Once you read the 1980 US Supreme Court decision below, you might understand why the farmers think they have an equitable water right held in trust by the Imperial Irrigation District for their benefit, not a mere right to service. While a right to service might be used to explain the condition, it lacks the condition imposed upon a trustee to look out and protect the held rights to the benefit of the ultimate beneficial owner the land of the District. This subtle difference prevents the District from selling or transferring the waters of the District to others.
The following is directly from the US Supreme Court decision in 1980:
It may be true, as the Court of Appeals said, that no individual farm in the District has a permanent right to any specific proportion of the water held in trust by the District. But there is no doubt that, prior to 1929, the District, in exercising its rights as trustee, delivered water to individual farmer beneficiaries without regard to the amount of land under single ownership. It has been doing so ever since. There is no suggestion, by the Court of Appeals or otherwise, that, as a matter of state law and absent the interposition of some federal duty, the District did not have the right and privilege to exercise and use its water right in this manner. Nor has it been suggested that the District, absent some duty or disability imposed by federal law, could have rightfully denied water to individual farmers owning more than 160 acres. Indeed, as a matter of state law, not only did the District's water right entitle it to deliver water to the farms in the District regardless of size, but also the right was equitably owned by the beneficiaries to whom the District was obligated to deliver water.