Location: Hydrology and Remote Sensing Laboratory
Title: Impact of conifer forest litter on microwave emission at L-band Authors
Submitted to: IEEE Transactions on Geoscience and Remote Sensing
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 1, 2011
Publication Date: September 29, 2011
Citation: Mehmet, K., O'Neill, P.E., Lang, R.H., Cosh, M.H., Joseph, A.T., Jackson, T.J. 2011. Impact of conifer forest litter on microwave emission at L-band. IEEE Transactions on Geoscience and Remote Sensing. 50(4):1071-1084. Interpretive Summary: The potential for using microwave remote sensing of soil moisture under tree canopies was evaluated using a ground-based simulator over a coniferous forest. This study focused on characterizing the contributions of the underlying forest floor and litter on the sensor response. Results of this study suggest that while it is possible to model the expected response, the sensitivity of the retrieval method will be reduced for forested conditions. The capability of this approach for crops and grasslands has been investigated but very few studies have focused on tree canopies, which make up 30% of the global land cover. Extending the technique to forests would lead to robust global soil moisture products, which will lead to better utilization in agricultural and hydrologic applications.
Technical Abstract: This study reports on the utilization of microwave modeling, together with ground truth and L-bank (1.4 GHz) brightness temperatures to investigate the characteristics of conifer forest floor. The microwave data were acquired over natural Virginia pine forest in Maryland by ComRAD, a ground-based microwave active/passive instrument system, in 2008/2009. Ground measurements of trees as well as forest floor characteristics were made during the year long field campaign. The test site consisted of a medium sized evergreen conifer with an average height of 12 m and average diameter at breast height (DBH) of 13 cm. The site is a typical pine forest site in that there is a surface layer of loose debris/needles and an organic transition layer above the mineral soil. In an effort to characterize and model the impact of the surface litter layer, a special experiment was conducted on April 23, 2009, which involved removal of surface litter layer from one half of the test site while keeping the other half undisturbed. The data shows that a detecable decrease in emissivity for both polarizations after the surface litter layer was removed. A first-order radiative transfer model of the forest stands including multi-layer nature of the forest floor in conjunction with the ground truth data are used to compute forest emission. The model calculations reproduced that major features of the experiment data observed over the entire experiment duration such as effect of surface litter and ground volumetric moisture content (VMC) on overall emission. Both theory and experimental results confirm that the litter layer increases the observed canopy brightness temperature and obscures the soil emission.