Combined Use of Sentinel‐1 SAR and Landsat Sensors Products for Residual Soil Moisture Retrieval over Agricultural Fields in the Upper Blue Nile Basin, Ethiopia

Authors

Getachew Ayehu, Ethioppian Space Science and Technology Institute
Tsegaye Tadesse, University of Nebraska - LincolnFollow
Berhan Gessesse, Ethiopian Space Science and Technology Institute
Yibeltal Yigrem, Bahir Dar University
Assefa M. Melesse, Florida International University

Date of this Version

6-9-2020

Comments

CC-BY

Abstract

The objective of this paper is to investigate the potential of sentinel‐1 SAR sensor products and the contribution of soil roughness parameters to estimate volumetric residual soil moisture (RSM) in the Upper Blue Nile (UBN) basin, Ethiopia. The backscatter contribution of crop residue water content was estimated using Landsat sensor product and the water cloud model (WCM). The surface roughness parameters were estimated from the Oh and Baghdadi models. A feed‐forward artificial neural network (ANN) method was tested for its potential to translate SAR backscattering and surface roughness input variables to RSM values. The model was trained for three inversion configurations: i) SAR backscattering from vertical transmit and vertical receive (SAR VV) polarization only; ii) using SAR VV and the standard deviation of surface heights (ℎ_rms, and iii) SAR VV, ℎ_rms, and optimal surface correlation length (𝑙_eff). Field‐ measured volumetric RSM data were used to train and validate the method. The results showed that the ANN soil moisture estimation model performed reasonably well for the estimation of RSM using the single input variable of SAR VV data only. The ANN prediction accuracy was slightly improved when SAR VV and the surface roughness parameters (ℎ_rmsand 𝑙_eff ) were incorporated into the prediction model. Consequently, the ANN’s prediction accuracy with root mean square error (RMSE) = 0.035 cm³/cm³, mean absolute error (MAE)= 0.026 cm³/cm³, and r= 0.73 was achieved using the third inversion configuration. The result implies the potential of Sentinel‐1 SAR data to accurately retrieve RSM content over an agricultural site covered by stubbles. The soil roughness parameters are also potentially an important variable to soil moisture estimation using SAR data although their contribution to the accuracy of RSM prediction is slight in this study. In addition, the result highlights the importance of combining Sentinel‐1 SAR and Landsat images based on an ANN approach for improving RSM content estimations over crop residue areas.