UZF: Unsaturated Zone Flow Package Pane

In the Unsaturated-Zone Flow (UZF) package, water is routed from the land surface, through the unsaturated zone to the water table. Evapotranspiration (ET) can be simulated in the UZF package and whatever is not removed by ET becomes recharge to the saturated zone. Infiltration excess, can be routed to streams (SFR) or lakes (LAK). The pane for this package is on the MODFLOW Packages and Programs dialog box under Flow Packages.

One of the data sets created for the UZF package is "UZF_Layer." To reduce memory usage, you should set this to zero in cells that will have no recharge or discharge in the UZF package.

Recharge and discharge location option (NUZTOP)

The recharge and discharge location indicates where recharge to the unsaturated zone originates and where discharge from the unsaturated zone is removed. If the second option (Specified layer) is selected, the UZF_Layer data set indicates the MODFLOW layer where recharge and discharge occur. Zero indicates that recharge and discharge will not be simulated. If Specified layer is not selected, non zero values of UZF_Layer data set indicates the MODFLOW layer where recharge and discharge will be simulated but do not indicate the layer in which these will occur..

Vertical hydraulic conductivity source (IUZFOPT)

The user chooses between using the same vertical hydraulic conductivity in the saturated and unsaturated zone or specifying them separately using IUZFOPT.

Number of trailing waves (NTRAIL2)

NTRAIL2 specifies the number of trailing waves used to define the water-content profile following a decrease in the infiltration rate. The number of trailing waves varies depending on the problem, but a range between 10 and 20 is usually adequate. More trailing waves may decrease mass-balance error and will increase computational requirements and memory usage.

Number of wave sets (NSETS2)

NSETS2 specifies the number of wave sets used to simulate multiple infiltration periods. The number of wave sets should be set to 20 for most problems involving time varying infiltration. The total number of waves allowed within an unsaturated zone cell is equal to NTRAIL2*NSETS2. An error will occur if the number of waves in a cell exceeds this value.

Route discharge to streams and lakes (IRUNFLG)

IRUNFLG specifies whether groundwater that discharges to land surface will be routed to stream segments or lakes or if groundwater discharge is removed from the model simulation and accounted for in the groundwater budget as a loss of water. Either the SFR2 or LAK package must be active for IRUNFLG to have an effect.

Simulate evapotranspiration (IETFLG)

IETFLG specifies whether or not evapotranspiration (ET) will be simulated

Print summary of UZF budget terms (IFTUNIT)

IFTUNIT is used to control whether a summary of the UZF budget terms will be printed or not. The summed output includes applied infiltration, runoff, actual infiltration, groundwater discharge to land surface, ET from the unsaturated zone, ET from groundwater, recharge, and change in unsaturated-zone storage.

The average height of undulations in the land surface altitude (SURFDEP)

SURFDEP is used to determine how much of the potential recharge is rejected if the head in a cell comes close to the land surface. See SUrFDEP in the UZF package documentation.

Specify residual water content (SPECIFYTHTR)

THTR originally was calculated internally by the UZF Package on the basis of the difference between the saturated water content (THTS) and the specific yield (SY) of the aquifer receiving recharge (Niswonger and others, 2006). However,the ability to specify THTR can be useful for some applications in which the maximum storage in the unsaturated zone (THTS-THTR)is different than the instantaneous drainage from the aquifer (SY). For these cases, specifying THTR based on external calculations provides greater flexibility for parameterizing the unsaturated zone. The option to specify THTR is activated using the key word SPECIFYTHTR.

Specify initial unsaturated water content (SPECIFYTHTI)

Originally, THTI was not specified for simulations that included both a steady-state and one or more transient stress periods. For this case, THTI was calculated internally by the code on the basis of the steady-state infiltration rate and the unsaturated-zone hydraulic properties (FINF, FKS, EPS, THTS, and THTR). However, in well-drained soils, the steady-state recharge rate corresponds to an initial water content that is too large for coarse sediments. Consequently, drainage from the unsaturated zone during the first transient stress period could result in an unrealistically large recharge rate. Additionally, the head dependency on recharge that does not occur during steady-state stress periods but that can occur during transient stress periods can cause a sudden increase in recharge during the transition between these stress periods. For these circumstances it is more realistic to specify the initial water content for the first transient stress period that follows a steady-state stress period. The option to specify THTI in simulations that include both steady-state and one or more transient stress periods is activated using the key word SPECIFYTHTI.

Calculate surface leakage (inverse of NOSURFLEAK)

UZF1 simulates surface leakage in the uppermost active cell if groundwater head is greater than the top of this cell. Surface leakage is a nonlinear boundary condition that can slow model convergence, and in some cases it is beneficial to inactive this boundary condition. Surface leakage is inactivated by the key word NOSURFLEAK that is specified on line 1a of the UZF1 input file.

ModelMuse has an option to determine how objects are used to define the infiltration rate. If Objects overwrite values of previous objects is selected and more than one object defines the infiltration rate for a cell, only the infiltration rate from the last object that assigns a value will be used. The rates from previous objects will be ignored. If Sum values of all objects is used, the infiltration rates from all the objects that assign a rate to a cell will be added together. The latter option is helpful for combining infiltration due to different processes. For example, it might be convenient to define infiltration due to rainfall with a polygon and infiltration from a river with a polyline and then have the two added together.