Water Balance

A lot of people think that drought means a lack of precipitation. However, precipitation is only part of the equation. Another important aspect about what we need to consider is the natural loss of liquid water to the atmosphere, the processes known as evaporation and transpiration (evapotranspiration). The higher the temperature, the higher the rate of evaporation in a particular area. Therefore, it is possible to have a drought in one region one year with a given amount of precipitation (35 inches), and to not have a drought the next year with less precipitation (30 inches). This is caused by the latter year have a lower rate of evaporation due to lower temperatures. For us to have a good understanding of the stresses that are placed on water resource it is necessary to have a good understanding of the hydro-climatic processes that are at work at different locations. Here we are looking at the periods of surplus, water utilization, deficit, and recharge. An analysis of these help indicate the severity of water needs in a particular area. In the mid-latitudes, the winter season is generally associated with surplus, when the soil is holding its capacity of water, partially due to low rates of evaporation. Spring is associated with water utilization, where the water stored in the soil from winter is being used up, at least until there is no water left in storage. Summer is associated with periods of deficit, when there is no water in storage, due to a lack of precipitation and high evaporation rates. The fall is associated with recharge, where moisture is being added to the soil due to declining levels of evaporation as the atmosphere begins to cool. Below we are looking at 2 very different locations, Berkeley, California, which is in a fairly dry environment, with a winter-time precipitation maximum, and Terre Haute, Indiana, which has a peak of precipitation in the summer months. Compare them for similarities and differences in their hydro-climatologic data.
A. Study the attached table in Figure 1 on of this Lab Exercise. This represents the Water Budget of Berkeley, California.
B. Using the Terre Haute, Indiana, data given below, complete a data tabulation of the average annual water budget of the area:
P 13.0 11.2 9.4 3.7 2.4 0.5 0.1 0.1 1.3 3.1 6.2 10.6
PE 2.6 3.2 4.5 5.6 7.1 8.4 8.8 8.2 7.5 6.3 4.3 2.8
P-PE 10.4 8.0 4.9 -1.9 -4.7 -7.9 -8.7 -8.1 -6.2 -3.2 1.9 7.8
Change in ST 0.0 0.0 0.0 -1.9 -4.7 -3.4 0.0 0.0 0.0 0.0 1.9 7.8
ST 10.0 10.0 10.0 8.1 3.4 0.0 0.0 0.0 0.0 0.0 1.9 9.7
AE 2.6 3.2 4.5 5.6 7.1 3.9 0.1 0.1 1.3 3.1 4.3 2.8
D 0.0 0.0 0.0 0.0 0.0 4.5 8.7 8.1 6.2 3.2 0.0 0.0
S 10.4 8.0 4.9 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
The data is in centimeters (cm)
The estimated soil moisture is at field capacity
In the data tabulation, complete one column at a time. In this case,
January serves as an acceptable starting point.
Consult the following web sites for information about water budgets and the following terminology:
P = Precipitation
Change in ST = change in 10.0 cm storage
ST = Actual storage (somewhere between 0 and 10 cm, inclusive)
AE = Actual Evapotranspiration (never greater than the Potential Evapotranspiration); evapotranspiration -> is the combined processes of evaporation and transpiration; the delivery of water to the atmosphere by vegetation and by direct evaporation from wet surfaces
D = Deficit (Will only occur when the soil has no moisture; ST = 0) ->
PE = Potential Evapotranspiration (the higher the temperature the greater; this is theamount that would be lost with an unlimited supply of water)
S = Surplus (will only occur at field capacity -> 10 cm -> soil is holding its maximum capacity of moisture
In terms of a water balance we generally look at 4 stages of water usage: surplus, usage, deficit and recharge. We are making an assumption, for the basis of this exercise, that the maximum storage capacity of the soil (field capacity) is 10cm. Using this value, a surplus can only occur when the soil is at field capacity in storage (10cm). Usage occurs as the soil water storage is reduced from 10 cm to none. A deficit will occur only when the soil has no water in storage. Recharge occurs as water is being added to storage, and the values of storage are increasing from 0 to 10 cm. Upon reaching 10cm, the soil will be back in a surplus situation. In the mid-latitudes, surplus is often associated with winter, usage with spring, deficit with summer and recharge with the Fall. One other assumption that we make here is that with the Berkeley data we are starting with 9.7 cm in storage at the beginning of the year and the Terre Haute starts off being at field capacity with a value of 10 cm of storage from the previous December.
Use Figure 2 for the Terre Haute data.
P 7.4 6.8 9.6 9.4 10.1 10.2 8.1 8.2 8.7 6.9 8.4 7.5
PE 0.0 0.0 1.8 4.9 10.2 13.4 15.8 13.8 9.9 5.2 1.7 0.1
Change in ST
STEP 1: P-PE is calculated by measuring Precipitation (P) minus Potential Evapotranspiration (PE) for each month.
Example: January at Berkeley is calculated as 13.0 minus 2.6, which equals 10.4.
STEP 2: Soil Storage (ST) will be a value between 0 and 10 cm. We will assume that in the previous December that the soil is saturated heading into January. It will remain saturated until P-PE is a negative value.
Example: At Berkeley, P-PE is positive in January through March, so ST remains 10.0 in January through March.
STEP 3: When P – PE becomes negative, that value is subtracted from soil storage, until ST reaches 0 or P – PE becomes positive again.
Example: In April at Berkeley, P – PE is -1.9 cm. 10 – 1.9 is 8.1 cm. In May at Berkeley, P – PE is -4.7 cm. 8.1 – 4.7 = 3.4 cm. In June at Berkeley, P – PE is -7.9. Since P – PE exceeds the Soil Storage of 3.4 cm from the previous month the Soil Storage (ST) goes down to 0. It can’t go down to any value less than 0.
STEP 4: The Soil Storage will remain at 0 until the P – PE becomes positive.
Example: At Berkeley, P – PE remains negative in the months of July, August, September, and October. Therefore, the storage remains 0.
STEP 5: When P-PE becomes positive, that positive value is added back to storage.
Example: At Berkeley in November P – PE is 1.9 and that added to the previous month’s storage of 0 gives a new storage value of 1.9. In December a P –PE of 7.8 added to the previoos month’s storage of 1.9 is 9.7 cm.
STEP 6: The change in storage is simply the change from the previous month’s value.
Example: At Berkeley in December, the change in storage from the previous month is 9.7 (December) minus 1.9 (November), which equals 7.8.
STEP 7: The difference between Potential Evapotranspiration (PE) and Actual Evapotranspiration (AE), is that PE represents the value that would exist with an unlimited amount of moisture at a given temperature, while AE represents the amount that could evaporate given the amount of precipitation (P) and water in storage (ST), that is actually available. As long as the soil is t full capacity (field capacity, which equals 10 cm), and P – PE is positive, AE and PE will be the same.
Example: At Berkeley, ST is at 10 and P – PE is positive in the months of January, February, and March. So, AE and PE are the same value.
STEP 8: When P – PE is negative, but ST is still above 0 for the entire month, AE and PE will still be the same value.
Example: At Berkeley, ST is still above 0, while P – PE is negative in the months of April and May, so the AE and PE are still the same.
STEP 9: Often we will have a month of transition where P – PE is a greater negative value than can be supplied by water in soil storage (ST). In this case AE is calculated by combining the precipitation from the current month and adding this with the storage that was available from the previous month:
Example: In Berkeley in the month of June P – PE was -7.9. There was only 3.4 cm of water in storage in the previous month May. To calculate the AE we combine the storage that was available in May and the precipitation (P) that fell in the month of June: 3.4 plus 0.5 equals 3.9 cm of AE. AE will always be equal to or less than PE, but never more.
STEP 10: If there is 0 water in storage and P – PE is a negative value, AE will be equal to only the amount of precipitation that falls.
Example: At Berkeley there is 0 water in storage and P – PE is negative in July, August, September, and October. So AE is equal to the precipitation that fell in each of these months only.
STEP 11: Once P – PE becomes positive AE and PE will be equal to one another.
Example: At Berkeley in November P – PE equals 1.9. This is also the value of AE since sufficient moisture is now available.
STEP 12: Deficits are only possible when there is 0 water in storage (ST), and is the difference between PE and AE.
Example: At Berkeley in June, ST is 0, PE is 8.4, and AE is 3.9. PE minus AE is 8.4 minus 3.9, which equals a deficit of 4.5 cm.
STEP 13: Surpluses are only possible when soil storage (ST) is at 10 cm, and is the difference between P and AE.
Example: At Berkeley in January P was 13.0 cm and AE was 2.6. The surplus is equal to P minis AE which is 13 minus 2.6, which equals a surplus of 10.4 cm.
Of course, Berkeley is a west-coast Mediterranean climate (distinct wet and dry seasons), and Terre Haute is a mid-latitude continental climate. How do these 2 locations compare in their surplus, deficit, usage, and recharge characteristics? Describe in detail, how and why these areas have differences in their characteristics. Remember to look at characteristics such as geographic position, topography, elevation, climate, prevailing winds, access to moisture, etc. T

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