A new remote sensing method to map field- scale time-series wheat without training data in Bulgaria

Accurate and up-to date wheat area estimate is a critical input for food security and policy related studies. Wheat as a staple crop covers fifty percent of agricultural lands in Bulgaria. In Bulgaria, current wheat area statistics is available at regional scale (equivalent to census regions in the United States) until year 2017 from government departments. Remote sensing provides extraordinary capability to map field-scale temporally consistent wheat area but, traditional large area mapping methods using remote sensing require large volume of training data and large computing resources. This study presents a new approach called "Peak Value Method" (PVM) to automatically map the wheat cultivated areas in Bulgaria without training data. This study mapped wheat areas between 2001 and 2020 using PVM through Landsat derived enhanced vegetation index (EVI) time series data and phenological understanding of crop. To our knowledge, this is the first study to map wall to wall high resolution time-series wheat areas with reliable accuracy assessment in this study area. This study offers three main outputs: 1. a new, simple but effective remote sensing method to map wheat crop without training data in the study area, 2. first field scale (30-m) time series (2001-2020) wheat maps with reliable accuracy covering Northcentral, Northeast, Southcentral and Southeast regions of Bulgaria and, 3. fills the gap in the knowledge about field-scale wheat information in the study area and it’s accessibility through remote sensing to study the environment related questions.