Page 286 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

250 part II The Water, Weather, and Climate Systems
Soil-moisture (increasing)
◀Figure 9.9 Types and availability of soil moisture. [(a) after D. Steila, The geog- raphy of Soils, © 1976, p. 45, © Pearson Prentice Hall, inc. (b) after U.S. Department of agriculture, 1955 yearbook of agriculture—Water, p. 120.]
Satellite
Global Water Balance Components
moisture can efficiently penetrate to recharge soil- moisture storage. You may have found yourself work- ing to improve soil perme- ability for a houseplant or garden—that is, working the soil to increase the rate of soil-moisture recharge.
Water Deficit and Surplus
A deficit, or moisture short- age, occurs at a given loca- tion when the PE demand cannot be satisfied by pre- cipitation inputs, by mois- ture stored in the soil, or through additional inputs of water by artificial irriga- tion. Deficits cause drought conditions (defined and discussed just ahead).
A surplus occurs where additional water exists after PE is satisfied by precipi- tation inputs; this surplus often becomes runoff, feed-
H2O unavailable for plants
H2O available for plants
H2O unavailable for plants
Soil particles with forms of soil moisture
Hygroscopic H2O* Capillary H2O Gravitational H2O
Wilting point
Field capacity
(a) Hygroscopic water bound to soil particles and gravitational water draining through the soil moisture zone are not available to plant roots.
Gravitational water
Saturation
30.0 22.5 15.0
7.5
0.0
Sand Sandy loam
Loam
Silt
loam loam
Clay
(b) The relationship between soil-moisture availability and soil texture determines the distance between the two curves that show field capacity and wilting point. A loam soil (one-third each of sand, silt, and clay) has roughly the most available water per vertical foot of soil exposed to plant roots.
*Note: Some capillary water is bound to hygroscopic water on soil particles and is also unavailable.
the soil, plants have increased difficulty extracting the amount of moisture they need. Eventually, even though a small amount of water may remain in the soil, plants may be unable to use it. In agriculture, farmers use irrigation to avoid a deficit and enhance plant growth with adequate amounts of available water.
When water infiltrates the soil and replenishes avail- able water, whether from natural precipitation or artificial irrigation, soil-moisture recharge occurs. The property of the soil that determines the rate of soil-moisture recharge is its permeability, which depends on particle sizes and the shape and packing of soil grains.
Water infiltration is rapid in the first minutes of precipitation and slows as the upper soil layers be- come saturated, even though the deeper soil may still be dry. Agricultural practices such as ploughing and adding sand or manure to loosen soil structure can im- prove both soil permeability and the depth to which
ing surface streams and lakes and recharging ground- water. Under ideal conditions for plants, potential and actual amounts of evapotranspiration are about the same, so plants do not experience a water shortage.
The Water-Budget Equation
The water-budget equation explained in Figure 9.10 states that, for a certain location or portion of the hydrologic cycle, water inputs are equal to the water outputs plus or minus the change in water storage. The delta symbol, D, means “change”—in this case, the change in soil-moisture storage, which includes both recharge and utilization.
In summary, precipitation (mostly rain and snow) provides the moisture input. This supply is distributed as actual water used for evaporation and plant transpi- ration, extra water running into streams and subsurface groundwater, and water that moves in and out of soil- moisture storage. As in all equations, the two sides must
Available water
Unavailable water
Clay
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Available water per vertical metre of soil (cm · m–1)
p
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F
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