Intermittent irrigation in the Waimea Field System, Hawai'i Island: A computational fluid dynamics model

In pre-European contact Hawai'i, flooded irrigated agricultural systems were developed in wet windward areas with rain-fed dryland systems dominating leeward zones. In select areas lacking sufficient rainfall for dryland production, irrigation from intermittent streams would have been a viable alternative. A number of intermittent irrigated agricultural systems have been recorded in leeward Hawaiian locations, with the ca. 33 km2 Waimea Field System being the most extensive. We use computational fluid dynamics modelling within a digital elevation model based on high resolution terrestrial laser scanning data to investigate intermittent irrigation within a 2.4 ha study area of the Waimea Field System. The analysis documents variation within channels and terraces. Some channels are gradual downslope ditches with the potential to irrigate large areas via garden plot flow through. Feeding off of these ditches are channels running parallel to slope with constrained flows of higher velocity that fed lower terraces without depositing water on upper terraces. The system also included channels running along the front of terraces that redirected flows horizontally across slope by collecting excess water. Terraces varied in terms of the amount of water that they could receive. Many terraces received moderate volumes of water, whereas select terraces in the valley bottom received much higher volumes and were probably used to grow water demanding crops. Other terraces were not fed by channels and were probably used for rain-fed agriculture or residential purposes. Documenting variation within this small section of the Waimea field system provides an understanding of diverse agricultural practices and how these were used for subsistence and surplus production.