiration and soil evaporation. The observed Chl–β_n relationshipprovides a functional mechanism for integrating remotely sensed Chl into theTSEB model, with the potential for improved mapping of coupled carbon, water,and energy fluxes across vegetated landscapes.