Mapping Crop Residue and Tillage Intensity Using WorldView-3 Satellite Shortwave Infrared Residue Indices

W Hively,Miguel Quemada,Jacob Shermeyer,Brian Lamb,Craig Daughtry,Gregory Mccarty

doi:10.3390/rs10101657

Abstract

Crop residues serve many important functions in agricultural conservation including preserving soil moisture, building soil organic carbon, and preventing erosion. Percent crop residue cover on a field surface reflects the outcome of tillage intensity and crop management practices. Previous studies using proximal hyperspectral remote sensing have demonstrated accurate measurement of percent residue cover using residue indices that characterize cellulose and lignin absorption features found between 2100 nm and 2300 nm in the shortwave infrared (SWIR) region of the electromagnetic spectrum. The 2014 launch of the WorldView-3 (WV3) satellite has now provided a space-borne platform for the collection of narrow band SWIR reflectance imagery capable of measuring these cellulose and lignin absorption features. In this study, WorldView-3 SWIR imagery (14 May 2015) was acquired over farmland on the Eastern Shore of Chesapeake Bay (Maryland, USA), was converted to surface reflectance, and eight different SWIR reflectance indices were calculated. On-farm photographic sampling was used to measure percent residue cover at a total of 174 locations in 10 agricultural fields, ranging from plow-till to continuous no-till management, and these in situ measurements were used to develop percent residue cover prediction models from the SWIR indices using both polynomial and linear least squares regressions. Analysis was limited to agricultural fields with minimal green vegetation (Normalized Difference Vegetation Index < 0.3) due to expected interference of vegetation with the SWIR indices. In the resulting residue prediction models, spectrally narrow residue indices including the Shortwave Infrared Normalized Difference Residue Index (SINDRI) and the Lignin Cellulose Absorption Index (LCA) were determined to be more accurate than spectrally broad Landsat-compatible indices such as the Normalized Difference Tillage Index (NDTI), as determined by respective R2 values of 0.94, 0.92, and 0.84 and respective residual mean squared errors (RMSE) of 7.15, 8.40, and 12.00. Additionally, SINDRI and LCA were more resistant to interference from low levels of green vegetation. The model with the highest correlation (2nd order polynomial SINDRI, R2 = 0.94) was used to convert the SWIR imagery into a map of crop residue cover for non-vegetated agricultural fields throughout the imagery extent, describing the distribution of tillage intensity within the farm landscape. WorldView-3 satellite imagery provides spectrally narrow SWIR reflectance measurements that show utility for a robust mapping of crop residue cover.

Highlights

Measurements of crop residue cover using a line-point transect and using photo analysis were linearly related with a high R2, low residual mean squared errors (RMSE), and a slope that was near a 1:1 ratio (Figure 4)
Mapping crop residue cover using shortwave infrared (SWIR) indices derived from WorldView-3 satellite imagery was successful, with narrow band indices specific to cellulose and lignin absorption features near 2100–2300
While the Normalized Difference Tillage Index (NDTI) index, when well-calibrated, can provide a reasonably accurate measurement of residue cover, especially when soil color and moisture conditions provide a strong contrast between residue and soil, the narrow band SWIR indices Shortwave Infrared Normalized Difference Residue Index (SINDRI) and Lignin Cellulose Absorption Index (LCA) are more accurate, and are more likely to provide a robust measurement of residue cover under late springtime conditions when summer crops have emerged and begun to provide green groundcover

Summary

Introduction

Crop residue is plant litter (non-photosynthetic vegetation) that accumulates on the surface of agricultural fields, generally after harvest has occurred and crops have senesced. The residues cover the soil surface with a mulch layer that plays an important role in soil conservation by reducing both water-based and wind-based erosion [1]. The amount of crop residue cover is directly linked to crop system management, responding to tillage practices, crop rotations, and harvest methods. With corresponding increases in crop residue cover, can increase soil water retention and help to control soil erosion, mitigating nutrient losses in runoff [4]. High-residue crop management practices are key components of conservation agriculture, and are crucial to promote sustainable cropping systems, complementing additional practices such as reduced chemical usage, improved nutrient management, and diversified crop rotations [1,6]

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