# Detecting Cover Crop End-Of-Season Using VEN\(\upmu\)S and Sentinel-2 Satellite Imagery
## 1 Introduction  
Cover crops are planted after a field is harvested to cover the soil for the benefit of the agroecosystem. Different from cash crop production, cover crops are mainly used to reduce soil erosion, increase soil fertility, and improve watershed management [1; 2; 3]. They can be used as \"green manures\" and plowed before reaching full maturity to provide nitrogen for cash crop production [4] can also increase soil organic matter levels and soil carbon sequestration [2]. Another benefit is that cover crops can retain soil nitrogen from chemical nitrogen fertilizer for cash crops and reduce leaching of farmland nitrogen to the groundwater and watershed surface waters [5]. In Maryland and Delaware, winter cover crops have been identified as an essential component of watershed conservation implementation plans toreduce nutrient and sediment losses from farmland [6; 7]. Cost-share programs have been created to encourage the planting of cover crops for improving water quality [8]. Cover crop termination timing is determined by cover crop growth conditions and regulations. The Chesapeake Bay Program Partnership water quality model defines varying environmental efficiencies for winter cover crop species depending on differences in timing and method of planting [9]. In the 2019-2020 winter cover crop management agreement, the Maryland Department of Agriculture cost-share program requires that spring termination occurs between 1 March 2020, and 14 June 2020 [9]. Delayed terminations after 1 May are eligible for receiving additional incentive payments for higher biomass production [9]. Traditionally, cover crop planting/emergence and termination dates are reported by farmers and confirmed through field survey by conservation district staff, which is labor-intensive. Cover crop planting and termination dates are also crucial for estimating cover crop biomass, nutrient uptake, and soil carbon sequestration [10]. Cover crops may be terminated by moving, tilling, plowing, rolling, or application of herbicides when cover crops are still green. Remote sensing data have been successfully used to estimate winter cover crop biomass and percent ground cover [6; 11; 12; 13]. Mapping the end-of-season (termination) dates for cover crops at the field scale, however, has been a challenge due to the lack of high temporal and spatial resolution remote sensing data. Hypothetically, high temporal remote sensing observations should capture the change of cover crop conditions if observations before and after termination are available. However, it is challenging to estimate termination dates within the season (or near-real-time) due to numerous reasons summarized below. First, cover crop termination depends on the cover crop conditions, management regulation, and farmer decisions. These factors can vary substantially from year-to-year and from field-to-field. The detection of cover crop termination at the field scale requires frequent and recent remote sensing observations at a high-to-medium spatial resolution (defined here as 5-30 m). However, such kinds of remote-sensing data products have only become available with adequate temporal frequency in recent years. For example, the two Sentinel-2 satellites provide 10-20 m resolution imagery in visible and near-infrared bands every 5 days [14]. The harmonized Landsat and Sentinel-2 (HLS) dataset provides global 30 m resolution surface reflectance products every 3-4 days [15] since 2017. The commercial PlanetScope constellation provides daily observations from a small satellite constellation at 3 m spatial resolution [16]. The pilot Vegetation and Environment monitoring New Micro Satellite (VEN\(\mu\)S) described in this paper provides a surface reflectance product at 5 m spatial resolution every 2 days since 2018 [17], and has demonstrated potential in various applications [18; 19]. These data products have different spatial and temporal resolutions. VEN\(\mu\)S provides a high-quality surface reflectance data product from a single sensor but is only available for selected sites.