**Syntax**

` >> COVARIANCE (FILE: file_name)`

**Parent Command**

` > OBSERVATION`

**Subcommand**

` -`

**Description**

This command reads the diagonal elements of the a priori covariance matrix C_{zz}. Usually the variances of the observations are specified separately for each data set using command `>>>> DEVIATION` (o), or they are provided as a separate column along with the data (see commands `>>>> COLUMN` and `>>>> DATA).` As an alternative, one can assign or overwrite variances using the second-level command `>> COVARIANCE,` followed by two columns holding index i and variance c . ii Since this command addresses elements of the assembled covariance matrix, the user must provide the index that corresponds to the position of the observation in vector z. This information is best retrieved from the iTOUGH2 output file after running one forward simulation. Note that the index changes whenever the number of observations, parameters, or calibration times is changed. The variances can also be read from a covariance file which must contain three columns, holding index i, index j, and the (co-)variance c ij (this is the same format as the one on the covariance file generated by command `>>> COVARIANCE).` Despite the fact that two indexes must be provided, only diagonal elements, i.e., c , will be accepted as input. ii

**Example**

` > OBSERVATION
>> : 30 EQUALLY space calibration TIMES in MINUTES between
3.0 90.0
>> plus : 1 steady-state TIME near
86400.0 seconds
>> PRESSURE
>>> ELEMENT: A1__1
>>>> transient and steady-state DATA are on FILE: pres.dat
>>>> standard DEVIATION: 2000.0 Pa
<<<<
<<<
>> change one element of COVARIANCE matrix to increase its weight
32 1.0E4
(provided that 2 parameters are estimated, the steady-state data point is
observation number 32)
<<`

**See Also**

` >>> COVARIANCE | >>>> COLUMN | >>>> DEVIATION | >>>> DATA
`