How Wet Is That Soil Actually?

The accuracy of the soil moisture measurements of SMAP are confirmed, or validated, by extensive comparisons to soil moisture values measured on the ground. Soil moisture can be measured by taking a core sample of the soil and determining the water content it withholds by drying the sample and measuring its weight difference to the original sample; the difference is the water content, or moisture, of the soil. This gives the standard for soil moisture measurements. There are also measurement probes which can be installed into the ground and used to log a continuous measurement of soil moisture at that location. At many places around the world there are organizations and investigators who conduct these measurements and have installed permanent soil moisture stations for different kinds of research and monitoring objectives (the stations cover only a tiny fraction of the globe, hence we need SMAP to cover the rest of the globe). SMAP is working together with the scientists of several of these activities for the validation of the mission’s soil moisture retrievals. [LINK] There are a few complicating factors related to comparison of the SMAP measurements to the ground observations. The main difficulty comes from the fact that the SMAP measurements corresponds to an area seen by the instrument (footprint) which is affected by much larger area than the ground samples. This can result in significant difference between the soil moisture estimates. However, the SMAP validation sites have multiple probe measurements within SMAP pixels which allows the determination of the reference soil moisture also for the SMAP footprint. Furthermore, the scientists working with these sites have a good understanding of the geophysical properties of the area that helps in interpreting the data for the SMAP validation purposes.

Validation Methods and Resources:

  • Homogenous and heterogeneous targets
  • In situ observations
  • Truck- and tower-based radiometers and radars
  • Aircraft-based radiometers and radars
  • Scaling methodologies
  • Modeling and data assimilation

Cal/Val Links


Links to Other Data Sources


  • Calibration: The set of operations that establish, under specified conditions, the relationship between sets of values of quantities indicated by a measuring instrument or measuring system and the corresponding values realized by standards.
  • Validation: The process of assessing by independent means the quality of the data products derived from the system outputs.

SMAP Cal/Val Program Objectives:

  • Pre-launch: Acquire and process data to calibrate, test, and improve models and algorithms for SMAP science data products. Develop and test techniques and protocols that will be used to validate SMAP data products after launch.
  • Post-launch: Verify and improve performance of the science algorithms. Validate accuracies of the science data products.

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