Sensitivity Analysis of the C and L bands SAR Data for Detecting Irrigation Events
Hassan Bazzi 1, Nicolas Baghdadi 1, Mehrez Zribi 2 and François Charron 3
1 INRAE, UMR TETIS, University of Montpellier, AgroParisTech, 500 rue François Breton, CEDEX 5, 34093 Montpellier, France; firstname.lastname@example.org (N.B.)
2 CESBIO (CNRS/UPS/IRD/CNES/INRAE), 18 av. Edouard Belin, bpi 2801, CEDEX 9, 31401 Toulouse, France; email@example.com (M.Z.)
3 G-EAU Unit, University of Montpellier, AgroParisTech, CIRAD, INRAE, Institut Agro, IRD, Domaine du Merle, 13300 Salon de Provence, France; firstname.lastname@example.org
For better management of water resources and water consumption, the detection of the irrigation timing at the agricultural plot scale is of great importance. With the availability of the Sentinel-1 (S1) SAR data in free and open access mode at 6 days revisit time, several studies have demonstrated the potential of the S1 data for detecting irrigation events (Bazzi et al., 2020a; Le Page et al., 2020). Current irrigation detection models built using S1 SAR data are based on the monitoring of the increase of the SAR backscattering signal between two consecutive SAR acquisitions (increase in soil moisture). However, the detection of irrigation events based on the soil moisture-SAR correlation using the C-band S1 data (wavelength ~ 6cm) is sometimes limited to the penetration capability of the C-band over some developed vegetation cover. For certain high vegetation cover, especially for some cereal crops such as wheat and barley, several studies have demonstrated that the sensitivity of the C-band SAR signal to soil moisture values becomes negligible when the vegetation is well-developed (El Hajj et al., 2018; Joseph et al., 2010; Nasrallah et al., 2019). To overcome the penetration limitation of the C-band, SAR data with a higher wavelength could be required such as the L-band (wavelength ~24 cm) SAR data.
The objective of this study is to compare the sensitivity of the S1 C-band and the ALOS-2 L-band images for irrigation detection over well-developed vegetation cover. A sensitivity analysis of both C and L bands for irrigation detection was performed over 45 references irrigated grassland plots located in the Crau plain of southeast France during two growth cycles. The first growth cycle is rich in grasses (coarse hay) and resembles wheat crops and the second grass cycle is richer in legumes with less percentage of coarse hay. The Normalized Difference Vegetation Index (NDVI) is used in the analysis to describe the vegetation cover.
In order to understand the capability of detecting irrigation events using the C-band SAR data, we studied first the response of the S1 backscattering signal (σ_C^0) following irrigation events by examining the temporal evolution of the σ_C^0 according to rainfall and irrigation events.
Two L-band images each acquired in each growth cycle were used to compare between the C and L bands sensitivity for irrigation detection. We analyze the relationship between the σ_C^0 and L-band backscattering coefficients (σ_L^0) as a function of the time difference between the SAR acquisition date and the irrigation date (∆t). The parameter ∆t is considered to be a proxy measure of the soil dryness-wetness.
The results showed that when the vegetation cover develops in the first growth cycle (coarse hay), the response of the σ_C^0 to the water supplements (either irrigation or rainfall) becomes negligible and the irrigation events could be hardly detected. The σ_C^0 in the first growth cycle shows no correlation with the ∆t value which means that the σ_C^0 are not sensitive to the dryness-wetness of the soil. The behavior of the σ_L^0 as a function of the time difference between the image acquisition date and the irrigation date in the first growth cycle indicates that in the presence of either low or high developed vegetation cover, the σ_L^0 in HH polarization could be still sensitive to the soil water content. Regardless of the NDVI values, wet soil conditions due to irrigation on the same day of the L-band acquisition induce high σ_L^0 (around -11 dB). In contrast, dry soil due to the absence of irrigation 5 to 6 days before the ALOS-2 acquisition shows low σ_L^0 values (around -15 dB).
In the second growth cycle of grass (rich in legumes), both C and L bands are sensitive to the soil moisture values in the presence of either high or low vegetation cover. In L-band, the σ_L^0 values decrease from -13 dB when the irrigation is on the same day of the ALOS-2 acquisition to less than -17 dB when the irrigation is 15 days before the ALOS-2 acquisition. Similar behavior with less tendency was observed for the C-band. Results showed that L-band is more sensitive than the C-band and the HH polarization in L-band is more sensitive than the HV polarization for detecting irrigation events.
Bazzi, H., Baghdadi, N., Fayad, I., Zribi, M., Belhouchette, H., and Demarez, V. (2020a). Near Real-Time Irrigation Detection at Plot Scale Using Sentinel-1 Data. Remote Sensing 12, 1456.
Bazzi, H., Baghdadi, N., Fayad, I., Charron, F., Zribi, M., and Belhouchette, H. (2020b). Irrigation Events Detection over Intensively Irrigated Grassland Plots Using Sentinel-1 Data. Remote Sensing 12, 4058.
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