Abstract
The presence of surface water on the canopy affects radar backscatter. However, its influence on the relationship between radar backscatter and crop biophysical parameters has not been investigated. The aim of this study was to quantify the influence of surface canopy water (SCW) on the relationship between L-band radar backscatter and biophysical variables of interest in agricultural monitoring. In this study, we investigated the effect of SCW on the relationship between co- and cross-polarized radar backscatter, cross ratios (VH/VV and HV/HH), and radar vegetation index (RVI) and dry biomass, vegetation water content (VWC), plant height and leaf area index (LAI). In addition, the effect of SCW on estimated vegetation optical depth (VOD) and its relationship with internal VWC was investigated. The analysis was based on data collected during a field experiment in Florida, USA in 2018. A corn field was scanned with a truck-mounted, fully polarimetric, L-band radar along with continuous monitoring of SCW (dew, interception) and soil moisture every 15min for 58 days. In addition, pre-dawn destructive sampling was conducted to measure internal vegetation water content and dry biomass. Results showed that the presence of SCW can increase the radar backscatter up to 2 dB and this effect was lower for cross ratios (CRs) and RVI. The Spearman's rank correlations between radar observables and biophysical parameters were, on average, 0.2 higher for dry vegetation compared to wet vegetation. The estimated VOD from wet vegetation was generally higher than those from dry vegetation, which led to different fitting parameter (so-called b) values in the linear fit between VOD and VWC. The results presented here underscore the importance of considering the influence of SCW on the retrieval of biophysical variables of interest in agricultural monitoring. In particular, they highlight the importance of overpass time, and the impact that daily patterns in dew and interception can have on the retrieval of biophysical variables of interest.
Highlights
Quantification of crop biophysical parameters is essential for many applications including agricultural management, yield forecasting, crop health monitoring and soil moisture estimation
We investigated the effect of surface canopy water (SCW) on the relationship between co- and cross-polarized radar backscatter, cross ratios (VH/VV and HV/HH), and radar vegetation index (RVI) and dry biomass, vegetation water content (VWC), plant height and leaf area index (LAI)
Continuous leaf wetness sensor data, combined with precipitation and irrigation data were used to chart the accumulation and dissipation of dew and interception throughout the growing season. These were combined with data from an L-band fully polarimetric tower-based radar to quantify the effect of SCW on L-band radar observables, as well as the relationship between these observables and biophysical parameters of a corn crop
Summary
Quantification of crop biophysical parameters is essential for many applications including agricultural management, yield forecasting, crop health monitoring and soil moisture estimation. Satellite data are increasingly used to estimate crop biophysical parameters such as leaf area index (LAI) (Brakke et al, 1981; Jiao et al, 2009; Gao et al, 2013; Hosseini et al, 2015; Chang, 2020), crop height (Fieuzal et al, 2012; Gao et al, 2013; Liao et al, 2018), dry biomass (Brakke et al, 1981; Ferrazzoli et al, 1992; Paloscia and Pampaloni, 1992; Gao et al, 2013; Chang, 2020) and vvegetation water content (VWC) (Saatchi et al, 1995; Steele-Dunne et al, 2017; Kim et al, 2018). Vegetation optical depth (VOD) is increasingly used for vegetation monitoring in agricultural applications and natural ecosystems (El Hajj et al, 2019; Konings et al, 2019; Frappart et al, 2020)
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