Submitted to: The Environmental Protection Agency Conference on Spectral Remote Sensing of Vegetation
Publication Type: Proceedings
Publication Acceptance Date: 5/15/2003
Publication Date: 7/1/2003
Citation: Proc. EPA Conf. on Spectral Remote Sensing of Vegetation, March 12-14, 2003, Las Vegas, NV, Unpaginated CD Rom. Interpretive Summary: Grasslands cover nearly 30% of the Earth¿s land surface and are an important sink for atmospheric carbon dioxide (CO2). Grassland uptake of CO2 is being measured in various places around the world as part of the ongoing effort to balance the global carbon budget. Current instrumentation can only obtain measurements spanning several hundred meters. However, the use of remote sensing could make it possible to measure CO2 uptake over several square kilometers. The Water Deficit Index (WDI), derived from Landsat satellite imagery can estimate regional transpiration rates for a point in time. CO2 plant uptake and transpiration are linked through photosynthesis. In this study, the relationships between WDI, transpiration, and CO2 were used to estimate large-scale CO2 uptake in semiarid grasslands. The study showed that the WDI derived from remote sensing was able to map CO2 uptake over 9 km2 of grassland in southeastern Arizona. The ability to estimate regional CO2 uptake shows great promise as a future tool in the effort to balance the global carbon budget.
Technical Abstract: Grasslands cover nearly 30% of the Earth¿s land surface and may be an important sink for atmospheric CO2. CO2 flux from semiarid grasslands could be mapped using sensors aboard orbiting satellites that measure surface reflectance and temperature. The relation between surface reflectance and temperature has been used to determine a Water Deficit Index (WDI) to estimate regional plant transpiration rates for a point in time. Given that transpiration and plant CO2 uptake are inextricably linked through the photosynthetic process, it follows that the WDI would be related directly to CO2 flux. Satellite images were acquired for a five-year period (1996-2000) during which transpiration and CO2 flux were measured for a grassland site in the Walnut Gulch Experimental Watershed (WGEW). Based on this dataset, we determined the relations between 1) WDI and plant transpiration; 2) transpiration and plant CO2 uptake; and 3) WDI and CO2 flux. The results indicated that there was a slightly curvilinear relation between WDI and transpiration (R2 = 0.73) and a strong linear relation (R2 = 0.90) between CO2 uptake and transpiration. Based on these results, an exponential relation between WDI and CO2 flux was determined and validated with an independent data set at WGEW. The mean absolute difference between measured and WDI-derived CO2 flux was 0.24 over a range of CO2 flux values from 0.10 to 1.10 (mg-2s-1). The WDI offers an operational, physically-based approach for estimating regional CO2 flux in semiarid grasslands.