## EXERCISE 10 EVAPOTRANSPIRATION

• Assigned 27 and 29 November
• Due 30 November and 3 December
• 40 possible points.
• WRITE THE QUESTION AT THE START OF EACH ANSWER.

### Evaporation Pans

Class A evaporation pans in the United States have the following dimensions: depth = 10 inches and diameter = 47.5 inches.

1. Calculate the cross-sectional area in square meters for a Class A evaporation pan.
2. Calculate the storage volume of the pan.
3. Calculate the depth of water in the pan, if the pan contains 10 US gallons of water. Remember that 1 gallon = 0.003785 m3 of water.
4. Calculate the mass of water in the pan, assuming a water density of 997.07 kg/m3.
5. After 24 hours in an open field the evaporation pan now has 9.25 gallons of water. There was no precipitation during this period. Calculate the average evaporation rate from the pan in mm/hr.
6. Let's calculate evaporation using the same pan but a different situation. Refill the pan to a volume of 10 gallons and again leave in the open field for 24 hrs. During this time precipitation fell for 3 hrs with an average intensity of 2.5 mm/hr. Ending volume in the pan was 11.5 gallons. What was the amount of evaporation? Show all your work, including formulas.
7. Calculate the flux of latent heat from the water in the pan to the atmosphere in the question where the ending volume in the pan was 9.25 gallons. Remember that the latent heat of evaporation is 2.45 x 10^6 J/kg and the density of water is 1000 kg/m3.

### More Evaporation Pan Problems

For the period April-November of an average year, we obtained the following information from a reservoir in Weld County, Colorado. Reservoir area is 400 hectares; Pan evaporation was 150 cm; and Average pan coefficient was 0.7.

1. Estimate water loss from the reservoir using the evaporation pan technique. Report your answer both as depth (mm) and total volume of water loss (m3).
2. Assuming municipal water use at 800 liters/person-day, how many person-days might have been supported by the evaporative losses? There are 1,000 L /m3.

### Energy Balance Approach to Calculating ET

A small catchment in Iowa (area = 300 ha) absorbs a mean net radiation of 330 W/m2 for the month of June. You will apply the energy balance approach to estimate evapotranspiration.
1. Write the energy balance equation for the catchment.
2. Assume conduction to the ground (G) and the change in energy stored (dQ/dt) are 0. Simplify your energy balance equation and then write the equation that would allow you to solve for latent heat flux.
3. Now, replace the sensible heat term (H) with the Bowen Ratio and rewrite the equation to solve for the latent heat.
4. Using a mean Bowen ratio of 0.20 for the month of June, calculate the mean latent heat flux and the mean ET rate. Use the density and latent heat of vaporization values from above.
5. Calculate the total ET from the catchment for the month of June as depth of water in mm.

### Blaney-Criddle Estimation of Evapotranspiration

The following data are derived from studies in alpine tundra in the Front Range in 1982

 July August September Air Temperature (oC) 6.47 6.96 6.20 Precipitation (mm) 43.0 57.5 56.5 Pan Evaporation (mm) 75 68 54 Evapotranspiration (mm) 38.9 49 44

(a) Estimate Et by the Blaney-Criddle procedure for each month:

Et = k*d*(0.142(1/oC)*T(oC) + 1.095)(T(oC) + 17.8(oC)) (cm/oC)

 July Aug Sept k (crop factor) 0.92 0.91 0.87 d (daylight hours) 0.101 0.094 0.083

(b) Compare these estimates of monthly Et with the field measurements of E and Et and account for any differences you define.