- Thank you for this elaborate. Will check the consistency of units for the parameters used in the model. However, would appreciate if you could elaborate your question/point number 5, 6 and 7 a bit more.

Sincerely,

Savera

"Richard B. Winston" <rbwinston@...> wrote:

The first thing to determine is the units of the budget terms. MODFLOW

uses consistent units so if your grid cells are measured in meters,

these numbers represent cubic meters. The values are for the entire

modeled area so if there was evapotranspiration outside the wetland that

would be included too. You could use ZONEBUDGET to get the

evapotranspiration in the wetland if evapotranspiration outside the

wetland is an issue. The evapotranspiration calculated by MODFLOW only

includes evapotranspiration from the saturated zone.

Evapotranspiration from the unsaturated zone might or might not be an

issue depending on whether or no an unsaturated zone is present. If an

unsaturated zone is present, the evapotranspiration calculated by

MODFLOW will only be part of the total evapotranspiration.

You can calculate the volume of evaporation by multiplying the pan

evaporation by the wetland area. In your case, 2.3 m X 3.8E6 m^2 =

8.7E6 m^2. This is about 4 times the evapotranspiration calculated by

MODFLOW. You will have to figure out why the numbers differ. Some

things you might want to consider include:

1: Is the pan evaporation for the same period as the model?

2. Is all the data in the model in consistent units?

3. Is evapotranspiration from the unsaturated zone an issue?

4. Are the stresses on the system represented correctly in the model?

5. Is evapotranspiration ever limited due to lack of water?

6. Does evapotranspiration from the water table occur outside the wetland?

7. Are there other processes that need to be include in the model?

SAVERINA KASHAIGILI wrote:

> Dear Groundwater group

>

> I have conducted a study to determine the component of water balance in a wetland using VISUAL MODFLOW.

>

> The table below show the cummulative mass balance for the last stress period. My problem is on interpretation for storage and evapotranspiration volumes. Note the extinction depth was set at 0.9 m. How is this evapotranspiration related to pan evaporation of 2300mm for an area of wetland of 3.8km2. Also what does storage term mean in actual sense?

>

>

> IN

> OUT

>

> Storage

> 840000

>

> 600000

>

> Constant head

> 17003

>

> 1001

>

> Drains

> -

>

> 7000

>

> River leakage

> 45000

>

> 1005

>

> Recharge

> 176000

>

> -

>

> Evapotranspiration

> -

>

> 2030000

>

>

>

>

>

> I will appreciate your assistance on the interpretation of such results from MODFLOW . Will also appreciate for a reference on the same.

>

> Looking forward for the assistance.

> Savera - One more comment to using pan evaporation data. Often the true open water

body evaporation is less then measured pan evaporation. There are a number

of reasons for this. In practical terms, if you have no other data, you can

probably assume pan evaporation factor of 0.7 (see Technical Note No 126,

"Comparison Between Pan and Lake Evaporation", by C.E. Hounam, World

Meteorological Organization, 1973), that is the true evaporation from the

lake surface will be 70% of pan evaporation value. This correction factor

gives you a standard error of 21% and a greatest expected error of 60%

according to the author.

The range of annual pan evaporation factors is roughly between 0.5 and 0.9

but on a monthly basis it can be as wide as form 0.13 to 2.04.

Re pt 5 - if there are cells in your model representing the wetland, that

went dry during simulation than they are excluded from further calculations

and consequently there will be evapotranspiration from these cells.

Cheers

Bron

-----Original Message-----

From: gwmodel@yahoogroups.com [mailto:gwmodel@yahoogroups.com] On Behalf Of

SAVERINA KASHAIGILI

Sent: Saturday, 1 April 2006 5:59 PM

To: gwmodel@yahoogroups.com

Subject: Re: [gwmodel] Re: cummulative mass balance interpretation

assistance

Thank you for this elaborate. Will check the consistency of units for the

parameters used in the model. However, would appreciate if you could

elaborate your question/point number 5, 6 and 7 a bit more.

Sincerely,

Savera

"Richard B. Winston" <rbwinston@...> wrote:

The first thing to determine is the units of the budget terms. MODFLOW

uses consistent units so if your grid cells are measured in meters,

these numbers represent cubic meters. The values are for the entire

modeled area so if there was evapotranspiration outside the wetland that

would be included too. You could use ZONEBUDGET to get the

evapotranspiration in the wetland if evapotranspiration outside the

wetland is an issue. The evapotranspiration calculated by MODFLOW only

includes evapotranspiration from the saturated zone.

Evapotranspiration from the unsaturated zone might or might not be an

issue depending on whether or no an unsaturated zone is present. If an

unsaturated zone is present, the evapotranspiration calculated by

MODFLOW will only be part of the total evapotranspiration.

You can calculate the volume of evaporation by multiplying the pan

evaporation by the wetland area. In your case, 2.3 m X 3.8E6 m^2 =

8.7E6 m^2. This is about 4 times the evapotranspiration calculated by

MODFLOW. You will have to figure out why the numbers differ. Some

things you might want to consider include:

1: Is the pan evaporation for the same period as the model?

2. Is all the data in the model in consistent units?

3. Is evapotranspiration from the unsaturated zone an issue?

4. Are the stresses on the system represented correctly in the model?

5. Is evapotranspiration ever limited due to lack of water?

6. Does evapotranspiration from the water table occur outside the wetland?

7. Are there other processes that need to be include in the model?

SAVERINA KASHAIGILI wrote:

> Dear Groundwater group

wetland using VISUAL MODFLOW.

>

> I have conducted a study to determine the component of water balance in a

>

period. My problem is on interpretation for storage and evapotranspiration

> The table below show the cummulative mass balance for the last stress

volumes. Note the extinction depth was set at 0.9 m. How is this

evapotranspiration related to pan evaporation of 2300mm for an area of

wetland of 3.8km2. Also what does storage term mean in actual sense?>

from MODFLOW . Will also appreciate for a reference on the same.

>

> IN

> OUT

>

> Storage

> 840000

>

> 600000

>

> Constant head

> 17003

>

> 1001

>

> Drains

> -

>

> 7000

>

> River leakage

> 45000

>

> 1005

>

> Recharge

> 176000

>

> -

>

> Evapotranspiration

> -

>

> 2030000

>

>

>

>

>

> I will appreciate your assistance on the interpretation of such results

>

> Looking forward for the assistance.

> Savera - 5. Pan evaporation is an estimate of potential evaporation. Actual

evaporation can be less if there is no water available to evaporate.

6. Trees in upland areas might have roots that reach the water table

and could transpire water from it. Daily cycles in groundwater levels

in upland might be caused by such a process.

7. Are there unknown sources or sinks of water? Failing to account for

all sources and sinks would throw off the model results.

SAVERINA KASHAIGILI wrote:

>Thank you for this elaborate. Will check the consistency of units for the parameters used in the model. However, would appreciate if you could elaborate your question/point number 5, 6 and 7 a bit more.

>

> Sincerely,

> Savera

>

>

>

>"Richard B. Winston" <rbwinston@...> wrote:

> The first thing to determine is the units of the budget terms. MODFLOW

>uses consistent units so if your grid cells are measured in meters,

>these numbers represent cubic meters. The values are for the entire

>modeled area so if there was evapotranspiration outside the wetland that

>would be included too. You could use ZONEBUDGET to get the

>evapotranspiration in the wetland if evapotranspiration outside the

>wetland is an issue. The evapotranspiration calculated by MODFLOW only

>includes evapotranspiration from the saturated zone.

>Evapotranspiration from the unsaturated zone might or might not be an

>issue depending on whether or no an unsaturated zone is present. If an

>unsaturated zone is present, the evapotranspiration calculated by

>MODFLOW will only be part of the total evapotranspiration.

>

>You can calculate the volume of evaporation by multiplying the pan

>evaporation by the wetland area. In your case, 2.3 m X 3.8E6 m^2 =

>8.7E6 m^2. This is about 4 times the evapotranspiration calculated by

>MODFLOW. You will have to figure out why the numbers differ. Some

>things you might want to consider include:

>

>1: Is the pan evaporation for the same period as the model?

>2. Is all the data in the model in consistent units?

>3. Is evapotranspiration from the unsaturated zone an issue?

>4. Are the stresses on the system represented correctly in the model?

>5. Is evapotranspiration ever limited due to lack of water?

>6. Does evapotranspiration from the water table occur outside the wetland?

>7. Are there other processes that need to be include in the model?

>

>SAVERINA KASHAIGILI wrote:

>

>

>

>> Dear Groundwater group

>>

>> I have conducted a study to determine the component of water balance in a wetland using VISUAL MODFLOW.

>>

>> The table below show the cummulative mass balance for the last stress period. My problem is on interpretation for storage and evapotranspiration volumes. Note the extinction depth was set at 0.9 m. How is this evapotranspiration related to pan evaporation of 2300mm for an area of wetland of 3.8km2. Also what does storage term mean in actual sense?

>>

>>

>> IN

>> OUT

>>

>> Storage

>> 840000

>>

>> 600000

>>

>> Constant head

>> 17003

>>

>> 1001

>>

>> Drains

>> -

>>

>> 7000

>>

>> River leakage

>> 45000

>>

>> 1005

>>

>> Recharge

>> 176000

>>

>> -

>>

>> Evapotranspiration

>> -

>>

>> 2030000

>>

>>

>>

>>

>>

>> I will appreciate your assistance on the interpretation of such results from MODFLOW . Will also appreciate for a reference on the same.

>>

>> Looking forward for the assistance.

>> Savera

>>

>>

>

>

>