Accurate knowledge of surface soil moisture is essential for near-term climate predictions, hydrological and agricultural studies, and effective water resources management. Passive microwave observations at the L-band (1.4 GHz) are highly sensitive to changes in soil moisture at the top few centimeters, known as the near-surface. The Soil Moisture and Ocean Salinity mission (SMOS), launched by ESA in 2009, uses an L-band radiometer to provide global estimates of soil moisture every 2-3 days at a spatial resolution of 50 km. In addition, the planned Soil Moisture Active Passive mission (SMAP), by NASA for a launch in 2014, will use a combination of radar and radiometer at L-band for soil moisture studies.

The goal of this study, conducted as part of the NSF-Research Experiences for Undergraduates (REU) Program, was to understand the soil moisture sensitivity of the microwave emission from growing Elephant grass. Elephant grass is a highly productive bio-energy crop, with a maximum height of about 5m. A microwave emission model for Elephant grass was developed and evaluated using the field observations during the ninth Microwave, Water and Energy Balance Experiment (MicroWEX-9). The MicroWEX-9 was conducted from March through December in 2010 in North Central Florida. Every 15-minute observations of soil temperature and moisture, canopy temperature, and microwave emission were conducted. Vegetation observations included leaf area index, dry biomass, crop height, and crop width.

The model estimates of brightness temperatures from the emission model were compared with the field observations. The Root Mean Square Differences (RMSDs) between the modeled and the observed brightness were high during the early growing season, and were significantly reduced during the late season, from mid-September through December, primarily due to increasing contribution from the vegetation. The sensitivities of the model microwave emission to the changes in soil moisture observed during MicroWEX-9 were compared with those from the observed brightness temperatures.