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<br />If the available head and/or the slope were less than that in the <br />previous example, either the flow rate to each tree would have to be <br />reduced, or pipe sizes would have to be increased. Halving the flow <br />rate decreases the head loss through the tapered lateral from 1.1 to <br />0.26 m (3.6 to 0.8 ft). Likewise, the head loss through the delivery <br />hoses is reduced from 0.59 to 0.2 m (1.9 to 0,64 ft). The minimum head <br />at the water source required for a level field would, therefore, be <br />about 0.5 m (1.5 ft). Heads of this magnitude are often available or <br />can be made available by enclosing a surface water supply in a pipeline <br />to a sufficiently high elevation. <br /> <br />A system with one hose to each tree could only be operated at this <br />low flow rate if the infiltrability of the soil were low enough to <br />permit some surface distribution of the water. Also, it may be neces- <br />sary to use a portable pump to provide sufficient pressure to flush <br />~he system. One further precaution that should be observed is that flow <br />velocity in the delivery hose should not fall below that corresponding <br />to a Reynolds number of 4000 (Karmeli and Keller, 1975) to avoid the <br />unstable flow regime. Avoiding this regime is desirable because within <br />it the friction coefficient can change appreciably because of minor <br />clogging and changes in wall roughness due to sedimentation. For 9.S-mill <br />(3/8-in) ID hose, the flow rate should exceed 0.028 ~/sec (0.44 gpm) to <br />avoid unstable flow.. <br /> <br />Cost for materials for the irrigation system will, of course, vary <br />Hith distance from manufacturing plants. The cost at Riverside, Cali- <br />fornia, for corrugated tubing, connectors, and drip hose was approxi- <br />mately $620/ha ($250/ac) for a system using 76-mm (3-in) ID corrugated <br />pipe and $740/ha ($300/ac) for 102-mm (4-in) ID pipe. Installation <br />costs on a commercial scale are difficult to estimate from my experi- <br />men~al systems. Cost of trenching and backfilling at Tacna, Arizona, <br />are estimated6 at $0.72/m ($0.22/ft). With row spacing of 6.1 m (20 <br />ft) and a la~eral between every two rows of trees, this would amount to <br />$590/ha ($240/ac). I estimate that installing the system by the pro- <br />cedure used at the Riverside site would require less than 10 man <br />days/ha (4 man days/ac). At a wage of $30/man day, this would add <br />$300/ha ($120/ac), for a total of $15l0/ha ($6l0/ac) for a 76 mm (3 in) <br />system and $1630/ha ($660/ac) for a 102 mm (4 in) system. <br /> <br />These costs are comparable with or lower than many complete drip <br />irrigation systems including pumps and filters. The advantage of the <br />longer life of a completely buried system, and the lower energy require- <br />ments may make this closed-conduit, gravity system an attractive <br />alternative, particularly for relatively level fields that can be con- <br />verted from surface irrigation methods. <br /> <br />6 <br />Personal communication, Soil Conservation Service, Well ton, <br />Arizona. <br /> <br />-11- <br /> <br />(; . I'I(FJl <br />.4v, <br />