Monitoring and Retrieving Rice Phenology by means of Satellite SAR Polarimetry at X-band

1. Monitoring and Retrieving Rice Phenology by means of Satellite SAR Polarimetry at X-band Juan M. Lopez-Sanchez J. David Ballester-Berman Shane R. Cloude AEL Consultants Signals, Systems & Telecommunications Group University of Alicante

2. Motivation • Motivation: examples of known demands from rice farmers in Spain • Timely information for: • Effective germination measurements • When all plants have emerged they count their number. If low, more seeds are added • Nitrogen fertillization stop • Once all panicles in a field have appeared, fertilization is not longer needed • Excessive fertilization may cause an increase in pests • Detection of cultivation problems due to water salinity: areas with delayed development • Objective: Is it possible to retrieve the current phenological stage from a single acquisition? • Approach: • Analysis and interpretation of the polarimetric behavior of rice at different phenological stages • If possible, proposal of a retrieval approach based on scattering properties

3. Site • Mouth of the Guadalquivir river, Sevilla (SW Spain) 30km x 30km

4. Ground campaign • Campaigns: 2008 and 2009 • Ground measurements over 5-8 parcels provided by the local association of rice farmers (Federación de Arroceros de Sevilla) • Weekly (defined at field level): • Phenology: BBCH stage (0-99) • Vegetation height • Additional information: • Sowing and harvest dates • Plantation density: plants/m2, panicles/m2 • Yield (kg/ha) • Important: • A water layer is always present at ground during the campaign • Sowing is carried out by spreading seeds (from a plane) randomly over flooded fields

5. Satellite data 2008 2009 Failed orders Available images TerraSAR-X images provided by DLR in the framework of projects LAN0021 and LAN0234

6. Analysis of observations • TerraSAR-X, 30 deg, 2009: Temporal evolution HH VV HH-VV

7. Analysis of observations TerraSAR-X HHVV dual-pol images: List of observables Backscattering coefficients and HH/VV ratio Backscattering coefficients at the Pauli basis (HH+VV, HH-VV) Correlation between HH and VV: magnitude and phase (PPD) Correlation between 1st and 2nd Pauli channels: mag. and phase Eigenvector decomposition (H2a): Entropy and alpha Model-based decomposition: Random volume + polarized term (rank1) Coherent acquisition of co-pol channels

8. Analysis of observations vs phenology HH and VV power • Power Nearly random volume Wind induced roughness Increasing randomness Development Vegetative phase Reproductive phase Maturation Double-bounce Vertical orientation: differential extinction

9. Analysis of observations vs phenology • Correlation between HH and VV Magnitude Phase (PPD) Vegetative phase Vegetative phase Reproductive phase Reproductive phase Maturation Maturation

10. Analysis of observations vs phenology • Eigenvalue decomposition Entropy Alpha (dominant) Vegetative phase Reproductive phase Maturation + Double-bounce Wind induced roughness

11. Retrieval of phenology from TSX data • Basic retrieval approach with a single acquisition (TSX) • Four parameters • HHVV coherence and phase difference • Entropy and alpha1

12. Retrieval of phenology from TSX data • Basic retrieval approach with a single acquisition (TSX) • Five phenological intervals • Decision plane

13. Retrieval of phenology from TSX data • Retrieval results (parcel F)

14. Retrieval of phenology from TSX data • Retrieval results: Comparison against ground data • Percentage of pixels assigned to each stage within a parcel Parcel B Parcel C

15. Retrieval of phenology from TSX data • Comments on the approach • Useful tracking of phenology: • At parcel level: BBCH agrees with the stage assigned to the majority of pixels inside the parcels (with some exceptions) • At (multi-looked) pixel level: parts with different development within a parcel are well identified • But not perfect.. • The algorithm is very ‘simple’: parameters and thresholds have been selected manually (it could be optimized) • An ambiguity between plant emergence (BBCH 18-21) and last stages (BBCH +50) is still present at some areas. Both are characterized by high entropies

16. Conclusions • Coherent dual-pol data provided by TerraSAR-X have been useful for retrieving phenology of rice fields with a single acquisition • Advantages when compared to other possible approaches: • 11-days revisit rate with the same sensor & mode • High spatial resolution • Retrieval with a single pass is possible (single-pol and incoherent dual-pol are not enough) • Limitations: • There remain some ambiguities that might be solved with full-pol data (e.g. using anisotropy), but not in operational mode with TSX • Low coverage: TSX dual-pol swath is 15 km on ground • Some measurements are below or close to the noise level of TSX (-19 dB)

17. Future lines of research • Multi-temporal approaches (time series) • Time coordinate provides extra information • Multi-angular (and multi-temporal) integration • Ideal to reduce refresh time or increase spatial coverage • Development of an operational scheme with farmers • Pending issues: • Presence of rain • Other species within the rice fields (mixture) • Application to rice under different farming practices: • Plantation procedures and arrangements • Dry ground at some moments