INTRODUCTION

A water budget describes the relationship between all of the water that moves into and out of a region. Incoming water is precipitation (P), which includes rain, snow, sleet, and hail. Outgoing water is water that leaves a region through use, as runoff, or as a result of evapotranspiration (E). Evapotranspirationincludes two processes: evaporation and transpiration. Evaporationoccurs when water enters the atmosphere as the result of an increase in air temperature. Transpiration is a process whereby water is released into the atmosphere by the leaves of a plant.

A water budget is balanced when the volume of precipitation over a region is equal to the volume of water that leaves the region. When the volume of precipitation exceeds water loss, there is a surplusin the region’s water budget. When the volume of precipitation is less than the volume of water that leaves a region, a deficitexists in the region’s water budget. In this laboratory activity, you will model the water budget of two cities. Then you will construct double-line graphs using water budget data from the cities.

OBJECTIVES

Model the water budget of two different cities.

Distinguish between a water budget surplus and a water budget deficit.

Graph water budget data from two cities.

MATERIALS

Calculator

colored pencils (4)

PROCEDURE

PART I—WATER BUDGET FOR CITY ALPHA

1. The data in Table 1 shows what is happening with the water, where precipitation (P) would be a positive value, and evapotranspiration (E) would be a negative value. For example, during January, 47mm of precipitation fell over the city, and there was 0mm evapotranspiration. So, 47 – 0 = 47, which is the number you would record for the water budget for January. As you work through each month, adding and subtracting according to P and E, rrecord the water budget total in the data table. The first two months have been filled in for you.

Table 1: Precipitation (P) and Evapotranspiration (E) for City Alpha

*This value is the water that could potentially enter the atmosphere in the given region during the given month.

PART II—WATER BUDGET FOR CITY BETA

2. Repeat step 1 using the data for City Beta from Table 2.

Table 2: Precipitation (P) and Evapotranspiration (E) for City Beta

*This value is the water that could potentially enter the atmosphere in the given region during the given month.

ANALYSIS

1. Examining Data. Describe the overall water budget for City Alpha during the year shown above (ie., did it have a surplus or a deficit?).

2. Recognizing Patterns. Did City Alpha experience any monthly water deficits? If so, when?

3. Examining Data. Describe the overall water budget for City Beta during the given year.

4. Recognizing Patterns. Did City Beta experience any monthly water surpluses? If so, when?

5. Constructing Graphs. Use colored pencils to create a double-line graph showing precipitation and evaporation for City Alpha. Plot the volume of water on the y-axis and the month of the year on the x-axis. Use different colors to shade the areas on the graph that represent times of surpluses and deficits. Create a key for your graph.

6. Constructing Graphs. Create a second graph of the water budget data for City Beta. Follow the same directions as in step 5.

CONCLUSIONS

7. During which month is City Alpha’s water deficit smallest? Largest?

8. Which month is City Alpha’s water surplus smallest? Largest?

9. During which month is City Beta’s water deficit smallest? Largest?

10. During which month is City Beta’s water surplus smallest? Largest?

11. City Beta is downstream from City Alpha. What might happen if City Alpha experienced a large water budget surplus during March and April?

12. What happened to the total water budget in your model for City Beta with your water additions and subtractions as you worked through the months?

13. If City Beta were an actual city, what would really happen once the water budget deficit exceeded the surplus?

14. In what ways might City Beta replenish its water budget supply?