Page 306 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

aQuantitativeSOlUTiOn Thornthwaite Water Budget
Table AQS 9.1 presents, in random order, monthly data (in mm) in rows for potential evapotranspiration (PE), actual evapotranspira- tion (AE), precipitation (P), soil-moisture storage (ST), water deficit (D), and water surplus (S) for a Northern Hemisphere station. Can you determine which of these variables each row (1–6) represents, and explain your reasoning?
To solve this problem, we apply the Thornthwaite water budget described earlier in this chapter. To identify the rows, we must follow a logical sequence of reasoning—and we do not have to proceed with identifying Row 1 to Row 6 in that order!
To begin, although soil moisture storage and field capacity can vary depending on soil type and the crop planted, we set it at 300 mm for this example. Therefore, Row 4 can be determined as ST (soil-moisture storage) because the value 300 does not change from January to April. It decreases from May to September and then increases through October to become fully recharged in November.
Now consider Row 5, immediately below the row identified
as storage. When ST falls below 300 mm, water from storage is being utilized. Water is removed from storage only when precipi- tation alone is unable to meet demand. Because of the decrease in storage from soil-moisture utilization and the increase in values in Row 5, this row is D (deficit)—the unmet demand. Note that soil-moisture recharge begins in October and continues
into November when field capacity is restored.
A moisture deficit commonly occurs in conjunction with a decrease in precipitation, and there are only two rows that show a decrease in values. Row 1 has non-zero values in all months and shows a decrease that begins the month before the deficit appears. This decrease continues through the summer months and then increases into the fall when the deficit ends. Row 3 also shows a similar pattern, but it has values of zero for some of the year. We can therefore conclude that Row 1 is P (precipitation) and Row 3 is S (water surplus).
This leaves two rows, numbers 2 and 6, to be labelled. These two rows contain values that are remarkably similar. They must represent AE and PE, the only two labels remaining. But which
TABLE AQS 9.1 Weather Station Data
one is which? Remember, potential evapotranspiration is the amount of evapotranspiration (evaporation + transpiration) that would occur if an unlimited water supply were available. It rep- resents the demand that the atmosphere makes on the available water supply. Remember also that if P > PE, then AE = PE,
and that D occurs when P < PE. Therefore, we can say that
D = PE − AE. An examination shows that the two remaining rows have equal values from January to April and from October to December. But when the deficit occurs in the summer months, val- uesinRow2arelessthaninRow6.SoRow6isPEandRow2isAE.
A graph of P, PE, and AE can be used to determine the type of climate depicted (Figure AQS 9.1). Keep in mind that we already know it is a Northern Hemisphere station and therefore which months are the winter and summer seasons. Here we see that
the precipitation is greatest during winter months. Both PE and AE increase in summer and both decrease in winter. We describe climate types in the next chapter. You will see that this graph is typical of a Marine West Coast climate, with all months averaging above freezing, a cool summer, and abundant precipitation.
200 150 100
50 0JFMAMJJASOND
▲Figure AQS 9.1 Sample water budget graph.
= Water surplus P = Soil-moisture utilization AE = Soil-moisture recharge PE = Water deficit
J
F
M
A
M
J
J
A
S
O
N
D
TOTAL
row 1 147 117 94 61 48 45 30 37 61 122 141 165 1068 row 3 139 102 64 13 0 0 0 0 0 0 35 154 507 row 5 0 0 0 0 2 6 24 29 5 0 0 0 66
Field capacity 5 300 mm
row 2
8
15
30
48
72
89
85
71
64
45
23
11
561
row 4
300
300
300
300
276
232
177
143
140
217
300
300
—
row 6
8
15
30
48
74
95
109
100
69
45
23
11
627
270
Amount of water (mm)