Microsoft Word - GEOG1106_Lab7_WaterBudget.docx
Name: Date:
Water Budget Lab Questions
***Please submit only this document at the end of the lab period***
INSTRUCTIONS: Please read the lab reader before beginning this exercise. The following lab questions will be based upon the material given in the lab reader. You will need a calculator for this exercise. You will also need colored pencils for this lab.
Part 1: Water Budge Calculation
1. [2.5] When there is a deficit in precipitation, where must the water for evapotranspiration come from?
2. [2.5] While raining, what happens to precipitation when it gets to the soil after field capacity is reached?
3. [20] Using the Table 1 and the formulas listed in Part III of the reader, please complete the table below. Hint: begin with the Ending Storage (E.S.) for May (note: the E.S. for December will ca
y over to January in this exercise).
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Table 1. Water budget in Dallas, TX
Water Balance for Dallas, TX
J
F
M
A
M
J
J
A
S
O
N
D
T (°C)
5
7
12
16
21
26
28
28
24
21
12
4
P (mm)
64
49
76
113
149
109
77
87
91
97
66
61
PE (mm)
5
10
31
62
105
152
177
171
117
87
26
8
P – PE
(mm)
I. S.
(mm)
150
150
E. S.
(mm)
150
150
Δ ST
(mm)
AE (mm)
Surplus (mm)
Deficit (mm)
4. [10] Complete the following using the Water Balance for Dallas, TX using the Table 1 that you just completed.
Total AE (mm) = Total Surplus (mm) = Total Deficit (mm) =
Total P (mm) = Total AE + Total Surplus. What is Total P? Total PE (mm) = Total AE + Total Deficit. What is Total PE?
5. [2.5] Which months had moisture surplus?
6. [2.5] Which months had moisture deficits?
7. [7.5] Using the chart below and the information from the Water Balance for Dallas, TX please plot the following variables:
Precipitation (P)
Potential Evapotranspiration (PE) Actual Evapotranspiration (AE).
Use a different color or style (pattern) of line to connect the annual data for each variable.
8. [10] Using the P, PE, and AE data that you plotted in Question 7, please shade and label the following polygons (spaces):
Moisture Surplus (P > PE)
Soil Moisture Utilization (AE > P) Moisture Deficit (PE > AE)
Soil Moisture Recharge after a period of deficit (P > PE).
Use a different color or style (pattern) to shade each polygon.
9. [2.5] Give the graph an appropriate tile and legend.
10. [5] When are the wet and dry seasons in Dallas (i.e. which month gets the most/least precipitation)?
Wet Season: Dry Season:
11. [5] How do these wet and dry seasons co
espond with the months of surplus deficit?
12. [2.5] What is the relationship between temperature and potential evapotranspiration?
Using the Water Balance for Washington, D.C. provided on the reader, complete the following questions.
13. [5] When are the wet and dry seasons in Washington, D.C. (i.e. which month gets the most/least precipitation)?
Wet Season: Dry Season:
14. [5] How do these wet and dry seasons co
espond with the months of surplus deficit?
15. [2.5] What is the relationship between temperature and actual evapotranspiration?
16. [15] Compare and contrast the local water balance of Dallas, Texas and Washington,
D.C. Name, at least, one difference or similarity (temporally, quantitatively, etc.) with respect to ALL of the following: soil moisture surplus, deficit, recharge, and utilization. Use the table below:
Water Surplus
Water Deficit
Soil Moisture and Aquifer Recharge
Soil Moisture Utilization
Microsoft Word - GEOG1106-Lab7_Reader.docx
Reader:
The Water Budget
Learning Outcomes: The purpose of this lab is to be able to identify and key soil-water budget components and to be able to calculate and plot the soil-water budget for a selected location. The lessons learned from this laboratory could, in the future, by applied by the student to water resource analyses of different hydro-systems including watersheds, reservoirs and regions as open systems with inputs, outputs and storages of water.
Materials: Pencil, colored pencils, lab handout, simple calculator.
Part I. What is the water budget?
Water is not always naturally available when and where it is needed. Hence, humans must rea
ange water resources. The maintenance of a houseplant, the distribution of local water supplies, and i
igation program on a farm, the rea
angement of river flows — all involve aspects of the water balance and water-resource management.
The water budget is an accounting of the incoming and outgoing components of water from a control volume (e.g. a watershed, a soil column, a farm, a lake, etc). Such a budget can be established for any given area on the Earth’s surface
— a continent, nation, region, or field. For a soil-water budget, it is important to measure all the precipitation input and its distribution to satisfy the “demands” of plants (transpiration), evaporation, runoff, soil moisture, groundwater, water bodies or snow (ice) storage in the area considered. Such a budget can examine any time frame, from minutes to years.
When there is not snow/ice or ponded water storage, then the only water storage occurs into the soil. So, think of a soil-water budget as a money budget: precipitation income must be balanced against expenditures of evaporation, transpiration, and runoff. Soil-moisture storage acts as a savings account, accepting deposits and withdrawals of water. Sometimes all expenditure demands are met, and any extra water results in a surplus. At other times, precipitation and soil moisture income are inadequate to meet demands, and a deficit, or water shortage, results. The water balance describes how the water supply is expended. Think of precipitation as “income” and evapotranspiration
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as “expenditure.” If income exceeds expenditures, then there is a surplus to account for in the budget. If income is not enough to meet demands, then we need to turn to savings (a storage account – if available) to meet these demands. When savings are not available, then we must record a deficit of unmet demand.
To understand the water-balance methodology and “accounting” or “bookkeeping” procedures, it is essential to understand the terms and concepts used in simple water-balance equations
Part II. Definitions
Precipitation (P) – rain, snow, sleet, or hail – the moisture supply.
Potential evapotranspiration (PE) – the water loss from a hypothetical, homogeneous, vegetated area that never suffers from a lack of water; the amount of water that would evaporate and transpire through plants if the moisture was available i.e. unlimited water availability in and on the ground).
Actual evapotranspiration (AE) –actual amount of evaporation and transpiration that occurs – the actual satisfied demand that is limited by water availability in and on the ground for evaporation and transpiration processes.
Soil moisture deficit – the amount of unsatisfied potential evapotranspiration; the amount of demand that is not met either by precipitation or by soil moisture storage – the moisture shortage. In mathematical terms it is Deficit = PE-AE
Soil moisture surplus – the amount of moisture that exceeds potential evapotranspiration, when soil moisture storage is at field capacity (full) – the moisture oversupply
Soil moisture storage change (∆ST) – the use (decrease) or recharge (increase) of soil moisture – the moisture savings.
Field capacity – the maximum amount of water the soil can hold against the pull of gravity (varies with soil types, but we will use 150 mm in this exercise) – field capacity can never exceed soil moisture storage value.
Part III. Formulations and Example
Calculating the soil-water budget is simply an accounting task. We simply have to keep track of the amount of water “income” and water “expenditures.” Below are the steps you need to follow to determine the soil-water budget for a given location. For application of the formulas, please refer to the soil moisture retention, water budget table and budget plot for Washington D.C. shown below.
1. (P-PE): Calculate the difference between precipitation (P) and potential evapotranspiration (PE) for each month and record the value in the table.
2. Initial storage (I.S.): is the previous month’s ending storage (E.S.)
Start computing these values at the end of a wet month (May in Dallas, December in Washington, for example). Use one of the following to complete the initial and ending storage values.
A. If P is less than PE, then use the soil moisture retention table (Table 1) to “count down” the P-PE value in the table.
Example: June P-PE = -43. So, start at 150 in the table and count 43 values down to 112 and enter it for June‘s ending storage value. Then, July P-PE = XXXXXXXXXXSo, start at 112 and count 100 values down to 57 and enter it for July’s ending storage.
B. If P is greater than PE, then initial storage + (P-PE) = ending storage.
When field capacity is reached, both the initial and ending storage values remain at 150 mm as long as (P-PE) is greater than or equal to 0.
3. Storage change, ∆ ST = absolute value of (initial storage – ending storage).
4. Actual evapotranspiration (AE): Use one of the following to calculate AE
A. If (P-PE) is greater than 0, then AE = PE
B. If (P-PE) is less than 0, then AE = P + ∆ST
5. Surplus and Deficit: Use one of the following to calculate.
A. If (P-PE) is less than 0, then deficit = PE – AE
B. If initial storage (IS) is at field capacity, then surplus = (P - PE)
C. If (P-PE) is greater than 0, initial storage is less than at field capacity, and ending storage is at field capacity, then surplus = (P - PE) – ∆ ST
D. If A, B, or C are not met then recharge is taking place (there is no surplus or deficit). Place an R in the surplus and deficit blank for that month
Table 1: Soil Moisture Retention Table
150
149
148
147
146
145
144
143
142
141
140
139
138
137
136
135
134
133
132
131
131
130
129
128
127
127
126
125
124
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122
122
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120
119
118
117
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115
114
114
113
113
112
111
111
110
109
108
107
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106
106
105
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103
103
102
101
100
100
99
98
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97
97
96
95
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93
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91
90
90
89
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87
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7
Source: Mather, R., ed. (1977): Workbook in applied Climatology. Elmer, N.J.: C.W. Thornthwaite Associates.
Example: Soil-water budget for Washington, D.C.
J
F
M
A
M
J
J
A
S
O
N
D
T (ºC)
3
3
7
13
19
24
26
25
21
15