A review of SMAPVEX2008 data: initial analysis

Microwave remote sensing is the only option for making global measurements of soil moisture. With the launch of the European Space Agency's Soil Moisture-Ocean Salinity (SMOS) mission in November, 2009, and the future launch of NASA's Soil Moisture Active Passive (SMAP) mission, we will have reliable and current maps of soil moisture across the world. These maps will be a great help to models ranging from short-range meteorological forecast models to regional and even global climate models. With the advent of these new satellites and other observing systems, it is important to use ground truth from smaller scale experiments to help us analyze the raw data and to produce calibrated output. In the case of microwave remote sensing of soil moisture, there are numerous ground conditions that can impact the measurement. These include: vegetation type, soil texture, water content of vegetation, soil thermal properties, and the roughness of the soil. We will present data from SMAPVEX2008 at the Iowa Validation Site (IVS). The IVS is a heavily instrumented and monitored 12 km field which was planted with soybeans. This experiment took place on September 23-25, 2008. The remote sensing data was collected using a passive-active L-band sensor (PALS) mounted on the underside of an airplane. The airplane made many passes over the IVS field at different altitudes of 3000 feet, 1000 feet, and 500 feet above ground level to achieve the different foot print sizes of 560 m by 420 m, 190 m by 140 m, and 95 m by 70 m, respectively. The flight lines were made across the field in the east/west direction across the field from just after sunset onward through late morning. The PALS airplane measured the horizontal and vertical polarized brightness temperature at 1.4GHz, and infrared canopy temperature at the aforementioned resolution sizes. Soil roughness data from SMAPVEX2008 will be presented along with the brightness temperature and IR temperature data. We will also collect new data during this coming summer to help us analyze the SMAPVEX2008 data. For example, the roughness measurements in 2008 were taken using an instantaneous-profile laser scanner at one location on the west end of the IVS in the end rows. The profiler has a vertical resolution of 0.1 mm and a horizontal resolution of 1 mm, with a scan area of 1392 mm by 870 mm. The scanner apparatus was placed in between soybean rows with the long portion parallel with the rows. The spacing between the soybean rows was 760 mm (30 inches). The roughness was determined by taking the standard deviation of the residuals of a linear best fit line for each line of data in the across row and row wise directions. Measurements in summer of 2010 will be taken with the same apparatus at other locations across the same field in order to assess the spatial variability of soil roughness, which is presently unknown. We will analyze the change in roughness in time and space as well as with the direction relative to row orientation. We hypothesize that the roughness will decrease as the summer progresses and will have little spatial variability in the field. Better understanding of the variability of soil roughness will help us interpret the SMAPVEX2008 remote sensing measurements.