Abstract: Differences in microenvironments---defined by geology, geomorphology, and hydrology-led to major differences in the nature of the environmental problems created by economic development in different parts of the Erhai catchment in southwestern China during the premodern period. In particular we show that the second half of the eighteenth century was the critical period for the onset of rapid environmental degradation in the northern part of the catchment. From this it is established that premodern Chinese irrigated farming cannot be defined as indefinitely 'sustainable' without major qualifications.