There are two files that are used to define the aquifer properties:

• General aquifer properties: aquifer.aqu

• Aquifer initialization: initial.aqu

The SWAT+ Editor assigns a name to every object, which is typically the abbreviation of the object type followed by the object number in QSWAT+. However, the name of the aquifer is not used by SWAT+, so the user may change it.

The value of dep_bot should be larger than the value of dep_wt.

The value of dep_wt should be smaller than the value of .

The ID of the aquifer is a primary key referenced by in .

The carbon content in the aquifer is used to compute a pesticide's Kd value, or distribution coefficient, which is a measure of how tightly a pesticide binds to soil particles. Kd values are important for understanding how pesticides move through the environment.

This variable is only used for calculating pesticide transport from the aquifer to the stream.

This is a fixed concentration used throughout the entire period of simulation. Accordingly, groundwater phosphorus loads only vary with groundwater flow.

Water may move from the aquifer into the overlying unsaturated zone. In periods when the material overlying the aquifer is dry, water in the capillary fringe that separates the saturated and unsaturated zones will evaporate and diffuse upward. As water is removed from the capillary fringe by evaporation, it is replaced by water from the underlying aquifer. Water may also be removed from the aquifer by deep-rooted plants that are able to uptake water directly from the aquifer. This process is significant in watersheds where the saturated zone is not very far below the surface or where deep-rooted plants are growing.

As revap approaches 0, movement of water from the aquifer to the root zone is restricted. As revap approaches 1, the rate of transfer from the aquifer to the root zone approaches the rate of potential evapotranspiration.

Nitrate nitrogen in the aquifer may be removed through uptake by bacteria present in the aquifer or through chemical conversion to other compounds in regions of the aquifer that are depleted of oxygen (reduced environment). The half-life is the time period required for the concentration of nitrate nitrogen to drop to half its original value. The reduction is a net reduction by all processes occurring in the aquifer.

Nitrate levels in the aquifer are modeled, allowing for variation in nitrate concentration of groundwater flowing to the receiving spatial object(s). Accordingly, groundwater nitrate loads vary with nitrate concentration and groundwater flow.

Groundwater flow is allowed only if the depth from the surface to the water table is equal to or smaller than flo_min.

The baseflow recession constant is a direct index of groundwater flow response to changes in recharge (Smedema and Rycroft, 1983). Values vary from 0.1-0.3 for land with a slow response to recharge to 0.9-1.0 for land with a rapid response. Although the baseflow recession constant may be calculated, the best estimates are obtained by analyzing measured streamflow during periods of no recharge in the watershed.

It is common to find the baseflow days reported for a stream gage or watershed. This is the number of days for base flow recession to decline through one log cycle. When baseflow days are known, the alpha factor can be calculated:

where $α_{gw}$ is the baseflow recession constant, and BFD is the number of baseflow days for the watershed.

Smedema, L.K. and D.W. Rycroft. 1983. Land drainage - planning and design of agricultural drainage systems. Cornell University Press, Ithaca, NY.

This parameter defines the fraction of root zone percolation reaching a deeper aquifer. If a deep aquifer is defined in aquifer.aqu and aquifer.con, its groundwater flow can be routed to any other spatial object using aquifer.con. If no deeper aquifer is defined, the water that recharges the deeper aquifer is assumed to be lost from the watershed.

The specific yield is defined as the ratio of the volume of water that drains by gravity to the total volume of rock. This variable is required to calculate groundwater height fluctuations.

The pointer to the pesticide initialization file is a foreign key referencing name in pest_water.ini.

The pointer to the salt initialization file is currently not active.

The pointer to the aquifer initialization file is a foreign key referencing name in initial.aqu.

The distance between the aquifer and the stream impacts the delivery of pesticides from the aquifer to the stream. The longer the distance, the smaller the pesticide load.

This variable is only used for calculating pesticide transport from the aquifer to the stream.

This parameter determines the daily groundwater flow amount needed for all channels linked to an aquifer to receive groundwater flow. If groundwater flow < flo_max, the channels with the smallest drainage areas will stop receiving groundwater flow. The amount of groundwater flow a channel receives depends on the channel length.

The name of the aquifer initialization record is a primary key referenced by aqu_init in aquifer.aqu.

The pointer to the organic-mineral initialization file is a foreign key referencing name in om_water.ini.

The pointer to the pathogen initialization file is currently not active.

The pointer to the heavy metal initialization file is currently not active.

Movement of water from the aquifer to the unsaturated zone is allowed only if the depth from the surface to the water table is equal to or smaller than revap_min.