Observing Vegetation Stress With Diurnal Variation in SMAP Polarization Index in Central Iowa Cropland

Soil Moisture Active Passive (SMAP) is an L-band Earth-orbiting satellite, launched in 2015 to measure soil moisture worldwide. SMAP is now exclusively a passive remote sensing satellite, which observes the brightness temperature of Earth's surface at 1.4 GHz, making it sensitive to the soil and land cover. Natural emission from the soil is polarized, being stronger vertically than horizontally. Vegetation decreases the degree of polarization. In the Corn Belt, row crops of corn and soybean are the dominant vegetation. A more polarized SMAP retrieval indicates less vegetation cover (specifically less water within vegetation tissue), and a horizontally polarized (h-pol) brightness temperature close to the vertically polarized (v-pol) temperature indicates more vegetation water. Polarization index (PI) is defined as the difference between v-pol and h-pol brightness temperature divided by their average. Using PI makes measurements independent of surface temperature. SMAP overpasses central Iowa around 6 am and 6 pm solar time about every other day; in a healthy canopy a higher polarization index is expected in the evening after transpiration reduces vegetation water content before the roots can recharge stem water. In a water stressed canopy stomata may not open and a similar polarization index may be observed at 6 am and 6 pm. Water stress can be characterized by evaporative stress index (ESI), a combination of surface temperature modeling and an infrared measurement. We found that when ESI is high (very hot canopy surface temperature and high stress), PI did not change significantly diurnally. We did find a significantly higher polarization index in the evening when the vegetation is not stressed. Overall, a higher absolute value of PI was found on stressed days than on days with low ESI. Environmental conditions that may limit transpiration and crop water loss are high vapor pressure deficit, cloud cover, and inadequate soil moisture. Consideration of the diurnal change in PI is proving to be an effective indirect measurement of the entire root zone soil moisture stats.