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

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  • SAVERINA KASHAIGILI
    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
    Message 1 of 5 , Mar 31, 2006
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      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
    • Bron Smolski
      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
      Message 2 of 5 , Apr 2, 2006
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        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
        >
        > 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
      • Richard B. Winston
        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
        Message 3 of 5 , Apr 3, 2006
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          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
          >>
          >>
          >
          >
          >
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