Abstract
Numerous studies have reported the use of multi-spectral and multi-temporal remote sensing images to map irrigated crops. Such maps are useful for water management. The recent availability of optical and radar image time series such as the Sentinel data offers new opportunities to map land cover with high spatial and temporal resolutions. Early identification of irrigated crops is of major importance for irrigation scheduling, but the cloud coverage might significantly reduce the number of available optical images, making crop identification difficult. SAR image time series such as those provided by Sentinel-1 offer the possibility of improving early crop mapping. This paper studies the impact of the Sentinel-1 images when used jointly with optical imagery (Landsat8) and a digital elevation model of the Shuttle Radar Topography Mission (SRTM). The study site is located in a temperate zone (southwest France) with irrigated maize crops. The classifier used is the Random Forest. The combined use of the different data (radar, optical, and SRTM) improves the early classifications of the irrigated crops (k = 0.89) compared to classifications obtained using each type of data separately (k = 0.84). The use of the DEM is significant for the early stages but becomes useless once crops have reached their full development. In conclusion, compared to a “full optical” approach, the “combined” method is more robust over time as radar images permit cloudy conditions to be overcome.
Highlights
More than 324 million hectares are equipped for irrigation in the world, among which about 85% are irrigated
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We investigate the usefulness of both radar and optical imagery combined with a importance for early crop detection
Summary
More than 324 million hectares are equipped for irrigation in the world (http://www.fao.org, 2012), among which about 85% are irrigated. In Europe, the percentage of land area equipped for irrigation is quite small (65%) compared to the rest of the world This is largely due to the moderate climate, where agriculture takes advantage of the available rainfall and constant irrigation can be avoided. In order to reduce these conflicts, national authorities have proposed various laws including the law on water and aquatic environments in 2006 and, in 2011, the first National Plan for Adaptation to Climate Change was adopted. These laws aim to help water resources managers. To successfully implement sustainable management, we need tools able to estimate actual water needs and supplies, which are frequently updated and cover large territories, to help the policy makers and water managers
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