Combination of Linear Regression Lines to Understand the Response of Sentinel-1 Dual Polarization SAR Data with Crop Phenology—Case Study in Miyazaki, Japan

Emal Wali,Masao Moriyama,Masahiro Tasumi

doi:10.3390/rs12010189

Abstract

This study investigated the relationship between backscattering coefficients of a synthetic aperture radar (SAR) and the four biophysical parameters of rice crops—plant height, green vegetation cover, leaf area index, and total dry biomass. A paddy rice field in Miyazaki, Japan was studied from April to July of 2018, which is the rice cultivation season. The SAR backscattering coefficients were provided by Sentinel-1 satellite. Backscattering coefficients of two polarization settings—VH (vertical transmitting, horizontal receiving) and VV (vertical transmitting, vertical receiving)—were investigated. Plant height, green vegetation cover, leaf area index, and total dry biomass were measured at ground level, on the same dates as satellite image acquisition. Polynomial regression lines indicated relationships between backscattering coefficients and plant biophysical parameters of the rice crop. The biophysical parameters had stronger relationship to VH than to VV polarization. A disadvantage of adopting polynomial regression equations is that the equation can have two biophysical parameter solutions for a particular backscattering coefficient value, which prevents simple conversion from backscattering coefficients to plant biophysical parameters. To overcome this disadvantage, the relationships between backscattering coefficients and the plant biophysical parameters were expressed using a combination of two linear regression lines, one line for the first sub-period and the other for the second sub-period during the entire cultivation period. Following this approach, all four plant biophysical parameters were accurately estimated from the SAR backscattering coefficient, especially with VH polarization, from the date of transplanting to about two months, until the mid-reproductive stage. However, backscattering coefficients saturate after two months from the transplanting, and became insensitive to the further developments in plant biophysical parameters. This research indicates that SAR can effectively and accurately monitor rice crop biophysical parameters, but only up to the mid reproductive stage.

Highlights

Rice is a staple food, feeding approximately half of the world’s population
The objectives of this research were to study the relationship between synthetic aperture radar (SAR) backscattering coefficients and rice crop biophysical parameters using Sentinel-1 satellite imagery, and to suggest an approach to evaluate plant biophysical parameters of rice crop using a combination of linear regression lines
Setting a study area in paddy rice fields in Miyazaki, Japan, ground measurements were conducted for plant height, green vegetation cover, leaf area index (LAI), and the total dry biomass of rice crop, every 12 days, simultaneously to the Sentinel-1A satellite SAR observations

Summary

Introduction

Rice is a staple food, feeding approximately half of the world’s population. Estimates indicate that, by the mid-twenty first century, one of the biggest challenges will be to feed the nine billion people of the world [1]. Optical remote sensing has been utilized for agricultural decision support systems via crops monitoring, such as by using the vegetation related products of Moderate Resolution Imaging Spectroradiometer (MODIS), combined with agro-metrology metrics such as precipitation, temperature, solar radiation, and soil moisture [5]. Examples of these tools are found in several literatures [6,7,8,9,10,11]

Objectives

Methods

Results

Conclusion