Abstract
Center pivot irrigation systems are used to enhance crop production in many countries around the world. Establishing the location and extent of such fields provides information that assists in describing a range of agricultural metrics, including crop identification, yield forecasts, monitoring of irrigation requirements and crop water use, as well as supporting national and regional auditing, licensing and compliance efforts. However, detailed information on the number, extent and changing dynamics of agricultural fields is often lacking in many countries: nowhere more so than in developing regions. To address this lack, we performed a national scale accounting of center pivot fields in Saudi Arabia, using a three year multi-temporal analysis of Landsat-8 satellite data. A geographic object-based image analysis approach was developed based on five 50 x 50 km sub-areas extracted from Landsat data for the year 2015, and applied to delineate individual center pivot fields at a national scale for 2013, 2014 and 2015. The extent of fields was determined via a map of the annual maximum Normalized Difference Vegetation Index (NDVI), while a 15 m spatial resolution map of annual panchromatic band minimums was used to produce an edge detection layer to delineate individual adjoining fields. Amongst a range of classification parameters that were included in the object-based mapping approach, shape information, such as the center pivot field length, length:width ratio, and elliptic fit, were identified as critical parameters. Applying the rule-set that was developed from the five 50 x 50 km sub-regions to the national scale resulted in the identification of 36,052 (11,103 km(2)), 38,114 (11,902 km(2)), and 37,254 (11,555 km(2)) individual fields in 2013, 2014, and 2015, respectively. Approximately 94% of these fields were correctly detected, while their individually measured area was mapped with >95% combined accuracy for fields >0.225 km(2) when evaluated against manually delineated fields. Smaller center pivot fields, and specifically those adjoining neighbouring fields, had lower area mapping accuracies (>91% in 75% of cases). The object-based approach allowed a national scale and multi-temporal assessment of center pivot field delineation and mapping, affording new insights into agricultural practice and providing a methodological basis for examining the impact of water management and related policy initiatives, amongst many other potential applications. Apart from filling a clear knowledge gap in Saudi Arabia, the approach has the potential to be expanded elsewhere: particularly to similar locations within the Middle East and North Africa.