Name ______
The Local Water Budget
Suppose you planned to move to a new area where you want to grow a large garden. If you could choose an area with high, moderate, or low rainfall, which one would you choose? Does rainfall alone determine the nature of climate or the suitability of an area for gardening? What happens to rainfall after it reaches the earth’s surface is as important as how much rain falls. Experience shows that, after rainwater reaches the earth’s surface, it can run off into streams and be carried out of the area, it can evaporate, or be transpirated by plants back to the atmosphere as water vapor, or it might infiltrate the ground and be stored. Regardless of where the water goes it must be accounted for.
Runoff = Evapotranspiration + Infiltration = Precipitation
But how much of the precipitation goes where? How can one measure all these separate factors?
Precipitation, runoff, and infiltration are fairly easy to measure, but the combination of evaporation and transpiration (evapotranspiration) is not. It is usually measured indirectly by changing the accounting equation around to read:
Evapotranspiration = Precipitation – Runoff – Infiltration
Data on evapotranspiration and precipitation are available for many places in the U.S. and the world. In this investigation you will use such data to compute a water budget for a particular area, account for the moisture quantities in the equation, and determine the suitability of an area for activities such as gardening.
Problem: How is a water budget computed?
Objectives: After you have completed the investigation you should be able to:
- Define the terms – potential evapotranspiration (Ep), actual evapotranspiratoin (Ea), storage (St), deficit (D), surplus (S), usage, and recharge as they relate to the water budget of an area.
- Determine, when given Ep and precipitation (P) data for an area, whether the climate is moist or dry, what times of year flooding is likely to occur, and what periods of the year are dry enough to make irrigation necessary.
- Graph the Ep, Ea, and P data for an area
- Label the zones of Deficit, Surplus, Usage, and Recharge on a water budget graph
P – Precipitation – Monthly values are given
Ep – Potential Evapotranspiration – Monthly values are given
P-Ep – Take the Precipitation value and subtract the Evapotranspiration value
∆St – Change in storage - same value as P-Ep
St – Storage – For each example the storage for January is zero. Add +∆St values Subtract -∆St values. The St may never be more than 100 any remainder above 100 becomes the surplus. The St may never be below 0 any remainder becomes a deficit.
Ea – Actual Evapotranspiration =Ep- D
Ea cannot be greater than the Ep
D – Deficit – Equal to any -∆St values below zero storage for the month
(Deficits do not add up month to month)
S – Surplus – Equal to any + ∆St values above 100 storage for the month
(Surpluses do not add up month to month)
Report Sheet
Alexandria, Louisiana
Month / J / F / M / A / M / J / J / A / S / O / N / DP / 127 / 133 / 142 / 134 / 119 / 115 / 136 / 106 / 73 / 85 / 111 / 152
Ep / 15 / 18 / 45 / 75 / 122 / 164 / 179 / 170 / 129 / 69 / 31 / 15
Month / J / F / M / A / M / J / J / A / S / O / N / D
P / 127 / 133 / 142 / 134 / 119 / 115 / 136 / 106 / 73 / 85 / 111 / 152
Ep / 15 / 18 / 45 / 75 / 122 / 164 / 179 / 170 / 129 / 69 / 31 / 15
P-Ep
∆St
St
Ea
D
S
Report Sheet
Syracuse, New York
Month / J / F / M / A / M / J / J / A / S / O / N / DP / 72 / 68 / 81 / 75 / 74 / 87 / 84 / 82 / 72 / 76 / 68 / 72
Ep / 0 / 0 / 3 / 34 / 83 / 115 / 134 / 122 / 84 / 46 / 15 / 0
Month / J / F / M / A / M / J / J / A / S / O / N / D
P / 72 / 68 / 81 / 75 / 74 / 87 / 84 / 82 / 72 / 76 / 68 / 72
Ep / 0 / 0 / 3 / 34 / 83 / 115 / 134 / 122 / 84 / 46 / 15 / 0
P-Ep
∆St
St
Ea
D
S
Report Sheet
El Paso, Texas
Month / J / F / M / A / M / J / J / A / S / O / N / DP / 10 / 11 / 8 / 7 / 8 / 18 / 40 / 41 / 33 / 17 / 13 / 12
Ep / 11 / 18 / 37 / 75 / 117 / 163 / 171 / 152 / 111 / 62 / 24 / 10
Month / J / F / M / A / M / J / J / A / S / O / N / D
P / 10 / 11 / 8 / 7 / 8 / 18 / 40 / 41 / 33 / 17 / 13 / 12
Ep / 11 / 18 / 37 / 75 / 117 / 163 / 171 / 152 / 111 / 62 / 24 / 10
P-Ep
∆St
St
Ea
D
S
FOLLOW UP QUESTIONS
- When P is than Ep, what is likely to happen to water stored as soil moisture?
- Under what condition does a moisture deficit exist?
- When P is greater than Ep, what is likely to happen to soil moisture storage?
- Under what conditions will surplus moisture exist?
- Which climate is most desert like: Alexandria, Louisiana, or El Paso, Texas? Why?
- How does the climate of Syracuse, NY compare with those of El Paso and Alexandria?
- Why are Ep values high in the summer and low in the winter?
- What factor probably accounts for the similarity between the Ea curves for El Paso and Alexandria, and their dissimilarity to the Ea curve for Syrcuse?