Field-scale assessment of Belgian winter cover crops biomass based on Sentinel-2 data

Abstract

Winter cover crops, used as green manure, can supply up to 45 units of nitrogen per hectare to the following summer crops. In order to contribute to the establishment of the nitrogen balance sheet for fertilisation recommendation of subsequent main crop at field scale, this supply is currently derived from the biomass production, classically estimated visually using 3 classes: 0–1, 1–3, 3+ tons of dry matter per hectare (Mg DM ha−1). The capabilities of Sentinel-2 satellite data to retrieve an operator-independent winter cover crop biomass have been assessed. Biomass samples were collected in 1 m quadrats for various types of winter cover fields classically used in Belgium (mustard, phacelia, oat and a range of mixed cover), in 2016 and 2017 (yield between 0.1 to 5 tons of dry matter per hectare). Empirical relationships between the winter cover crop biomass and a wide range of vegetation indices (VIs) derived from Sentinel-2 have been defined, and the most performant VI identified. For pure stands of winter cover crops, the cross-validation RMSE (CVRMSE) of the best model is 0.36 Mg DM ha−1 for mustard and 0.3 Mg DM ha−1 for phacelia. The CVRMSE observed for mixed stands, around 0.61 Mg DM ha−1, is roughly two times higher than the CVRMSE observed for pure stands. The added-value of objective satellite-based estimation of winter cover biomass was also assessed by comparing respective estimations with regards to an independent reference dataset made of sample measurements on the ground. Models based on Earth observation showed better results than farmer visual assessment for mustard crops, and were as good as farmers for phacelia crops.