Development, Intensification and Sustainability of the Hālawa Valley Agroecosystem, Moloka‘i, Hawaiian Islands, AD 1200–1850

Abstract

One of four large valleys on the windward side of East Moloka'i, Hālawa was a major centre of traditional irrigated taro cultivation in the 19th and early 20th centuries. Leveraging prior results of archaeological research undertaken in 1969–1970, from 2020 to 2022 we investigated the valley's irrigated and dryland agricultural features in order to better understand the longue durée of agricultural development, and to assess the long-term sustainability of the agroecosystem. Extensive sampling and analysis of soil nutrient properties reveal that the colluvial slopes have the greatest potential to support intensive cultivation, and that the irrigated alluvial flats are likely enriched by periodic transport of colluvial sediments due to flooding and overland mass transport. The earliest evidence for agricultural activities, dated to ca. AD 1200, is suggestive of shifting cultivation on the colluvial slopes. Stone-faced irrigated terraces were in place by at least by the late 13th or early 14th centuries, and a stone-lined irrigation canal by the mid-15th century. Extensive pondfields on the lower alluvial flats near the river mouth were constructed later, in the mid-17th century, presumably after the marshy terrain was elevated through deposition of sediments eroded from the interior valley slopes. Extensive dryland (non-irrigated) terraces were constructed on the colluvial slopes in the 17th and 18th centuries. Geochemical analysis of stratified sediments in pondfield terraces is indicative of some nutrient draw-down through intensive cultivation over long time periods, however the agroecosystem remained sustainable due to continued rejuvenation of soil nutrients through regular down-valley transport of fertile colluvium. Potential drivers of agricultural intensification are discussed.