Ancient South-East Asia

Cambodia is situated in southeast Asia on the coast of the Gulf of Thailand and shares borders with Vietnam to the east, Thailand to the west, and Laos to the north. Lake Tonle Sap occupies ~2.5% of Cambodia’s land area and plays a vital role in the rice agriculture of the country. The total cultivatable area is about 2.1 million hectares, of which 1.8 million is devoted to rice agriculture. The growing season is largely coupled to the monsoon cycles: the bimodal wet season starts in May and ends in October with peaks in June and September/ October resulting from diVerent rainfall origins. Rainfall levels vary around the country: although average levels are about 1.5 m, amounts vary from about 1.0m at Svay Check in the western province of Banteay Meanchey to nearly 4.7m in the southern province of Kampot. The Tonle Sap River reverses flow twice each year: from July to October, water flows into Tonle Sap Lake from branches of the Mekong River, swelling its area from 2,600 to 10,500 km²; in November when the flow rate of the Mekong River decreases, the Tonle Sap River reverses flow and water flows into the Mekong once again. Since 85% of Cambodia’s land area is included in the Mekong River basin, river water levels coupled to groundwater levels play a role in agricultural systems. The dry season from November to April requires irrigation to support rice agriculture making water storage and high groundwater levels important. Based on recent research (FAO 2005), the net renewable water balance (volume in flows minus volume) is equal to about 120km³ with about 18 km³ stored in groundwater reduced by 13 km³ per year by river drainage. Of the total amount of water withdrawal per year (520_10⁶m³), about 94% is devoted to agriculture; given the dependence on rice farming through the ages, it is likely that a similar percentage was used for agriculture in ancient times as now to support like-sized agrarian populations. In the 10th to 14th centuries ce, Angkor’s water supply system was based on four (baray) reservoirs (not all functioning simultaneously) with a total capacity of 100–150_10⁶m³.