Page 288 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

252 part II The Water, Weather, and Climate Systems
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running in the tens of billions of dollars each year in North America.
Hydrological drought refers to the effects of precipita- tion shortages (both rain and snow) on water supply, such as when streamflow decreases, reservoir levels drop, mountain snowpack declines, and groundwater mining increases.
(a)
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Vancouver, British Columbia
• Socioeconomic drought results when reduced water supply causes the demand for goods or services to ex- ceed the supply, such as when hydroelectric power production declines with reservoir depletion. This is a more comprehensive measure that considers water rationing, wildfire events, loss of life, and other wide- spread impacts of water shortfalls.
Drought is a natural and recurrent feature of climate. In the southwestern United States, drought conditions have existed since early 2000, one of several such droughts evident in the region’s climatic record over the last 1000 years. However, scientists are finding mounting evidence that this increased aridity, or climatic dryness, links not only to natural factors but also to global climate change and a poleward expansion of the subtropical dry zones. Thus, human-caused warming is combining with natural climate variability to create a trend toward lasting drought.
Studies suggest that droughts such as those occur- ring previously in the U.S. Southwest, which were re- lated to sea-surface temperature changes in the distant tropical Pacific Ocean, will still occur, but they will be worsened by climate change and the further expansion of the hot, dry, subtropical high-pressure system and the summertime continental tropical (cT) air mass into the region. The spatial implications of such shifting of Earth’s primary circulation systems and semi-permanent drought into a region of steady population growth and urbanization are serious and suggest the immediate need for water-resource planning.
Droughts of various intensities occurred on every continent throughout 2014. Australia is in a decade-long drought, its worst in 110 years, correlating with record high temperatures, as discussed in Chapter 5. In 2010 and 2011, the Horn of Africa (including Somalia and Ethiopia) experienced the worst drought in 60 years and an associ- ated food crisis. Research suggests that ongoing drought patterns in east Africa are tied to sea-surface temperature patterns in the Indian Ocean.
In the United States, in July of 2012, the U.S. Department of Agriculture declared almost one-third of all U.S. counties federal disaster areas owing to drought conditions—the largest natural disaster area ever de- clared. According to the National Climatic Data Center, 16 of the droughts that occurred from 1980 to 2011 cost over $1 billion each, making drought one of the costli- est U.S. weather events. (See www.drought.unl.edu/ for a weekly Drought Monitor map and other resources for as- sessing and mitigating drought.) Although Canada is not experiencing this level of drought, a Drought Watch page exists at the Agriculture Canada website (www.agr.gc.ca/ eng/?id=1326402878459).
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= Surplus
= Soil-moisture utilization = Soil-moisture recharge = Deficit
PRECIP POTET ACTET
▲Figure 9.12 Sample water budgets for stations near two North American cities.
only precipitation, temperature, and soil-moisture con- tent, but also the water-resource demand.
• Meteorological drought is defined by the degree of dryness, as compared to a regional average, and the duration of dry conditions. This definition is region- specific, since it relates to atmospheric conditions that differ from area to area.
• Agricultural drought occurs when shortages of pre- cipitation and soil moisture affect crop yields. Al- though agricultural drought evolves slowly and gets little media coverage, losses can be significant and are
Soil moisture (mm*) Soil moisture (mm)