037-1 Wyo. Code R. § 1-7
a. A reservoir is entitled to be filled in order of priority once each year if water is available. The water year is defined as from October 1 through September 30 of the following year. If water remains unused in the reservoir at the end of the water year, this water is termed "carry-over storage" and is counted toward providing the water to meet the following year's supply for the appropriation. For example, if a reservoir which has a right to store one thousand acre-feet has two hundred acre-feet remaining in storage which is carried over into the following year, then this appropriation would be entitled to again accrue only the remaining appropriated capacity (800 a.f.) as the storage right comes into priority on the permitted source of supply.
b. In order to conserve the waters of the state, it is a necessary requirement that all reservoirs be filled at times that will not interfere with or that will provide the least interference with the use of water by direct flow appropriators and thereby prevent a waste of water. W.S. 41-3-603 provides authority to the hydrographer commissioner or water commissioner to control reservoir filling and reads in part as follows:
Such water commissioner shall have authority to require the filling of any reservoir whenever practical and water is available for storage from the stream or streams for which the appropriations for such reservoir are established.
All water which is allowed to flow past the reservoir diversion point after receipt of a notice or order to store from the hydrographer commissioner or water commissioner will be chargeable to the storage in the reservoir and the right to store may be reduced by that amount.
037-1 Wyo. Code R. § 1-7