Water management in irrigated rice - Rice Knowledge Bank ...

Table 5.1. Area, water use, and total production of flooded rice and of rice grown under AWD, in ahypothetical irrigation scheme of 10,000 ha with 134 10 6 m 3 of water available.Land use Area (ha) Water use (10 6 m 3 ) Production (tons)Scenario I (80% farmers, flooded rice; 20% farmers, no crop)Flooded 8,000 134 46,400AWD 0 0 0Fallow 2,000 0 0Sum 10,000 134 46,400Scenario II (65% of farmers, flooded rice; 35% farmers, AWD)Flooded 6,560 110 38,048AWD 3,440 24 14,852Fallow 0 0 0Sum 10,000 134 52,900Table 5.2. Yield, water productivity with respect to total water input (WP IR), and water input of floodedrice and rice grown under AWD (from Table 3.2, Guimba 1989).Yield (t ha −1 ) WP IR(kg m −3 ) Water use (m 3 ha −1 )Flooded 5.8 0.345 1,679AWD 4.3 0.614 700Table 5.3. Area under rice production, and total water use, rice production, and number of farmers,with flooded rice and with AWD as design criteria.Design criterionRice area Water use Production Farmers(ha) (10 6 m 3 ) (tons) (number)Flooded 10,000 168 58,000 10,000AWD 23,986 168 103,000 23,986but illustrates the effect of including the conceptof water productivity (besides yield) in irrigationsystem management.5.5 Irrigation system designAWD irrigation can be taken as a starting point inthe design of a new irrigation system. Suppose thata hypothetical reservoir can be constructed with acapacity of 168 10 6 m 3 , and that a command areawill be designed for rice farmers getting 1 ha each.If AWD irrigation is the design criterion ratherthan flooded rice, the irrigated rice area and totalrice production are larger, and more farmers willbenefit from irrigation development (Table 5.3).If reuse of percolation and drainage water is takeninto consideration in the design, the rice area underirrigation can increase further in both scenarios.With 100% reuse of water, the irrigated rice areaswould be similar in both scenarios since AWDmostly saves water by reducing percolation flows(that can be reused) and only a little by reducingevaporation (which cannot be reduced). However,100% water reuse is in practice not attainable andthe implementation of AWD would lead to thelargest area under irrigation.This example demonstrates the impact thatwater-saving technologies can have on irrigationsystem design. Rather than taking continuous floodingof fields as a design criterion, a rotation of waterdelivery can be used that is designed to let the fieldsdry out for a few days during each rotation cycle.43