DRAIN PACKAGE The Drain Package is based on

DRAIN PACKAGE • The Drain Package is based on the assumption that a drain removes water from the aquifer at a rate proportional to the difference between the head in the aquifer and some fixed head or elevation, provided the head in the aquifer is above that elevation. • If the head in the aquifer is below the elevation, there is no flow to or from the drain. • In actual practice, wells near drains can actually remove water from the drain and even dry the drain up. • If wells are capable of disrupting drain flow, it is better to use the Stream-Aquifer Package to simulate drain behavior. • The Drain Package is quite useful in simulating small DRN 6 1 spring behavior.

DRAIN PACKAGE • The drain is assumed partially full so that the head within the drain is approximately equal to the median drain elevation, dijk • The head computed by the model for cell i, j, k is hijk. • The drain head, dijk prevails only with in the drain. • Flow to the drain generates a head loss which is assumed proportional to the discharge, QD DRN 6 2

DRAIN PACKAGE • • The discharge QD is given by the equation, CDijk(dijk – hijk) QDijk= 0 The CDijk is a function of, – Hydraulic conductivity near the drain, – Distribution of fill material and its hydraulic properties, – Amount of clogging material and its hydraulic properties, and drain slot openings. • The coefficient CDijk is a conductance measured by seepage runs and adjusted during the model calibration. DRN 6 3

DRAIN PACKAGE • Agricultural drains (figure top-left) will reverse flow if the head in the aquifer drops below the drain elevation. • If the head in the aquifer remains above or, at the minimum, equal to the drain elevation, the flows shown in the bottom-right figure prevails, and the previous equation is valid; and the Drain Package can be used to simulate DRN 6 agricultural drains. 4

DRAIN PACKAGE NPDRN—is the number of drain parameters. MXL—is the maximum number of drain cells that will be defined using parameters. MXACTR—is the maximum number of drain cells in use during any stress period, including those that are defined with parameters. DRN 6 5

DRAIN PACKAGE IDRNCB—is a flag and unit number. IDRNCB > 0, it is the unit number to which cell-by-cell flow terms will be written when SAVE BUDGET or a non-zero value for ICBCFL is specified in output control. IDRNCB = 0, cell-by-cell terms will not be written. IDRNCB < 0, drain leakage for each drain cell will be written to the List File when SAVE BUDGET or a non-zero value for ICBCFL is specified in output control. DRN 6 6

DRAIN PACKAGE [Option]—is an optional list of character values. AUXILIARY abc or AUX abc— defines an auxiliary variable named abc, which is read for each well as part of Items 4 and 6. Up to 5 variable can be specified, each preceded by AUXLIARY or AUX. CBCALLOCATE or CBC— indicates that memory should be allocated to store cell-bycell flows for each well in order to make these flows available for use in other packages. DRN 6 7

DRAIN PACKAGE PARNAM—is the name of a parameter. This name can consist of 1 to 10 characters, and is not case specific. PARTYP—is parameter type. There is only one parameter type for the DRN Package— the drain hydraulic conductance, DRN. Parval—is the parameter value. NLST—is the number of drain cells that are included in the parameter. DRN 6 8

DRAIN PACKAGE ITMP—is a flag and a counter. ITMP<0, non-parameter drain data from last stress period will be reused. ITMP≥ 0, ITMP will be the number of non-parameter drain cells read for current stress period. NP—is the number of parameters in use in current stress period. Layer—is the layer number of the cell that contains the drain. Row—is the row number of the cell that contains the drain. Column—is the column number of the cell that the contains the drain. DRN 6 9

DRAIN PACKAGE Elevation—is the elevation of the drain. Condfact—is the factor used to calculate the drain hydraulic conductance from the parameter value. The conductance is the product of Condfact and the parameter value. Cond—is the hydraulic conductance of the interface between the aquifer and the drain. DRN 6 10

DRAIN PACKAGE [xyz]—represent the value of any auxiliary variable for a river reach that has been defined in Item 2. The auxiliary variable must be present in each repetition of of Item 4 and 6 if they are defined in Item 2. Pname—is the name of the parameter being used in the current stress period. NP parameter names are DRN 6 read. 11

DRAIN PACKAGE. No Parameters DRN 6 12

DRAIN PACKAGE. Parameters DRN 6 13