Assessing the operational capabilities TerraSAR-X for monitoring summer crop biophysical parameters

Timely information about agricultural crops (e.g. typology, phenology, productivity, health) is crucial for proper agronomic planning and management by farmers and public administrations. SAR data at different frequencies have been used for retrieving biophysical crop parameters pertinent to the crops and for monitoring their intra-annual cycles, e.g. for the estimation of biomass and growth status. Here we present an experimental analysis based on the investigation of X-band backscattering derived from multitemporal TerraSAR-X (TSX) data over three different types of summer crops (rice, maize, soybean). The study area is located in south-eastern portion of Lombardy region, Northern Italy, framed within the Po river Plain and the Ticino river basin, and covers two farms located in Rosasco municipality (45°15'00" N, 8°35'00" E). In situ campaigns have been conducted along the summer crop season (May-September) on this area in order to measure biophysical parameters related to: i) agronomy (crop type and variety, agro-practices, seeding date, seeding scheme and density), ii) substrate (soil roughness, soil moisture, flooding conditions), iii) crop phenology (BBCH scale stage, context and detail photos), iv) morphology (plant height, number of leaves, leaves size), and v) biomass and density (biomass, plant water content, LAI). Field observations and measurements of crop parameters were carried out over 30 fields (14 for rice, 11 for maize, 5 for soybean), on 14 dates spanning over three years: 2014, 2015 and 2016. Contextually, the acquisition of TerraSAR-X Stripmap dual-pol HH/HV (2014-2015) and VV/HH (2016) scenes was planned and carried out in the context of TSX scientific proposals LAN2412 (2014), LAN2984 (2015) and LAN3228 (2016). TSX images acquired for HH, VV, and HV polarizations were radiometrically calibrated, including multilooking (7R x 4A) and terrain correction, to obtain sigma nought (?°) maps in different polarizations. The correlation of ?°HH, VV and HV with measured biophysical parameters was assessed using experimental data collected on test fields. Analysis of preliminary results led to some interesting remarks. Specifically, we observed a significant sensitivity of ?°HH to fresh and dry biomass, with different behaviour for the different crops. Rice ?°HH and VV derived from TSX data show a sensible decrement over the whole range of biomass values, while a different trend is observed for maize, with a first steep increment of ?°HH during the initial biomass growth phase, followed by a flat trend for values above 2.0 kg m-2. A reduced sensitivity to biomass changes has been observed for HV polarization case. According to these preliminary analyses, backscatter over soybean fields does not show a significant relationship with the measured parameters. A radiative transfer model (Paloscia et al., 2014) was used in order to evaluate the contribution of different components of the vegetation-soil system to total backscattering under different observation