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28 Cards in this Set
- Front
- Back
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How to Measure Actual ET
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1) Water Balance Approach
2) Soil Water Depletion 3) Lysimeter 4) Energy Budget Approach 5) Mass Transfer Approach |
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Lysimeter
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Artificially enclosed volume of soil for which
the inflows and outflows of liquid water can be measured and changes in storage can be monitored by weighing. |
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Mass Transfer Equation
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E = KeVa(es – ed)
E = evaporation (in/hr) KE = coefficient function of numerous factors Va= wind speed es = vapor pressure at evaporating surface (function of temperature) ed = vapor pressure of overlying air (function of relative humidity and temperature) |
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Climatic Data Needs for ET Estimation
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1) Saturation Vapor Pressure (property of air, f(temp)
2) Wind speed (slower at surface) 3) Solar Radiation 4) Temperature |
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Solar Radiation (needed for ET estimates)
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Rn = (1 – α)Rs - Rb
Rn = net radiation (MJ/m2/d) Rs = solar radiation received (measured) Rb = net outgoing thermal radiation α = albedo (0.23 for green crops) |
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Open Water Evaporation
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E = C(еs – ed)(1 + u 2 5/10 )
E = evaporation (in/month) es = saturation vapor pressure (inches of mercury) of air at water temperature at 1 ft deep ed = actual vapor pressure (inches of mercury) u25 = average wind velocity (mph) at 25 ft above lake C = coefficient (11 small lakes; 15 shallow ponds) |
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Thornthwaite Equation
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1) Lowest level: Based on annual and monthly Temps
E = 16 [ 10 T/I ]^a What do you need to know? T = mean monthly temperature (C) |
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SCS Blaney-Criddle
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2) Seasonal actual ET (uses Temp, Lat.) [Used by farmers]
f = tp/100 What do you need to know? -mean monthly air temperature -monthly percent of annual daytime hours -crop coefficient |
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Jensen-Haise
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3) What do you need to know?
Avg. air Temp Elevation, Incoming Solar Radiation Max and Min Temp for warmest Month |
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Penman
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4) Most complex method of measuring PET
What do you need to know? -Temp -Rel. Humidity -Wind speed -Elevation -Net radiation Adaptability and pragmatic use of available data Utilizes existing meteorological measurements |
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Hydrograph
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Plots discharge through time
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Lag Time
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time interval from the center
of mass of rainfall excess to the peak of the resulting hydrograph |
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Time to Peak
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time interval from the start of
runoff to the peak of the resulting hydrograph |
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Time Base
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time from the beginning to
the end of surface runoff |
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Discharge
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Discharge is a measure of the volume of water passing a given point over a period of time.
=cross-sectional area of the channel multiplied by the velocity of the water |
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Streamflow
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Surface Runoff + Baseflow
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Cross-sectional Stream Area
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area of the stream perpendicular to flow
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Rating Curve
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relates stage to discharge. Empirical relationship
from observations. Measure discharge at different flows. |
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Design Floods
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flood of a specified return period
used in design of culverts, bridges, levees, dams 10-year flood, 25-year flood, 100 year flood |
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Curve Number
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Function of land cover, soil type, antecedent moisture. (CN) range from 0 to 100. They are assigned
based on soil type (hydrologic soil group), land use and cover, and management and conservation treatment, and are modified depending on a soil’s moisture content at the time of the precipitation or snowmelt event. The lower the curve number, the lower the runoff potential. |
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Rational Method
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Predicts peak runoff rate as a fixed proportion of rainfall
Qpk = ciA Qpk = peak flow in cfs i = rainfall intensity (spatially uniform) (in/hr) c = runoff coefficient (function of land use and soil type) A = drainage area (in acres) |
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Unit Hydrographs
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Hydrograph that results from 1 inch of excess precipitation spread uniformly in space and time over
a watershed for a given duration. The shape of the unit hydrograph is a function of land, soil, and watershed characteristics that are assumed to be repeated for similar duration storms. |
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Total Dissolved Solids (TDS)
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total amount of solids (mg/L) that remain
when a water is evaporated to dryness |
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Conductivity Probe
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to determine the ability of the
dissolved salts and their resulting ions to conduct an electrical current |
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Henry Darcy
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First systematic study of the movement of water
through a porous medium. Investigated the flow of water in a column of sand. |
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Darcy's Law
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Q = -KA (Change in h / L)
Q= flow rate K=Hydraulic conductivity/ coefficient of permeability A= Cross sectional area Change in H= Change in head L= length of column |
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Aquifer
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geologic unit that can store and
transmit water at rates fast enough to supply reasonable amounts to wells |
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Aquitard
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geologic unit of little to no permeability
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