William Crosson, Ashutosh Limaye, Charles Laymon National Space Science and Technology Center Huntsville, Alabama, USA Soil Moisture Retrievals Using C- and X-band Radiometer Observations in SMEX02: Successes and Challenges

Motivation and Objective Motivation: Data from NASA’s Advanced Microwave Scanning Radiometer (AMSR) are being used to estimate near-surface soil moisture. Analysis of the retrieval algorithms is needed to better understand the sensitivity of retrieved soil moisture to model parameters. Objective: To utilize C- and X-band horizontally polarized brightness temperature data collected from an aircraft platform during the SMEX02 field experiment to evaluate the feasibility and limitations of soil moisture retrieval under conditions of significant vegetation biomass.

Soil Moisture Experiments in 2002 (SMEX02) Location: Near Ames, IowaTime: 24 June – 13 July 2002 Regional Area: ~5000 km 2 Walnut Creek Watershed Area: ~ 400 km 2 31 ground sampling sites for measuring gravimetric soil moisture, surface and soil temperatures (daily, AM), and vegetation properties (~weekly) Surface energy flux stations, lidar, and radiosonde measurements Aircraft microwave remote sensing data Iowa Walnut Creek Watershed Soybeans (40% of area) Corn (50% of area)

Polarimetric Scanning Radiometer (PSR) 10 km Platform: NASA P-3B Altitude: 7300 m regional, 1800 m Walnut Creek Watershed Incidence angle: 55 o 3 dB footprint resolution (watershed): 750 m (C), 500 m (X) C-band frequencies (GHz): , , , X-band frequency (GHz): – Walnut Creek Watershed coverage PSR TB measurements Corn Soybeans

Daily mean volumetric soil moisture based on field site gravimetric ‘scoop’ measurements with 2*standard error bars Vegetation water content based on in situ destructive sampling PSR days Daily Soil Moisture and Vegetation Water Content

Soil Moisture Retrieval Algorithm Volumetric Soil Moisture Volumetric Soil Moisture Smooth Surface Reflectivity Smooth Surface Reflectivity Effective Dielectric Constant Effective Dielectric Constant Vegetation Transmissivity Vegetation Transmissivity Surface Temperature Surface Temperature Soil Properties Soil Properties Brightness Temperature Brightness Temperature Land Cover Type Land Cover Type Soil Temperature Soil Temperature Rough Bare Surface Reflectivity Rough Bare Surface Reflectivity Surface Roughness Surface Roughness Vegetated? Yes Apparent Reflectivity Effective Temperature No Veg Water Content Veg Water Content Parameterizations: Dobson Mixing Model Choudhury et al. (1979) surface roughness correction Choudhury et al. (1982) effective temperature Jackson and Schmugge (1991) vegetation correction Fresnel Reflectance

Experiment Design Retrieve soil moisture: Using input data characterizing mean conditions over the SMEX02 Walnut Creek Watershed on each of 7 days, apply the retrieval algorithm for various points across a 3-dimensional parameter space (surface roughness, vegetation B parameter and single scattering albedo) Validate: Using observed soil moisture conditions for each day, determine the parameter combinations that yield soil moisture estimates within a defined error range about the observed mean Inputs representing mean conditions for given day: Brightness temperature – Mean of PSR C- or X-band ‘postings’ Surface and soil temperatures – Mean of observed temperatures Vegetation water content – Mean of values measured at field sites (corn and soybeans), interpolated between ~weekly measurements

Parameter Space Comparison C-band, H-Pol Single Scattering Albedo = 0.03 Shaded regions indicate combinations of surface roughness and vegetation B parameter that produce soil moisture values within 2 standard errors of the observed mean for the given day. The observed mean is for the 0-1 cm layer, intended to represent the effective C-band emitting depth. There is no overlap in valid soil moisture retrievals for July 9 and July 11. July 1/July 11 (Wet) Overlap July 1 (Very Dry) July 1/July 9 (Moderately Wet) Overlap Roughness (cm)

Parameter Space Comparison X-band, H-Pol Single Scattering Albedo = 0.03 Shaded regions indicate combinations of surface roughness and vegetation B parameter that produce soil moisture values within 2 standard errors of the observed mean for the given day. The observed mean is for the 0-1 cm layer, intended to represent the effective X-band emitting depth. July 1/July 11 (Wet) Overlap July 1 (Very Dry) July 1/July 9 (Moderately Wet) Overlap Roughness (cm) All Days Overlap

Sensitivity of Retrieved VWC to Surface Roughness C-band, H-pol SS Albedo =.03, Vegetation B = 0.16 The range of roughness values for which retrieved soil moisture is within.04 of the observed values for each day is represented here by the horizontal bars.

Sensitivity of Retrieved VWC to Surface Roughness C-band, H-pol SS Albedo =.03, Vegetation B = 0.16 Soil moisture retrievals for wet, moderate and dry days have distinctly different sensitivity to roughness.

Sensitivity of Retrieved VWC to Surface Roughness C-band, H-pol SS Albedo =.03, Various B Parameter Values

Sensitivity of Retrieved VWC to Surface Roughness X-band, H-pol SS Albedo =.03, Vegetation B = 0.16

Sensitivity of Retrieved VWC to Vegetation C-band, H-pol SS Albedo =.03, Roughness = 0.05 cm The horizontal bars indicate the range of Vegetation B parameter values for which the retrieved soil moisture is within.04 VWC of the observed value.

Sensitivity of Retrieved VWC to Single Scatter Albedo C-band, H-pol Vegetation B = 0.16, Roughness = 0.05 cm Wet Dry

Sensitivity of Retrieved VWC to Surface Roughness C-band, V-pol SS Albedo =.03, Vegetation B = 0.07

Sensitivity of Retrieved VWC to Vegetation C-band, V-pol SS Albedo =.03, Roughness = 0.05 cm

Sensitivity of Retrieved VWC to Single Scatter Albedo C-band, V-pol Vegetation B = 0.07, Roughness = 0.05 cm

Soil Moisture Retrievals Using Constant Parameters C-band, H-pol

Soil Moisture Retrievals Using Constant Parameters C-band, V-pol

Soil Moisture Retrievals Using Constant Parameters X-band, H-pol

July 1 July 9 July 8 June 25 July 12 July 7 June 25 Acceptable Parameter Ranges as Function of Soil Moisture C-band, H-pol Roughness = 0.05 cm Vegetation B parameter =.16 Due to increasing sensitivity, valid parameter ranges tend to narrow with increasing VWC. June 27 July 11 July 1 July 9 July 12 July 11 June 27 July 8 Illustrations of the range of parameter values that produce retrievals within.04 of the observed VWC. Single scattering albedo =.03. Lower roughness values are indicated for wetter soils.

Acceptable Parameter Ranges as Function of Soil Moisture X-band, H-pol Due to increasing sensitivity, valid parameter ranges tend to narrow with increasing VWC. Roughness = 0.2 cm July 1 July 9 July 11 June 27 July 1 July 9 Vegetation B parameter =.16 June 27 July 11 Illustrations of the range of parameter values that produce retrievals within.04 of the observed VWC. Single scattering albedo =.03. Lower roughness values are indicated for wetter soils.

Conclusions Using a single-frequency, single-polarization soil moisture retrieval algorithm, there is no parameter vector (surface roughness, vegetation B parameter, single scattering albedo) that produces soil moisture values for aggregate corn and soybean fields that are within two standard errors of the observed values for each of the five days for which complete PALS L-band coverage is available. For three days that are relatively wet, excellent retrievals were obtained with a single parameter vector. For each of the two dry days, different sets of parameter vectors were required to produce acceptable results. The inability to obtain accurate soil moisture retrievals with a single parameter vector does not appear to be related to errors in brightness temperatures. Under wet conditions, retrieved soil moisture for corn is seen to be very sensitive to all three parameters. For dry conditions, sensitivity is lower, but not negligible.