# 11.7 Problem Set

1. A young corn plant in a growth chamber produced 31 g of above-ground biomass with a cumulative transpiration of 4500 g of water.
1. What was its water use efficiency (WUE) in g kg-1?
2. For the same growing conditions, would you expect higher or lower WUE for a native C3 plant? Explain why in one sentence.
3. If the vapor pressure deficit during the growth period were increased, would the WUE be higher or lower? Explain why in one sentence.
1. Imagine you are responsible for scheduling irrigation for a corn crop. Your management goal it to keep the corn well-watered so that the evapotranspiration from the field is equal to the reference (or potential) evapotranspiration rate. Create a spreadsheet to perform a simple daily soil water balance for the field to determine the number of irrigation events and total irrigation amount required in the 120-d growing season assuming:
• The reference evapotranspiration rate starts at 2.0 mm d-1 on day 1 of the season and increases by 0.1 mm d-1 each day until day 60, after which it decreases by 0.1 mm d-1 each day.
• Rainfall of 12 mm occurs early in the morning on day 7, day 14, and every 7th day after that for the duration of the season, and all the rain infiltrates into the soil.
• The root zone is 1-m deep silt loam with a “field capacity” of 0.23 cm3 cm-3 and permanent wilting point of 0.09 cm3 cm-3 and is at field capacity on day 0.
• Each irrigation event occurs on the last possible day to maintain FAW > 0.5. The water added in each irrigation event is the amount required to restore the profile to field capacity based on the soil water storage at the end of the previous day.
1. Repeat problem 2, but this time for a loamy sand with a “field capacity” of 0.09 cm3 cm-3 and permanent wilting point of 0.02 cm3 cm-3. Report the number of irrigation events and total irrigation amount required.