In the Lam Sieo Yai basin, 83% of the total area is agricultural
land, of which 96% is covered with paddy fields (Fig.1). In the basin,
75% of paddy fields fall under the Sieo Yai Irrigation Project and
benefit from controlled water supply. The other 25% are rain-fed
paddy fields of individual farmers. Lowland rain-fed rice is grown
only during the wet season, while irrigated rice may be cultivated
during both seasons. However, approximately only half of irrigated
land is cropped during the dry season, due to uncertain water
supply. Rain-fed conditions refer to conditions of lowland rice that
is cropped under flooding conditions with no control of water
supply. Rainfall, soil moisture, and natural runoff alone (green
water) provide water to the paddy fields. Fig. 2 shows a simplified
sketch of water flows in a paddy field. In Fig. 2, the outflow
(drainage) is hardly happening because farmers let the water
evapo-transpirate and percolate well before the end of the cycle,
and usually do not have to pump water off the fields.