Data View, Branch: Supply
and Resources \ Transmission Links, Category: Linking Rules, Tabs: Supply
Preference, Demand Priority, Maximum Flow Volume, Maximum Flow Percent
of Demand Transmission links carry water from local and river supplies, as well as wastewater from demand sites and wastewater treatment plants, to demand sites, subject to losses and physical capacity, contractual and other constraints. A transmission link is also required to bring water to satisfy irrigation requirements in Catchments that have been indicated to have irrigation. Transmission links only transfer water in response to a demand. For example, if a transmission link connects a desalination plant (Other Supply) to a Demand Site, water will flow from the plant through the transmission link to the Demand Site only to satisfy demand, and only if the Demand Site did not get is supply requirement satisfied from other supplies.
Primarily, WEAP allocates water according to the demand priority associated with each demand site. The sites with the highest priorities (lowest numbers) get water first, followed by sites with lower priorities (higher numbers), as availability allows. This system is useful in times of shortage to ensure that the highest priority water uses (e.g., municipal or minimum instream flows) are satisfied. When there is plenty of water to satisfy everyone, demand priorities are unnecessary.
A secondary concern, in cases where a demand site is connected to more than one supply source, is determining the mix of supply from various sources. Perhaps a city prefers groundwater to surface water because of its quality, or a farmer prefers surface water to groundwater because of the pumping expense, yet they are connected to both sources to ensure reliability of supply. However, in many cases, you may not know the underlying reasons to explain a particular observed mix (e.g., 20% from groundwater, 80% from surface water), but you want to reproduce it.
WEAP includes linking rules to specify the mix of supply from multiple sources. These rules enable the analyst to match observed allocation patterns in the Current Accounts and model future changes in the scenarios.
Each demand site (and catchments with irrigation) with multiple sources can specify its preference for a source, due to economic, environmental, historical, legal or political reasons. In the above example, the agricultural site would have a preference of 1 for the river source, and 2 for the groundwater source. With no other constraints, the site would pull everything it could from the river, falling back on the aquifer only if there was a shortage of river water. See Demand Priority, Supply Preferences and Allocation Order for more information. If there is no preference for sources (or a demand site has only one source), set the preference to 1. If a demand site has two sources with the same preference, WEAP will NOT necessarily draw an equal amount from each sources . 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 (below). 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, 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?". In this case, the variable tab will be named "Demand Priority," and you would enter the transmission link's priority, relative to all other demands in the system.
You can restrict the supply from a source, to model contractual or physical capacity limitations, or merely to match observations. For example, an agricultural site has a fixed allotment of river water, beyond which it must pump from groundwater. In this case, the demand site supply preference would be 1 for the river and 2 for the aquifer, and the allotment would be entered for the river source under Maximum Flow: Volume.
The rate of restriction can be entered for any time scale. For example, physical capacities would be entered as cubic meters per second, while contractual limitations might be entered as million cubic meters per month or per year. If the time scale is year, then the demand site's monthly variation will be used to distribute the allotment monthly. If no constraint, leave blank. To turn off flow completely, set to 0.0 (because 0 means no constraint).
You can also restrict the monthly flow along a transmission link by a percentage of the demand site's total monthly supply requirement. For instance, you might only know that a demand site got 20% of its yearly flow from one source and 80% from another. In this case, set the supply preferences for the sources to 1 and 2, respectively, then set the Maximum Flow: % of Demand for the preference 1 source to be its observed share (either 20% or 80%), and leave the preference 2 source unlimited. In general, you would choose the source more likely to experience shortages as the preference 1 source, in which cases the preference 2 source would meet the shortfall. To turn off flow completely, set to 0.0 (because 0 means no constraint).
Another example for restricting flow as a percentage of demand would be for quality considerations. Perhaps one source is cheaper than another, but of inferior quality. You could estimate the maximum fraction of the poorer quality water you could use and still meet your water quality criteria. In this case, the cheaper source would have a higher preference than the more expensive one, and you would set its Maximum Flow: % of Demand accordingly.
In some cases, you might have restrictions both on Volume and % of Demand. For example, the volume constraint might represent a physical capacity, while the % of demand would model quality criteria (as mentioned above).
Entered on: Data View, Branch: Supply
and Resources \ Transmission Links, Category: Linking Rules, Tabs: Supply
Preference, Demand Priority, Maximum Flow Volume, Maximum Flow Percent
of Demand