Sentinel-2 for Assessing Winter Cover Crop Growth and Its Influential Factors in Maize Cropping System in the Netherlands

An effective establishment of a winter cover crop is important for reducing nitrogen leaching to groundwater in the maize-based cropping system of the Netherlands. Cover crop establishment after maize cultivation is obliged by law for sandy soils and consequently implemented in nearly all maize fields, but the vegetative ground cover in winter varies significantly between fields. The objective of this study is to evaluate to what extent differences in winter vegetative cover can be explained by the timing of cover crop establishment and weather conditions in two growing seasons (2017-18). We used Sentinel-2 satellite imagery, in combination with the Dutch Basic Registration Parcels (BRP) data, to construct NDVI time-series profiles for parcels known to be cultivated with maize within Province of Overijssel in 2017. We developed a piecewise NDVI time-series smoothing method to fit the time profile of maize-cover crop rotation, from which parameters were then retrieved, including the cover crop sowing date and the fitted NDVI value for 1 December 2017 (NDVIdec). The fitted NDVIdec was used to denote the quality of cover crop establishment in the winter season. A comparison against ground reference data for eight parcels showed that the Sentinel-2 retrieved sowing date was an efficient proxy of on-farm actual sowing date, with low root-mean-squared error (RMSE=6.6 days). To eliminate spatial autocorrelation effects, sowing dates for 500 out of 13980 randomly selected maize parcels were then retrieved, which could explain 38% of NDVIdec variability. Finally, we mapped the spatial patterns of sowing time and its impact on winter cover crop growth for all maize parcels in Overijssel at fine resolution (Figure 1). Besides sowing date, we also explored various other spatially-variable predictors for difference in NDVIdec, such as soil type and parcel planting history. Weather conditions in 2018 were substantially different from 2017 because of low levels of rainfall and high temperatures. As a result, maize harvesting times and cover crop sowing times shifted forward significantly. Although at the time of writing we cannot yet perform a full assessment of the 2018 winter season, our presentation will include also an assessment of the effect of this drought on estimated cover crop sowing dates and the resulting quality of the cover crop establishment. Our results demonstrate the potential of Sentinel-2 image time series for retrieving seasonality parameters for small agricultural parcels, allowing for effective analysis of the effect of farmer practices (such as timing of maize harvest and sowing of cover crop) on cover crop establishment. As such, these provide actionable information both for farmers as well as regulatory bodies aiming to reduce nitrogen leaching.