Mae Tao, a small (8573 ha) but important watershed in Mae Sot District, Tak province, northwest Thailand (Fig. 1), has mineral-rich, fertile soils and good water resources. Reputedly the best rice-growing area in Mae Sot district, the >3500 ha of cascade-irrigated rice fields yield almost 76 000 t rice y−1 (Lertlakanawong, 2005 and Simmons et al., 2008). The watershed is on the western side of the Thanon-Thongchai mountain ranges, near the Thai-Myanmar border. Its geographical features include mountains with valuable metal-rich areas in the east and Mae Tao Creek, which flows from the east and feeds a network of hundreds of canals, taking water through to the floodplain fields in the west. The rice-growing areas on the floodplains in the western lowlands are ∼4–15 km from the largest Zn mining area (Padaeng deposit) in Southeast Asia. Rice production commences at the beginning of the rainy season (May/June) and ends in December every year (Lertlakanawong, 2005).