Page 287 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 9 Water resources 251
Moisture supply
Actual moisture demand
Moisture oversupply
Precipitation (P)
Actual evapotranspiration (AE)
Surplus (S)
Δ ST Soil-moisture utilization (–) or Soil-moisture recharge (+)
Satellite
Global Water Balance Components
▲Figure 9.10 The water-budget equation explained.
balance; that is, the precipitation input (left side) must
equal the outputs (right side).
Sample Water Budgets
As an example, study the water-budget graph for the city of Hamilton, Ontario. Figure 9.11 plots P, AE, and PE, using monthly averages, which smooth the actual
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90 80 70 60 50 40 30 20 10
0
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
Month
▲Figure 9.11 Sample water budget for Hamilton, Ontario. Compare the average monthly values of precipitation inputs and potential evapotranspiration outputs to determine the condition of the soil-moisture environment. a typical pattern of spring surplus, summer soil-moisture utilization, a small summer deficit, autumn soil-moisture recharge, and early winter surplus highlights the year. Snow accumulation occurs in January and February when average temperatures are below freezing. During this time, snow is stored on top of the ground and does not recharge soil moisture. in March, when temperatures warm, snow melts, the ground thaws, and soil- moisture storage returns to 300 mm.
daily and hourly variability. The cooler time from October to May shows a net surplus (blue areas), as pre- cipitation is higher than potential evapotranspiration. The warm days from June to September create a net water demand. If we assume a soil-moisture storage capacity of 100 mm typical of shallow-rooted plants, this water demand is satisfied through soil-moisture utilization (green area), with a small summer soil- moisture deficit (orange area).
Hamilton experiences water supply-and-demand patterns typical of a humid continental region. In other climatic regimes, the relationships between water- budget components are different. Figure 9.12 presents water-budget graphs for the cities of Vancouver, British Columbia, which has a summer minimum in precipita- tion; and Phoenix, Arizona, which has low precipitation throughout the year. Compare the size and timing of the water deficit at these locations with the Hamilton graph.
Drought: The Water Deficit
Drought is a commonly used term, and might seem sim- ple to define: Less precipitation and higher temperatures make for drier conditions over an extended period of time. However, scientists and resource managers use four distinct technical definitions for drought based on not
= Water surplus P = Soil-moisture utilization AE = Soil-moisture recharge PE = Water deficit
= Surface storage, snow/ice
=+±
Moisture demand
Potential evapotranspiration (PE)
Deficit (D)
–
Moisture shortage
Moisture savings
Hamilton, Ontario
CRiTiCALthinking 9.1 Your Local Water Budget
Select an area of interest, such as your campus, yard, or even a houseplant container, and apply the water-budget concepts to it. Where does the water supply originate—its source? estimate the ultimate water supply and demand for the area you selected. For a general idea of P and Pe, find your locale on Figures 9.7 and 9.8. Consider the seasonal tim- ing of this supply and demand, and estimate water needs and how they vary as components of the water-budget change. •
Soil moisture (mm)