An assessment of the differences between spatial resolution and grid size for the SMAP enhanced soil moisture product over homogeneous sites

Authors: COLLIANDER, A. - Jet Propulsion Laboratory; Jackson, Thomas; CHAN, S. - Jet Propulsion Laboratory; O'NEIL, PEGGY.E. - Goddard Space Flight Center; BINDLISH, R. - Goddard Space Flight Center; Cosh, Michael; CALDWELL, T. - University Of Texas; WALKER, J. - Monash University; BERG, A. - University Of Guelph; MCNAIRN, H. - Agriculture And Agri-Food Canada; THIBEAULT, M. - Universidad De Buenos Aires; MARTINEZ-FERNANDEZ, J. - University Of Salamanca; JENSEN, K.H. - Copenhagen University; ASANUMA, J. - University Of Tsukuba; Seyfried, Mark; Bosch, David; Starks, Patrick; Holifield Collins, Chandra; Prueger, John; SU, Z. - University Of Twente; LOPEZ-BEEZA, E. - University Of Valencia; YEUH, S. - Jet Propulsion Laboratory

Submitted to: Remote Sensing of Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/1/2018
Publication Date: 4/1/2018
Publication URL: http://handle.nal.usda.gov/10113/5906746
Citation: Colliander, A., Jackson, T.J., Chan, S., O'Neill, P., Bindlish, R., Cosh, M.H., Caldwell, T., Walker, J., Berg, A., McNairn, H., Thibeault, M., Martinez-Fernandez, J., Jensen, K., Asanuma, J., Seyfried, M.S., Bosch, D.D., Starks, P., Holifield Collins, C.D., Prueger, J.H., Su, Z., Lopez-Beeza, E., Yeuh, S. 2018. An assessment of the differences between spatial resolution and grid size for the SMAP enhanced soil moisture product over homogeneous sites. Remote Sensing of Environment. 207:65-70.

Interpretive Summary: The impact of assuming that satellite soil moisture products represent the grid resolution as provided instead of the actual domain that contributes to the measurement was assessed. Soil moisture products derived from satellite-based passive microwave remote sensing are typically resampled to a fixed Earth grid that facilitates their use in applications. In many cases the grid size is finer than the actual spatial resolution of the observation, and often this difference is not well understood by the user. Here, this issue was examined for the Soil Moisture Active Passive (SMAP) enhanced version of the passive-based soil moisture product, which has a grid size of 9-km and a nominal spatial resolution of 33-km. In situ observations from core validation sites were used to compute comparison metrics. The impact of validating the 9-km grid product with in situ data collected over a 9-km versus a 33-km domain was found to be very small. The implication is that using the enhanced SMAP product at its grid resolution of 9-km should not introduce large errors in most applications.

Technical Abstract: Satellite-based passive microwave remote sensing typically involves a scanning antenna that makes measurements at irregularly spaced locations. These locations can change on a day to day basis. Soil moisture products derived from satellite-based passive microwave remote sensing are usually resampled to a fixed Earth grid that facilitates their use in applications. In many cases the grid size is finer than the actual spatial resolution of the observation, and often this difference is not well understood by the user. Here, this issue was examined for the Soil Moisture Active Passive (SMAP) enhanced version of the passive-based soil moisture product, which has a grid size of 9-km and a nominal spatial resolution of 33-km. In situ observations from core validation sites were used to compute comparison metrics. For sites that satisfied the established reliability and scaling criteria, the impact of validating the 9-km grid product with in situ data collected over a 9-km versus a 33-km domain was very small (0.039 m3/m3 unbiased root mean square difference for the 9-km case versus 0.037 m3/m3 for the 33-km case for the sites studied). This result does not mean that the resolution of the product is 9-km but that for the conditions studied here the soil moisture estimated from in situ observations over 9-km is a close approximation of the soil moisture estimated from in situ observations over the 33-km resolution. The implication is that using the enhanced SMAP product at its grid resolution of 9-km should not introduce large errors in most applications.