Irrigation systems pipe networks are often branched and possible approach to increase their hydraulic capacity (while keeping capital investment at minimum) is to convert branched systems to looped. However, optimal design of looped hydraulic pipe networks belongs to the class of large combinatorial optimisation problems, which are difficult to handle using conventional operations research techniques. In this study a classical optimisation methodology was applied, linear programming (LP), and the genetic algorithm (GA) to network rehabilitation. The LP method was first used for changing some diameters on the existing pipe network so it could satisfy the increased demand conditions with an unchanged topology (network remains branched). Such a solution was compared with a solution obtained using the GA, with a good agreement of results. When the GA was used to design the looped version of the network rehabilitation, the full advantage of this approach was achieved. Economical comparison of branched and looped options shows that the looped option is for studied irrigation system about 17 % cheaper.