WEAP uses three user-defined priority systems to determine allocations from supplies to demand sites and catchments (for irrigation), for instream flow requirements, and for filling reservoirs and generating hydropower: Demand Priority, Supply Preference, and Distribution Order.
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 Conservation Zone, filling Buffer Zone, or generating hydropower), or flow requirement, and can be changed by right clicking on it and selecting General Info. By default, priorities can range from 1 to 99, with 1 being the highest priority and 99 the lowest, although you can change the lowest priority in Basic Parameters. 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. If a demand site has no preference, set Supply Preference to 1 on all its transmission links. You may change the preferences over time or from one scenario to another. For example, a demand site might prefer to pump groundwater in the winter and withdraw water from the river in the summer. In this case, you would use the Monthly Time-Series Wizard to separately specify the preferences for these two transmission links (from groundwater and from the river) in each month. If a demand site has two supplies with the same preference, WEAP will NOT necessarily draw an equal amount from each supply. It could draw all from one, or all from the other, or some mix of the two. If you want to specify the mix from the two (or more) sources, e.g., 50% from each, you must use the "Maximum Flow: % of Demand" variable on the two (or more) transmission links. However, keep in mind that if one of the supplies cannot supply the required % of demand, the other supply WILL NOT supply more, and there will be unmet demand (unless you have other supplies at a lower preference).
Alternatively, for demand sites and catchments, you also have the option of setting the demand priority directly on the transmission link, instead of the demand priority on the demand site and the supply preference on the transmission link. This could be useful if a demand site represents several demands which have different priorities. To select this option, right click on the demand site or catchment, choose General Info, then check the box for "Priority on Transmission Link?". You may also change the default setting for new demand sites and catchments on the Basic Parameters screen.
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, reservoirs 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, supply preferences may be inconsistent with this rule. In those cases, the allocation order will be the same for all transmission links with the same demand priority and supply preference. (In this case for the previous example, the allocation order would be 1 both for DS1's link to the groundwater and for DS2's link to the river, and DS1's link to the river would be allocation order 2.)
You may switch among viewing demand priorities, supply preferences or allocation orders on the schematic: from the Main Menu, select Schematic, Change Priority View. Demand priorities are shown inside parentheses, e.g., (1); allocation orders are shown inside square brackets, e.g., ; and supply preferences are shown by themselves, e.g., 1. If a demand site or catchment has the demand priority on the transmission link, there will be no priority shown next to the demand site or catchment -- instead, the demand priority will be shown next to the transmission link. Also, when WEAP calculates, it create a file named Result_Priorities.csv that shows the demand priority, supply preference and allocation order for every demand site, catchment with irrigation, transmission link, flow requirement and reservoir.
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, or view the Result_Priorities.csv file created during calculations.
Additionally, if you want to be able to distribute differing amounts to branches within a demand site or catchment in case of shortage, you can set a distribution order for each branch. The distribution order does not affect how much water gets allocated to each demand site and catchment, only how the demand site or catchment distributes its allocation internally to its sub-branches. By default, this option is turned off; to turn it on, go to Basic Parameters. Then, set the distribution order for each demand branch. Enter data at any level of the Data Tree. A value entered at one level will be used by all branches at lower levels, unless a value has also been entered at the lower level. Leave all blank to allocate to all branches equally. If coverage < 100%, WEAP will allocate water first to branches with distribution order 1, then second to order = 2, third to order 3, etc. If multiple branches have the same order, they will get the same percentage of their demand met. For catchments, these allocations will be used to calculated the soil moisture balance. If the demand site as a whole has 100% coverage, then each branch will also have 100% coverage. If the "Distribution Order" option has been turned on in Basic Parameters, three new reports will be available in the Results View, detailing how the water has been distributed within the demand sites: Supply Delivered by Branch, Unmet Demand by Branch, and Coverage by Branch.