This short video from the New York Times describes the economic and environmental impacts of the severe drought that occurred in California in 2014.
Identifying an area as ‘in drought’ is different from identifying it as ‘arid’. While the two may seem related, the subtle difference is important. Aridity is defined as the “degree to which a climate lacks effective, life-promoting moisture” (Glossary of Meteorology, American Meteorological Society). Drought, on the other hand, is ‘a prolonged period of abnormally dry conditions.’ Thus, aridity is a quasi-permanent condition (persistent over human timescales), while drought is a temporary condition (which may persist for weeks, years, or in some cases, decades). The Sahara Desert is an arid environment. The Hoh rainforest in western Washington State is a very humid place that occasionally experiences drought.
Droughts tend to be somewhat elusive phenomena, with severity gradually increasing over many days, weeks, months, or even years. The spatial extent of drought is also quite difficult to delineate, due to the spatial variability in precipitation. Therefore, they are much harder to define, monitor, and identify (relative to floods) within the ‘noisy’ background of natural wet and dry cycles. Yet the impacts of drought can be significant on many facets of the economy and environment. All types of drought originate from a deficiency of precipitation from an unusual weather pattern. If the weather pattern persists for a few to several weeks, it is said to be a short-term drought. However, if precipitation remains well below average for several months to years, the drought is considered to be a long-term drought.
Related to the difficulty in defining drought, economic damages related to drought are also difficult to define. But only considering economic damages that can be directly related to drought, it is clear that they too can be costly natural disasters. In 2013, EM-DAT claims only 9 deaths worldwide that were directly attributed to drought (15 is global annual average from 2003-2012), but a total of nearly 8 million people were significantly affected by drought in 2013 (average is over 36 million per year from 2003-2012). Damages related directly to drought in 2013 were estimated in excess of $1 billion (average of nearly $5 billion per year from 2003-2012). However, these numbers do not include related effects of wildfire and indirect effects of decreased food production, water quality, etc.
Phenomenona and direction of trend | Likelihood that trend occurred in late 20th century (typically post 1960) | Likelihood of a human contribution to observed trendb | Likelihood of future trends based on projections for 21st century using SRES scenarios |
---|---|---|---|
Warmer and fewer cold days and nights over most land areas | Very likelyc | Likelyd | Virtually certaind |
Warmer and more frequent hot days and nights over most land areas | Very likelye | Likely (nights)d | Virtually certaind |
Warm spells/heat waves. Frequency increases over most land areas | Likely | More likely than notf | Very likely |
Heavy precipitation events. Frequency (or portion of total rainfall from heavy falls) increases over most areas | Likely | More likely then notf | Very likely |
Area affected by drought increases | Likely in many regions since 1970s | More likely than not | Likely |
Intense tropical cyclone activity increases | Likely in some regions since 1970 | More likely than notf | Likely |
Increased incidence of extreme high sea level (excludes tsunamisg | Likely | More likely than notf,h | Likelyi |
Table notes:
a See table 3.7 for further details regarding definitions.
b See table TS.4, Box TS.5 and table 9.4.
c Decreased frequency of cold days and nights (coldest 10%).
d Warming of the most extreme days and nights of each year.
e Increased frequency of hot days and nights (hottest 10%).
f Magnitude of anthropogenic contributions not assessed. Attribution for these phenomena based on expert judgment rather than formal attribution studies.
g Extreme high sea level depends on average sea level and on regional weather systems. It is defined here as the highest 1% of hourly values of observed sea level at a station for a given reference period.
h Changes in observed extreme high sea level closely follow the changes in average sea level. {5.5} It is very likely that anthropogenic activity contributed to a rise in average sea level. {9.5}
i In all scenarios, the projected global average sea level at 2100 is higher than in the reference period. {10.6} The effect of changes in regional weather systems on sea level extremes has not been assessed.
There are four different kinds of drought.
Go to the US Drought Monitor webpage [2] and answer the following questions:
1. Is the place where you live currently in a drought?
2. Looking back through historical maps, when was the last time your home town was in a drought?
ANSWER: The answer to this question will be different for everyone. Write your answers down and be prepared to talk about it with the class if it comes up.
Many different indices have been developed over the past several decades to indicate the occurrence and severity of drought. The simplest index relates precipitation amounts during a specific period of time to the historical average during that same time period. For example, precipitation for the month of June 2014 was 15% below the historical average for Wenatchee, Washington. While this statement conveys some useful information, it is not possible to determine whether or not that 15% deficit qualifies for any of the definitions of drought. The number of days with no precipitation is another simple index, but again must be considered in the context of historical data or water demand, and there is no standard definition for what number of days without precipitation would necessarily qualify under any of the four types of drought. Also, if an area receives a very small amount of precipitation (< 0.1 cm) during an otherwise unusually dry time period, a strict interpretation of this index would ‘reset the clock’, but in reality, the severity of the water deficit remains essentially unchanged. Complex phenomena, such as drought, require somewhat complex metrics to be measured in a meaningful way.
The Standardized Precipitation Index (SPI) is a slightly more complex measure of precipitation deficit that compares measured precipitation to the median historical precipitation over multiple timescales, ranging from one month to 24 months. As dry or wet conditions become more severe, SPI becomes more negative or positive, respectively. Several different indices of varying complexity have been developed to assess drought based on both water supply and demand using multiple environmental criteria. The most common index used to define and monitor drought is the Palmer Drought Severity Index (PDSI), which attempts to measure the duration and intensity of long-term, spatially extensive drought, based on precipitation, temperature, and available water content data. PDSI ranges from values exceeding 4.0, which are considered extremely wet, to values below -4.0, which are considered extreme drought (see Figure 12). Weekly maps of PDSI for the entire US (current and historical) can be viewed on the web page maintained by the National Weather Service Climate Prediction Center [3].
Related indices are the Palmer Z Index, which attempts to measure short-term drought on a monthly timescale, the Palmer Crop Moisture Index, which attempts to measure short-term drought and quantify impacts on agricultural productivity, the Palmer Hydrological Drought Index, which attempts to estimate the long-term effects of drought on reservoir levels and groundwater levels. An immense compilation of current and historical drought information for the entire US is freely available on the US Drought Monitor web page [2], maintained by the University of Nebraska National Drought Mitigation Center.
Increasingly, government and industry groups are using ‘cloud seeding’ techniques to induce precipitation and reduce the severity of a drought. One of the potentially limiting steps in the formation of precipitation is the presence of tiny particles (nuclei) on which water can condense and coalesce to form raindrops or ice crystals large enough to begin falling through the air. Cloud seeding is the practice of injecting nucleating agents, such as silver iodide (AgI), into clouds in an attempt to form precipitation. The effectiveness of these approaches is questionable, but under the right conditions, cloud seeding may increase the probability of rain and therefore it is practiced in some semi-arid regions, including the western US. However, questions remain regarding environmental and human health impacts as well as concerns regarding ‘stealing’ atmospheric moisture from would-be recipients downwind.
1. What was the Palmer Drought Severity Index for the week ending on Aug 30, 2014, for the following locations (see Figure 12 above):
St. Louis, Missouri
ANSWER: 0
Davis, California
ANSWER: -4
Miami, Florida
ANSWER: -3
2. Which of these three locations were likely experiencing socio-economic drought during this time, forcing them to actually change water use/management practices, at least temporarily?
St. Louis, Missouri
David, California
Miami, Florida
ANSWER: Definitely Davis, CA, probably Miami, Fl
Variation in river flow (i.e., the river flow regime – see Module 3) exerts a strong influence on river and riparian ecosystem function. In particular, floods and droughts control the creation and maintenance of river and floodplain habitats and the sustainability of the high biodiversity observed along river systems. The temporal pattern of floods interacts with channel and floodplain topography to create a highly heterogeneous landscape of depressions, oxbows, gravel bars, and terraces (Figure 13). The hydro-geomorphic diversity means that the inundation frequency varies strongly over short distances on river floodplains, and creates habitats for a diverse suite of organisms adapted to a wide range of flooding frequencies.
Both riparian and aquatic organisms have adapted to take advantage of flood-drought cycles in river ecosystems. For example, many fish species time spawning runs to coincide with predictable floods, because this allows large adult fish to access small streams that provide optimal habitat for egg development and growth and survival of young fish. In the Amazon River, many fish species can almost be considered forest-dwelling fish, because they feed directly on leaves, fruits, seeds, and insects that fall into the river when it floods surrounding forests during the annual rainy season. Trees of these seasonally flooded forests have in turn developed fruits and seeds that mature during the flooding season and that can survive fish digestive systems in order to take advantage of the seed dispersal ability of mobile fish species. In the western U.S., cottonwood trees time the release of seeds to coincide with the recession of flood peaks in order to access fresh sediment deposits with elevated water tables that provide ideal habitats for germination.
It may be less obvious that droughts could be beneficial for aquatic and riparian biota, but when coupled with periodic flooding, droughts play an important role in the survival of many river organisms. During droughts, resources such as organic material and nutrients can accumulate on floodplain surfaces, and when a flood does occur there is a pulse of greater resource availability than would occur under regular flooding, and this period of high resource availability can ensure the quick growth and survival of organisms, including young fish. In addition, periodic drying of rivers and floodplain wetlands eliminates competitors and predators for organisms that can quickly colonize areas when water returns. Such areas of refuge from predators are critical for the persistence of many aquatic organisms and would not exist without periods of drought.
The importance of floods and droughts to the integrity of river-floodplain ecosystems is apparent when alterations to the natural flow regime occur. Riverine organisms are often closely adapted to the local magnitude, frequency, duration, and predictability of extreme events, such that alteration of any one component can threaten species persistence. For example, recruitment of cottonwood trees along many dammed rivers in the western U.S. has essentially ceased, because the dams prevent flooding and creation of germination sites during the spring when cottonwood trees release their seeds. Excessive drought is also highly detrimental to river systems. One of the most famous examples of drought impacts is seen in the Colorado River delta in Mexico, which was once a highly productive floodplain forest and swamp, but due to prolonged drought conditions in the river basin and water infrastructure development, is now a dry desert.