ALPHa_BF.docx

8/9/2019 ALPHa_BF.docx

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Hi all,

I calculated my ALPHA_BF for a certain gaging station using the SWAT

baseflo filter! Hoe"er, hen I a##ly the resulting "alue to themodel, the simulated flos are $%&' different from the obser"ed flos

(using the same data ) same time #eriod that #roduced the ALPHA_BF*!I+m ondering if it+s bad calibration not to use the calculated

ALPHA_BF "alue (i!e! should the ALPHA_BF ma-e the simulationbetter, and since it+s not ma-ing it better, could that mean my

other #arameters are rong.*! /r if I shouldn+t orry at all about

the fact that the best simulation is #roduced ith a substantiallydifferent ALPHA_BF!

Than-s0

In my study basin, I found ALPHA_BF is the most important parameter that can

inuence the calibration sucessful or not. if its value is too lare, theshallo! a"uifer can not hold enouh !ater !hich can totally lose the a"uifer

characteristics. It is not very sensitive to streamo! calibration. #ou are

usin baseo! to et the parameter, I thin$ it !ould be more reliable.

Hi,

than-s so much for the feedbac-! If I understand you correctly,

you+re suggesting it is im#ortant that I use the ALPHA_BF that the

baseflo filter calculated. %"en though the simulations are #oor henthis ALPHA_BF is used.

&osie,

it all de#ends on your hydrological understanding of the catchment! 'ou can

alays fit the streamflo ith some combination of #arameters, there are

#lenty of #arameters to tea- in SWAT! Hoe"er this does not alays mean

that this is a true re#resentation of the catchment #rocesses! We ill

mostly ne"er -no e1actly! It is u# to you as a hydrologist to ma-e a e1#ert

decision on hat you thin- is the most reasonable based on your

understanding of the actual data!

What does the real catchment tell you! Is it a ty#ical high basefloatershed. High flo al year round ith only small effects of rainfall ith

slo res#onses and slo recessions! /r a more flashy res#onse ith ra#id

rise of #ea-s and ra#id res#onses and little or no baseflo in beteen the

storms or in summer!



In the end mathematical baseflo se#aration is a rather une1act science! 'ou

are basically filtering the real data and loo-ing for the lo fre2uency

signal! Hoe"er if most of your atershed res#onse is through groundater

than you might be loo-ing at in#ut from a different a2uifer or some less

connected #art of the a2uifer! There are 2uite a fe #a#ers ritten on

baseflo se#aration and most ac-noledge that the mathematical se#aration is

only an estimate!

If you ha"e chemcial (such as tracer or cations* data than you could try

using that to constrain your baseflo analysis (see the #a#ers by Hoo#er*!

It might be also orth your hile reading some of Be"en+s theory about

e2uifinality, or ho you can ha"e se"eral sets of calibrated #arameters ith

e2ually good fits!

So all calibration also re2uires some common hydrological sense! If you

thin- that this is the best re#resentation and you can argue your case than

that is the best calibration!

Hi &osie,

I ould orry about my ALPHA_BF as it is the constant hich determines

the slo#e of the stream flo decline! If using the calculated ALPHA_BF

gi"es you a rong result! I thin- you need to loo- at the other groundater #arameters (3W456, &7H&3_8P, 3W_&%$AP, &%$AP56!!!* and also the

the soil #ro#erties li-e the AW7 and the saturated hydraulicconducti"ity of the soil as they affect the amount of ater reacingthe shallo a2uifer! But first of all che-ing the total amount of

ater contributed by the baseflo is e2ual or nearly e2ual to the

amount of baseflo you calculated from the measured streamflo ould be a good start!

Hi Willem and Biniam,

than- you "ery much for the ad"ice99it+s greatly a##reciated!

The stream a##ears to ha"e a more flashy hydrogra#h ith relati"elyra#id rise of #ea-s and little baseflo in summer after snomeltrecharge ends! The cumulati"e amount of total baseflo I estimated

o"er a :9year #eriod is ithin ;< of the obser"ed total cumulati"e

baseflo, and the ater balance and baseflo inde1 calculated from theout#ut!std files comes in "ery closely to re#orted "alues for this

stream! I am re9running the baseflo9filter calculated ALPHA_BF on a



sa"ed co#y of a simulation ithout any other SWAT #arameters ad=usted

to try to get at the effect of the ALPHA_BF alone!

Than-s, e"eryone0

Hi all,

so it loo-s li-e simulations ith the ALPHA_BF calculated ith the

TA5> baseflo filter #rogram actually fits much of the obser"ed

baseflo ell, hen other SWAT #arameters ha"e not yet been ad=usted!

Hoe"er, it results in a much #oorer 6ash9Sutcliffe for surface andtotal flos! 5y surface flos still ha"e much better 6ash9Sutcliffe

and &; ith other simulations in hich the ALPHA_BF is higher! In

these simulations ith the higher ALPHA_BF, the total amount of baseflo, ater balance, and baseflo inde1 a##ear to be =ust about

right, and the 6ash9Sutcliffe and &; are still 2uite good, e"en

though I+m not using the calculated ALPHA_BF!

>sing the ALPHA_BF calculated ith TA5> ould mean re9tracing my ste#sand or-ing "ery hard to get 6ash9Sutcliffes and &; as good as the

ones I had #re"iously ith the rong ALPHA_BF! 5y 2uestion is, is

it orth it to re9trace. Will using the rong ALPHA_BF cause me #roblems don the line99for e1am#le, if I+m modeling nitrate or other

groundater9trans#orted contaminants. /r is the fact that my baseflo

is modeled #retty ell, e"en ith the rong ALPHA_BF, mean that thisshould be o-ay.

Than- you, all000

%osie &&

I don't thin$ I'd re&do everythin && if baseo! is about riht, then presumably

you've ot the "uantity of round!ater about riht. If the rate of hydroraph

recession is a bit o(, that may be a small price to pay. I've never seen a lot of e(ect

of chanin ALPHA_BF in my models && I believe the reports that it can be a sensitive

parameter, but I )ust haven't seen in my models. I'm not sure !hy...

*heers,

7an I res#ectfully disagree ith ?im. I thin- that for ater 2uality

simulations the right baseflo could be "ery im#ortant! Baseflo is =ust oneof the ater #ath ays but since the ater balance is closed it means ater

is not going somehere else!

I agree ith ?im that if the difference is small there should not be a

#roblme, but if there are large differences than the ater #athays are

2uite different and this means that it also trans#orts things li-e 6itrate



differently! All this might not be a #roblem in your calibration if you

calibrate on both 6itrate and flo, but it could be a #roblem if you are

"alidating or ma-ing forard #redictions!

In the end it is u# to you! 'ou should assess hether the #ea- flos are

more im#ortant from a ater 2uality #ers#ecti"e (greater load* or that the

baseflo is more im#ortant (continuous and delayed res#onse of #ollutants*!

Willem

yes, I aree !ith you +illem. streamo! is combination of baseo! and

overland o!, the t!o components could be totally opposite to the real

situation even thouh the streamo! simulation is riht. I am not sure if

nutrients incorporation !ould improve the calibration since its obeservation

is also from streamo!. he baseo! seperation seems a direct !ay to

contrain the baseo! component. but as disscussed above, people doubt aboutthe calcualtion accuracy. !hat I used is !ell elevation ucuations, !hich

is -t !ith a"u-er storae variations. y clabration e/clude a lot of

models !hich ive e/cellent streamo! but !ith unacceptable baseo!

simulations.

ll 99?ust to follo u# 99 I thin- e all agree that the 2uantity of baseflo is "ery im#ortant to try and

match, hich is essentially the same as saying it+s im#ortant to get the right #artitioning beteen

groundater and surface runoff contributions to streamflo! This is es#ecially true for ater2uality considerations, as Willem noted!

/"er an annual ater9cycle under e2uilibrated conditions (no net changes in ater storage*,ALPHA_BF says nothing about the total 2uantity of baseflo 99 the total 2uantity of baseflo

ill be controlled by the net infiltration that e"entually recharges the a2uifer! To a large degree,out#ut (as baseflo* ill e2ual in#ut (as recharge*!

ALPHA_BF does control the rate of release of groundater from shallo a2uifer storage 99

and so it therefore influences the mi1 of groundater @ other flo (o"erland runoff and lateralflo* in the channel at any one time! If you+"e got enough ater92uality sam#les to follo ho

the mi1 of groundater and surface ater changes during hydrogra#h recession, then fine9tuning

ALPHA_BF could be critical! I add that 3W_8%LA' also influences hen baseflo is released(by delaying hen infiltration is tallied as recharge*, and thus also must influence the mi1 of

groundater and surface ater in the channel as ell!

So 99 does that ma-e sense. 8id I misre#resent anything.

Hi =im and others,

It is a com#licated #roblem, becasue a lot of #arameters ould im#act the

baseflo, finally to the streamflo! As far as the ALPHA9BF and 3W98%LA' are

concerned, the former is more im#ortant to baseflo generation! Because it



is alays or-ing and influencing the baseflo 2uantity untill there is no

ater left in a2uifer! 3W98%LA' is different, it lose im#action during dry

seasons hen there is no ater recharge to ac2uifers! Besides,

ALPHA9BF is in a e1#ontential terms, it im#acts more than

a linear im#actor! It de#ends on ho e understand our study area! I ould

rather to ad=ust ALPHA9BF because it is more sensiti"e!

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ALPHa_BF.docx