Hydrology Exam 2

What are the 4 limitations of the phi-index approach?

It assumes infiltration rate is constant, it assumes that the infiltration always proceeds at the rate of phi-index irrespective of rainfall amount, requires the quick response runoff to be known before the infiltration can be calculated, and a phi-index determined from a single storm is not generally applicable to other storms

What component is regarded as the largest loss in the water budget?

Evapotranspiration

Definition of Evaporation

the process by which water is transferred from land/bodies of water to the atmosphere

Definition of Transpiration

the evaporation process from plants, trees, and grass

What are the two main factors influencing ET?

Supply of energy and ability to transport vapor away

What subfactors have a proportional relationship with ET (they increase/decrease together)

Air Temp, Water Temp, Wind Velocity, Geometry of Water Surface, Freshness of water

What subfactors have an invers relationship with ET (they increase/decrease opposite of each other)

Air pressure, Smaller geometry, Saline in water

What are the five methods of estimating evaporation?

Water budget, Energy Budget (complex and expensive), Mass transfer (complex), Use of evaporation pan (EL = K*Ep), and Graphical Regression (Combined theory)

What are the four instruments and what do they measure with an evaporation pan?

Thermometer (temp), Anemometer (wind speed), Rainfall gauge (rainfall), and Psychrometer (relative humidity)

When do we pick the shortest tc for runoff calculations?

When we want to be conservative and when it is only from one point to another.

When do we pick the longest tc for runoff calculations?

Whenever it is a watershed made up of numerous areas and coefficients of runoff with multiple path options from point to point.

How do we find ea (meyers and dunne eq)?

We find eo for the air temp and we multiply it with RH (decimal)

Methods of evaporation control

Cover water, underground storage, minimal surface area, closed conduits, BMP’s

What are some examples of Best Management Practices?

Permeable Pavement and Infiltration Roadside Ditches. They allow rainwater tot soak into (infiltrate) the ground instead of running off into storm drains.

What are the types of runoffs and what do they stem into?

Total runoff is known as Overland flow going into two runoff types known as Surface Runoff (direct runoff over surface of ground) and Subsurface Runoff (under the ground, interflow from unsaturated zone, Base flow discharging into stream channel)

Where does the runoff journey start and end?

The runoff starts at overland flow/sheet flow and ends at stream flow when it reaches a defined stream

Hydrograph Definition

a plot of streamflow vs time at a section of a stream channel

What are the three segments in a Hydrograph?

Rising limb, peak, and falling limb

If a Hydrograph starts at zero, what is it representing?

It represents a direct runoff hydrograph DRH “Stormflow”

If a hydrograph does not start at zero, what is it representing?

It shows the Total Runoff Hydrograph TRH “Streamflow”

What method gives the peak flow?

Rational method (Q = CiA)

What method calculates total runoff?

SCS-CN Method, Q (depth)

How to calculate S in SCS-CN method?

S = (1000/CN)-10

How to find composite CN? (CCN)

Sum of CN*A/ Sum of A

How to calculate peak flow of multi-area?

Q = i (sum of C*A)

What are some descriptors of Ia?

Initial Abstraction, all losses before runoff begins, highly variable, correlated with soil and cover parameters

What three things does Runoff CN depend on?

Soil Type (A-D, A is sand with lowest CN, D is Clay with highest CN), Land use/cover (pervious has lower CN, impervious has higher CN), AMC (I is dry low CN, II is average, and III is wet with highest CN of three)

What is the relationship between CN and runoff potential?

CN is proportional with Runoff potential, higher CN means higher R potential

When is rational method used?

Small urban areas ,<200 acres, to design drainage systems

Definition of time of concentration

The time it takes a drop of water to flow from the furthest point in watershed to outlet

Method of Streamflow measurements?

Direct methods and Indirect methods

What is stream gauging?

measuring discharges and water levels in open channels

Direct Streamflow Measurements?

Structural, Flumes and weirs, gauging station, depth is relative to set datum

Indirect Streamflow methods?

Manning’s Eq and Stage-Discharge Rating Curve