Evapotranspiration and GroundwaterA Brief Overview

John WheatonMontana Bureau of Mines and GeologyPresented to:Water Policy Interim CommitteeHelena  March 11, 2010

Some People we are working with:

MBMG – Elizabeth MeredithGary Icopini

MSU – Clayton Marlow

Monitoring Well In Salt Cedar stand

Evaporation :  Process  of  water  going  from  a  liquid  to  gas  state

Transpiration :  Water  moving  through  plants  and  evaporating  

Both  Energy  and  Mass  are  removed  to  the  atmosphere

Evapotranspiration  affects  groundwater  by  intercepting  potential  rechargeand by removing  water  directly  from  aquifer

In Montana,  the  Potential Evapotranspiration  far  exceeds  Precipitation.

NATIONAL WEATHER SERVICE OFFICE OF HYDROLOGY

HYDROLOGIC RESEARCH LABORATORY

ANNUALFREE WATER SURFACE EVAPORATION

(SHALLOW LAKE)1956‐1970INCHES

Source: NOAA Technical Bulletin 33

35

40

40

45

35

35

35

30

30

30

Source: Western Regional Climate Center

http://www.wrcc.dri.edu/pcpn/mt.gif

10 ‐ 14

14 ‐ 18

Floating  Class A  pan

Measuring  Free – Water

Surface  Evaporation

Free  Water  Surface  Evaporation  ExampleMine  Pit  Impoundment  in  eastern  MontanaMeasured  with  floating  pan  Annual  Total :  3.19  ft

Free  Water  Surface  Evaporation  ExampleMine  Pit  Impoundment  in  eastern  MontanaSurface  area = 4.2  acresEvaporation  rates  shown  are  monthly  averages  :

12  hours  per  day  during  evaporating  months

Some  results  of  impact  to  surface  water   and  groundwater reservoirs

Stream  flow:  Rosebud  Creek  near  Kirby, MT

Specific  conductivity  of  groundwater

Missouri Breaks (Roy)Clayton Marlow (MSU)

Average Seasonal Groundwater Response to Prescribed Fire

-160.0

-140.0

-120.0

-100.0

-80.0

-60.0

-40.0

-20.0

0.0

2002 2003 2004 2005 2006

Date

Dep

th to

Gro

undw

ater

(cm

)

Up ControlLow ControlBU1BU2BU3BU4

Ashland Ranger DistrictSpring Inventory : 2002 & 2003435,000 acresPrecipitation : 16 inches per yearPotential Evaporation : 40 inches per yearLivestock : 20,500

Spring Discharge Count, Ashland Ranger District : 409

125  Or  31 %  Q = 0 gpm

Ashland  Ranger  District  Water  

Evapotranspiration Precipitation

Groundwater (in)Groundwater (out)

Surface  water (out)

Recharge

Livestock Streams and springs

ETPrecip

Livestock Streams and springs

ETPrecip

Conclusion:

We  need  to  recognize  the  role  of  transpiration in:

Stream  flow

Spring  flow

Aquifer  recharge

Aquifer  depletion