Calibration and Modelling of Vegetation Structure Parameter B for L-Meb Models Over Agricultural Sites

Abstract

Passive microwave observations of vegetation water content (VWC) are important for monitoring vegetation growth and aboveground biomass. To constrain the L-band Microwave Emission of the Biosphere (L-MEB) model for NASA’s Soil Moisture Active Passive (SMAP) mission, vegetation structure parameter b is used to estimate VWC fro m the climatological Normalized Difference Vegetation Index (NDVI). However, little is known about what and how the driving factors impact the parameter b and its modeling due to the lack of quantitative analysis. Using the Passive Active L-band Sensor (PALS) data from the SMAPVEX16-MB campaign, this study firstly calibrated parameter b for canola, wheat and soybean. Afterwards, a Random Forest algorithm was employed to model b-values using both dual-polarized observations with input features screened by correlation analysis and stepwise regression, the R2 of which exceeds 0.82. To enhance interpretability of the model, multivariable-linear regression was also employed, achieving an R2 exceeding 0.75. The results showed three crops had increasing trends in b-values with growth, except for wheat which exhibited a decreasing trend at v-polarization. Wheat and soybean indicated sharp changes in b-values particularly during late growth stages, while canola exhibited slower changes. The b-values were higher at v-polarization than h-polarization, except for wheat during early growth stage and at the end of late growth stages. The multivariable regression with vegetation type as dummy variables indicated that crop type significantly affected b-values, with canola and wheat positively impacting them at h-polarization but negatively at v-polarization. Additionally, key factors affecting b -values included VWC, biomass and LAI at h-polarization, while VWC and biomass dominated at v-polarization. Notably, VWC and brightness temperature at h-polarization (TBh) negatively impacted b-values at both polarizations. Overall, this study enhances our understanding of parameter b and the VWC retrieval and also contribute to model parameter b at the pixel scale of the satellite observations.

Keywords: L-MEB model, Parameter b, Passive microwave, SMAPVEX16