Balancing the Water Budget Handout
Water cycles through the environment. According to the hydrologic cycle, energy from the sun evaporates water from the surface
of the earth. As the atmosphere becomes saturated with water vapor, it cools, causing the vapor to condense. The water then returns to the
earth’s surface via some form of precipitation. On a global scale, the earth’s water budget is balanced. Water evaporated from the earth is
returned to its surface through precipitation. On a local level, however, the water budget may be completely unbalanced, depending upon
precipitation levels, evaporation rates, soil types, vegetation, and additional factors. In this lesson you will examine the factors that
influence the water balance for a specific location. Using your water budget calculations, you will then create and interpret graphical
representations for the water situation in each area.
Part I: Calculating water Budgets
In order to calculate the water budget, you need the monthly precipitation and potential evapotranspiration values for a given location.
You have in this packet the Texas Average Evapotranspiration (ETo) and the Texas Rainfall worksheet.
Follow steps 1-7 to calculate a water budget for Port Arthur. Record your results in Table 1 (Final page of this packet).
1. Row 1: Precipitation (P): The data on the worksheet is expressed in inches. You will need to convert inches of precipitation to
millimeters of precipitation. You may use the conversion sheet that I gave you or the textbook appendix. Round to the nearest
2. Row 2: Potential Evapotranspiration (E p): Potential evapotranspiration is the amount of water that would be lost through
evapotranspiration if there were no limit to the water supply. Use the conversion sheet or textbook appendix to convert inches
into millimeters. Round to the nearest tenth place.
3. Row 3:Water Excess/Shortage (P-Ep): Subtract Potential Evapotranspiration (Row 2) from Precipitation (Row 1). If the
difference is positive, there is more than enough water (excess) to meet the monthly demand. If the difference is negative, there is
not enough water (shortage) to meet the monthly demand.
4. Row 4: Stored Water (St): for these calculations, assume that the soil can hold, at most, 100 mm of water.
a. For January, add 100 (storage estimate for the previous month) to the Water Excess/Shortage (Row 3)
b. For each subsequent month, add the Water Excess/Storage for that month (Row 3) to the Stored Water for the previous
month (Row 4).
Stored Water can never be greater than 100 or less than 0.
c. Enter the calculated value in Row 4 if it falls between 0 and 100.
d. Enter 0 in Row 4 if calculated value is less than 0. Also, enter the difference (between the calculated value and 0) in
Row 6: Deficit (D).
e. Enter 100 in Row 4 if the calculated value is greater than 100. Also, enter the difference (between the calculated value
and 100) in Row 7: Surplus (S).
5. Row 5: Change in Stored Water (∆St): Subtract Stored Water (Row 4) for the previous month from the Stored Water (Row 4)
for the current month. For January, use 100 as the Stored Water value for the previous month.
6. Rows 6 & 7: Deficit and Surplus: See the instructions provided in Step 4: for months in which a deficit or surplus does not
occur, enter zeros.
7. Row 8: Actual Evapotranspiration (Ea): Actual evapotranspiration is the amount of water that is actually lost through
evapotranspiration, based on local conditions.
a. If Precipitation (Row 1, current month) + Stored Water (Row 4, previous month) > Potential Evapotranspiration (Row
2), then Actual Evapotranspiration = Potential Evapotranspiration (Row 2)
b. If Precipitation (Row 1, current month) + Stored Water (Row 4, previous month) < Potential Evapotranspiration (Row
2), than Acutal Evapotranspiration = Precipitation (Row 1) + Stored Water (Row 4, previous month).
Part II: Interpreting Water Budgets
You have performed calculations to determine the water status of Port Arthur. Table 2 and 3 provide similar data for
Brownsville and Wichita Falls. It is helpful to put such data in graph form for analysis. Use a spreadsheet program, such as
Excel 2003, to create individual line charts for each of the cities. Plot millimeters (y-axis) versus months (x-axis). Plot the
Precipitation (P), Potential Evapotranspiration (Ep) and the Actual Evapotranspiration (Ea) values as three different data series
on the same chart. When formatting the x-axis, deselect the “Value (Y) axis crosses between categories” option, which is
located under the Scale menu. Label the charts with the location names and print out one copy of each chart.
Use your data tables and charts to answer the following questions. You will need to answer the following questions in
complete sentences and on a separate sheet of paper typied. Use data to prove your point.
1. Recharge occurs when water is being added to the soil. In other words, the change in the stored soil moisture (∆S t) during
recharge is positive. On each chart, color the sections of the graph for which ∆St is positive, as indicated by the data charts.
2. Utilization occurs when water is being removed from the soil. In other words, the change in the stored soil moisture (∆S t) during
utilization is negative. On each chart, color the sections of the graph for which ∆S t is positive, as indicated by the data chart.
3. Color the deficit portion of each graph. A deficit occurs when the potential evapotranspiration (E p) is greater than the actual
4. Color the surplus portion of each graph. A surplus occurs when the potential evapotranspiration (E p) equals the actual
evapotranspiration (Ea), precipitation (P) is greater than the potential evapotranspiration (E p), and the moisture storage capacity of
the soil is met.
5. On each graph, include a legend that indicates what information is represented by each color.
6. Analyze the water budget information for each city. Do the climates have distinct wet and dry periods, or can they be classified
as deserts, where water deficits occur consistently throughout the year? Explain your answer
7. Use data from the lab to prove which city experiences the most severe period of drought.
8. Use data from the lab to prove which city experiences the longest period of drought.
9. Use data from the lab to prove which city receives the most precipitation.
10. Use the data from the lab to prove which city has the greatest potential evapotranspiration.
11. All of the cities have periods in which there is a zero deficit, yet the potential evapotranspiration is greater than the precipitation.
Explain this observation using data that you have obtained in the lab.
12. Using the data that you collected from the lab during which months is the potential for evapotranspiration the greatest? Why is
the potential for evapotranspiration greatest during during those months
13. Wichita Falls receives precipitation each month, yet it never experiences a surplus. Why does this occur?
14. For water budget calculations, it was assumed that the field capacity (initial storage), or amount of water that the soil can hold,
was the same for each city (100 mm). Might there be factors that would cause the field capacity to vary in different location?
15. What effects would these water budgets have upon agriculture in these three locations?
16. Compare and Contrast the water budgets for each of the cities. What factors might account for budget variations?
Created by: SAS Institute Inc.
Modified by: Joshua Abernethy