In the present study, we evaluate the potential of multi-incidence L-band and C-band data to retrieve soil moisture. In -situ measurements were acquired during satellite acquisitions over cereal fields in the Kairouan plain in central Tunisia (semi-arid area). Analysing radar data, L-band Advanced Land Observing Satellite-2 multi-incidence data (28°, 32.5° and 36°) in HH (L-HH) and HV (L-HV) polarizations and C-band like-polarization Sentinel-1data, with an incidence angle of approximately 39°, (C-VV) are strongly impacted by soil roughness. In addition, results highlight the sensitivity of L-band data to soil moisture in dense cover class where Normalized Vegetation Difference Index (NDVI) values are higher than 0.6. Two options of Water Cloud Model (WCM) were used (with and without the integration of soil-vegetation interaction component) to simulate radar signal over cereal fields. Each option of WCM was coupled to the best performance bare soil backscattering models. By inverting WCM, results underline the important contribution of soil-vegetation interaction component to estimate soil moisture with L-HV data compared to a neglected impact on C-band data inversion accuracy and stable accuracy in L-HH.
This article aims to analyze agronomic drought in a highly anthropogenic semi-arid region. This is the western Mediterranean region. The study uses satellite data from the Moderate Resolution Imaging Spectroradiometer (MODIS) and Advanced Scatterometer (ASCAT) describing the dynamics of vegetation cover and soil water content through the Normalized Difference Vegetation Index (NDVI) and the Soil Water Index (SWI). An analysis of the vegetation anomaly index (VAI) highlights the difference between agricultural and natural areas. Thus, two land use classes are considered for the analysis of drought indices, agricultural areas and natural areas. The contribution of vegetation cover (VAI) was combined with the effect of soil water content using the moisture anomaly index (MAI) through a new drought index called the global drought index (GDI). This index considers the seasonal effect of the development of vegetation cover and soil water content with variable weightings over time for the two indices VAI and MAI.
Quantification of Root-Zone Soil Moisture (RZSM) is crucial for agricultural applications. It impacts processes like vegetation transpiration and water percolation. The surface soil moisture (SSM) can be assessed through active and passive microwave remote sensing, but no current sensor enables direct retrieval of RZSM. Spatial maps of RZSM can be retrieved via proxy observations (vegetation stress, water storage change, surface soil moisture) or from land surface model predictions. Recently, more interest has risen in the use of data-driven methods to predict RZSM. In this study, we investigated the use of physical-process based features in the context of Artificial Neural Networks (ANN). We integrated the infiltration process information into an ANN model through the use of the recursive exponential filter. We also used a remote sensing-based evaporative efficiency as an input feature. It is important to note that these two processes depend on surface soil moisture which can be assessed through remote sensing. The impact of the use of geophysical variables was also assessed through the use of surface soil temperature and Normalized Difference Vegetation Index (NDVI). At each step of the study, the ANN models were trained using either only in-situ surface soil moisture data provided by the International Soil Moisture Network (ISMN) or an additional geophysical or processbased feature. The results show that the use of more features in addition to SSM information improves the prediction accuracy in specific cases when compared to an ANN model that predicts RZSM based on only SSM. The ability of the developed models to predict RZSM over larger areas will be assessed in the future.
The goal of this paper is to analyze the sensitivity of X-band SAR (TerraSAR-X) signals as a function of different physical bare soil parameters (soil moisture, soil roughness), and to demonstrate that it is possible to estimate of both soil moisture and texture from the same experimental campaign, using a single radar signal configuration (one incidence angle, one polarization). Firstly, we analyzed statistically the relationships between X-band SAR (TerraSAR-X) backscattering signals function of soil moisture and different roughness parameters (the root mean square height Hrms, the Zs parameter and the Zg parameter) at HH polarization and for an incidence angle about 36°, over a semi-arid site in Tunisia (North Africa). Results have shown a high sensitivity of real radar data to the two soil parameters: roughness and moisture. A linear relationship is obtained between volumetric soil moisture and radar signal. A logarithmic correlation is observed between backscattering coefficient and all roughness parameters. The highest dynamic sensitivity is obtained with Zg parameter. Then, we proposed to retrieve of both soil moisture and texture using these multi-temporal X-band SAR images. Our approach is based on the change detection method and combines the seven radar images with different continuous thetaprobe measurements. To estimate soil moisture from X-band SAR data, we analyzed statistically the sensitivity between radar measurements and ground soil moisture derived from permanent thetaprobe stations. Our approaches are applied over bare soil class identified from an optical image SPOT / HRV acquired in the same period of measurements. Results have shown linear relationship for the radar signals as a function of volumetric soil moisture with high sensitivity about 0.21 dB/vol%. For estimation of change in soil moisture, we considered two options: (1) roughness variations during the three-month radar acquisition campaigns were not accounted for; (2) a simple correction for temporal variations in roughness was included. The results reveal a small improvement in the estimation of soil moisture when a correction for temporal variations in roughness is introduced.
Finally, by considering the estimated temporal dynamics of soil moisture, a methodology is proposed for the retrieval of clay and sand content (expressed as percentages) in soil. Two empirical relationships were established between the mean moisture values retrieved from the seven acquired radar images and the two soil texture components over 36 test fields. Validation of the proposed approach was carried out over a second set of 34 fields, showing that highly accurate clay estimations can be achieved.
In semi-arid areas, an operational grain yield forecasting system, which could help decision-makers to plan annual imports, is needed. It can be challenging to monitor the crop canopy and production capacity of plants, especially cereals. Many models, based on the use of remote sensing or agro-meteorological models, have been developed to estimate the biomass and grain yield of cereals. Remote sensing has demonstrated its strong potential for the monitoring of the vegetation's dynamics and temporal variations. Through the use of a rich database, acquired over a period of two years for more than 60 test fields, and from 20 optical satellite SPOT/HRV images, the aim of the present study is to evaluate the feasibility of two approaches to estimate the dynamics and yields of cereals in the context of semi-arid, low productivity regions in North Africa.
The first approach is based on the application of the semi-empirical growth model SAFY “Simple Algorithm For Yield estimation”, developed to simulate the dynamics of the leaf area index and the grain yield, at the field scale. The model is able to reproduce the time evolution of the LAI of all fields. However, the yields are under-estimated. Therefore, we developed a new approach to improve the SAFY model. The grain yield is function of LAI area in the growth period between 25 March and 5 April. This approach is robust, the measured and estimated grain yield are well correlated. Finally, this model is used in combination with remotely sensed LAI measurements to estimate yield for the entire studied site.
From the beginning of the 1990s, the use of Global Navigation Satellite System (GNSS) reflected signals have been identified as a as source of opportunity for remote sensing applications. In the last two decades, the potential of the technique have been demonstrated for ocean and continental surfaces studies, and several applications have been proposed in the context of high availability of GNSS signals. The GNSS-R technique is generally based on the use of a passive receiver simultaneously acquiring the direct and reflected signals from various GNSS satellites to estimate geophysical parameters from the scattering surface. In the last years, several ground-based [2], [3], airborne [4] and space-borne [5]–[8] experiments have been proposed. The most considered application foreseen for GNSS-R is ocean altimetry for a precise determination of sea-surface heights as well as roughness and wind direction. For continental surfaces, because of direct relationship between surface permittivity and reflected signal, different approaches [6], [9], [10] have been proposed to estimate surface parameters (soil moisture, vegetation biomass, snow). Different observables have been proposed to analyze GNSS signals: the Delay-Doppler Map, the direct and reflected complex waveforms bistatic signal, the ratio between the direct and reflected waveform’s peak time series (Interferometric Complex Field). In this context, the airborne instrument GLORI is proposed to demonstrate contribution of GNSS-R to estimate soil moisture over agricultural soils and biomass of forests or annual cultures. A secondary goal is the feasibility of centimeter-precision altimetry above continental water bodies. The second section describes the characteristics of GLORI instrument. The third section presents airborne campaigns realized over the south West of France and fourth sections discusses the first results. Conclusions are gathered in section 5.
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