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Title: Validation and scaling of soil moisture in a semi-arid environment: SMAP Validation Experiment 2015 (SMAPVEX15)

item COLLIANDER, A. - Jet Propulsion Laboratory
item Cosh, Michael
item MISRA, S. - Jet Propulsion Laboratory
item Jackson, Thomas
item Crow, Wade
item CHAN, S. - Jet Propulsion Laboratory
item BINDLISH, R - Science Application International Corporation(SAIC)
item CHAE, C. - Jet Propulsion Laboratory
item Holifield Collins, Chandra
item YUEH, S. - Jet Propulsion Laboratory

Submitted to: Remote Sensing of Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/8/2017
Publication Date: 7/1/2017
Citation: Colliander, A., Cosh, M.H., Misra, S., Jackson, T.J., Crow, W.T., Chan, S., Bindlish, R., Chae, C., Holifield Collins, C.D., Yueh, S. 2017. Validation and scaling of soil moisture in a semi-arid environment: SMAP Validation Experiment 2015 (SMAPVEX15). Remote Sensing of Environment. 196:101-112.

Interpretive Summary: Satellite remote sensing requires ground truth in many forms to verify the accuracy of the products they provide. Soil moisture remote sensing is a prime example of how this can be accomplished, as demonstrated by the recent NASA Soil Moisture Active Passive Mission (SMAP). In the summer of 2015, the SMAP Validation Experiment was conducted in Tombstone, AZ with the intent of calibrating and validating the SMAP soil moisture products at several scales. This methodology included coincident ground sampling, or truthing, and aerial surveys with an instrument similar to SMAP during SMAP overflight days. Arizona is subject to a monsoonal system, so there were many favorable conditions for this study, including frequent rain events, fast drying conditions, and a long term in situ network which has been recording soil moisture since 2001. As a result of this study, it was determined that for this domain, SMAP can provide highly accurate moisture estimates. This information is useful for land managers in the western U.S. who need to understand moisture conditions for foraging rates as well as watershed managers who need to partition water resources among many customers.

Technical Abstract: The NASA SMAP (Soil Moisture Active Passive) mission conducted the SMAP Validation Experiment 2015 (SMAPVEX15) in order to support the calibration and validation activities of SMAP soil moisture data product.The main goals of the experiment were to address issues regarding the spatial disaggregation methodologies forimprovement of soil moisture products and validation of the in situ measurement upscaling techniques.To support these objectives high-resolution soil moisture maps were acquired with the airborne PALS (Passive Active L-band Sensor) instrument over an area in southeast Arizona that includes the Walnut Gulch experimental watershed, and intensive ground sampling was carried out to augment the permanent in situ instrumentation. The in situ measurements were used to validate the PALS soil moisture acquired at 1-km resolution. Comparisons showed that the in situ soil moisture measurements did not capture all the precipitation events accurately, based on the information from a dense network of rain gages in the area. This is attributed to the relatively small spatial extents of the typical convective storms in this region. After removing those cases (approximately 10% of the observations) the following metrics were obtained: RMSD (root mean square difference) of 0.016 m3/m3 and correlation of 0.83. The PALS soil moisture was also compared to SMAP and in situ soil moisture at the 36-km scale, which is the SMAP grid size. PALS and SMAP soil moistures were found to be very similar owing to the close match of the brightness temperature measurements and the use of a common soil moisture retrieval algorithm. However, there were discrepancies between these and the in situ soil moisture measurements. These differences were associated with discrepancies between and the in situ soil moisture sensor measurements and the detection of precipitation from the rain gages. That is, the PALS and SMAP soil moistures corresponded to precipitation events detected by rain gages, which were in some cases left undetected by the in situ soil moisture sensors. Spatial heterogeneity, which was identified using the high-resolution PALS soil moisture and the intensive ground sampling, also contributed to differences. In general, discrepancies found between the L-band soil moisture estimates and the 5- cm depth in situ measurements require methodologies to mitigate the impact on their interpretations in soil moisture validation and algorithm development. Specifically, the metrics computed for the SMAP radiometer-based soil moisture product over Walnut Gulch will include errors resulting from rainfall, particularly during the monsoon season when the spatial distribution of soil moisture is especially heterogeneous.