1. CE 3354 Engineering Hydrology
Lecture 11: Watershed Loss Processes Evapotranspiration

2. Outline
Loss Processes
Evapotranspiration

3. Hydrologic Cycle Precipitation (Input) Loss Runoff (Output) Module 2
Subsurface storm sewer network if appropriate and available.
Runoff (Output)
Module 2

4. Loss Processes – Evapotranspiration
Process Concepts
Useful Models:
Blaney-Criddle
Thornwaithe

5. Loss Processes – Evapotranspiration
Process Concepts
Heat Energy
Vapor transport
Relative humidity
Wind speed

6. Loss Processes – Evaporation
Process Concepts
Thermodynamics
Fluid Mechanics
Energy Method
Aerodynamic Method
Combined Method
Data Requirements
Extensive
CMM pp 80-91

7. Loss Processes – Evaporation
Measurement
Evaporation Pans
Used worldwide
Flux Instruments
Eddy Covariance Instruments

8. Evaporation Pans
Used in conjunction with lysimeter instruments to calibrate to crop type.
Then make measurements with a pan or EC instrument
The global standard for measuring evaporation is the evaporation pan.
A glorified stock tank that thousands of cooperative observers (volunteers!) have operated for about 150 years.
Most of the Pan section is lifted directly from the UN Training manual for irrigation water management (Brouwer and Heibloem, 1986.)
Evaporation pans provide a measurement of the combined effect of temperature, humidity, windspeed and sunshine on the reference crop evapotranspiration ET0.
Figure is a schematic of the relationship of the pan to a crop.
Generally the idea is that pan measurements are correlated to a particular crop - usually in a lysimeter setting.
Once the pan is 'calibrated' it can then be used to manage crop watering.

9. Evaporation Pans Class A - Circular. Colorado Sunken
Dug into ground, rectangular
Evaporation pans have other uses and their data is crucial to understanding the evaporative component of any regional water budget.
Many different types of evaporation pans are being used.
The best known pans are the Class A evaporation pan (circular pan), and the Sunken Colorado pan (square pan)

10. Evaporation Pan Operation (1 of 2)
The pan is installed in the field
The pan is filled with a known quantity of water
The water is allowed to evaporate during a certain period of time (usually 24 hours).
The rainfall, if any, is measured simultaneously
Every 24 hours, the remaining quantity of water (i.e. water depth) is measured

11. Evaporation Pan Operation (2 of 2)
The amount of evaporation per time unit (the dffierence between the two measured water depths) is calculated; this is the pan evaporation: Epan (in mm=24 hours)
The Epan is multiplied by a pan coecient, Kpan, to obtain the ETo.
Reset the pan for next time interval to desired level
Don’t forget to dress well for the measurement. You are a scientist/engineer. STEM == TIES

12. Pan Constants
Need to be determined by lysimeter or Eddy Covariance instruments

13. Evapotranspiration – Models
Models are used to estimate ET for practical cases where measurements are not available
Blaney-Criddle
Turk
Thornwaithe
All similar in that they are correlations to averaged measurements at different locations
All are just approximations, but are used in practice and when ET matters they may be only tool available

14. Blaney-Criddle Model
Simple formula – Temperature and latitude driven only!
Estimates daily rate for a particular month
Temperature is an average from daily values for a month

15. Blaney-Criddle Model
P- value by latitude and month

16. Next Time
Loss Processes
Evapotranspiration