1. The Water Budget

2. Evapotranspiration
Climate is determined by conditions of temperature and humidity.
The amount of moisture entering the atmosphere depends on evaporation and transpiration.
The term evapotranspiration refers to all of the water vapor released into the atmosphere.

3. The Water Budget
The amount of water gained, stored, or lost in a specific location varies from day to day and from month to month.
A monthly account of what happens to the water in a particular location over the course of the year is called the water budget.

4. Income and Expenses
The income in a water budget is the amount of precipitation.
The monthly expenses (“bill”) is potential evapotranspiration.
Potential evapotranspiration in a monthly water budget is the amount of water that would evaporate or transpire if the water were available.

5. Potential Evapotranspiration
The potential evapotranspiration of an area is directly proportional to the amount of energy available.
The potential evapotranspiration is highest in summer – temperatures are hottest – and lowest in winter.
Forests, with their large leaf surface area, give off more evapotranspiration than deserts or grasslands.

6. Climate and the Water Budget
Each place has its own unique water budget.
Rainfall is abundant all year; evapotranspiration exceeds precipitation only during summer months.
So NYC has a moisture surplus most of the year.

8. Climate Regions and Water Budgets
P = precipitation
PE = potential evapotranspiration
If total P< total PE, the climate is arid.
If P> PE, the climate is humid.

9. Stream Discharge
In areas where there is a great surplus of moisture, there is a large amount of runoff into streams – stream discharge.
During dry periods, runoff diminishes, streams take water from groundwater, lowering water table.
Steam discharge fed by groundwater is called base flow.

10. Climate and the Water Budget
Climates can be distinguished mathematically by determining the P/PE ratio.
If the ratio is greater than 1, humid climate;
Less than 1, arid climate.

11. Water Budget Lab
Below is monthly water budget data for Syracuse, New York and Fresno, California.
“P” stands for precipitation, Ep is the potential evapotranspiration.
When P>Ep, the location has a surplus of water; when P<Ep, the location is becoming dryer, losing more water than it receives via the water cycle.

12. Syracuse NY Water Budget
Month J F M A S O N D
Totals P 72 68 81 75 74 87 84 82 76 Ep 3 34 83
115
134
122 46 15
P-Ep
Fresno CA Water Budget
Month J F M A S O N D
Totals P 44 40 41 24 10 3 4 13 22 Ep 20 37 63 99
139
180
165
114 70 31 12
P-Ep

13. Procedure
Calculate the total precipitation and total potential evapotranspiration for both cities.
Calculate the monthly difference between precipitation and potential evapotranspiration for both cities
Graph both P and Ep using a double line graph. Make separate graphs for both cities, on separate sheets of graph paper.
Millimeters of water is the dependent variable (y axis); the independent variable, months, is on the x axis. Give each graph an appropriate title.

14. Questions
What causes potential evapotranspiration to vary from month to month? Why do some months have no evapotranspiration?
When precipitation is less than potential evapotranspiration, what is likely to happen to moisture stored in the soil?
When precipitation is more than potential evapotranspiration, what will happen to the amount of moisture contained in the soil?
If total annual precipitation is less than the potential evapotranspiration, the climate is said to be arid. If the total precipitation is greater than the potential evapotranspiration, the climate is said to be humid. Using your data, explain why Syracuse is considered to have a humid climate.

15. During which months is moisture being lost in the Syracuse area?
During which months is moisture being gained in the Syracuse area?
During which months is moisture being lost in the Fresno area?
During which months is moisture being gained in the Fresno area?
Syracuse has a smaller Ep curve than Houston, Texas, or Miami Florida. Explain why in terms of latitude, insolation, and monthly temperatures.
In areas like the Catskill watershed, P >>Ep during most years. Suppose that we experience a severe drought. What would New York City need to do to respond to this situation?