PRMS Groundwater Module

Steve Markstrom: This is Steve Markstrom again.

I’m talking about the gwflow module that
simulates the groundwater process in PRMS.

Groundwater in PRMS is calculation of the
baseflow component of the streamflow hydrograph.

That’s what it does.

If we think about baseflow as being an exponential
decay, essentially that’s the way it’s

modeled in PRMS.

Think about it in terms of this equation here,
characteristic depletion equation.

If you plot a hydrograph on a semi-log piece
of paper and do a separation, what we’re

interested in, OK if you look at this equation,
t is the time step, that’s the value on

the x axis.

Q zero is the initial amount of storage, then
this Kr is essentially the groundwater flow

coefficient.

And if you do this hydrograph separation on
semi-log paper, here you can see the groundwater

flow separation.

In this case you can see the slope of this
line is this value Kr.

To get it in the form that we use in PRMS,
if you integrate it to get it into an expression

in terms of storage, it comes out looking
like this.

For every HRU, we have a groundwater tank
and every time step that tank has some amount

of storage and the flow out of that tank,
which is essentially the amount that that

HRU is contributing to baseflow to the streams
that it’s connected to, is described by

this right here.

We can compute q, the baseflow, at any given
time using this groundwater flow coefficient.

You’ve got the q at this time step, subtract
that q from the storage at this time step

and you get the storage for the next time
step.

This is the same system, one time step later.

So, basically what you have to do is, the
parameter for the gwflow module is this gwflow_coef,

and really what it is it’s this value from
this slope in this plot.

So, of course, it’s not quite that easy.

If you look at this tank, there’s going
to be something coming into that tank, right?

It gets filled up and if you listen to the
soilzone presentation, you’ll know there’s

going to be some inflow from the soilzone.

And in fact this is the equation from the
PRMS users’ manual.

The soilzone is contributing to groundwater.

The subsurface reservoir is cascading groundwater
from upslope.

And also if there is any storage from depressions.

All of those things could contribute water
into this tank.

Likewise, we’ve got a feature called the
groundwater res sink.

This equation is essentially the same as the
baseflow, but there’s a parameter gwsink_coef.

You set this in the parameter file and some
portion of the storage can come out as the

sink.

And this represents gage underflow or groundwater
moving across the basin boundary that is not

accounted for any other way in the model.

So that’s the groundwater sink.

And that’s it.