Mapping carbon dioxide flux in semiarid grasslands using optical remote sensing

Increasing atmospheric levels of carbon dioxide (CO2) and the potential impact on climate change has caused an increased effort to more accurately quantify terrestrial sources and sinks. Semiarid grasslands cover a significant portion of the Earth's land surface and may be an important sink for atmospheric CO2. This study was conducted to examine the role semiarid grasslands play in the carbon cycle. The relation between surface reflectance and temperature obtained from satellite imagery was used to determine a Water Deficit Index (WDI) to estimate distributed plant transpiration rates for a point in time. Due to the relationship between transpiration and plant CO2 uptake, WDI was directly related to CO2 flux. Satellite images were acquired for a five-year period (1996-2000) during which transpiration and net CO2 flux were measured for a semiarid grassland site in southeastern Arizona. Manual and automatic chamber data were also collected in 2005 and 2006 and used to assess the spatial variability of nighttime soil respiration. Spatial analysis showed the most influential factor affecting nighttime respiration was aspect, where flux from North-facing slopes was significantly (P < 0.05) higher than on South-facing slopes. A strong linear relationship (R2 = 0.97) existed between WDI-derived instantaneous net CO2 flux and daytime net CO2 flux estimates, and was used to generate maps of distributed daytime net CO2 flux. A linear relationship (R2 = 0.88) was also found between daytime and nighttime net CO2 flux, and used in combination with maps of daytime net CO2 flux to create maps of daily net CO2 flux. This study indicated that remote sensing offers an operational, physically-based means of obtaining daily net CO2 flux in semiarid grasslands.