Geography 511: Hydrology Exam 1
Hydrograph
A graph plotting flow (usually cfs) against time. Some include precipitation on a third axis.
Longitudinal vs. paired catchment studies
A longitudinal study would be very useful if looking streamflow of a watershed before and after a large fire. Common for environmental studies and accuracy depends on repeatability and data storage. The period before the fire is the 'calibration period' and after is the 'simulation period'. A paired watershed could be useful in looking at effects of forest management on water yield, nutrient export, etc. Sometimes they strip trees from a whole area.
No-analogue conditions
Alternately novel, climatic conditions" (no-analog climates) or biological communities (no-analog communities) in paleoecology and ecological forecasting are those without current equivalents. Projections based on climate models and species-distribution-models
AEP
Annual exceedance probability for a 1-in-100 year event: 1/100=.01 (or 1% chance of that happening). For a 1-in-500 year event: 1/500=0.002 (multiply by 100, so a 0.2% chance of that happening).
Describe today's global temperature, and rate of temperature change, in comparison to global temperatures over the last 800,000 years.
As the Earth moved out of ice ages over the past million years, the global temperature rose a total of 4 to 7 degrees Celsius over about 5,000 years. In the past century alone, the temperature has climbed 0.7 degrees Celsius, roughly ten times faster than the average rate of ice-age-recovery warming.
DESCRIPTOR: Spatial scale of change
Global: Systematic change Local: Cumulative changes may eventually lead to global impacts (ex: wetlands, rivers, aquifer depletion, urban heat islands)
Interglacial Maximum
Period of time between glacial periods where sea level is highest
Flow Duration curve
Plot that shows the percentage of time that flow in a stream is likely to equal or exceed some specified value of interest.
Interception
Precip that lands on leaves, branches and forest floors, not making the soil. Occurs in canopy and floor or litter layer
Degree day
a) converts the number of degree days into snowmelt depths b) M= DDF x T c) M = DDF x (T-Tref) d) As snow ages, snow water content and hence density increases, albedo decreases.
Return period
Return period- recurrence interval. Estimate of likelihood of an event happening. T=(N+1)/m T = Return Period N= Number of years in record m= Rank
characteristics and differences between river floods, coastal floods and urban floods
River Flood: Flooding along rivers is a natural and inevitable part of life. Some floods occur seasonally when winter or spring rains, coupled with melting snows, fill river basins with too much water, too quickly. Torrential rains from decaying hurricanes or tropical systems can also produce river flooding. Coastal Flood: Winds generated from tropical storms and hurricanes or intense offshore low pressure systems can drive ocean water inland and cause significant flooding. Escape routes can be cut off and blocked by high water. Coastal flooding can also be produced by sea waves called tsunamis , sometimes referred to as tidal waves. These waves are produced by earthquakes or volcanic activity. Urban Flood: As land is converted from fields or woodlands to roads and parking lots, it loses its ability to absorb rainfall. Urbanization increases runoff 2 to 6 times over what would occur on natural terrain. During periods of urban flooding, streets can become swift moving rivers, while basements can become death traps as they fill with water. In urban floods, water is flowing towards the river; in river floods, water is flowing out of the river. Others: Ice Jam, Bridge Jam, Dam burst
Explain what is meant by a "1-in-N year flood", and why these may not occur exactly N years apart. Explain why 1-in-N year floods occur more often than 1-in-N years when considering multiple catchments.
See above. When considering multiple catchments, a 1-in-100 year flood might occur in a neighboring catchment upstream of the one we are studying, in which case that flood would impact the catchment
3 components of Milankovitch cycles
Slight change in Earths' orbital eccentricity (shape of orbit) + obliquity (angle earth's axis makes with plane of the earth's orbit) and precession (direction of Earths' axis of rotation) Cause of climate variation historically.
Milankovitch cycles
Slight change in Earths' orbital eccentricity (shape of orbit) + obliquity (angle earth's axis makes with plane of the earth's orbit) and precession (direction of Earths' axis of rotation) Cause of climate variation historically.
Explain how accumulation and melting of snow change patterns of river flow at annual and daily timescales.
Snow cover changes with: Seasons, elevations. Snow accumulation depends on the slope of terrain Highest accumulation between 10-25* slope steep slopes don't hold snow Accumulation differences prolong melt season
SWE
Snow water equivalent. Allows us to specify total water even during thawing and refreezing of snowpack. Temperature: during and after snowfall affects the size of snow grains, compaction and moisture content Snow density: directly affects the SWE value. Light, powdery snow has a low density and lower SWE value. Snow that falls mixed with rain in near-freezing temperatures can have a high density and higher SWE How to calculate: inches of snow X snow density % = SWE
Paired watershed
Two watersheds where one is altered and the other is a control. -SAME CLIMATE OR SIMILAR CATCHMENT -Ex: Forest management effect on water yield, sediment yield, nutrient transport, water temp, etc.
Define "accuracy", "precision", "systematic error", "random error"
-Accuracy: Closeness of a measurement to true value. The higher the accuracy, the lower the error -Precision: Closeness of multiple observations to one another, or the repeatability of measurement -Systematic error: An error having a nonzero mean, so that its effect is not reduced when observations are averaged -Random error: An error in measurement caused by factors that vary from one measurement to another
Give three examples of why it is useful to divide an area into similar "hydrologic regions".
-Better experimental and monitoring networks to cover all catchments -identify groups of catchments for similar management -choose appropriate computer simulation for poorly understood regions -Potential impacts of land use and climate changes on catchment scale hydrologic response in different parts of the world.
Longitudinal study
-Data is collected for a long period of time. Many environmental studies are longitudinal Ex: -Following stream flow below, during, and after a rain -Measuring light penetration on a forest floor during the day or throughout the year -Doing transect/quadrant studies to determine levels of invasive plants over many years. -Longitudinal studies may include sudden events or gradual changes, and the changes may be planned or unexpected. -SAME CATCHMENT OR SIMILAR CLIMATE
DESCRIPTOR: Time scale of change
-Duration (temp/permanent & reversibility) -Speed of change/sensitivity (linear, threshold response such as algal blooms, bistable range ex invasive species) -Lag (time between making a change and seeing impact on system. ex: Tree roots take 30 years to rot before land becomes unstable)
Describe the following components of timescales in hydrologic change: duration of change, speed and sensitivity of change, lag time; and give examples from hydrologic systems
-Duration of change: Temporary or permanent? Reversibility? -Speed and sensitivity of change: Less sensitive with larger area. -Threshold response- no change and then it happens all at once, such as with algal blooms. Bistable range- system can be in good or bad state with the same drivers, such as invasive species. -Lag time: Time between making a change and seeing an impact on the system.
) Give a brief overview of global trends in temperature and precipitation that have already occurred over the past few decades
-General increase of precipitation in most areas in mid-and high latitude -Decreased precipitation in the Western, Southern Africa and Sahel -With mixed signs in Eurasia -Precipitation increases in Northwest India
DESCRIPTOR: Magnitude of change
-High-intensity/low-intensity farming -Use of good management practices such as fencing cattle out of water bodies
DESCRIPTOR: Proximity of change impacts to sources
-Intra-regional change (within region) -Inter or transregional (take place in neighboring region like acid rain or contamination by upstream sources) -Regional change (global change occurs when activity in region contributes to systematic global change. Ex: Emission of methane from wetlands, emission of CFCs)
Give examples of hydrologic changes from small spatial scales to large spatial scales, and describe what controls the spatial scale of a hydrologic change.
-Large scale: Global (systematic change) Using large numbers of watersheds (ex: all gauged watersheds in a country) we can test how different hydrologic indices (ex: Mean annual low flow) relate to different physical characteristics of the watershed. Those physical characteristics might include precipitation, temperature, vegetation, % urban land use, etc. -Small scale: local- cumulative leads to global. Looking at SDSU change. Regional.
Explain how the effects of climate change are modified by positive and negative feedbacks. Give examples and describe the most significant feedbacks in the global climate system.
-Negative feedbacks act to resist and slow down climate change. As the earth heats up, more of its energy will be lost to space. Increased CO2 levels stimulate faster plant growth, which absorbs more CO2. Secondary effect that increased CO2 causes plants to close their stomata and therefore reduce transpiration. Also increased CO2 means more carbon dioxide dissolved into the ocean. -Positive feedbacks in the Earth system act to reinforce and speed up climate change. "Runaway climate change". Ex: Melting of permafrost in Western Siberia could release up to 70k tonnes of methane, a potent GHG. Release of methane from hydrates/clathrates, forms of underground water ice. Unlikely but could cause extreme warming up to 5 degrees Celsius. Some suggest this positive feedback was responsible for mass extinctions in Earth's history. Ex2: Ecosystem change- peat decomposition and burning due to water table changes. Rainforest drying and replacement by scrub. Increased forest fires as droughts intensify. Ex 3: Ice albedo feedback: as ice melts, it is replaced by land or water. Each of these are less reflective and absorb more heat from the sun. This effect is one of the main reasons that northern polar regions are warming faster than the rest of the planet. Ex 4: Water vapor feedback: as the atmosphere warms, saturation vapor pressure increases and the amount of water vapor in the atmosphere increases. Water vapor is a greenhouse gas so this amplifies the existing warming.
Give three examples of climatic oscillations and briefly describe their impacts on global weather patterns.
1) Pacific Decadal Oscillation: The PDO spatial pattern and impacts are similar to those associated with ENSO events. During the positive phase the wintertime Aleutian low is deepened and shifted southward, warm/humid air is advected along the North American west coast and temperatures are higher than usual from the Pacific Northwest to Alaska but below normal in Mexico and the Southeastern United States. Winter precipitation is higher than usual in the Alaska Coast Range, Mexico and the Southwestern United States but reduced over Canada, Eastern Siberia and Australia. 2) North Atlantic Oscillation: Positive phase- Strong Atlantic pressure gradient. Eastern US experiences mild and wet winter conditions. Negative phase- weak Atlantic pressure gradient. Eastern US experiences more cold air and outbreaks and snowy weather conditions. 3) Atlantic Multidecadal Oscillation: (30-40 years) pattern of North Atlantic sea surface temperature variability between the equator and Greenland. When the AMO is positive (warm Atlantic), rainfall is lower than average over most of the United States. During warm phases of the AMO, the numbers of tropical storms that mature into severe hurricanes is much greater than during cool phases. Two of the most severe droughts of the 20th century occurred during the positive AMO between 1925-1965. Dust Bowl of the 1930s and the 1950s drought.
List four different components of hydrologic errors, and give examples of each one for measurements of rainfall or flow. Describe possible methods to estimate the size of each uncertainty component.
1) Random errors +- 5% = Small variations in measurement instrument 2) Systematic errors +-15% = Better calibration of rain gauge 3) bias errors +-15% = Errors in windy conditions 4) Lack of knowledge errors +-40%
Explain how to ensure repeatability and accuracy in longitudinal studies:
1) Repeatability: It is important to develop a set of protocols that can be accurately repeated. (same equipment, reliable, participants know how to use equipment, map skills, time of day, establish units and degree of accuracy of the measurements). 2) Accuracy: a- data storage; maintaining databases (create way to store data in format that can be easily added to with each new data set. Labeling. Metadata. Store data in several formats).
DESCRIPTOR: Cause of hydrologic change
Climate change, land use change, land cover change
Run-off coefficient
Coefficient of variation: Describes variability independent of magnitude of flow. Can be used to calculate how big a reservoir is needed to maintain a given flow, even during drought years. The bigger the CV, the bigger the reservoir needed. CV=(SD/Mean Flow)
Flow variability
Consistency of water supply. Changes in annual flow sometimes show clear impacts of human-caused changes.
Sublimation
Conversion between solid and gaseous phase of matter, with no intermediate liquid stage Most often used to describe process of snow and ice changing into water vapor in the air without first melting water
Negative feedback
Dampens or buffers changes; holds system to equilibrium
precipitation elasticity": (Ɛp)
Defined as proportional change in mean annual streamflow divided by proportional change in mean annual precipitation. "HOW SENSITIVE THE WATERSHED IS TO CHANGES IN RAINFALL" - helps make predictions of changes in flow and water availability under future climates with more or less rainfall.
Explain what an emission scenario is and why IPCC reports use multiple different emissions scenarios.
Describing different potential pathways for CO2 emissions and greenhouse gases to analyze potential states of the planet with various levels of actions
Water year
Designated by calendar year in which it ends. 12 month period from October through September. In order to capture the rainy season without interruption.
Explain why we should be worried about sea-level rise, when sea levels have been higher in the past?
Development along shores
Describe the main causes of change in the hydrology of watersheds, and relationships between different types of changes
Drought, floods, fire, removal of native vegetation, tree planting, development, pollution runoff, urban heat island effects, river re-routing, river channels, roads on flood plains.
ENSO
El Nino Southern Oscillation based on trade winds.
Explain the impacts of an El Niño event on sea temperatures and global rainfall and temperature patterns.
El Niño is the periodic warming of water in the Pacific Ocean every few years. When it occurs, it means more energy is available for storms to form there. El Niño also affects wind shear, which is when air currents at a lower altitude blow in a different direction from winds higher in the atmosphere.
Positive Feedback
Enhances or amplifies changes; moves system away from equilibrium and makes it more unstable.
DESCRIPTOR: Affected part of the water cycle
Ex: Climate change, melting, soil compaction, artificial drainage, urbanization, groundwater extraction
Hydrologic changes impacting different parts of the water cycle
Ex: Reduced rainfall --> Reduced soil moisture content --> Reduced stomatal conducance --> Reduced rate of tree water use <--Reduced soil moisture content <--Increased rate of evaporation from soil wet canopies <--Increased air temp and decreased humidity -->Increased leaf temperature <--> Reduced stomatal conducance <--Increased CO2 in atmosphere
DESCRIPTOR: Human influence on change
Human induced trends
Climate analogue
Identifies other areas of the globe that experience statistically similar climatic conditions to predicted future of your location. Good for crop prediction and mitigation efforts. Also seeing how other areas have adapted.
Hydrologic Indices Examples
Indices= "statistics of a catchment" Mean flow (describes water availability) Mean flow per month (when water is available) Flow variability (consistency of water supply) Floods/droughts - how often / how severe Source of water (importance of groundwater / snowpack) Smaller area = smaller graph Less slope = lower graph Oval valley vs. round valley = more spread out.
Biome
Interactions between climate and land cover. Ex: Hot to cold vs. wet to dry.
IPCC Emissions scenario
Intergovernmental panel on climate change- special report indicating what will happen with various levels of climate change due to CO2 emissions.
Describe at least four reasons why it is difficult to identify trends in hydrologic variables caused by climate change, and explain how the IPCC tries to solve this problem.
It is difficult to simulate impacts of climate change on a watershed because it requires manipulation of temperature and/or CO2 levels. IPCC looks at various combinations of those factors.
Give an example of how the paired watershed approach can be used to study the impacts of climate change on hydrology, and identify any limitations of this approach.
It is difficult to simulate impacts of climate change on a watershed because it requires manipulation of temperature and/or CO2 levels. IPCC looks at various combinations of those factors. -Heated forest studies: Simulate climate warming in 100 years. Measure indirect effects of how the environment responds, such as trees ability to take up and retain nitrogen, carbon and other elements.
Why do managers need accurate descriptions?
Mitigation, adaptation, urgency, stakeholders
Describe the concept of a Global Climate Model and how it works, including the types of physical quantities and processes that it simulates, how it uses grid cells, types of decisions that must be made by the modeler.
Take into account atmosphere, oceans, cryosphere, vegetation. At every grid cell they calculate temperature (T), pressure (P), winds (U, V), and humidity (Q). This is in three spatial planes. Utilizes conservation of momentum, energy, mass, H20, equation of state. Increased resolution requires increased computing resources. Getting more and more detailed. Can answer 'what if' questions on climate change, land use change. Have to specify theoretical effect of each change (write equation for change in plant growth with change in CO2 levels), which processes to include (Too many to include all- ex: which gases in atmosphere, effects of soil moisture variability on low atmosphere, shifts in gravity due to Antarctic ice melt, etc). Grid cell size tradeoff and # of processes and number of runs that can be done to test sensitivity.
Explain how a flow duration curve can be used to study impacts of human changes to a river, and the impact on flora/fauna of the river channel.
The flow duration curve is a plot that shows the percentage of time that flow in a stream is likely to equal or exceed some specified value of interest.
Albedo
The ice albedo effect is simply a name for how ice and snow reflect solar radiation, and thus help keep the Earth cool. Since a cool Earth also tends to have more ice and snow, the ice albedo effect is an example of a positive climate feedback.
Hydrologic Indices
Using indices to draw global maps of flow regimes. 1) Climate (precipitation, evapotranspiration- mainly temperature). 2) catchment (geology, topography, land use). 3) Human alterations (land-use, water infrastructure) FLOW Indices: Global mean flow, latitudinal / regional mean flow / floods / regional floods . minimum daily flows/drought indices. OTHER Indices: Total evapotranspiration / evapotranspiration over land / mean surface humidity / snowpack / peak snowmelt / groundwater recharge.
Explain what is meant by the concept of mountains as "water towers" and give examples of how climate-driven changes in snow hydrology impacts on downstream water resources.
Water is stored up on mountains in the forms of ice and snow. During the summer it melts and provides water to downstream areas when it is needed.
Infiltration
Water on ground surface enters soil
Describe what instruments or methods are used to measure snow depth, snow water equivalent, snow extent, and river flow under ice, and how they work. Given a scenario, decide which method you would use and justify your decision.
a) Ground penetrating radar (GPR) : uses radar pulses to image the subsurface along with high-frequency radio waves. b) Moderate resolution imaging spectro-radiometer (MODIS): Uses visible, near infrared and thermal infrared imagery.
Explain why the hydrology of cold climates is particularly sensitive to climate change.
a) One month advance in snowmelt timing could threaten storage efficiencies for many reservoirs in Western US b) Besides providing water supply, these reservoirs are operated for flood-protection, and so may release large amounts of otherwise useful water during the winter and early spring c) Earlier flows place more of the year's runoff into the category of hazard rather than resource