PREHISTORIC SOLAR INNOVATION AND WATER MANAGEMENT ON RAPA NUI

摘要

The prehistoric Rapa Nui (Easter Island, East Polynesia) population radically transformed the island environment as it was changed to accommodate an agricultural system based primarily on taro, yam and sweet potato production. These changes included the extinction of several bird species, the heavy exploitation of coastal and near-shore marine resources, and the near total deforestation of the island's palm forest by AD 1350. The removal of the forest exposed the agricultural fields to increased wind velocities, promoted evapotranspiration, moisture loss in the soils, and led to erosion in some sloping areas. Archaeological survey and excavation of the Rapa Nui landscape has documented that prehistoric agriculturalists reduced the possibility of crop failure by careful management of the soil moisture regime. This was accomplished by the development of gardens covered with a dense layer of stone, or lithic mulch, where rocks were accumulated from the open landscape, removed from outcrops, and placed over growing areas. What appeared to be random strewn volcanic rocks have recently been discovered to be a solar innovation. It is hypothesized that a rock layer captures the intense solar radiation on a cool soil to manufacture dew, retain soil moisture, facilitate moisture percolation, reduce wind velocities and maintain higher soil temperatures. We evaluate this hypothesis with some preliminary data gathered from monitoring the soil temperature and moisture in the near surface region.