There is evidence of pre-modern agriculture on the leeward slopes of Kohala. From 1400 to 1800, the principal crop grown at Kohala was sweet potato "(Ipomoea batatas)", although there is also evidence of yams "(Dioscorea sp.)", taro "(Colocasia esculenta)", bananas "(Musa hybrids)", sugarcane "(Saccharum officinarum)", and gourds of the family Cucurbitaceae. The optimal rainfall level for the sweet potato lies between per year. A combination of factors makes the rainfall at Kohala variable both from location to location and from year to year. In addition, Kohala is buffeted by strong winds, which are directly correlated to soil erosion; ancient farmers utilized a series of earthen embarkments and stone walls to protect their crops. This technique has been shown to reduce wind by at least 20–30 percent.