Soil Water Budgets

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part 1: Bioclimatic Transects

Six sites comprise a transect (a transect is simply a route along which you make observations) across latitude, as shown in Figure 13.2. The latitudinal transect crosses almost 10,000 kilometers, but all six sites are within 550 meters of sea level.

Figure 13.2 Data Collection Sites

1. Complete Table 13.2 for sites along the middle longitudes of North America. Climatic data are found in Table 13.3. Calculate the climatic index variables to the nearest .01, then use the Life Zone Triangle to predict vegetation and other zones. Use an atlas vegetation map and photographs (Figure 13.4) to confirm or refute the initial predictions. Kugluktuk is complete as an example.

TABLE 13.2 Latitude Transect for Central North America

	Kugluktuk	Hay River	Williston	Concordia	New Orleans	Merida
T_B	2.08	4.49°C	8.05°C	12.32	20.58°C	25.83°C
PE	121.53	262 mm	470 mm	720 mm	1203 mm	1509 mm
PE/P	0.51	0.80	1.31	1.06	0.79	0.95
Life Zone	tundra	Forest	Grassland	Savanna	Forest	Savanna
Growing Zone	frigid	Cold	Cool	Warm	Warm	Hot
Altitude Zone	alpine	Subalpine	Montane	Foothills	Foothills	Lowland
Latitude Zone	subarctic	Boreal	Temperate	Subtropical	Subtropical	Tropical
Map Community	tundra	Taiga	Grassland	Grassland	Broadleaf forest	Savanna
Photograph Community	tundra	Taiga	Grassland	Grassland	Broadleaf forest	Woodland Savanna

TABLE 13.3 Mean Monthly Climate Data for the Latitude Transect

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Hay River, Northwest Territories (61°N, 116°W, elevation 166m)

T (°C) -25 -22 -17 -4 5 11 16 14 8 0 -12 -21

P (mm) 19 17 16 16 22 31 40 43 38 21 31 22

Williston, North Dakota (48°N, 104°W, elevation 549m)

T (°C) -13 -11 -4 6 12 17 21 20 14 7 -2 -9

P (mm) 13 11 19 29 47 77 49 37 31 19 14 13

Concordia, Kansas (40°N, 098°W, elevation 454m)

T (°C) -3 0 5 12 17 23 26 25 20 14 5 0

P (mm) 15 22 40 57 105 111 86 82 69 47 26 18

New Orleans, Louisiana (30°N, 090°W, elevation 1m)

T (°C) 12 14 17 20 24 27 28 28 26 22 16 13

P (mm) 118 119 129 124 117 145 171 156 132 84 100 122

Merida, Yucatan (21°N, 090°W, elevation 22m)

T (°C) 23 24 26 27 28 28 27 27 27 26 24 23

P (mm) 63 92 24 47 96 189 130 150 223 269 174 130

2. How consistently are the predicted vegetation communities correct, as verified by documentary data, for the latitudinal transect?

a. Number of sites correct (out of six): 4/6

b. List any erroneous sites: Concordia, possibly New Orleans (marsh)

c. What might account for the mispredictions at any erroneous sites?

Concordia: marginal life zone position near savanna/grassland/forest boundary

New Orleans: marginal location near marsh on map

3. How consistently are the biotemperature zones (to left side of the triangle) correct, as verified by documentary data, for the latitudinal transect?

a. Number of sites correct (out of six): 6/6

b. List any erroneous sites: none

c. What might account for the mispredictions at any erroneous sites?

4. a. What is the gradient of mean annual temperature (MAT) across the 47° of latitude difference between Kugluktuk and Merida? (Retain any signs!)

MAT₂ – MAT₁ =

Latitude₂ – Latitude₁

(-11.4 – 25.8)/(68 – 21) = - 0.79°C [per1°latitude]

b. What is the gradient of mean annual biotemperature (T_B) across the 47° of latitude difference between Kugluktuk and Merida? (Retain any signs!)

T_{B 2} – T_{B 1} =

Latitude₂ – Latitude₁

(2.04-25.8)/(68-21) = - 0.51°C [per1°latitude]

c. Why is the biotemperature gradient less than the MAT gradient?

TB is only for the “growing season”, and ignores temperatures <0. During the summer growing season temperature fradients across latitude are smaller than in winter.

5. What generalization can you make about the progression of life zones from low to high latitude along this central North America transect?

Tropical/subtropical forests, then continental interior grasslands, then subarctic needleleaf forest (taiga), and finally tundra

6. Table 13.4 provides data for a latitude transect along the west coast of North America.

TABLE 13.4 Latitude Transect for Western North America

	Fairbanks, Alaska (65°N, 148°W, elevation 147m)	Sitka, Alaska (57°N, 136°W, elevation 20m)	Vancouver, British Columbia (49°N, 123°W, elevation 3m)	Eureka, California (41°N, 124°W, elevation 13m)	El Rosario, Baja California (30°N, 115°W, elevation 82m)	Manzanillo, Colima (19°N, 105°W, elevation 3m)
T_B	4.92	6.33	9.82	11.14	22.46	26.48
PE	287.28	369.57	573.79	651.01	1312.24	1547.42
PE/P	0.91	0.16	0.52	0.67	7.36	1.52
Life Zone	forest	rainforest	forest	forest	hot desert	savanna
Growing Zone	cold	cool	cool	cool	warm	hot
Altitude Zone	subalpine	montane	montane	montane	foothills	lowland
Latitude Zone	boreal	temperate	temperate	temperate	subtropical	tropical
Map Community	taiga forest	temperate rainforest	temperate rainforest	temperate rainforest	hot desert	tropical forest
Photograph Community	taiga forest	temperate rainforest	temperate rainforest	temperate rainforest	hot desert	tropical forest

a. Describe the main similarities and differences between this and the central North America transect. Overall, the west coast transect has more humid biomes, except where coastal deserts are drier due to the combination of subtropical high pressure and cold ocean currents.

b. What generalization can you make about the progression of life zones from low to high latitude along this west coast transect? From low to high latitude, the progression is forest, then desert, then forest; there is no grassland or tundra along the west coast

c. Plot T_B against latitude for the six west coast locations on the graph. Use a ruler to draw a trend line through the center of these points. Then plot T_B against latitude for the six central locations and draw a second trend line through the center of these points. Label the two lines.

d. Notice that T_B usually is higher at comparable latitudes on the west coast, except at about 40°N. What might account for the higher T_B (thus more favorable growing season) for the central transect site at this latitude? Continenal interior in midlatitude westerlies has drier air, so more heat energy is used to raise temperature. Cold ocean currents and greater humidity along coasts at this latitude reduces heat energy available to raise temperature.

e. The T_B gradient for the west coast is 0.47 degrees celsius per degree of latitude, which is quite similar to the gradient for the central North America transect. The lines, while nearly parallel, do have some offset, and the vegetation sequence is quite dramatically different along the two transects. What does the offset signify, and how might this relate to the community differences? In general, the coast has greater moisture availability, higher TB (except for Eureka), less severe winters, and longer growing seasons

Just as temperature and moisture conditions differ with altitude, so too the vegetation communities at unlike elevations above sea level also vary. A good illustration of such altitudinal (sometimes called "vertical") zonation occurs along the Front Range of Colorado. At this continental interior location the high plains steppe environment abruptly ends, and there is a rapid transition through several distinctive bioclimatic zones as one moves into the Rocky Mountains. The altitudinal transect here consists of six data collection stations [NcH1] along an east-west line that is only 43 kilometers (27 miles) long, but entails a vertical rise of 2234 meters (7330 feet), with the lowest site (Longmont) being over 1500 meters above sea level. Figure 13.3 is a stylized vegetation profile configuration of the data stations, viewed from the south.

Figure 13.3 Front Range Data Sites Profile

Adobe Systems

7. Complete Table 13.5 for sites along the Colorado transect. Climate data are found in Table 13.6. Use Figure 13.3 instead of the vegetation maps and the photographs (Figure 13.5) to confirm or refute the initial predictions.

TABLE 13.5 Altitude Transect

	Niwot Ridge	Como Creek	Sugarloaf	Ponderosa	Boulder	Longmont
T_B	2.25°C	3.92°C	6.58°C	8.33°C	10.23°C	9.75°C
PE	131 mm	229 mm	385 mm	487 mm	597 mm	570 mm
PE/P	0.14	0.30	0.66	0.84	1.26	1.74
Life Zone	Tundra	Forest	Forest	Forest	Grassland	Grassland
Growing Zone	Frigid	Cold	Cool	Cool	Cool	Cool
Altitude Zone	Subarctic	Boreal	Temperate	Temperate	Temperate	Temperate
Latitude Zone	Alpine	Subalpine	Montane	Montane	Montane	Montane
Graph Community	Tundra	Pine Forest	Fir Forest	Pine Forest	Steppe	Steppe
Photograph Community	Tundra	Pine Forest	Fir Forest	Pine Forest	Elm Forest	Parkland

TABLE 13.6 Mean Monthly Climate Data for the Altitude Transect

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Niwot Ridge, Colorado (40.1N/105.6W, elevation 3743 m)

T (°C) -13 -13 -12 -7 -1 5 10 9 3 -6 -9 -12

P (mm) 92 91 105 102 68 70 80 57 72 39 72 78

Como Creek, Colorado (40.1N/105.5W, elevation 3048m)

T (°C) -7 -7 -6 -2 4 9 12 11 8 3 -3 -6

P (mm) 48 49 71 77 96 84 82 62 56 47 54 45

Sugarloaf, Colorado (40.1N/105.4W, elevation 2591m)

T (°C) -4 -3 -2 3 8 13 18 17 12 7 1 -3

P (mm) 14 28 48 50 96 74 49 56 58 59 24 26

Ponderosa, Colorado (40.1N/105.3W, elevation 2195m)

T (°C) -1 -1 1 7 11 16 20 19 14 9 3 0

P (mm) 13 25 53 51 105 73 38 48 56 60 26 31

Boulder, Colorado (40.1N/105.2W, elevation 1652m)

T (°C) 1 2 4 10 16 19 24 22 17 12 6 1

P (mm) 14 17 38 33 72 71 54 36 53 31 21 8

Longmont, Colorado (40.2N/105.1W, elevation 1509m)

T (°C) -3 0 2 9 14 18 22 21 16 10 4 -2

P (mm) 9 10 26 31 51 68 31 28 38 24 14 8

8. How consistently are the predicted vegetation communities correct, as verified by documentary data, for the altitudinal transect?

a. Number of sites correct (out of six): 4/6

b. List any erroneous sites: Boulder, Longmont

c. What might account for the mispredictions at any erroneous sites?

Marginal position in life zone graph; non-precipitation water sources, urban heat island

9. Notice that Boulder has higher temperature-related conditions (MAT, T_B, and PE) than Longmont, even though Boulder is at a somewhat higher elevation. The vegetation at Boulder also does not conform with the life zone prediction. What factors might help account for both the anomalously high temperatures and the apparently erroneous vegetation prediction for Boulder?

In a single word, humans. Boulder by 1987 (the end of the climatic data record) was a metropolitan area of some 80,000 persons, and the pavements, rooftops, and thermal emissions from various sources created an urban heat island with frequent inversions and a “dust dome”. Nearby (<10 km) semi-natural steppe locations without the urbanization are as much 2°C cooler for mean annual temperature. Boulder has significantly higher potential evapotranspiration (PE). However, humans have also imported water from the nearby mountains to raise the actual evapotranspiration (AE) by non-climatic moisture supply, which supports an artificial density of broadleaf trees within the city. Historical photographs of the city in 1876 show that its local landscape was classic steppe before human manipulation.

10. a. What is the gradient of mean annual temperature (MAT) across the 2243 meters of altitude difference between Niwot Ridge and Longmont? (Retain any signs!)

MAT₂ – MAT₁ =

Elevation₂ – Elevation₁

(-3.8-9.3)/2243 = - 0.006°C [per1 m of altitude]

b. What is the gradient of mean annual biotemperature (T_B) across the 2243 meters of altitude difference between Niwot Ridge and Longmont? (Retain any signs!)

T_{B 2} – T_{B 1} =

Elevation₂ – Elevation₁

(3.92-9.75)/2243 = - 0.003°C [per1 m of altitude]

c. Divide the temperature gradient from the central North America latitude transect by that of the altitude transect. How many meters of altitude change have the same effect on MAT as changing one degree of latitude?

-0.79/-0.006 = 131.67 m of altitude change are the equivalent of one degree of latitude change

d. Divide the biotemperature gradient from the central North America latitude transect with that of the altitude transect. How many meters of altitude change are the equivalent of changing one degree of latitude for T_B?

-0.51/-0.003 = 170.00 m of altitude change are the equivalent of one degree of latitude change