Hydrometeorology Flashcards
(105 cards)
1
Q
Describe Evaporation / Transpiration:
A
Evaporation is the movement of water from a liquid to a vapor state, and is the opposite of condensation. Transpiration is the process whereby soil moisture is taken up by a plant’s root system to drive photosynthesis. The combined effect of evaporation and transpiration is often called evapotranspiration, or ET, and is generally constitutes the largest removal of water from the soil water system.
2
Q
Describe Condensation:
A
Condensation is the movement of water from a vapor to a liquid state, and is the opposite of evaporation.
3
Q
Describe Precipitation:
A
Precipitation is generally described as water falling to the surface of the Earth, either in the form of liquid or frozen water; therefore, it is also known as hydrometeors (hence, meteorology!).
4
Q
Describe Runoff/Infiltration:
A
Runoff is the portion of rainfall that does not infiltrate into the soil. As water infiltrates, some water will flow just below the surface. This is called interflow or through-flow.Infiltration is defined as the downward movement of water through the soil surface into the soil profile.
5
Q
Describe Storage
A
Storage is the general amount of water in a particular location and it can be calculated using an accounting budget approach. (Infow) – (Outflow) = (Change in Storage)
6
Q
Describe Groundwater Discharge
A
Groundwater discharge occurs when water seeps from aquifers into rivers, streams, and lakes.
7
Q
Where and in what percentages is out ground water stored?
A
• The oceans store over 97% of the Earth’s water supply in the form of
saltwater.
• Polar icecaps and glaciers account for slightly more than 2% of the Earth’s
water, and comprise the largest percentage of freshwater on the planet.
• Surface water storage in freshwater lakes, ponds, rivers, and streams
account for less than 0.01% of the total water on Earth.
• Groundwater is normally stored within aquifers, which are subsurface
regions comprised of unconsolidated rock and soil particles. Less than
1% of the Earth’s total water is stored as groundwater or soil moisture.
8
Q
How does water enter and leave out atmosphere?
A
Water reaches the atmosphere through transpiration, evaporation, and sublimation, meaning that water vapor is the primary form by which water enters the atmospheric system. Water leaves the atmosphere almost solely through precipitation, either solid (snow, hail, etc.) or liquid (rain, dew).
9
Q
How does surface water become ground water and how does surface water enter the atmosphere?
A
Infiltration to become groundwater. Evaporation to enter the atmosophere.
10
Q
What process inhibits water from draining from soil even when there may still be water present?
A
Capillary tension.
11
Q
Define wilting point.
A
There is a point where the tension of the water to the soil particle becomes so tight that the water cannot be used by plant roots. This is called the wilting point.
12
Q
Name the soil textures. (Hint 3)
A
Clay, sand and loam.
13
Q
What is a confined aquifer?
A
. In confined aquifers the groundwater is restricted by a nonporous or very low porous layer termed an aquiclude and is not in contact with the atmosphere.
14
Q
What is an unconfined aquifer?
A
In unconfined aquifers, the groundwater is in contact with the atmosphere through the pores of the overlaying soil. The top of the groundwater is termed the water table.
15
Q
Define recharge.
A
Recharge is the introduction of surface water to the groundwater system.
16
Q
Define withdrawl.
A
. Withdrawal is the artificial extraction of groundwater through a well or network of wells. When groundwater withdrawal rates are greater than the recharge of water into the ground, a lowering of the local water table occurs.
17
Q
Name the two ways that a cloud droplet can form.
A
homogenous nucleation and heterogeneous nucleation.
18
Q
Define homogenous nucleation.
A
Homogeneous nucleation occurs when water droplets form by the chance collision and bonding of water vapor molecules under supersaturated conditions. In other words, water vapor molecules bond to other water vapor molecules with no condensation nuclei involved. This is possible under the absence of atmospheric aerosols and the droplets are inherently small with a high degree of curvature.
19
Q
Define heterogeneous nucleation.
A
Heterogeneous nucleation occurs when water droplets form on external hygroscopic (a.k.a., water attracting known as condensation nuclei) particles. Heterogeneous nucleation can occur under saturated or minimally supersaturated conditions. Haze forms under unsaturated conditions.
20
Q
Explain Condensation Nuclei.
A
The hygroscopic particles over which droplets are formed are called condensation nuclei. When condensation occurs, the condensation nuclei dissolve to form a solution. The resulting solution further reduces the saturation necessary for condensation to occur. This is because the surface area of the droplet becomes populated by solute, not water.
21
Q
What is required to get supper cooled water to freeze at temps near 0?
A
Saturation can occur at below-freezing temperatures, but this does not necessarily lead to the formation of ice crystals. In order for water to freeze at temperatures just below 0°C, an ice nucleus is required. The ice nucleus performs a similar role to condensation nuclei. Ice nuclei are far rarer than condensation nuclei. They must have a six-sided structure, just like ice. Ice crystals themselves act as efficient ice nuclei.
22
Q
How do cloud droplets form, grow in above freezing conditions?
A
In above-freezing conditions, cloud droplet growth through condensation is the dominant formation processes. Condensation occurs when air is lifted adiabatically past the lifted condensation level (LCL). Above the LCL, most of the water is drawn to condensation nuclei. In general, there is relatively little water but a lot of nuclei. This creates a large number of smaller particles competing for a limited amount of water. Growth through condensation by itself cannot lead to large water droplets; therefore, other processes must come into play.
23
Q
In warm clouds, what process leads to precipitation?
A
collision-coalescence
24
Q
Explain the collision-coalescence process.
A
The collision-coalescence process depends on the differing fall speeds of different sized droplets. The process begins with droplets falling through a cloud where large droplets fall faster than smaller droplets, and eventually overtake smaller droplets. As the droplets collide they form bigger droplets, which fall even faster.
As a collector drop falls, it only collides with some of the drops in its path. The Likelihood of a collision depends on the size of the collector drop and the size of the drops in its path. The larger the collector drop, the lower the collision efficiency, and vice versa. Efficiencies are low for droplets near the same size. When the droplets have the same terminal velocity, it is difficult for the particles to catch up to each other and collide. Surface tension causes droplets to “bounce” off of one another.
Turbulence can cause collision efficiencies over 100%. This is because falling drops entrains particles from outside of the fall path. The centrifugal force of a spinning cloud tends to sort droplets by size with smaller droplets toward the center and larger droplets on the outside. The larger droplets get slung away from the center which allows it to encounter smaller droplets along as it moves.
When a collector and a smaller drop collide, one of two things can happen: The two drops can bounce apart due to surface tension or the two drops can stick together, forming a single larger
droplet (a.k.a., coalescence). Coalescence is the process of two or more drops combining upon collision. Coalescence efficiency is the percentage of drops that coalesce upon collision. Most collisions result in coalescence and coalescence efficiencies are assumed to be near 100%.
25
Name the temperature layers and corresponding forms of water present in cool/cold clouds.
-4 C: Water droplets make up the lower portion.
26
Define a cool cloud and a cold cloud.
Cool clouds are characterized as having temperatures both above and below freezing in the region of precipitation generation.
Cold clouds are characterized as having subfreezing temperatures throughout their entire structure. A cold cloud can be composed of both super-cooled water droplets and ice crystals.
27
How do ice crystals from in cool/cold clouds? (Hint 2 ways)
homogeneous or heterogeneous ice nucleation
28
Name the ways that ice crystals grow in cool/cold clouds. (Hint 3)
* Diffusion-deposition (Bergeron process).
* Riming (accretion).
* Aggregation.
29
Define Diffusion-Deposition (Bergeron) Process.
In the diffusion-deposition process, the coexistence of ice and supercooled water is essential to precipitation development. Saturation vapor pressure over ice is less than that over supercooled water at the same temperature. The water vapor necessary to keep a supercooled water droplet from evaporating is more than enough to maintain an ice crystal. Water vapor is preferentially deposited onto ice nuclei, causing the super-cooled water to evaporate due to lower humidity levels
30
Define Riming.
When ice crystals fall through clouds, supercooled water droplets freeze onto them. This process is called riming, or accretion. The process leads to rapid crystal growth, increased mass, and increased terminal velocity.
31
Define Aggregation.
Aggregation occurs when two or more ice crystals join to form a single, larger crystal. This is an important process in the development of frozen precipitation. It occurs most easily when the crystals have a thin layer of liquid water on them; therefore, the process is most efficient at temperature just above 0°C. The liquid water acts as a sort of “glue” to make them stick together. This is why large snowflakes normally occur during warm, early season snow events.
32
What are the advantages of using radar based QPE and what relationship does the radar use to estimate rainfall?
Radar is a remote sensing QPE tool with excellent spatial and temporal resolution. Despite the regional and seasonal inconsistencies in radar-derived precipitation estimates, radar guidance is generally considered superior to satellite guidance of QPE. The is primarily due to the superior spatial and temporal resolution and overall better quantitative guidance.
Radar reflectivity (Z), expressed in units of dBZ, is used to compute rainfall rates (R) in mm/h using a reflectivity to rainfall rate relationship, known as a Z-R relationship.
33
What factors influence the Z-R relationship? (Hint 4)
· Droplet size.
· Droplet size distribution (DSD).
· Phase of the droplets (liquid or solid).
· Droplet shape.
34
Name some limitations of using radar to estimate QPE. (Hint 8)
Radar coverage may be inconsistent from place to place and from storm to storm.
Radar assumes a random droplet distribution, which is not always true. Radar is more sensitive to the horizontal diameter of hydrometeors than it is to the concentration of hydrometeors; therefore, a small number of large hydrometeors can result in the same reflectivity value as a large number of smaller drops. As a result, more than one Z-R relationship may be necessary to accurately measure precipitation accumulations.
Ice has a vastly different reflectivity profile than liquid water due to the crystalline nature of the hydrometeors. Pure snowflakes and crystals violate the assumption of liquid hydrometeors that go into the Z-R equation. This means that frozen precipitation may require a Z-S, or reflectivity-snowfall rate relationships.
As snowflakes begin to melt, a coating of water can make them appear as very large raindrops to the radar. This can lead to high reflectivity and overestimated rainfall rates where the radar is sampling the melting layer. Hail results in anomalously high derived-rainfall rates as well.
Radar may inaccurately believe that non-hydrometeors like insects or ground targets are hydrometeor targets.
Terrain blocking limits the sampling distance.
Because stratiform clouds are generally not as deep as convective clouds, radar may overshoot stratiform clouds at closer distances to the radar than for convective clouds. Sampling of stratiform precipitation is almost always poor beyond 100 km from the radar. Shallow convective clouds may be under sampled as well do to over shooting. Convective cloud tends to be deeper so deep convection may be sampled at distances of 150 km or less from the radar.
Since the radar beam gets higher in the atmosphere with distance from the radar, even a low tilt angle like 0.5° is ~1.5 km (5,000 ft) above the ground at 100 km from the radar, and 5.2 km (17,000 ft) high at 230 km.
Higher elevation samples may not be as representative of surface rainfall, especially if the precipitation is falling through a dry boundary layer.
35
What are the advantages of using satellite derived QPE? (Hint 2)
Satellite estimates of precipitation are more regionally consistent than radar estimates.
Satellite estimation of precipitation is potentially useful in areas with poor coverage from radars and rain gauges.
36
When does satellite QPE preform best and what can be done to improve estimates?
Satellite-derived rainfall products perform best in the tropics and in the middle latitudes during the warm season. Verifies best when wind shear is minimal and precipitation is dominated by convection.
There is increasing use of additional sensor capabilities, such as microwave satellite sensing and lightning detection, to improve satellite QPE.
Terrain enhancement helps with spatial resolution of precipitation. NOAA product known as the hydro-estimator, based on geostationary satellite observations, applies a terrain factor to the precipitation estimate using 700 mb winds.
Other improvements to precipitation rates may be achieved by applying factors related to cloud, moisture, and stability characteristics.
37
What are limitations of satellite derived QPE? (hint 2)
Satellite is a remote sensing QPE tool with much coarser resolution spatially and temporally than radar. This is especially true in terms of temporal resolution with polar orbiting satellites.
38
Name the advantages of using gauge based QPE. (Hint 4)
Surface-based precipitation estimates are often considered “ground truth” since they directly measure precipitation at the surface.
Manual gauges may allow for more accurate liquid equivalent measurements with frozen precipitation. Snow and hail are melted and measured manually. The observer may collect a “core” measurement of snow from the ground to give a more representative sample.
Weighting sensors, such as snow pillows used at SNOTEL sites in the western US, generally provide better estimates of snow water equivalent than automated gauges.
Manual, or recording, gauges require physical interaction to obtain a precipitation estimate, which has advantages and disadvantages. Biggest advantage is that errors can be identified and fixed more quickly. Insect or bird nests, leakage, overflow, blockage, vandalism.
39
Name and describe the types of rain gauges.
· Tipping bucket: Measures precipitation as weight of rain causes a pendulum to tip. Measurement resolution dependent on the weight required to tip the pendulum.
· Weighing: Measures precipitation based on the change in weight of water in an enclosed volume. Best used to measure precipitation accumulation.
· Recording or manual: Rainfall is held in an enclosed volume, which has a graduated display to manually describe the amount of rainfall (or liquid equivalent) that fell since the last time the gauge was checked.
40
Name the limitations of gage based QPE. (10 with 2 concerning precipitation rate and 4 concerning phase.)
Rain gauges are direct ground-based measurements, but cannot resolve the spatial detail of precipitation patterns.
Nearby obstructions can alter precipitation reaching the surface.
Rain gages are limited to point measurements that are valid for the area of the gauge.
Rain which falls at an angle due to wind reduces the effective size of the rain gage opening which may lead to an underestimation of rainfall. Frozen precipitation, especially snow, is more severely impacted by wind than liquid precipitation. The magnitude of the under-catch will vary with snowflake characteristics. Denser crystals will have fewer gauge catch errors than low density crystals.
The low temporal resolution (usually daily estimates) with varying recording times makes manual reports less suitable for software programs that need fast access to high resolution gauge estimates.
Precipitation rate:
· If the precipitation rate is too fast, then splash and overflow effects can cause substantial under-catch.
· If the precipitation rate is too slow, then evaporation and surface tension can reduce estimates.
Precipitation phase:
· Freezing rain or frozen precipitation can clog apertures.
· Tipping mechanism may become inoperable due to frozen water.
· To obtain a liquid equivalent measurement of frozen precipitation with a tipping bucket gauge, the gauge must be heated to turn the ice to liquid. A heated gage can lead to enhanced sublimation/evaporation of precipitation which reduces estimates.
· If the snowfall rate is too high, the melting will not occur fast enough to prevent gauge overflow.
41
What is good about precipitation climatology?
Precipitation climatology guidance can be used to help fill in the gaps where estimates of observed precipitation are poor. Also, it can be very useful in regions where precipitation distribution, and the ability to observe it, is greatly affected by terrain features.
42
What climate tool is often used to create precipitation climatologies and what is this tool based on?
The Parameter-elevation Regressions on an Independent Slopes Model (PRISM) provides a commonly-used precipitation climatology tool.
PRISM precipitation climatologies are based on:
· Historic record of measured precipitation at point locations.
· Geographic input, especially terrain information.
· Prevailing wind direction (in some cases).
· Stream flow discharge measurements.
43
What are the disadvantages of using PRISM? (Hint 6)
Keep in mind that a precipitation climatology is generated using the same observations it is being compared to, so biases may still be inherent.
Usefulness is limited by period of record and inherent limitations in the QPE products.
Climatology data average out individual events.
Individual events can show large departures from climatology. The is especially in locations with extreme values or sharp gradients.
Observations tend to be closer to climatology with orographically-driven precipitation and widespread, stratiform precipitation.
Departures from climatological norms will likely have greater impacts as one looks at smaller basins.
44
Describe the basic methodology for MPE with good radar coverage. (Hint 5)
1. Locate and differentiate areas of general rain, convective rain, hail, snow, virga and melting precipitation.
2. Evaluate gauge data and correct bad or suspicious reports.
3. Re-generate and re-evaluate the different radar-based QPE fields after the gauge quality control is performed.
4. Correct remaining problem areas with editing tools, including polygon edits.
5. Correct inaccuracy that may become apparent later when more data are available.
For frozen precipitation, it’s likely that none of the radar-based products provide suitable
QPE. So, after steps 1-2, the most efficient option may be to gather as many liquid equivalent reports as possible and produce a gauge-based QPE.
45
Name the radar mosaic products.
Radar Mosaic, Average Radar Mosaic, Max Radar Mosaic and NMQ’s Q2
46
What condition must be met to use the Radar Mosaic and how does it handle overlapping coverage?
1. The Radar Climatology guidance indicates good coverage.
2. For areas with more than one radar coverage, the radar with the lowest
elevation coverage is used.
47
How does the Average Radar Mosaic handle overlapping coverage?
average of overlapping coverage.
48
How does the Max Radar handle overlapping coverage?
max value.
49
What is special about the NMQ's Q2?
dynamic Z-R relationships that are based on the vertical profile of radar reflectivity.
50
What are the gauge-only products?
The Gage Only Analysis results in a field that’s based on only the gauge reports, along with a precipitation climatology adjustment.
51
How is the radii of influence set? What is assigned for values beyond the radii? What happens when there are overlapping radii of influence? How are problems with terrain handled?
Radii of influence can be set by the user. “Missing” values are assigned to bins outside the radius of influence of any gauge. When the gauge density increases such that there are overlapping radii of influence, distance weighting is applied to the gauge value.
The Gage Only Analysis is adjusted by precipitation climatology, such as the PRISM data. This will be more apparent in the West where orographic influences have a greater impact on precipitation distribution.
52
Name the two Multisensor products discussed in the class.
Multisensor Mosaic and the Local Bias Multisensor Mosaic
53
What two products are used to create the Multisensor Mosaic?
The Multisensor Mosaic is the merging of the Field Bias Radar Mosaic and the Gage Only Analysis.
54
What two products are used to create the Local Bias Multisensor Mosaic.
The Local Bias Multisensor Mosaic is the merging of the Local Bias Radar Mosaic and the Gage Only Analysis
55
How are gauge values used int he multi-sensor products? What happens for bins without a gauge but are under the influence of a gauge? What about for bins with no gauge and are not under the influence of a gauge? So, Does the mutisensor product preserve the radar pattern?
The gauge values in a multi-sensor mosaic are used as anchoring points at the gauge locations. This means that the grid bin associated with the gauge location must be the gauge value. For grid bins not directly associated with a gauge report, the value is based on blending the radar QPE value and the distance-weighted value from the gauge analysis. As you get further from the gauge location, the radar QPE value will become more dominant until you are beyond the influence of the gauge and use only the radar QPE. With the merging of the gauge data, the multi-sensor mosaic can alter the relative values and spatial distribution of the radar QPE.
Precipitation climatology can impact gauge QPE and thus the multi-sensor mosaic. Currently, the PRISM-adjusted gauge analyses have a strong impact on multisensor analyses in regions with sharp terrain features and poor radar coverage.
So the question is, when should multi-sensor products be used and when should bias adjusted products be used? Bias-adjusted fields are often viewed as an intermediate step in the generation of the multi-sensor products. Multi-sensor techniques have the advantage of using gauge data to fill in areas of poor, or missing, radar coverage while using information from bias-adjusted radar QPE in other areas.
The multi-sensor approach forces the QPE grid to match the gauge report at gauge locations and can be performed with fewer gauges. A single bad gauge report can have a large impact on a multi-sensor mosaic, so careful gauge QC is essential.
56
Does Prism play a role in the Multisensor product?
Yes, I think for all products that include gauge data.
57
What is the primary advantage of a multi-sensor product and a primary disadvantage?
Multi-sensor techniques have the advantage of using gauge data to fill in areas of poor, or missing, radar coverage while using information from bias-adjusted radar QPE in other areas. The multi-sensor approach forces the QPE grid to match the gauge report at gauge locations and can be performed with fewer gauges. A single bad gauge report can have a large impact on a multi-sensor mosaic, so careful gauge QC is essential.
58
How can a user QC rain gauge data when creating MPE. (Hint 4)
MPE offers a suite of tools for assessing, adjusting, and if necessary, removing gauge data. General options are divided into four groups:
1. The Daily QC tool. Daily QC is a component of the Mountain Mapper Program, which is used
primarily in the western US to produce precipitation analyses for input to river models.
2. Viewing the gauge data.
3. Interrogating and altering the gauge data.
4. Using pseudo gauges.
59
How does the Daily QC tool help to QC gauge reports?
Daily QC tool, uses the available gauge data and performs spatial and temporal checks. Gauges are flagged “questionable” but not removed if they don’t pass these checks. The forecaster may choose to further evaluate these questionable val
60
How does viewing gauge data help to QC reports?
Viewing gauge data allows the forecaster to make a quick assessment of the gauge field. Although not entirely scientific, one approach can be as simple as looking at the plotted gauge values to pick out any outliers. Overlaying a radar-derived precipitation plot can help in the visual QA of gauge data.
61
How is interrogating and altering gauge data done?
Both the Gage Table and Display 7X7 allow for subjective interrogation and adjustment of gauge data. Forecaster can remove a gauge estimate from subsequent computations, change the gauge value, or set the value to missing.
A gauge value can be directly compared with any MPE precipi– Replacing an area from one analysis field with values from another analysis field.
– Setting an area to a given accumulation value.
– Establishing a minimum accumulation value where all values that are lower are raised to that value.
– Establishing a maximum accumulation value where all higher values are lowered.
– Applying a multiplicative correction to the accumulation values.tation field at the grid bin where the gauge is located. Display 7X7 allows a forecaster to view the neighborhood of gridded radar-based precipitation field values relative to the gauge.
62
What is typically the final step in the QC process and what can this be used for?
Polygon edits:
– Replacing an area from one analysis field with values from another analysis field.
– Setting an area to a given accumulation value.
– Establishing a minimum accumulation value where all values that are lower are raised to that value.
– Establishing a maximum accumulation value where all higher values are lowered.
– Applying a multiplicative correction to the accumulation values.
63
What is the Field Bias Radar Mosaic and the Local Bias Radar Mosaic?
Field bias: one bias correction for entire radar coverage.
| Local bias: bias correction for each grid bin.
64
List and describe the three ways in which overlapping radar data are treated in the MPE radar mosaic products. In your description, discuss the environmental and/or atmospheric conditions when each method is most appropriate.
Radar Mosaic uses the lowest elevation scan, which would be most appropriate in mountainous terrain when radar installations are at different elevations. Average radar mosaic uses the average of the overlapping radar scans, which would be most appropriate when the radar coverages are at roughly the same elevation or during stratiform rainfall events. Max Radar Mosaic uses the maximum value of the overlapping radar scans, which would be most appropriate during convective rainfall when precipitation intensity changes rapidly with height.
65
What are the major differences between multi-sensor and bias-adjusted MPE products, and what advantages does each product offer in terms of QPE accuracy and spatial consistency?
The multi-sensor MPE product merges the radar mosaic (either field or local bias) with the gauge-only analysis, generating an adjustment factor that is then applied to the radar mosaic. With the multi-sensor products, the radar data are fully adjusted to match the gauge values, with a distance weighting algorithm used to adjust the bias further from the gauge. In this way, the multi-sensor MPE products can actually fill in areas with no radar coverage. The bias-adjusted MPE products use gauge-radar pairs to calculate either a field or local bias, which is then applied to the entire radar coverage.
66
For the past several years I have led research trips to the Grand Tetons in Wyoming during late June and early July, where I usually lead a group of students up a steep-walled glacial valley to a small lake called Lake Solitude. More times than not, despite the temperature being in the 60s or 70s, the lake is covered in snow and we are able to have a rather vicious snowball fight. It is now up to you to answer the question that all the students ask: Why is the snow still there? HINT: Lake Solitude is at ~9500’ in the semi-arid western US at approximately 45°N latitude.
There are several reasons why the snow still exists, but the primary reason is sublimation. Since the area is semi-arid the rate of sublimation is relatively high, which causes the surface of the snowpack to lose substantial latent heat and remain cold. Second, since the walls of the valley are steep, the surface wind speed is relatively low, which minimizes turbulent heat transfer. The steep walls also act to shield the snow from radiation for a longer part of the day, which is already minimal due to the low sun angle. Lastly, the high elevation and low atmospheric humidity causes temperatures to drop at night, minimizing snowmelt.
67
Although radar provides good spatial coverage of precipitation, it has been shown to exhibit definite biases relative to gauge observations during specific atmospheric conditions. Describe these conditions with respect to over-estimation and under-estimation of radar QPE relative to gauge QPE, and explain what causes these biases.
Radar tends to over-estimate precipitation during light rainfall events due to increased evaporation and wind undercatch of the rainfall at the surface. Over-estimation can also occur during deep convection due to hail or frozen precip contamination. Radar tends to under-estimate precipitation during convective rainfall events because it doesn’t recognize varying drop size distribution and drop shape.
68
The “Gage Table” and “Display 7x7” programs are used to help NWS hydrometeorologists analyze the quality of gauge estimates; however, the information they present allows for varying types of analysis. Briefly describe the information that each tool shows and how it is used for data QC. In addition, discuss the primary types of gauge error that each tool can assess, and how it can be recognized.
The “Gage Table” allows a single gauge estimate to be compared to all other precip estimates over the same location. In this way, the relative difference between the gauge, radar, multi-sensor, bias-corrected, and satellite data (if available) can be quickly calculated to see if the gauge reading is valid. This type of analysis is best used for picking up gauges that are clogged, inoperable, or are reporting heavily biased values. The “Display 7x7” program compares a single gauge value to surrounding values from any of the radar-based QPE products. This allows for spatial errors such as precipitation drift or localized rainfall to be recognized and accounted for.
69
Describe the reasons and associated mechanisms by which forested areas generally have less snow volume per area (both on the surface and in the canopy) than non-forested areas?
Forest canopies tend to intercept snowfall, minimizing the amount of snow reaching the surface. The intercepted snow is now more susceptible to sublimation and melt due to its exposure to the sun and wind and the resulting enhanced heat fluxes.
70
What are the major differences between a field bias and a local bias? Be sure to include details regarding how each bias is calculated, as well as the conditions when each bias is most appropriate.
A field bias is applied to an entire radar coverage, while a local bias is only applied to the radius of influence of a single gauge. Each bias is calculated by averaging the ratios between radar and gauge precip estimates over each valid gauge/radar pair. A field bias is most appropriate when a uniform bias exists across the entire area of good radar coverage, which is usually associated with stratiform-type precipitation. A local bias is able to account for changing storm conditions; therefore, it is most appropriate for convective precipitation events.
71
Much of my master’s research focused on spring flooding of the Red River of the North, which is a northward-flowing river that forms the border between North Dakota and Minnesota. If you’ve never been there, it is an extremely flat region with very few trees. This river goes into flood (to some extent) nearly every spring due to snowmelt runoff. What factors lead to this annual flooding, and what specific conditions of the area and the river do you think lead to the enhanced flood conditions?
First, there is a lot of snow on the ground since it is a high latitude, continental location. Second, as spring progresses and snowmelt is initiated, the runoff must travel north into still-frozen soils and river channels. As the melting process continues north, it only acts to augment the already existing flood conditions. Third, the extremely flat terrain and saturated and/or frozen soils minimized the flow and infiltration rates, causing water to remain suspended on the surface and become runoff into the hydrologic system.
72
Explain Infiltration excess over land flow and what is another name for it?
Infiltration excess overland flow occurs with soil that is not saturated. In fact the soil can be quite dry, but soil properties or land cover do not allow for infiltration to keep up with high rainfall or snowmelt rates.Infiltration excess occurs when the rate of rainfall or snowmelt is greater than the infiltration capacity, such that the water that cannot infiltrate becomes surface runoff. Infiltration excess overland flow is sometimes called Hortonian flow.
73
When is infiltration excess overland flow most likely (Rainfall characteristics and soil type).
Infiltration excess is most commonly observed with short-duration but high intensity rainfall. It occurs most often in areas with high clay content or where the surface has been altered by soil compaction, urbanization, or fire.
74
Explain saturation excess overland flow.
Saturation excess overland flow occurs when the soil becomes saturated and there is no longer any space for water to infiltrate. This can occur even with soil that would typically allow for large amounts of infiltration in sub-saturated conditions.
Saturation excess occurs when the soil layers have become saturated and no further water
can infiltrate.
75
When is saturation excess overland flow most likely (Rainfall characeristics and soil type).
It is most common with long-duration, low to moderate intensity rainfall, or with
a series of successive precipitation and or snowmelt events. It can occur anywhere the soil is wet. It is most common in humid climates with gently sloped or flat basins.
76
Explain interfow and how the spped compares to that of baseflow and surface runoff.
Interflow (a.k.a., subsurface stormflow), is relatively rapid flow toward the stream
channel that occurs below the surface. It occurs more rapidly than baseflow, but typically more slowly than surface runoff.
77
What land, soil and climatological characteristics encourage interflow?
In regions with high infiltration rates and steep terrain, interflow may be the dominant process by which streams react quickly to rainfall or snowmelt. The process is most likely to occur in humid, deep-soil areas; however, significant interflow contribution may occur in thin-soiled regions when there is an impermeable layer such as bedrock beneath the more permeable surface soil layer.
78
What is transmissivity feedback?
Transmissivity feedback occurs when a network of macropores is activated following rapid infiltration, often contributing substantial water volume to the hydrologic system through interflow.
79
What are macropores, what creates them and where do they most commonly occur?
Macropores and natural pipes are void spaces in the soil that provide preferential pathways for water to move downslope. Decayed plant roots, burrowing insects and animals, and chemical reactions between water and soil minerals are a few ways that macropores form. Macropore networks are more common in deep-soiled areas with considerable organic materials; therefore, humid climates are more likely to have substantial interflow through macropore networks.
80
What is the soil-bedrock interface, where is it most common and how does it relate to interflow?
The presence of a soil-bedrock interface enhances interflow by acting as a material surface for water to flow along. The soil-bedrock interface typically occurs in steep terrain where the soil layer is considerably more permeable than the underlying bedrock. Rainwater or snowmelt infiltrates rapidly to the bedrock interface and then moves rapidly downslope along the interface.
81
What is fraigpan and how does is effect both interflow and surface runoff?
Sometimes a feature called a fragipan can focus the lateral subsurface flow, similar to a soil-bedrock interface. A fragipan is a low permeability layer, like rock or clay, which can exist at relatively shallow depths. It can play an important role in enhancing both interflow and even surface runoff after the soil layers above the fragipan are saturated.
82
What is groundwater ridging.
Groundwater ridging is a process that occurs in sloped drainage basins where the water table is much closer to the surface near the stream channel than it is further away from the stream.
83
Explain how groundwater ridging works.
Rainwater or snowmelt reaches the groundwater level near the stream channel more quickly than it does further up the hill away from the stream. The water table begins to rise near the stream channel more quickly than it does further away, creating a groundwater ridge close to the stream. The gradient between the groundwater ridge and the stream channel results in more rapid interflow to the stream. In some cases the groundwater ridge can reach the soil surface and contribute to surface runoff through saturation excess overland flow.
84
What is pre-event water?
In some cases there is considerable water already in the soil layers that gets displaced as new water infiltrates, such that the water that appears in the stream immediately following a rainfall or rapid snowmelt period may be from previous precipitation events.
85
What is pre-event water imortant and where is it most significant?
Called pre-event water, this is often a rapid source of interflow to stream channels since the water does not have to travel through the entire shallow sub-surface system. In humid climates, studies have shown that pre-event water is often the greatest contributor to rapid rises in stream level.
86
List the basin characertistics which effect runoff. (HInt 3)
Size, shape, slope.
87
How does basin size effect runoff? (Hint 2 main ways)
The size of the contributing area of the rainfall in a basin has a direct influence on the total volume of runoff that drains from that basin. All things being equal, a drainage area that is twice as large can generate twice as much runoff volume as its smaller counterpart. For most situations the runoff volume will be determined by the contributing area of the rainfall event, not the total size of a basin.
Basin size plays a role in runoff timing, such that runoff traveling from the most upstream point of a larger basin will travel a longer path, and therefore take longer to reach the basin outlet, than runoff traveling from the farthest point in a smaller basin. In addition, a single thunderstorm will likely only impact a portion of the large basin at any given time, but it may envelope the entire small basin.
88
How does shape effect runoff?
Basin shape has an influence on magnitude and timing of the peak flow at the basin outlet. Consider two basins of equal area where one is long and narrow, and the other is more round. The runoff in the more round basin will arrive more quickly at the basin outlet due to the smaller distance traveled. In addition, water from multiple locations in a more round basin is more likely to arrive at the outlet at the same time, resulting in a greater peak flow.
89
How does basin slope effect runoff?
The slope of a basin affects the amount and the timing of runoff by changing the gradient over which the runoff flows, and also influences the erosional potential of the runoff. As slope increases, the gravitational pull of water into the ground decreases; therefore, water is more likely to become surface runoff. Water will move faster and will have less time in contact with the ground surface, reducing the time during which it could infiltrate. Although erosion is highly dependent on soil type and ground cover (i.e., surface roughness), erosion generally increases with slope and water velocity. With higher amounts of sediment in the water, the surface pores in the soil can become plugged, reducing infiltration. In general, the steeper the hill slope and the steeper the drainage channels, the quicker the flow response and the higher the peak discharges.
90
List the stream channel properties which effect runoff. (Hint 3)
roughness, meanders and density.
91
How does stream roughness effect runoff?
Stream roughness has a direct impact on water velocity in the channel and how high the peak stage will be. Manning’s equation is often used in hydrology to account for the roughness factor. Roughness of a stream channel increases due to the presence of rocks, vegetation, and debris. The higher the stream roughness, the more turbulent the flow, resulting in slower runoff and streamflow velocities. This allows more time for infiltration, and it also results in a broader flood wave with lower peak discharges than rapid runoff events.
92
How do meanders effect runoff?
Meanders in the stream channel add to the distance that water must travel from upstream to downstream, increasing the travel time of runoff through the basin and reducing the overall runoff volume. Also increases the time over which water may infiltrate the ground through the bottom of the stream channel.
93
How does stream denisty effect runoff?
Stream density is the length of all channels within a basin divided by the area of the basin, and is one of the most important characteristics for evaluating potential runoff. Higher stream density allows the landscape to drain more efficiently following a storm event. More efficient drainage means that water moves into streams and creeks faster, causing peak storm flows to be higher and to occur sooner. A basin with a lower stream density usually indicates a deep, well-developed soil. In this case, water is more likely to infiltrate into the soil rather than become surface runoff and enter into the channel network.
94
List the soil properties which effect runoff.
Texture, Composition and profile.
95
How does soil texture effect runoff?
Soil texture refers to the size of the soil grains. There is a spectrum with clay on one end and sand on the other. Clay particles are the smallest and sand is the largest. Silt falls between the two. The combination of these various sizes determines the type of soil.
96
How does soil composition effect runoff?
Soil Composition describes the soil texture. Soil containing a high sand composition contains larger particles and has increased infiltration rates. However, sandy soils have lower pore volume and decreased storage. Soil composition with a high clay content means that the soil contains smaller particles and has decreased infiltration rates. However, clay soils have a high pore volume and thus more storage. A high silt composition means that the soil is mostly composed of particle sizes between the size of sand and clay. So, infiltration rates are greater than clay but less than sand.
97
How does the soil profile effect runoff?
The soil profile tells us about the depth to the bedrock or fragipan. Deep soil depths usually result in increased storage and interflow. This is mostly due to the increase in pore space available over a larger depth. Shallow soils often saturate rapidly since there is less pore space and thus less storage capacity. This leads to an increased runoff potential.
98
How does deforestation - and deforestation from fire effect runoff? (hint 3)
Deforestation can have an impact on infiltration and runoff by reducing friction at the surface. Higher water velocities mean less time for water to infiltrate the soil. Higher water velocities also mean enhanced runoff and erosion with large sediment loading into the stream channels. The sediment load can take up space in the stream channel that would otherwise be available for increased flow. Fires are a type of deforestation that can have more severe consequences on subsequent runoff. In addition to the typical deforestation problems, fires can alter the soil surface and make it temporarily hydrophobic, that is, unable to absorb water. This is especially noted in pine forest areas because the oils and resins from the trees vaporize and get infused into the soil. This creates a hydrophobic layer at or near the surface. The severity of runoff and sediment loading can often be seen by the scour on the tree trunks and the alluvial deposits left after the floodwater passes.
99
How does urbanization effect runoff? (Hint 5)
City materials such as concrete and asphalt as well as compact soils reduce infiltration.
Certain urban structures break down large basins into a series of small basins. This could be through the construction of berms or embankments. These artificially small basins respond to localized rainfall much faster than their larger, parent basin.
Urban structures such as the road network or storm sewers provide organized pathways for runoff to flow which effectively increases the stream density in the urban environment. An increase in density results in a more rapid runoff.
Urban structures decrease roughness as well. Surfaces such as roadways, sidewalks, and concrete lined streams (channelization) are much more smooth than a vegetation covered natural surface. This decrease in roughness allows runoff to flow at a higher velocity over these surfaces.
In addition, cities often straighten stream meanders. This decreases the distance that runoff has to travel which decreases the timing of the flood response after a rainfall event. It also increases the slope by forcing the water to flow from higher elevation to lower elevation over a smaller distance.
100
Define flash flood guidance.
Flash flood guidance is an estimate of the amount of temporally and spatially averaged rainfall necessary to produce flooding in small streams. In the US, it is provided in terms of inches of 1, 3 and 6 hour timescales. In other words, if the 3-hour flash flood guidance is 1.50 inches (38 millimeters), then flooding should begin on small streams if that amount or more falls in a 3-hour period.
101
Explain headwater guidance.
Headwater guidance is an estimate of the basin averaged rainfall that would cause flooding at the basin outlet displayed in tabular form.
102
Explain grided guidance.
Gridded guidance is viewed in a gridded form and provides an estimate of rainfall necessary to produce flooding in a given grid cell. Since gridded guidance is very dependent on the basin the grid is contained within, areas of similar gridded guidance often mimic the basin they are contained within.
103
Explain county guidance.
County guidance is simply the average of the gridded flash flood guidance contained within a given county. Since the county can span many grid cells, the average value across the county may be very different from the FFG provided for any particular cell. Further, the smoothing associated with county guidance tends to eleiminate important small scale details. Also, a county may span several basins which may lead to unrepresentative FFG.
104
What is ThreshR and gridded ThreshR? What is used to make the Gridded ThreshR values?
ThreshR is the numeric threshold value of runoff that is necessary to produce small stream flooding. An important assumption is that the basin averaged rainfall is uniformly distributed. Gridded ThreshR values take on the value calculated for the entire basin, so each grid cell contains the same value.
105
What is the rainfall/runoff curve and how is it used with the ThreshR value?
Once a TheshR value is calculated, rainfall/runoff curves are used to determine the amount of rainfall that would cause the determined threshold runoff to be met. It is important to know that rainfall runoff curves can be changed based upon soil conditions such as soil moisture. Headwater guidance (which represents values for the entire basin) is used to create the gridded guidance.
