hydrology final

what 2 things are needed to estimate streamflow?

amount of precipitation that infiltrates into the soils

amount of precipitation that flows over the land surface as runoff

model infiltration with which models?

horton

green-ampt

model runoff with which model?

SCS curve number

runoff equation

Runoff = Precipitation - Infiltration

Horton Model def

infiltration model developed as an approximate solution of an exact theory

Green-Ampt Model def

infiltration model developed as an exact solution to an approximate theory

infiltration def

process of water penetrating from ground into soil

four zones of infiltration (highest to lowest)

1. saturation zone

2. transition zone

3. transmission zone

4. wetting zone

infiltration rate, f(t)

rate at which water enters the soil at the surface

cumulative infiltration, F(t)

accumulated depth of water infiltrating during a given time period

infiltration capacity, fp(t)

ability of the soil to infiltrate water regardless of precipitation rate

horton model limitations

initial infiltration capacity has to be adjusted based on antecedent soil moisture

can be difficult to estimate all the needed parameters bc not based on soil properties that are easily measured.

ponding time, tp

elapsed time between the time rainfall begins and the time water begins to pond on the soil surface

Fp eqn

tp*i

SCS Curve Number abbreviation

soil conservation service curve number

SCS curve number def

most widely used rainfall-runoff model for routine design purposes in the US

2 basic computation elements in the SCS curve number method

1. estimation of effective rainfall or flow volume

2. estimation of peak discharge

before ponding time, rainfall rate is _____ the infiltration capacity

less than

T/F: before ponding time, all the rainfall has infiltrated

true

SCS curve number depends on what 2 things?

1. soil condition

2. land use

what is the default antecedent soil wetness condition for the CN adjustments for the SCS method?

Condition II

how to adjust the curve number based on antecedent soil condition?

use the table

CN II

average

CN I

dry

CN III

wet

streamflow hydrograph def

continuous graph of streamflow discharge vs. time

what are hydrographs a function of?

precipitation

interception

infiltration

groundwater

basin flow characteristics

unit hydrograph goal

quantify overall watershed response without consideration of detailed watershed flow processes

D-hr UH

characteristic response of a given watershed to a unit volume (1in or 1cm) of effective water input (direct runoff) applied at a constant rate for D hours

unit hydrograph def

result of a unit depth of runoff occuring uniformly over a watershed at a constant rate for a specified duration

unit pulse response is the function of a _____ hydrologic system

linear

the unit response follows which 2 principals?

proportionality

superstition

UH assumptions

1. excess rainfall has constant intensity during duration

2. excess rainfall is uniformly distributed over the watershed

3. base time (time of direct runoff) is constant

4. ordinates of unit hydrograph are proportional to total runoff (linearity)

5. UH represents all characteristics of watershed (lumped parameter) and is time invariant (stationarity)

given a 2.5-hr UH, how to get to a 1-hr UH?

1. create S-hydrograph by lagging the given UH by the duration (D)

2. lag S-hydrograph by the new duration (t)

3. calculate the difference in the lagged S-hydrographs

4. multiply the ordinates of step 3 by D/t

baseflow def

portion of streamflow that is presumed to have entered the watershed in previous events and is derived from persistent, slowly varying sources

water that is slowly being released from previous events

baseflow separation method used in class

assume constant baseflow based on streamflow prior to storm response

key step to created UH from observations

remove baseflow

steps to create a UH from observations

1. select multiple hydrographs from intense storms of approximately equal duration

2. separate event response from baseflow

3. for each hydrograph determine the total effective precipitation (P*total) from the event

4. multiply selected ordinates of each hydrograph by 1/P*total

5. determine the average peak and time to peak and average shape of all the hydrographs

6. adjust the curve to make sure the area under the curve is 1 unit

where is UH developed from observations applicable?

watershed at the point on the stream where the data are measured

gauged watersheds

watersheds where data on precipitation, streamflow, and other variables are available

ungauged watersheds

watersheds w no data on precipitation, streamflow and other variables

how are synthetic UHs derived?

relating hydrograph characteristics with watershed characteristics

3 common methods for generating synthetic unit hydrographs

gamma distribution

snyder's method

SCS method

SCS method def

based on using basin properties to estimate tl and empirically derived relationships for tp and Qp and an assumed hydrograph shape

hydrograph routing def

procedure to determine the flow hydrograph at a point on a watershed downstream from a known hydrograph

what happens as hydrograph shifts?

it gets weaker and delays as a result of storage

what hydrograph is used to create a unit hydrograph of a different variation?

S-hydrograph

4-hr UH with peak discharge of 100 m3/s, what would be the peak discharge for a 4-hr storm w 2.5cm of effective precipitation?

250m3/s

when to use a synthetic unit hydrograph?

ungauged watersheds

when to use hydrograph routing?

when modeling the streamflow response of a basin that is broken up into subbasins or when the water moves through a reservoir

2 flow routing methods

hydrologic routing (lumped)

hydraulic routing (distributed)

hydrologic routing (lumped) def

uses the continuity equation to relate inflows, outflows, and storage to solve for outflows at a point

hydraulic routing (distributed) def

uses continuity and momentum to solve open channel flow equations in space and time

hydrologic routing method

muskingum method

what is muskingum method used for?

river reach

muskingum method is function of?

inflow (I)

outflow (Q)

K

travel time through the reach

level pool method use

reservoirs

level pool routing assumptions

water only flows over the spillway, no flow through emergency spillway or gate

horizontal water surface over the reservoir

relationship between stage (height) and flow

level pool method - what is storage a function of?

Q (outflow)

solving level pool method

given:

inflow hydrograph

Q-H-A relationship (outflow-stage-area eqn) reservoir specific

solve:

1. develop Q-H-A vs. "right side" relationship using reservoir table

2. solve for Q, storage and stage for each time step

a) compute "left side" using inflow hydrograph and previous time step

b) use "left side" with the reservoir table to interpolate Q, S, and H for current time step

c) repeat for next time step

for muskingum method, what is the ravel time through the reach?

K

for the muskingum method, what is the weight on inflow vs. outflow

X

affect of urbanization on hydrology of the land

results in accelerated removal of storm water and increase in the volume and peak runoff

when is rational method used?

urban basins with areas less than 600 acres

rational method

based on the premise that uniform rainfall intensity over a basin will produce a maximum peak flow when all areas over the basin are contributing to the outflow at the point of design

time of concentration, tc

the time required, with uniform rain, for 100 percent of a tract of land to contribute to the direct runoff at the outlet

rational method assumptions

precipitation return period is the same as streamflow return period

antecedent moisture conditions are inherent in the the C coefficient and is typically developed for frequent storms of 2-10 year return periods

C coefficient is assumed to be constant during a storm and from storm to storm

rational method requires what kind of table?

short duration precipitation return periods with fine temporal resolution

often developed by local agencies

What is HEC-HMC used for?

designed to simulate the complete hydrologic processes of dendritic watershed systems

Which organization developed the HEC-HMS model?

Army Corps of Engineers

how is groundwater used in natural systems?

linked to surface water through infiltration (input) and baseflow (output)

how is groundwater used in engineered systems?

wells tap into deep groundwater sources that are unsustainably used

effects of unsustainable use of groundwater

impacts to land surface

depletions of the groundwater resource

land subsidence cause

excessive groundwater pumping allows upper soil layers to dry out, compress, and compact

Aquifer under US Great Plains

Ogallala Aquifer

aquifer def

geologic formation that can store enough water and transmit it at a rate fast enough to be a significant source of water

aquitard def

formation that does not transmit water at a hydrologically significant rate.

unconfined aquifer def

upper boundary of aquifer is the water surface at atmospheric pressure (water table)

confined aquifer def

aquifer bounded above and below by aquitards (aka confining layers)

piezometric surface def

imaginary surface of a confined aquifer

similar to the water table for an unconfined aquifer

fundamental transmission property of aquifer

saturated hydraulic conductivity (Kh)

homogenous conductivity

conductivity is the same at all points

heterogenous conductivity

conductivity is different at all point

isotropic conductivity

conductivity at a point is the same in all directions

anisotropic conductivity

conductivity at a point is not the same in all directions

specific storage (Ss) of aquifer

the increase/decrease in volume of water stored beneath the unit area per unit increase/decrease in response to the change in pressure (head)

change of head in an unconfined aquifer produces...

a change in volume of water in the medum

specific yield (Sy) def

amount of content change in an unconfined aquifer

volume of water released per unit surface area per unit decline of water table

decrease in head of a confined aquifer

piezometric surface lowers

aquifer beneath the unit surface area remains saturated

decrease of confined aquifer storage from the head decrease is due to which 2 reasons?

1. compaction of the aquifer as part of the weight of the overlying material is transferred from the liquid to the solid grains, increasing effective stress and decreasing porosity

2. expansion of water due to lowered pressure.

storativity def

storage coefficient for a confined aquifer

function of the compressibility of medium and water

well hydraulics def

understanding of the processes in effect when one or more wells are pumping from an aquifer

why are well hydraulics important to hydrology?

groundwater withdrawals from aquifers are important to meet water demand

well hydraulics assumptions

well is pumped at a uniform rate

well is fully penetrating across the aquifer thickness

aquifer is homogenous and isotropic

well/screen losses are ignored

drawdown (s) def

difference in initial elevation and the current water table or piezometric surface

drawdown is dependent on what?

time of pumping

pumping test is used to...

estimate the transmissivity and storage coefficient.

how does pumping test work?

pump the well at a constant rate and measure the change in drawdown in an observation well over time

which method is used to solve complex well problems?

method of superposition