Spectral Indices to Improve Crop Residue Cover Estimation under Varying Moisture Conditions

Miguel Quemada,Craig Daughtry

doi:10.3390/rs8080660

Abstract

Crop residues on the soil surface protect the soil against erosion, increase water infiltration and reduce agrochemicals in runoff water. Crop residues and soils are spectrally different in the absorption features associated with cellulose and lignin. Our objectives were to: (1) assess the impact of water on the spectral indices for estimating crop residue cover (fR); (2) evaluate spectral water indices for estimating the relative water content (RWC) of crop residues and soils; and (3) propose methods that mitigate the uncertainty caused by variable moisture conditions on estimates of fR. Reflectance spectra of diverse crops and soils were acquired in the laboratory over the 400–2400-nm wavelength region. Using the laboratory data, a linear mixture model simulated the reflectance of scenes with various fR and levels of RWC. Additional reflectance spectra were acquired over agricultural fields with a wide range of crop residue covers and scene moisture conditions. Spectral indices for estimating crop residue cover that were evaluated in this study included the Normalized Difference Tillage Index (NDTI), the Shortwave Infrared Normalized Difference Residue Index (SINDRI) and the Cellulose Absorption Index (CAI). Multivariate linear models that used pairs of spectral indices—one for RWC and one for fR—significantly improved estimates of fR using CAI and SINDRI. For NDTI to reliably assess fR, scene RWC should be relatively dry (RWC < 0.25). These techniques provide the tools needed to monitor the spatial and temporal changes in crop residue cover and help determine where additional conservation practices may be required.

Highlights

Best management practices for croplands often include maintaining crop residues on the soil surface [1]
The absorption feature associated with the −OH bond in the cellulose exhibited two linear phases with a breakpoint near relative water content (RWC) = 0.6 (Figure 4c)
Reflectance of the soils in the OLI6 and OLI7 bands decreased continuously with RWC, but at slightly different rates (Figure 4f), which accounted for the increase in Normalized Difference Tillage Index (NDTI) as a function of RWC

Summary

Introduction

Best management practices for croplands often include maintaining crop residues on the soil surface [1]. Crop residues often completely cover the soil surface after harvest, but residue cover decreases as the soil is tilled or the residues are harvested for fuel or feed Simulation models, such as the Environmental Policy. Integrated Climate (EPIC) [3] and Soil and Water Assessment Tool (SWAT) [4], can predict the overall impact of crop and soil management practices on soil organic carbon, greenhouse gas emissions and water quality. These models require geospatial information on landscape topography, soil properties, weather and climate, crop type, crop management practices and soil tillage intensity.

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Methods

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Discussion

Conclusion