Potential evapotranspiration potet refers to

GEOG 101 Physical Geography

LAB 6: The Water Balance and Water Resources

(based on Christopherson with major modifications by D. Fairbanks)

Name ___Answers___ Lab Section __________ Date ____________

Materials and sources that will help you
· Color pencils

· Calculator

· Writing Assignment Data

· An internet connection

Introduction

Because water is not always naturally available when and where it is needed, humans must rearrange water resources. The maintenance of a houseplant, the distribution of local water supplies, an irrigation program on a farm, the rearrangement of river flows – all involve aspects of the water balance and water-resource management.

The water-balance is an examination of the hydrologic cycle at a specific site or area for any period of time, including estimation of stream flow, accurately determining irrigation quantity and timing, and as an important climatic element, that is, the relationship between a given supply of water and the local demand.

A water balance can be established for any defined area of Earth’s surface – a continent, nation, region, or field – by calculating the total precipitation input and the total water output.

In this lab you will work with a water-balance equation and accounting procedure to determine moisture conditions for two cities – Indianapolis, Indiana, and Chico, California (both lie on the same latitude), and Oroville reservoir which is a key piece of the California State Water Project which moves water from the Feather River watershed to the California Aqueduct to supply southern California’s water requirements. Given this data you will prepare graphs that illustrate these water balance relationships. Also, this lab examines the broader issues of water resources in the United States.

Key Terms:

actual evapotranspiration evaporation soil moisture storage

available water evaporation soil moisture utilization

capillary water field capacity surplus

consumptive uses potential evapotranspiration transpiration

deficit precipitation wilting point withdrawal

evapotranspiration soil moisture recharge

Section 1: Water Balance Components

A soil-water budget can be established for any area of Earth’s surface – a continent, country, region, field, or front yard. Key is measuring the precipitation input and its distribution to satisfy the “demands” of plants, evaporation, and soil moisture storage in the area considered. Such a budget can examine any time frame, from minutes to years.

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 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. In the water balance these budgetary components are presented as follows:

· Precipitation = supply

· Potential evapotranspiration = demand

· Deficit = shortages

· Surplus = oversupply

· Soil Storage = savings

To understand the water-balance methodology and “accounting” or “bookkeeping” procedures, we must first understand the terms and concepts in simple water-balance equation.

The objective is to account for the ways in which this supply is distributed: actual water taken by evaporation and plant transpiration, extra water that exits in streams and subsurface groundwater, and recharge or utilization of soil-moisture storage. All the while, remember the objective of the water balance is to account for the expenditure of precipitation.

Water Balance Equation

PRECIP = (POTET – DEFIC) + SURPL ± ∆STRGE

Supply demand shortage oversupply soil-moisture

utilization or recharge

ACTET

actual

evapotranspiration

· PRECIP (precipitation) is rain, sleet, snow, and hail – the moisture supply.

· POTET (potential evapotranspiration) is the amount of moisture that would evaporate and transpire through plants if the moisture were available; the amount that would become output under optimum moisture conditions – the moisture demand.

· DEFIC (deficit) is the amount of unsatisfied POTET; the amount of demand that is not met either by PRECIP or by soil moisture storage – the moisture shortage.

· ACTET (actual evapotranspiration) is the actual amount of evaporation and transpiration that occurs.

· POTET – DEFIC; thus, if all the demand is satisfied, POTET will equal ACTET – the actual satisfied demand.

· SURPL (surplus) is the amount of moisture that exceeds POTET, when soil moisture storage is at field capacity (full) – the moisture oversupply.

· ± ∆STRGE (soil moisture storage change) is the use (decrease) or recharge (increase) of soil moisture, snow pack, or lake and surface storage or detention of water – the moisture savings.

Key to the water balance is determining the amount of water that would evaporate and transpire if it were available (POTET). Now, examine and compare the PRECIP (supply) map in Figure 1a to the POTET (demand) map in Figure 1b for the continental United States. The relationship between PRECIP supplies and POTET demands determines the remaining components of the water balance equation water resources.

USppt

USevapotran

Figure 1. (a) average annual precipitation in inches; and (b) potential evapotranspiration in inches. 1. Can you identify from the two maps regions where PRECIP (Figure 1a) is higher than POTET demand (Figure 1b)? Describe these regions.

The Pacific Northwest (Olympic Peninsula) receives over 80 inches of rain, but has a potential evapotranspiration of 24-36 inches. New Orleans receives 60-80 inches of rain, but has a potential evapotranspiration of 36-48 inches

2. Can you identify from the two maps regions where POTET demand is higher than PRECIP supply? Describe these regions.

Las Vegas receives less than 10 inches of rain, but has a potential evapotranspiration of 36 – 48 inches. Los Angeles receives 10 – 20 inches of rain, but has a potential evapotranspiration of 24-36 inches.

3. Based on these maps, why does 95% of the irrigated agriculture in the United States occur west of the 100th meridian?

Rain doesn’t fall consistently throughout the year in the west, varied topography (mountains, valleys) also has an effect on precipitation amounts. The land must be irrigated to ensure that crop get the water when they need them.

4. In the Sacramento River valley, is the natural water demand usually met by the natural precipitation supply? Or, does this region experience a natural shortage?

The region experiences a natural shortage* during the summer months.

*To say “shortage” is a very human-centric (read: farmer) way of looking at water. From a native plant’s perspective there is no “shortage,” this is just the way things are and native plants have adaptations to survive these conditions!

Section 2: Water Balance Supply and Demand for Indianapolis, Indiana

Soil-moisture storage is a “savings account” of water that can receive deposits and allow withdrawals as conditions change in the water balance. Soil-moisture storage (∆STRGE) refers to the amount of water that is stored in the soil and is accessible to plant roots. Soil is said to be at the wilting point (withdrawal) when all that is left in the soil is unextractable water (hygroscopic water); the plants wilt and eventually die after a prolonged period of such moisture stress.

The soil moisture that is generally accessible to plant roots is capillary water, held in the soil by surface tension and hydrogen-bonding between the water and the soil. Almost all capillary water is available water in soil moisture storage and is removable for POTET demands through the action of plant roots and surface evaporation. After water drains from the larger pore spaces, the available water remaining for plants is termed field capacity, or storage capacity. This water is held in the soil by hydrogen bonding against the pull of gravity. Field capacity is specific to each soil type and is an amount that can be determined by soil surveys.

Assuming a soil moisture storage capacity of 100 mm for Indianapolis, Indiana, typical of shallow-rooted plants, the months of net demand for moisture are satisfied through soil-moisture utilization. Various plant types send roots to different depths and therefore are exposed to varying amounts of soil moisture.

For this exercise we assume that soil moisture utilization occurs at 100%, that is, if there is a net water demand, the plants will be able to extract moisture as needed. Actually, in nature as the available soil water is reduced by soil-moisture utilization, the plants must exert greater effort to extract the same amount of moisture. As a result, even though a small amount of water may remain in the soil, plants may be unable to exert enough pressure to utilize it. The unsatisfied demand resulting from this situation is calculated as a deficit. Avoiding such deficit inefficiencies and reduction in plant growth are the goals of a proper irrigation program, for the harder plants must work to get water, the less their yield and growth will be.

Likewise, relative to soil moisture recharge we assume a 100% rate if the soil moisture storage is less than field capacity, then excess moisture beyond POTET demand will go to soil-moisture recharge. We assume in this exercise a soil moisture recharge rate as 100% efficient as long as the soil is below field capacity and above a temperature of –1 °C. Under real conditions we know that infiltration actually proceeds rapidly in the first minutes of a storm, slowing as the upper layers of soil become saturated even though the soil below is still dry.

Table 1. Water budget calculations table for Indianapolis, Indiana. All quantities in millimeters.

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Total

PRECIP

76

59

90

104

123

100

108

92

82

67

91

80

1072

POTET

0

0

14

50

92

128

148

129

89

48

15

0

713

PRECIP – POTET

+ 76

+ 59

+ 76

+ 54

+ 31

– 28

– 40

– 37

– 76

+ 19

+ 76

+ 807

--

STRGE

100

1001

100

100

100

722

32

04

0

19

95

100

--

∆STRGE

0

0

0

0

0

– 283

– 40

– 32

06

+ 19

+ 76

+ 57

0

ACTET

0

0

14

50

92

128

148

129

82

48

15

0

706

DEFIC

0

0

0

0

0

0

0

55

76

0

0

0

12

SURPL

768

59

76

54

31

0

0

0

0

0

0

757

371

1 There is no storage change (from Jan to Feb) because the balance of (PRECIP – POTET) is positive.

2 The water demand (POTET) is greater than supply (PRECIP). How can we satisfy this deficit? Use the stored water (STRGE). Since the balance of (PRECIP – POTET) is negative in Jun, this supply shortage is balanced out by using water from STRGE.

3 As a result, there is a change in STRGE (∆STRGE = – 28) in Jun. You see how much change took place in STRGE from May to Jun (this is shown in ∆STRGE).

4 The maximum STRGE is 100, while the minimum STRGE is 0. In Aug, the balance of (PRECIP – POTET) is again negative (– 37), but this shortage of supply is balanced out by using water from STRGE (whatever remaining…that is 32). In this month, you use up all the water in the storage, and there is still a shortage of water demand by 5 (which cannot be satisfied).

5 Thus, for this month, you have water deficit (DEFIC) of 5.

6 In Sep, there is again a negative balance of (PRECIP – POTET). Since the STRGE has been depleted, there is no change in ∆STRGE and this balance of (– 7) is recorded as DEFIC.

7 Beginning in Oct, the balance of (PRECIP – POTET) has become positive. Any positive value of (PRECIP – POTET) can contribute to the storage (recharging the storage), if it is under 100 (maximum capacity). If it is already 100, any positive value of (PRECIP – POTET) becomes surplus (SURPL). December begins with the STRGE value 95. Given that (PRECIP – POTET) for this month is +80, 5 out of this 80 is used to fill the STRGE to the maximum of 100, and the remaining 75 is considered SURPL.

8 For the months of Jan through May, the amount of supply exceeds the amount of demand. That is, there is no shortage of water. In addition, the storage is full (100), and there is no need for this storage of water to be used (again, there is no water shortage), any positive balance of (PRECIP – POTET) is considered SURPL.

5. Soil moisture remains at field capacity (full) through which month? May

6. How much surplus is accumulated through these first five months? 265 mm

7. What is the net demand for water in June? 28 mm

8. After you satisfy this demand through soil moisture utilization, what is the remaining water in soil moisture at the end of June, to begin the month of July?

72 mm

Calculate the actual evapotranspiration for each month of the year for Indianapolis and note this in the table. By subtracting DEFIC from POTET, you determine the actual evapotranspiration, or ACTET, that takes place for each month. Under ideal moisture conditions, POTET and ACTET are about the same, so that plants do not experience a water shortage. Prolonged deficits could lead to drought conditions, in which POTET exceeds ACTET.

9. According to your calculations, do the soils of Indianapolis return to field capacity (full storage) by the end of the year? Are any surpluses generated in December? What is the amount?

Yes, there is a surplus in December, 75 mm

10. What is the total ACTET, DEFIC and SURPL for the year?

ACTET= 706 mm DEFIC = 12 mm SURPL = 371

Section 3: Water Budget Calculations for Chico, California

For comparison let’s work with Chico (on same latitude as Indianapolis), which experiences large seasonal deficits in its annual water balance. Chico, California, (39.78° N, 121.85° W, at 59 m elevation) has a Mediterranean dry, warm summer climate. The data for Chico is in Table 3. Please assume the same soil-moisture storage capacity of 100 mm, typical of shallow-rooted plants. The months of net demand for moisture are satisfied through soil-moisture utilization , as long as the soil moisture is available. Chico does experience a wilting point each year.

Table 3. Water budget calculations table for Chico, California. All quantities in millimeters.

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Total

PRECIP

197

157

132

69

32

14

1

4

15

51

120

151

943

POTET

13

21

33

53

87

121

152

132

99

62

27

13

813

PRECIP – POTET

184

136

99

16

-55

-107

-151

-128

-84

-11

93

138

--

STRGE

100

100

100

100

45

0

0

0

0

0

0

100

--

∆STRGE

0

0

0

0

-55

-45

0

0

0

0

93

7

0

ACTET

13

21

33

53

87

59

1

4

15

51

27

13

374

DEFIC

0

0

0

0

0

62

151

128

84

11

0

0

436

SURPL

184

136

99

16

0

0

0

0

0

0

0

131

566

13. For Chico, how many months does POTET exceed PRECIP?

6 months

14. Water resources, the “water crop,” are harvested from water surplus. If you were a water resource manager for the Chico region, what strategies would you recommend to meet agricultural and urban water demands? (Discuss this among others in your lab before you begin writing. Note that there is a mountain range east of the Chico region that accumulates a snow pack in winter; make this part of your consideration.)

· Water conservation strategies (drip irrigation, water only at night)

· Xeriscaping/rockscaping in place of water-intensive front yard lawns

· High water rates and/or penalties for folks who use excess water

· Plant varieties of crops that require less water

· Eliminate clear cutting in the forest as a strategy to decrease water runoff

· Use of low flow toilets, faucets

· Advocate for more dams to store water

Section 4: Water Balance Graphs

A useful way to visualize the water balance for a location is to graph the data. The following activity will allow you to graph and then compare the water balances for Indianapolis and Chico.

15702

Surplus

Soil-moisture recharge

Soil-moisture utilization

Precipitation

Potential Evapotranspiration

Deficit

Surplus

Take the PRECIP and POTET data presented for Chico and Indianapolis and prepare a water balance graph for each location. Prepare the graphs as line graphs by month. Using your colored pencils make PRECIP a blue line and POTET a red line. (See the graph above for an example).

Identify with shading the areas between the PRECIP and POTET line-graph plots that represent various aspects of each water balance. For the four relations possible between moisture supply and demand, utilize the following key colors for shading the appropriate portions of your graphs:

· Surplus: blue shading (PRECIP exceeds POTET)

· Soil moisture utilization: brown shading (POTET exceeds PRECIP with soil moisture available to meet some of the demand)

· Deficit: orange shading (POTET exceeds PRECIP with inadequate soil moisture available)

· Soil moisture recharge: green shading (PRECIP exceeds POTET, until soil reaches field capacity)

Macintosh HD:Users:jeremy:Desktop:IMG_1228.jpg waterbalance_graph

Thank you to Krystal!

Section 5: Your Cities Water Balance Analysis

ANSWERS VARY DEPENDING ON YOUR CITY

First City:______________________

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Total

PRECIP (mm)

56

41

50

20

5

2

1

2

5

9

30

35

256

POTET (mm)

29

31

40

51

68

81

106

104

85

65

43

29

732

PRECIP - POTET

27

10

10

-31

-63

-79

-105

-102

-80

-56

-13

6

--

Storage

100

100

100

69

6

0

0

0

0

0

13

100

--

Storage

0

0

0

-31

-63

-6

0

0

0

0

13

87

0

Deficit

0

0

0

0

0

73

105

102

80

56

0

0

416

Surplus

27

10

10

0

0

0

0

0

0

0

0

81

128

Second City:______________________

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Total

PRECIP (mm)

POTET (mm)

PRECIP - POTET

--

Storage

100

--

Storage

Deficit

Surplus