Identifying Water Stress in Cropland by Diurnal Variation in SMAP Polarization Index

SMAP’s polarization index (PI) is correlated to the mass of water in vegetation above the soil surface. Vegetation emits an unpolarized signal, while the soil emission is stronger in vertical polarization than horizontal. Thus, a more polarized SMAP measurement is indicative of less water in the vegetation canopy. We hypothesize that less vegetation canopy water in the afternoon is indicative of crops that are not water-stressed, as water in vegetation is transpired in the morning and then recharges at night. We tested the diurnal variation in SMAP polarization index in the Corn Belt state of Iowa in 2020 and 2021 at two locations: one under severe drought and an area not under drought conditions. The mean diurnal change in PI was significantly positive at the site not under drought conditions (p = 0.0250), while the diurnal cycle at the site experiencing drought was not significant (p = 0.1587). These tests were repeated for the eight surrounding SMAP pixels at both sites, and similar results were found. Filtering out rainy periods resulted in a sample size that was too small to support significant results.