Table 5.1. Area, water use, and total production of flooded <strong>rice</strong> and of <strong>rice</strong> grown under AWD, <strong>in</strong> ahypothetical irrigation scheme of 10,000 ha with 134 10 6 m 3 of water available.Land use Area (ha) <strong>Water</strong> use (10 6 m 3 ) Production (tons)Scenario I (80% farmers, flooded <strong>rice</strong>; 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 <strong>rice</strong>; 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 <strong>in</strong>put (WP IR), and water <strong>in</strong>put of flooded<strong>rice</strong> and <strong>rice</strong> grown under AWD (from Table 3.2, Guimba 1989).Yield (t ha −1 ) WP IR(kg m −3 ) <strong>Water</strong> use (m 3 ha −1 )Flooded 5.8 0.345 1,679AWD 4.3 0.614 700Table 5.3. Area under <strong>rice</strong> production, and total water use, <strong>rice</strong> production, and number of farmers,with flooded <strong>rice</strong> and with AWD as design criteria.Design criterion<strong>Rice</strong> area <strong>Water</strong> 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 <strong>in</strong>clud<strong>in</strong>g the conceptof water productivity (besides yield) <strong>in</strong> irrigationsystem <strong>management</strong>.5.5 Irrigation system designAWD irrigation can be taken as a start<strong>in</strong>g po<strong>in</strong>t <strong>in</strong>the 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 <strong>rice</strong> farmers gett<strong>in</strong>g 1 ha each.If AWD irrigation is the design criterion ratherthan flooded <strong>rice</strong>, the <strong>irrigated</strong> <strong>rice</strong> area and total<strong>rice</strong> production are larger, and more farmers willbenefit from irrigation development (Table 5.3).If reuse of percolation and dra<strong>in</strong>age water is taken<strong>in</strong>to consideration <strong>in</strong> the design, the <strong>rice</strong> area underirrigation can <strong>in</strong>crease further <strong>in</strong> both scenarios.With 100% reuse of water, the <strong>irrigated</strong> <strong>rice</strong> areaswould be similar <strong>in</strong> both scenarios s<strong>in</strong>ce AWDmostly saves water by reduc<strong>in</strong>g percolation flows(that can be reused) and only a little by reduc<strong>in</strong>gevaporation (which cannot be reduced). However,100% water reuse is <strong>in</strong> practice not atta<strong>in</strong>able andthe implementation of AWD would lead to thelargest area under irrigation.This example demonstrates the impact thatwater-sav<strong>in</strong>g technologies can have on irrigationsystem design. Rather than tak<strong>in</strong>g cont<strong>in</strong>uous flood<strong>in</strong>gof fields as a design criterion, a rotation of waterdelivery can be used that is designed to let the fieldsdry out for a few days dur<strong>in</strong>g each rotation cycle.43
Field water requirements dur<strong>in</strong>g the flooded daysshould be based on evapotranspiration, seepage, andpercolation, whereas, dur<strong>in</strong>g the nonflooded days,they can be based on evapotranspiration alone.A system can also be designed to maximizethe reuse of seepage and percolation water as muchas possible through check dams and pump<strong>in</strong>g. Ifthere is a sal<strong>in</strong>ization hazard, care should be takento avoid us<strong>in</strong>g water that has become too sal<strong>in</strong>e byreuse through judicious mix<strong>in</strong>g of dra<strong>in</strong>age andfresh water.Construction of field channels (irrigation,dra<strong>in</strong>age) and land level<strong>in</strong>g should be considered tofacilitate good water <strong>management</strong>. Farmers <strong>in</strong> a newirrigation system should be tra<strong>in</strong>ed <strong>in</strong> sound water<strong>management</strong> practices such as those detailed <strong>in</strong> thismanual (bund ma<strong>in</strong>tenance, crack plow<strong>in</strong>g, etc.).44