Two user-defined priority systems are used to determine allocations from supplies to demand sites and catchments (for irrigation), for instream flow requirements, and for filling reservoirs and generating hydropower.
Competing demand sites and catchments, reservoir filling and hydropower generation, and flow requirements are allocated water according to their demand priorities. The demand priority is attached to the demand site, catchment, reservoir (priority for filling or hydropower), or flow requirement, and can be changed by right clicking on it and selecting General Info. Priorities can range from 1 to 99, with 1 being the highest priority and 99 the lowest. Reservoir filling priorities default to 99, meaning that they will fill only if water remains after satisfying all other higher priority demands. (Hydropower priorities for individual reservoirs are set in the Data View under Reservoir, Hydropower. To set a system hydropower priority, go to the Supply and Resources branch in the Data View.) Many demand sites can share the same priority. These priorities are useful in representing a system of water rights, and are also important during a water shortage, in which case higher priorities are satisfied as fully as possible before lower priorities are considered. If priorities are the same, shortages will be equally shared. Typically, you would assign the highest priorities (lowest priority number) to critical demands that must be satisfied during a shortfall, such as a municipal water supply. You may change the priorities over time or from one scenario to another.
If a demand site or catchment is connected to more than one supply source, you may rank its choices for supply with supply preferences. The supply preferences are attached to transmission links, and can be changed by right clicking on a link in the Schematic View and selecting General Info, or Edit Data, Supply Preference.
Using the demand priorities and supply preferences, WEAP determines the allocation order to follow when allocating the water. The allocation order represents the actual calculation order used by WEAP for allocating water. All transmission links and instream flow requirements with the same allocation order are handled at the same time. For example, flows through transmission links with allocation order 1 are computed, while temporarily holding the flows in other transmission links (with higher allocation order numbers) at zero flow. Then, after order 1 flows have been determined, compute flows in links with allocation order 2, while temporarily setting to zero flows in links ordered 3 and higher.
In general, if a source is connected to many demand sites with the same demand priority, WEAP attempts to allocate these flows simultaneously, regardless of the supply preferences on the links. For example, demand site DS1 is connected to both a river and a groundwater source, with preference for the groundwater, while demand site DS2 is only connected to the river. Both demand sites have the same demand priority. The allocation orders would be 1 for DS1's link to the groundwater, and 2 for both demand sites' links to the river. In calculations, first DS1 is allocated water from groundwater and then both DS1 and DS2 are allocated water from the river. In this way, both demand sites have an equal chance to receive water from the river in the case of a water shortage. Note: in some unusual configurations, the supply preferences may be inconsistent with this rule. In those cases, a supply preference of 1 is used for all demand sites.
You may switch among viewing demand priorities, supply preferences or allocation orders on the schematic: from the Main Menu, select Schematic, Change Priority View.
Tip: If WEAP is not allocating water as you would expect, change the priority view on the Schematic to "Allocation Order" to make sure that it is allocating in the order you intend.