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When the workers can irrigate the fields without water sources (river)?

They can't. An adjacent tile to the one you're irrigating must have river, lake, ocean, or irrigation -- and real irrigation, the automatic irrigation bonus of cities doesn't count, though you can irrigate a city tile to bring the water explicitly through.

mfans wrote:When the workers can irrigate the fields without water sources (river)?

That depends on what rules you play with. The classic (default) ruleset requires that a water source (oceanic terrain, an irrigation or a river) is present no matter how much tech you research. The alien ruleset requires you to know Water Flow or Manufacturing to be able to irrigate anything at all but no water source is required once Deep Pumping is discovered.

If you wish to play using the classic rules except that a water source no longer is required once Electricity is discovered you can create a new ruleset your self. First make a copy of the classic ruleset (called default in 2.4). See the section "How to create a new ruleset - step by step" in http://freeciv.wikia.com/wiki/Editing_rulesets for details on how you do it. Then add the following code to the file effects.ruleset in your copy.