Extreme Events Extreme events are defined as “most unusual” climatic events at a given place (natural and anthropogenic causes) “Most unusual” can be categorized as occurring less than 10 percent of the times based on historical observations Climatic events: Flooding Droughts/heat waves Tropical cyclones Severe local storms (tornado, hail, wildfires, crop freeze, winter storms, straight-line wind damage, etc.) Can cause severe socio-economic and environmental damage Most often, flood occurs due to natural weather/climate variability Anthropogenic reasons includes human impacts that contribute to change in climate including increase in greenhouse gas emission, land use change, urbanization, increase demand of water, water rights, water allocation, etc. Focus of this unit is only “flood and drought”

What is flood? Excess water caused by heavy rains and/or rapid snowmelt Usually happens on a flat or low-lying areas when the ground is saturated and water cannot run off quickly or not at all Types: areal, riverine, estuarine and coastal, urban, catastrophic Look at some pictures from: https://en.wikipedia.org/wiki/Flood Details on type of flooding can be found at: http://en.wikipedia.org/wiki/Flood

Magnitude-Frequency Analysis for Flood Direct observations of stream flow can be used for magnitude-frequency analysis Frequency of floods are useful information in designing water resources infrastructures. For example, a flood of 5 to 10 yr. frequency (or return period) is used in the design of highway culvers for low traffic. Similarly: Culverts (high-traffic): 50-100 yrs. RP Highway bridge: 10-100 yrs. RP Dam: 50-100 yrs. RP Urban drainage: 2-25 yrs. (small cities) and 25-50 yrs. (large cities)

Flood Frequency Analysis Best fit line (normally fitted using Log-Pearson Type III Distribution) Flood frequency distributions can take on many forms according to the equations used to carry out the statistical analyses. Four of the common forms are: Normal; Log-Normal; Gumbel; and Log-Pearson Type III Distribution. Each distribution can be used to predict design floods; however, there are advantages and disadvantages of each technique. Click on the above links to learn more about each technique. According to the U.S. Water Advisory Committee on Water Data, the Log-Pearson Type III Distribution is the recommended technique for flood frequency analysis.  Detail on using Log-Pearson Type III Distribution frequency analysis can be found at: http://streamflow.engr.oregonstate.edu/analysis/floodfreq/index.htm Answer: Select a point on the curve corresponding to RP of 50 years in x-axis, and read off the value in Y-axis: 56,500 cfs Select a point on the curve corresponding to discharge 45,000 cfs in Y-axis, and read off the value in X-axis: 11 years What is the magnitude of flood that has a return period of 50-years? What is the frequency of a flood with a magnitude of 45,000 cfs?

DDF: Depth Duration Frequency curves Answer: Choose 12-hr. in x-axis and find graph representing 25 years of return period (Average recurrence interval) and read off Y-axis for precipitation depth in inches: 5 inches (PMP) Generated from http://hdsc.nws.noaa.gov for Greensboro, North Carolina What is the precipitation depth for a 12-hr. duration storm that is exceeded on average once in in 25 years (return period = 25 years)?

IDF: Intensity Duration Frequency curves Answer: Choose 12-hr. in x-axis and find graph representing 25 years of return period (Average recurrence interval) and read off Y-axis for precipitation intensity: 0.16 inches per hour Generated from http://hdsc.nws.noaa.gov for Greensboro, North Carolina What is the intensity of precipitation of 12-hr. duration that has a 25-year return period?

What is drought? Deficiency of rain (low to none) for an extended period of time Look at some pictures from: https://en.wikipedia.org/wiki/Drought https://en.wikipedia.org/wiki/Drought Read: http://www.nws.noaa.gov/os/brochures/climate/DroughtPublic2.pdf

Drought Characteristics Droughts are among the most costly weather-related events in terms of economics and loss of life Key aspects of drought: severity, duration, time of occurrence and spatial extent Defined in a different number of ways: Meteorological drought: Deficit in precipitation; It represents degree of dryness compared to the average condition and duration of the dry period Agricultural drought: Deficit in soil moisture; It precedes precipitation deficit (meteorological drought); Reduce agricultural production due to lack of water for irrigation; Increase wildfires Hydrological drought: Deficit in surface and ground water; Impacts water supply for drinking water, irrigation, industrial needs, hydropower, etc.; Significant societal impact Droughts differ from one another in three essential characteristics: intensity, duration, and spatial coverage. Moreover, many disciplinary perspectives of drought exist. Because of these numerous and diverse disciplinary views, considerable confusion often exists over exactly what constitutes a drought.  Read more: http://drought.unl.edu/DroughtBasics/TypesofDrought.aspx How do the following levels of drought impact the urban water system (answers in parentheses)? 1. Meteorological drought (affects rainwater harvesting and outdoor water use) 2. Agricultural drought (might not affect urban water system, but there are feedbacks between drought and ag productivity)