Under inundated paddy field conditions, it proves impossible to
measure directly real consumptions at field level; also, losses
incurred at both plot and conveyance system levels are not
measurable. As a consequence, water use was first modelled with
CropWat (FAO, 1992), based on the concepts of climatic demand
and crop evapotranspiration. Since the whole of Lam Sieo Yai basin
is under the same climatic station, hence the same available data,
crop water requirements (CWR ¼ blue water þ green water) and
irrigationwater requirements (IWR ¼ bluewater) calculated inmm
(¼0.001 cubic meter per square meter) are the same between
systems in any given season. There are only differences between
dry and wet season. Second, we had to further analyse water use
with ad-hocwater balance to take account of specific traits of paddy
fields (i.e. quasi-permanent ponding conditions, deep and lateral
infiltrations, direct evaporation). Also, losses incurred at system
level were modelled. Finally, marginal additional uses such as
mixing and dilution of agrochemicals were included as blue water
components. Results on IWR were ultimately cross-checked with
observations on water pumping practices and specs, and water
levels, and proved consistent