Page 283 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 9 Water resources 247
storage acts as a savings account, accepting deposits and yielding withdrawals of water. Sometimes all expenditure demands are met, and any extra water results in a surplus. At other times, precipitation and soil-moisture savings are inadequate to meet demands, and a deficit, or water short- age, results.
Geographer Charles W. Thornthwaite (1899–1963) pioneered applied water-resource analysis using water budgets, working with others to develop a methodology for solving real-world problems related to irrigation and water use for maximizing crop yields. Thornthwaite also recognized the important relationship between water supply and water demand as it varies with climate.
Components of the Water Budget
To understand Thornthwaite’s water-budget methodology and “accounting” procedures, we must first define certain terms and concepts. We begin by discussing water supply, demand, and storage as components of the water-budget equation.
Water Supply: Precipitation The moisture supply to Earth’s surface is precipitation (P) in all its forms, such as rain, snow, or hail. A summary table of the different types of precipitation is on the MasteringGeography web- site. As you read through this table, recall the forms of precipitation you have experienced.
One way to measure precipitation is with a rain gauge, essentially a large measuring cup that collects rainfall and snowfall so the water can be measured by depth, weight, or volume (Figure 9.6). Wind causes un- derestimation because the drops or snowflakes are not falling vertically; the wind shield reduces the undercatch by catching raindrops that arrive at an angle.
▲Figure 9.6 A rain gauge. a funnel guides water into a bucket sitting on an electronic weighing device. The gauge minimizes evaporation, which would cause low readings. The wind shield around the top of the gauge minimizes the undercatch produced by wind. [Bobbé Christopherson.]
Regular precipitation measurements are made at more than 100000 locations worldwide. A global map of annual precipitation averages is displayed in Chapter 10. Figure 9.7 shows precipitation patterns in Canada and the United States, which relate to the air masses and lift- ing mechanisms presented in Chapter 8.
Water Demand: Potential Evapotranspiration Evapo- transpiration is an actual expenditure of water to the atmosphere. In contrast, potential evapotranspiration (PE) is the amount of water that would evaporate and transpire under optimum moisture conditions when adequate precipitation and soil moisture are present. Filling a bowl with water and letting the water evaporate illustrates this concept: When the bowl becomes dry, some degree of evaporation demand remains. If the bowl could be constantly replenished with water, the amount of water that would evaporate given this constant sup- ply is the PE—the total water demand. If the bowl dries out, the amount of PE that is not met is the water deficit. If we subtract the deficit from the potential evapotrans- piration, we derive what actually happened—actual evapotranspiration (AE).
Precise measurement of evapotranspiration is dif- ficult. One method employs an evaporation pan, or evaporimeter. As evaporation occurs, water in measured amounts is automatically replaced in the pan, equaling the amount that evaporated. A more elaborate measure- ment device is a lysimeter, which isolates a represen- tative volume of soil, subsoil, and plant cover to allow measurement of the moisture moving through the sam- pled area (an illustration is on the MasteringGeography website). A rain gauge next to the lysimeter measures the precipitation input.
Thornthwaite developed an easy and fairly accurate indirect method of estimating PE for most midlatitude lo- cations using mean air temperature and daylength to ap- proximate PE. (Remember from Chapter 2 that daylength is a function of a station’s latitude.) His method works well for large-scale regional applications using data from hourly, daily, monthly, or annual time frames, and works better in some climates than in others.
Figure 9.8 presents PE values for the United States and Canada derived using Thornthwaite’s approach. Note that higher values occur in the South, with highest read- ings in the Southwest, where higher average air tempera- ture and lower relative humidity exist. Lower PE values are found at higher latitudes and elevations, which have lower average temperatures.
Compare Figures 9.7 and 9.8, the maps of water sup- ply in the form of precipitation and water demand in the form of potential evapotranspiration. Can you identify regions where P is greater than PE (for example, the lower Great Lakes Basin)? Or where PE is greater than P (for ex- ample, the southwestern United States)? Where you live, is the water demand usually met by the precipitation supply? Or does your area experience a natural shortage? How might you find out?