remote sensing Article Extracting Irrigation Structure Networks from Pre-Landsat 4 Satellite Imagery Using Vegetation Indices Corrine Coakley 1, *, Mandy Munro-Stasiuk 1 , James A. Tyner 1 , Sokvisal Kimsroy 1 , Chhunly Chhay 1 and Stian Rice 2,3 1 Department of Geography, Kent State University, Kent, OH 44240, USA; mmunrost@kent.edu (M.M.-S.); Jtyner@kent.edu (J.A.T.); skimsroy@kent.edu (S.K.); Cchhay@kent.edu (C.C.) 2 Center for Urban Environmental Research and Education, University of Maryland, Baltimore County, Baltimore, MD 21250, USA; stianrice@gmail.com 3 Department of Geography, College of Arts and Sciences, Kent State University, P.O. Box 5190, Kent, OH 44242-0001, USA * Correspondence: ccoakley@kent.edu; Tel.: +1-440-715-5125 Received: 17 August 2019; Accepted: 11 October 2019; Published: 16 October 2019   Abstract: Often discussed, the spatial extent and scope of the Khmer Rouge irrigation network has not been previously mapped on a national scale. Although low resolution, early Landsat images can identify water features accurately when using vegetation indices. We discuss the methods involved in mapping historic irrigation on a national scale, as well as comparing the performance of several vegetation indices at irrigation detection. Irrigation was a critical component of the Communist Part of Kampuchea (CPK)’s plan to transform Cambodia into an ideal communist society, aimed at providing surplus for the nation by tripling rice production. Of the three indices used, normalized difference, corrected transformed, and Thiam’s transformed vegetation indexes, (NDVI, CTVI, and TTVI respectively), the CTVI provided the clearest images of water storage and transport. This method for identifying anthropogenic water features proved highly accurate, despite low spatial resolution. We were successful in locating and identifying both water storage and irrigation canals from the time that the CPK regime was in power. In many areas these canals and reservoirs are no longer visible, even with high resolution modern satellites. Most of the structures built at this time experienced some collapse, either during the CPK regime or soon after, however many have been rehabilitated and are still in use, in at least a partial capacity. Keywords: Khmer Rouge; irrigation; vegetation index; remote sensing; Landsat MSS 1. Introduction It is well established that Landsat and other remotely sensed imagery can be used to study land use change [1,2], agriculture [3], the impacts of climate change [4], and many other physical characteristics of the Earth’s surface. Less emphasis has been placed on using older Landsat data to apply remote sensing analyses to historical and political questions. Much as current satellite information can be used to track destruction in Syria and genocide in Myanmar [5–7], remote sensing techniques can be applied to older Earth observation data to enhance our understanding of political situations of the past. This paper illustrates the benefits and drawbacks of using older, relatively low-resolution satellite imagery from Landsats 1–3 to fill in gaps in the historical record. The information extracted from these images will enhance our understanding of what was (literally) happening “on the ground” during times when very little information was available to those outside of Cambodia’s borders. Remote Sens. 2019, 11, 2397; doi:10.3390/rs11202397 www.mdpi.com/journal/remotesensing