Is secretly irrigating a farm with water from a residential meter considered theft, given that the government expropriated the farm's water source for a pittance?

The interest of the community takes precedence over the interest of the individual. It is permissible to appropriate a water source for the public good, provided that compensation is given for the damage. One must first claim their right from the authorities. If that is not possible, then it is permissible for someone who is unable to obtain their right to take from their debtor's wealth an amount equivalent to their right without their permission. This is known as the issue of al-zhafar (taking one's right by force).