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# Residence Time - PowerPoint PPT Presentation

Residence Time. Residence Time. Mean Water Residence Time (aka: turnover time, age of water leaving a system, exit age, mean transit time, travel time, hydraulic age, flushing time, or kinematic age) T = V / Q = turnover time or age of water leaving a system

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### Residence Time

• Mean Water Residence Time (aka: turnover time, age of water leaving a system, exit age, mean transit time, travel time, hydraulic age, flushing time, or kinematic age)

• T= V/ Q = turnover time or age of water leaving a system

• For a 10 L capped bucket with a steady state flow through of 2 L/hr, T = 5 hours

• Assumes all water is mobile

• Assumes complete mixing

• For watersheds, we don’t know V or Q

• Mean Tracer Residence Time (MRT) considers variations in flow path length and mobile and immobile flow

• Geomorphology controls fait of water molecule

• Soils

• Type

• Depth

• Bedrock

• Permeability

• Fracturing

• Slope

• Elevation

• Signal processing technique common in

• Electronics

• Seismology

• Anything with waves

• Hydrology

• Brief reminder of transfer function HYDROGRAPH model before returning to

Precipitation

Hydrologic Model

time

time

Hydrograph Modeling

• Goal: Simulate the shape of a hydrograph given a known or designed water input (rain or snowmelt)

Precipitation

Hydrologic Model

time

time

Hydrograph Modeling: The input signal

• Hyetograph can be

• A future “design” event

• What happens in response to a rainstorm of a hypothetical magnitude and duration

• See http://hdsc.nws.noaa.gov/hdsc/pfds/

• A past storm

• Simulate what happened in the past

• Can serve as a calibration data set

Precipitation

Hydrologic Model

time

time

Hydrograph Modeling: The Model

• What do we do with the input signal?

• We mathematically manipulate the signal in a way that represents how the watershed actually manipulates the water

• Q= f(P, landscape properties)

• What is a model?

• What is the purpose of a model?

• Types of Models

• Physical

• http://uwrl.usu.edu/facilities/hydraulics/projects/projects.html

• Analog

• Ohm’s law analogous to Darcy’s law

• Mathematical

• Equations to represent hydrologic process

• Process representation

• Physically Based

• Derived from equations representing actual physics of process

• i.e. energy balance snowmelt models

• Conceptual

• Short cuts full physics to capture essential processes

• Linear reservoir model

• Empirical/Regression

• i.e temperature index snowmelt model

• Stochastic

• Evaluates historical time series, based on probability

• Spatial representation

• Lumped

• Distributed

Integrated Hydrologic Models Are Used toUnderstandandPredict(Quantify) the Movement of Water

REW 2

REW 3

REW 4

REW 1

REW 5

p

REW 7

REW 6

q

Small

Large

Coarser

Parametric

Physics-Based

Fine

How ?Formalizationof hydrologic process equations

Semi-DistributedModel

DistributedModel

Lumped Model

e.g: Stanford Watershed Model

e.g: HSPF, LASCAM

e.g: ModHMS, PIHM, FIHM, InHM

Process Representation:

Predicted States Resolution:

Data Requirement:

Computational Requirement:

• Physically Based, distributed

Physics-based equations for each process in each grid cell

See dhsvm.pdf

Kelleners et al., 2009

Pros and cons?

• Motivation: How can we retain the theory behind the physically based model while avoiding the computational difficulty? Identify the most important driving features and shortcut the rest.

• Beven, K., R. Lamb, P. Quinn, R. Romanowicz and J. Freer, (1995), "TOPMODEL," Chapter 18 in Computer Models of Watershed Hydrology, Edited by V. P. Singh, Water Resources Publications, Highlands Ranch, Colorado, p.627-668.

• “TOPMODEL is not a hydrological modeling package. It is rather a set of conceptual tools that can be used to reproduce the hydrological behaviour of catchments in a distributed or semi-distributed way, in particular the dynamics of surface or subsurface contributing areas.”

• Surface saturation and soil moisture deficits based on topography

• Slope

• Specific Catchment Area

• Topographic Convergence

• Partial contributing area concept

• Saturation from below (Dunne) runoff generation mechanism

• Recognizes that topography is the dominant control on water flow

• Predicts watershed streamflow by identifying areas that are topographically similar, computing the average subsurface and overland flow for those regions, then adding it all up. It is therefore a quasi-distributed model.

Key Assumptionsfrom Beven, Rainfall-Runoff Modeling

• There is a saturated zone in equilibrium with a steady recharge rate over an upslope contributing area a

• The water table is almost parallel to the surface such that the effective hydraulic gradient is equal to the local surface slope, tanβ

• The Transmissivity profile may be described by and exponential function of storage deficit, with a value of To whe the soil is just staurated to the surface (zero deficit

P

a

c

asat

qoverland

β

qsubsurface

We need equations based on topography to calculate qsub (9.6) and qoverland (9.5)

qtotal = qsub + q overland

• qsub = Tctanβ

• What is the origin of this equation?

• What are the assumptions?

• How do we obtain tanβ

• How do we obtain T?

a

c

asat

qoverland

β

qsubsurface

• Recall that one goal of TOPMODEL is to simplify the data required to run a watershed model.

• We know that subsurface flow is highly dependent on the vertical distribution of K. We can not easily measure K at depth, but we can measure or estimate K at the surface.

• We can then incorporate some assumption about how K varies with depth (equation 9.7). From equation 9.7 we can derive an expression for T based on surface K (9.9). Note that z is now the depth to the water table.

a

c

asat

qoverland

z

β

qsubsurface

Transmissivity of Saturated Zone required to run a watershed model.

• K at any depth

• Transmissivity of a saturated thickness z-D

a

c

asat

D

qoverland

z

β

qsubsurface

Equations required to run a watershed model.

Subsurface

Assume Subsurface flow = recharge rate

Saturation deficit for similar topography regions

Surface

Topographic Index

Saturation Deficit required to run a watershed model.

• Element as a function of local TI

• Catchment Average

• Element as a function of average

A transfer function represents the lumped processes operating in a watershed

-Transforms numerical inputs through simplified paramters that “lump” processes to numerical outputs

-Modeled is calibrated to obtain proper parameters

-Predictions at outlet only

### Hydrologic ModelingSystems Approach

P

Mathematical Transfer Function

Q

t

t

Q operating in a watershed

t

Transfer Functions

• 2 Basic steps to rainfall-runoff transfer functions

1. Estimate “losses”.

• W minus losses = effective precipitation (Weff) (eqns 9-43, 9-44)

• Determines the volume of streamflow response

2. Distribute Weff in time

• Gives shape to the hydrograph

Recall that Qef = Weff

Event flow (Weff)

Base Flow

Transfer Functions operating in a watershed

• General Concept

Draw a line through the hyetograph separating loss and Weff volumes (Figure 9-40)

W

Weff = Qef

W

?

Losses

t

Q operating in a watershed

t

Loss Methods

• Methods to estimate effective precipitation

• You have already done it one way…how?

• However, …

Loss Methods operating in a watershed

• Physically-based infiltration equations

• Chapter 6

• Green-ampt, Richards equation, Darcy…

• Kinematic approximations of infiltration and storage

• Exponential: Weff(t) = W0e-ct

c is unique to each site

W

Uniform: Werr(t) = W(t) - constant

Examples of Transfer Function Models operating in a watershed

• Rational Method (p443)

• No loss method

• Duration of rainfall is the time of concentration

• Flood peak only

• Used for urban watersheds (see table 9-10)

• SCS Curve Number

• Estimates losses by surface properties

• Routes to stream with empirical equations

SCS Loss Method operating in a watershed

• SCS curve # (page 445-447)

• Calculates the VOLUME of effective precipitation based on watershed properties (soils)

• Assumes that this volume is “lost”

SCS Concepts operating in a watershed

• Precipitation (W) is partitioned into 3 fates

• Vi = initial abstraction = storage that must be satisfied before event flow can begin

• Vr = retention = W that falls after initial abstraction is satisfied but that does not contribute to event flow

• Qef = Weff = event flow

• Method is based on an assumption that there is a relationship between the runoff ratio and the amount of storage that is filled:

• Vr/ Vmax. = Weff/(W-Vi)

• where Vmax is the maximum storage capacity of the watershed

• If Vr = W-Vi-Weff,

SCS Concept operating in a watershed

• Assuming Vi = 0.2Vmax (??)

• Vmax is determined by a Curve Number

Curve Number operating in a watershed

The SCS classified 8500 soils into four hydrologic groups according to their infiltration characteristics

Curve Number operating in a watershed

• Related to Land Use

Q operating in a watershed

Base flow

t

Transfer Function

1. Estimate effective precipitation

• SCS method gives us Weff

2. Estimate temporal distribution

Volume of effective Precipitation or event flow

-What actually gives shape to the hydrograph?

Transfer Function operating in a watershed

2. Estimate temporal distribution of effective precipitation

• Various methods “route” water to stream channel

• Many are based on a “time of concentration” and many other “rules”

• SCS method

• Assumes that the runoff hydrograph is a triangle

On top of base flow

Tw = duration of effective P

Tc= time concentration

Q

How were these equations developed?

Tb=2.67Tr

t

Transfer Functions operating in a watershed

• Time of concentration equations attempt to relate residence time of water to watershed properties

• The time it takes water to travel from the hydraulically most distant part of the watershed to the outlet

• Empically derived, based on watershed properties

Once again, consider the assumptions…

Transfer Functions operating in a watershed

2. Temporal distribution of effective precipitation

• Unit Hydrograph

• An X (1,2,3,…) hour unit hydrograph is the characteristic response (hydrograph) of a watershed to a unit volume of effective water input applied at a constant rate for x hours.

• 1 inch of effective rain in 6 hours produces a 6 hour unit hydrograph

Unit Hydrograph operating in a watershed

• The event hydrograph that would result from 1 unit (cm, in,…) of effective precipitation (Weff=1)

• A watershed has a “characteristic” response

• This characteristic response is the model

• Many methods to construct the shape

1

Qef

1

t

Unit Hydrograph operating in a watershed

• How do we Develop the “characteristic response” for the duration of interest – the transfer function ?

• Empirical – page 451

• Synthetic – page 453

• How do we Apply the UH?:

• For a storm of an appropriate duration, simply multiply the y-axis of the unit hydrograph by the depth of the actual storm (this is based convolution integral theory)

Unit Hydrograph operating in a watershed

• Apply: For a storm of an appropriate duration, simply multiply the y-axis of the unit hydrograph by the depth of the actual storm.

• Assumes one burst of precipitation during the duration of the storm

In this picture, what duration is 2.5 hours Referring to?

Where does 2.4 come from?

U(t-t) = time distributed Unit Hydrograph

Weff(t)= effective precipitation

t=time lag between beginning time series of rainfall excess and the UH

Convolution operating in a watershed

• Convolution is a mathematical operation

• Whereas addition takes two numbers to make a third number, convolution takes two functions to make a third function

x(t)

U(t)

y(t)

x(t) = input function

U(t) = system response function

τ = dummy variable of integration

Convolution operating in a watershed

• http://en.wikipedia.org/wiki/Convolution

Convolution operating in a watershed

• Convolution is a mathematical operation

• Whereas addition takes two numbers to make a third number, convolution takes two functions to make a third function

x(t)

U(t)

y(t)

x(t) = input function

U(t) = system response function

τ = dummy variable of integration

For Unit Hydrograph (see pdf notes)

J=n-i+1

### Catchment Scale Mean Residence Time: An Example from Wimbachtal, Germany

Wimbach Watershed Wimbachtal, Germany

• Drainage area = 33.4 km2

• Mean annual precipitation = 250 cm

• Absent of streams in most areas

• Mean annual runoff (subsurface discharge to the topographic low) = 167 cm

Streamflow Gaging Station

Major Spring Discharge

Precipitation Station

Maloszewski et. al. (1992)

Geology of Wimbach Wimbachtal, Germany

Many springs discharge at the base of the Limestone unit

Maloszewski, Rauert, Trimborn, Herrmann, Rau (1992)

3 aquifer types – Porous, Karstic, Fractured

300 meter thick Pleistocene glacial deposits with Holocene alluvial fans above

Fractured Triassic Limestone and Karstic Triassic Dolomite

d Wimbachtal, Germany18O in Precipitation and Springflow

• Seasonal variation of 18O in precipitation and springflow

• Variation becomes progressively more muted as residence time increases

• These variations generally fit a model that incorporates assumptions about subsurface water flow

Watershed/Aquifer Processes Wimbachtal, Germany

Filter/

Transfer

Function

Modeling Approach

• Lumped-parameter models (black-box models):

• Origanilly adopted from linear systems and signal processing theory and involves a convolution or filtering

• System is treated as a whole & flow pattern is assumed constant over the modeling period (can have many system too)

1

Weight

0

Normalized Time

Modeling by Wimbachtal, GermanyConvolution

• A convolution is an integral which expresses the amount of overlap of one function g as it is shifted over another function Cin. It therefore "blends" one function with another

where

C(t) = output signature

Cin(t) = input signature

t = exit time from system

t = integration variable that describes the entry time into the system

g(t-t) = travel time probability distribution for tracer molecules in the system

• It’s a frequency filter, i.e., it attenuates specific frequencies of the input to produce the result

C Wimbachtal, Germanyin(t)

g(t) = e -at

t

g(-t)

e -(-at)

t

e -a(t-t)

g(t-t)

t

Cin(t)g(t-t)

t

C(t)

t

t

t

Folding

Multiplication

Displacement

Integration

Convolution Illustration

Step

1

2

3

4

Transfer Functions - Piston Flow (PFM) Wimbachtal, Germany

• Assumes all flow paths have same residence time

• All water moves with advection (no dispersion or diffusion)

• Represented by a delta function

• This means the output signal at a given time is equal to the input concentration at the mean residence time T earlier.

Maloszewski and Zuber

PFM

PFM

DM Wimbachtal, Germany

Transfer Functions - Exponential (EM)

• Assumes contribution from all flow paths lengths and heavy weighting of young portion.

• Similar to the concept of a “well-mixed” system in a linear reservoir model

EM

EM

EPM

EM

Maloszewski and Zuber

DM Wimbachtal, Germany

Exponential-piston Flow (EPM)

• Combination of exponential and piston flow to allow for a delay of shortest flow paths

• This model is somewhat more realistic than the exponential model because it allows for the existence of a delay

Maloszewski and Zuber

DM Wimbachtal, Germany

Dispersion (DM)

• Assumes that flow paths are effected by hydrodynamic dispersion or geomorphological dispersion

• Geomorphological dispersion is a measure of the dispersion of a disturbance by the drainage network structure

Maloszewski and Zuber

(White et al. 2004)

Input Function Wimbachtal, Germany

• We must represent precipitation tracer flux to what actually goes into the soil and groundwater

• Weighting functions are used to “amount-weight” the tracer values according recharge: mass balance

• where

• Pi = the monthly depth of precipitation

• N = number of months with observations

• = summer/winter infiltration coefficient

Cout = mean output 18O composition (mean infiltration composition)

Infiltration Coefficient Wimbachtal, Germany

• a was calculated using 18O data from precipitation and springflow following Grabczak et al., 1984

• Application of this equation yielded an a value of 0.2, which means that winter infiltration exceeds summer infiltration by five times

where

Cout (1988-1990) = -12.82o/oo (spring water)

Mean Weighted Precipitation (1978-1990) = -8.90o/oo and -13.30o/oo, for summer and winter, respectively

Grabczak, J., Maloszewski, P., Rozanski, K. ans Zuber, A., 1984. Estimation of the tritium input function with the aid of stable isotopes. Catena, 11: 105-114

Input Function Wimbachtal, Germany

Convolution using FLOWPC

Application of FLOWPC to estimate MRT for the Wimbach Spring Wimbachtal, Germany

Maloszewski, P., and Zuber, A., 1996. Lumped parameter models for interpretation of environmental tracer data. Manual on Mathematical Models in Isotope Hydrogeology, IAEA:9-58

Convolution Summation in Wimbachtal, GermanyEXcel

• Work in progress