|Lakshmi, Venkat -|
|Walker, Jeffrey -|
|Merlin, Olivier -|
|DE Jeu, Richard -|
Submitted to: Remote Sensing of Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 11, 2011
Publication Date: August 1, 2011
Citation: Mladenova, I., Lakshmi, V., Jackson, T.J., Walker, J.P., Merlin, O., De Jeu, R.A. 2011. Validation of AMSR-E soil moisture using L-band airborne radiometer data from National Airborne Field Experiment 2006 (NAFE'06). Remote Sensing of Environment. 115:2096-2103. Interpretive Summary: Passive microwave remote sensing can provide information of soil moisture conditions; however, when applied globally using satellite instruments, localized factors can introduce potential errors in the retrievals. In many parts of the world standing water associated with small water bodies, irrigated fields, and rice paddies can be important. An assessment of the significance of saturated soils and standing water was conducted using data collected in a large scale field campaign involving aircraft and ground-based observations. Comparisons of the high resolution aircraft and low resolution satellite observations were conducted. Results of analyses showed that the satellite products adequately reflected changes in soil moisture caused by both precipitation and irrigation, however, they exhibited a bias toward wet conditions that was due to not accounting for the the standing water. These results indicate that information on saturated soils/standing water should be incorporated into the retrieval algorithm. The improved large scale satellite-based soil moisture products that will result from the knowledge gained in this study will contribute to more accurate and reliable hydrologic, weather and climate model forecasts and assessments for agricultural appications that include irrigation planning, water resource management, and crop growth/crop stress monitoring.
Technical Abstract: AMSR-E has been extensively evaluated under a wide range of ground and climate conditions using in situ and aircraft data, where the later were primarily used for assessing the TB calibration accuracy. However, none of the previous work evaluates AMSR-E performance under the conditions of flood irrigation or other forms of standing water. Also, it should be mentioned that global soil moisture retrievals from AMSR-E typically utilize X-band data. Here, C-band based AMSR-E soil moisture estimates are evaluated using 1 km resolution retrievals derived from L-band aircraft data collected during the National Airborne Field Experiment (NAFE’06) field campaign in November 2006. NAFE’06 was conducted in the Murrumbidgee catchment area in southeastern Australia, which offers diverse ground conditions, including extensive areas with dryland, irrigation, and rice fields. The data allowed us to examine the impact of irrigation and standing water on the accuracy of satellite-derived soil moisture estimates from AMSR-E using passive microwave remote sensing. Results showed significant AMSR-E sensitivity to changes in soil moisture caused by precipitation and irrigation, as well as good spatial (average R = 0.92 and RSMD = 0.049 m3/m3) and temporal (R = 0.94 and RMSD = 0.04 m3/m3) agreement between the satellite and aircraft soil moisture retrievals. It was expected that in fields with standing water, the satellite estimates would have a lower accuracy as compared to soil moisture values over the rest of the domain, and the retrieval error would exceed the desired 0.04 m3/m3 at as little as 2.5% coverage of standing water. Under the NAFE'06 ground conditions, the satellite retrievals exhibited wet bias and overestimated the soil moisture conditions compared to the aircraft; the difference between the two products was consistent in magnitude, with the expected error at the percent water coverage encountered in the domain.