Water Balance Exercises

Introduction: Water balance in the root zone of a soil is of interest for at least 3 reasons. (1) Water is needed for plant growth. (2) Chemicals move through soil primarily in response to water movement. Chemicals movement below the root zone depends upon the extent of water movement below the root zone or the amount of drainage from the root zone. (3) Irrigation management practices affect the amount of water available for plant growth and the amount of drainage below the root zone. These exercises are designed to examine the influence of the soils water holding capacity, different weather distributions at different sites, differences in historical rainfall patterns at a site, and different irrigation practices upon the water stored in the root zone over time, the amount of drainage taking place, and the amount of irrigation water used.

1. Consider a soil with a water holding capacity of 200 mm and with 100 mm of available water on January 1. Select a rainfall and PET station.
1. What is the name of the rainfall and PET station selected?
2. Examine the daily rainfall pattern for this station for year 1.

i. What is the maximum rainfall amount occurring on any day during that year?

ii. What is the total amount of rainfall that occurred during year 1?

3. Examine the graph of available water

i. What is the maximum available water on any day during the year? Is this value reached repeatedly? During what months of the year?

ii. What is the minimum amount of available water at any day during the year? Does it reach zero on any days? Why might it be zero?

4. Examine the daily drainage amounts for this soil.

i. What is the maximum drainage on any day during the year?

ii. What is the total amount of drainage during the year?

iii. Does drainage occur on as many days as rainfall? Why or why not?

iv. Is there a relationship between the days when the available water is at its maximum and the days at which drainage occurs? What is the reason for this relationship?

2. Consider now a soil with a water holding capacity of 150 mm.
1. Examine the graph of available water

i. What is the maximum available water on any day during the year? Is this value reached repeatedly? During what months of the year? Is this maximum reached more frequently than the maximum was reached in problem 1?

ii. What is the minimum amount of available water at any day during the year? Does it reach zero on any days? Does it become zero more frequently than it did in problem 1?

2. Examine the daily drainage amounts for this soil.

i. What is the maximum drainage on any day during the year? How does this compare with that in question 1?

ii. What is the total amount of drainage during the year? How does this compare with that in question 1?

3. Explain any differences between results for the soil with a water holding capacity of 200 mm and those for the soil with 150 mm.
2. Repeat exercise 2 for a soil with water holding capacity of 250 mm.
3. Complete the following table. Draw graphs of total drainage vs Rainfall for each water holding capacity. Discuss the results. Does an increase in rainfall always correspond to an increase in drainage? Why?

Rainfall Station:__________________ PET Station __________________

Irrigation: None

Year	Total Rainfall (mm)	Total Drainage (mm)
		Water Holding Capacity = 100 mm	Water Holding Capacity = 200 mm	Water Holding Capacity = 300 mm
1
2
3
4
5
6
7
8
9

5. Complete the following table for the same rainfall and PET stations as used in problem 4. Draw graphs of Total Drainage vs Rainfall+Irrigation for each water holding capacity. Compare these results with those in question 4.

Rainfall Station:__________________ PET Station __________________

Irrigation: Demand with 100% efficiency

Year	Total Rainfall + Irrigation (mm)	Total Drainage (mm)
		Water Holding Capacity = 100 mm	Water Holding Capacity = 200 mm	Water Holding Capacity = 300 mm
1
2
3
4
5
6
7
8
9

6. Compare drainage amounts for the different types of irrigation. Which type produces the largest amount of drainage? Which produces the smallest?


7. Fallow is sometimes used to build up water in the soil for planting the following year. During fallow, no crop is grown and weeds are controlled to reduce water use. Suppose the growing season for the crop is from May to September. Assume that the fallow practices reduce evapotranspiration equivalent to having a crop coefficient of 0.6 from October 1 to the following Sept 30. Furthermore assume that the soil has only 10 mm of available water on October 1 when fallow begins. Complete the following table for a soil with a water holding capacity of 250 mm. Does the fallow result in increased water stored at the end of the year?

Year	Available water (mm) on September 30 following fallow
1
2
3
4
5
6
7
8
9

8. We will now examine the impact of the available water at the end of a fallow period upon the available water in the next growing season. Reset the crop coefficients to the values entered when the program is started. These values approximate the water use by a crop in the northern hemisphere. Ideally, the available water in the soil will not be zero during the growing season since available water of zero will lead to moisture stress in the plants. Set the water holding capacity to 250 mm. Do the following for each of the 9 years for which weather is available.
1. Set the available water to be 250 mm on October 1
2. Examine the available water graph looking for days of zero or near zero available water during the months of May to September. Note the approximate dates when the water is depleted.
3. Set the available water to zero on October 1.
4. Examine the available graph again. Has the number of days within the growing season for which the available water is zero increased? What does this imply about the value of having a fallow period for this soil-weather system?