The user can choose whether all the branches within a demand site will have the same monthly variation in demand, or whether each branch can have a different monthly variation.
Separately, the user can choose whether all land use branches within a catchment will have the same climate data (Soil Moisture Method Climate, Simplified Coefficient Method Climate or MABIA Climate), or whether each branch can have different climate data. This second option might be necessary if there is a large variation in the elevation among different land uses within a catchment. Alternatively, the catchment could be divided into several different catchment nodes according to elevation, so that the climate within each catchment did not vary by land use. If you use Automatic Catchment Delineation with elevation bands and loaded climate data, WEAP will automatically set this to "Each branch within a catchment can have different climate data."
Before December 2010, WEAP incorrectly used the Latent Heat of Vaporization (2260 kJ/kg, at 100 C) instead of the Latent Heat of Fusion (334 kJ/kg) for calculating snow melt in the soil moisture method. Although we recommend that you use Latent Heat of Fusion, if you have a previously calibrated model that used Latent Heat of Vaporization, you might want to use that setting instead. NOTE: Only models with a timestep smaller than monthly use latent heat in the snow melt model. Monthly models do not use it.
If your model includes any catchments that use the MABIA catchment method, you can choose whether to use one or two vertically stratified "buckets" (compartments) to compute the water balance. The top bucket is defined by the rooting zone, and includes the surface layer (the layer that is subject to drying by evaporation). The bottom bucket, if the two bucket method is used, is the remainder of the soil below the rooting depth down to the Total Soil Thickness. The size of each bucket changes with the rooting depth, but the sum remains constant (Total Soil Thickness). Infiltration takes place at the top bucket only, groundwater recharge from the bottom bucket only. Flow from bucket one to bucket two, or from bucket two to groundwater, only occurs if the bucket's field capacity is exceeded.
We strongly recommend using the two bucket method because it will give more realistic results. (The one bucket method is included for backward compatibility with datasets that were created in older versions of WEAP, before the two bucket method was added.)
If you have linked your WEAP model to a MODFLOW model, you can choose whether all demand site subbranches will pump from the same MODFLOW layer or set of layers (see the Pump Layer variable under Soil Moisture Method Irrigation or Simplified Coefficient Method Irrigation), or whether each subbranch can pump from a different layer or set of layers. In addition, if you are using injections wells for artificial recharge of demand site return flow, this setting will determine if all of a demand site's branches will inject into the same layer(s), or if each branch can inject into different layers.
In addition, you can choose whether all land use branches within a catchment will pump from the same MODFLOW layer or set of layers (see the Pump Layer variable under Soil Moisture Method Irrigation or Simplified Coefficient Method Irrigation), or whether each land use branch can pump from a different layer or set of layers.
In cases where some of the cells being pumped become dry or inactive, this setting determines whether WEAP will pump more from the other cells (Reapportion), or reduce pumping proportionally (Do not reapportion) . For example, if a demand site is pumping equally from 10 cells (10% of demand from each), and 2 cells become dry, WEAP will either pump 12.5% from each of the other 8 cells (Reapportion the 20% from the 2 dry cells to the other 8 cells) or pump 10% of demand from each of the other 8 cells, which would mean that at most 80% of the demand could be satisfied. Determining the correct setting for this option depends on the characteristics of your system, for example, if individual pumps have the ability to increase their pumping or not to pick up the slack for pumps that cannot pump due to dry cells. Also, if you are using MODFLOW-NWT and the UPW (Upstream Weighting) package, MODFLOW will reduce the amount that can be pumped from a cell as the cell's head approaches the bottom of the cell. If your setting is "Do not reapportion," WEAP will reduce groundwater pumping by demand sites and catchments from these drying cells accordingly, resulting in unmet demand. See the Pumping Reduction by Demand Site and Pumping Reduction by Groundwater Node reports to see how much pumping has been reduced due to dry or drying cells.
Note: if all of the cells that a demand site or branch (if subbranches are linked to cells) pumps from are dry, then WEAP is not able to reapportion to other cells and there will be unmet demand.
By default, demand priorities in WEAP range from 1 (Highest) to 99 (Lowest), but you can change the Lowest Allowed Demand Priority as high as 999,999,999. Although even the most complex model should not have more than a few hundred distinct priorities, it may be convenient to use larger numbers to represent real-world information, such as the date that a given water right was assigned for a seniority-based water rights system (in YYYYMMDD format). This will work as long as smaller numerical values correspond to higher priorities.
By default, a reservoir's filling priority is the lowest priority; if you change the lowest allowed demand priority, WEAP will automatically change any reservoir that uses the lowest priority to the new lowest value. In expressions, you can use "LowestPriority" to refer to the lowest allowed demand priority, so that it will automatically change if you subsequently change the value for the lowest allowed demand priority. For example, here's an expression that will change a reservoir's filling priority to the lowest priority if its storage is more than 95% of the Top of Conservation, and 5 otherwise: If( PrevTSValue(Storage) > (0.95 * Top of Conservation), LowestPriority, 5)
For demand sites and catchments, you can choose between two options: setting the demand priority on the demand site and the supply preference on the transmission link or setting the demand priority on the transmission link. (See Demand Priority, Supply Preferences and Allocation Order for more information about these two options.) You can make this choice independently for each demand site and catchment on their General Info screen, but on the Basic Parameters screen you can choose the default for all new demand sites and catchments. Tip: If you change the default setting for "Priority on Transmission Link," WEAP will give you the option (when you exit the Basic Parameters screen) to change the setting for all existing demand sites and catchments.
If you want to be able to distribute differing amounts of water to branches within a demand site or catchment in case of shortage, you can set a Distribution Order for each branch. (See Demand Priority, Supply Preferences and Allocation Order for more information.)
WEAP can store results using either single precision or double precision floating point numbers. Single precision uses half the space as double precision with little loss of meaningful precision (7-8 significant digits vs 15-16 significant digits), and is therefore the recommended option. During calculations, all results are stored using double precision to prevent round-off errors. If the single precision option is chosen, after calculations are done all results are converted from double to single precision in a quick post processing step for each scenario calculated.
Menu Option: General: Basic Parameters