HRU level water balance

[Noel]

Hello SWAT-users,

I want to check the water balance at HRU level. I would like to know if the following formulation is appropriate?

  Precip = ET + avg. Soil water + Water yield + Deep aquifer recharge,           where

  Water yield = SURQ_CNT + LATQ + GW_Q

Am I double counting LATQ by including it in the water balance since it comes from the soil storage?

Thanks,
Noel

[sbegueria]

Hi Noel,

That is exactly what I asked a few weeks ago. I cannot believe nobody is interested in performing a water balance / checking the mass balance of SWAT at the HRU level!

This is the water budget equation I am using:

PRECIP + IRR  =  ET + WYLD - Delta(SNOFALL) - Delta(SW_END) - Delta(DA_ST),

where Delta(SNOFALL) is the result of SNOFALL-SNOMELT, Delta(SW_END) is the result of SW_END(t-1)-SW_END(t), and Delta(DA_ST) is the result of DA_ST(t-q)-DA_ST(t), being t the current month and t-1 the month before.

My problem is this equation never holds true, i.e. the two sides of the equality give different values.

I hope you'll be luckier than I with the responses.

Cheers,

Santiago

[Ann van Griensven]

Hi,

 

try to include the shallow aquifer SA_ST in your equation.

Kind regards

Ann
_________________________________________
Ann van Griensven, PhD MSc

Associate Professor
Core of Hydrology and Water Resources
UNESCO-IHE Institute for Water Education
P.O. Box 3015, 2601 DA DELFT, The Netherlands
Visiting address: Office W.316, UNESCO-IHE, Westvest 7, 2611 AX Delft, NL
tel +31 15 215 18 12
e-mail a.vangr...@unesco-ihe.org
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Department of Hydrology and Hydraulic Engineering
Vrije Universiteit Brussel
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Date: 02/25/2013 11:39AM
Subject: [SWAT-user:3804] Re: HRU level water balance

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[Jim Almendinger]

Well -- the ability to respond is generally based more on time constraints of the responder and other such obligations -- not really in the lack of interest.  And if one doesn't really know the answer, then it's important to be cautious in perhaps giving a wrong answer that could be propagated.  

Water balances can be constructed from different viewpoints (e.g., what's the boundary of your system?) so it's not always easy to give a general response that holds true.  

In your equation -- you've got water sources on the left, and water destinations on the right:

-- Doesn't precip already include snow?  If so, then you've double-counted it.

-- If soil water and the deep aquifer are destinations, then they should be added to, not subtracted from, the other destinations.  

-- Because of time lags in the system, especially for the groundwater part of WYLD, you shouldn't expect a perfect balance of the equation.

All this said -- I think there can be problems with water balance in SWAT, so it's genuinely important to check it out.  You're doing the right thing.  

-- Jim

Dr. James E. Almendinger, Senior Scientist
St. Croix Watershed Research Station
Science Museum of Minnesota
16910  152nd St. N
Marine on St. Croix, MN  55047
tel: 651-433-5953 X 19
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[sbegueria]

Dear Jim,

Sorry if my email sound offensive, it was not my intention. I understand everyone's schedules are too tight. 

To continue with the debate, I expressed my water balance equation as: inputs = outputs + variations in internal storages. I use the naming conventions used in the .hru files.

With respect to snowfall, Delta(SNOFALL) represents the variation in the snowpack, i.e. an internal storage and not an input flux. Similarly, Delta(DA_ST) and Delta(SW_END) are variations in the deep acquifer and soil water, i.e. internal storages and not output fluxes. I defined Delta(SW_END) as the result of SW_END(t-1)-SW_END(t), SW_END being (SWAT io manual, p. 410) the 'amount of water in the sol profile at the end of the time period'. I defined Delta(DA_ST) as the result of DA_ST(t-q)-DA_ST(t), DA_ST being (SWAT io manual, p. 410) the 'amount of water in the deep acquirer at the end of the time period'.

I don't understand your statement about the time lags. I know there are time lags controlling the fluxes across internal storages and out of the HRU, but that water needs to be accounted for in some place, it does not 'evaporate' from the system to re-apear later...

To finish, something I don't fully understand are the true boundaries of the HRU's. From the documentation (SWAT io manual again) it seems that both the shallow and deep aquifers are part of the HRU system, but this seems a bit odd to me.

Cheers,

Santiago

PRECIP + IRR  =  ET + WYLD - Delta(SNOFALL) - Delta(SW_END) - Delta(DA_ST),

where Delta(SNOFALL) is the result of SNOFALL-SNOMELT, Delta(SW_END) is the result of SW_END(t-1)-SW_END(t), and Delta(DA_ST) is the result of DA_ST(t-q)-DA_ST(t), being t the current month and t-1 the month before.

[sbegueria]

Thank you Ann for your response.

You're right the shallow aquifer should be included in the equation. This would yield:

PRECIP + IRR  =  ET + WYLD - Delta(SNOFALL) - Delta(SW_END) - Delta(DA_ST) - Delta(SA_ST),

where Delta(SA_ST) is SA_ST(t-1)-SA_ST(t).

LULC  HRU PRECIPmm SNOFALLmm SNOMELTmm     IRRmm     ETmm SW_ENDmm SA_STmm DA_STmm WYLDmm

POPL 1      19.500     0.000     0.000     0.000    43.538    21.414    28.719    26.059    14.904    43.821    21.208     0.000     0.000     0.000   319.267  1298.954     0.251     0.251     0.000     0.055     0.000     0.306    89.419     8.468    14.774     2.161     9.480     0.087     0.011     0.005     0.000     0.000     0.000     0.000     0.000     0.000     0.000     0.000     0.195     0.000     0.000     0.324    -0.170     0.000     0.055     0.052     0.032     0.000     0.000     0.000     0.016     0.002     0.001     0.002     0.001     0.705     0.000     0.000     0.000     0.000     0.000     0.000     0.000     0.267     0.000     0.000 .00000E+00 .00000E+00 0.000E+00 0.000E+00

[sbegueria]

Excuse my last (unfinished) email. I was trying to put an example straight from one of my countless output.hru files and press the 'publish now' button accidentally. Here's the example data (I removed the unneded columns):

LULC  HRU MON PRECIPmm SNOFALLmm SNOMELTmm IRRmm ETmm   SW_ENDmm SA_STmm DA_STmm  WYLDmm

POPL  1   1   19.500   0.000     0.000     0.000 21.414 26.059   319.267 1298.954 0.306

POPL  1   2   0.000    0.000     0.000     0.000 14.495 11.564   319.560 1299.230 0.045

Putting these numbers into my WB equation (for month 2) gives:

ET     + WYLD - Delta(SNOFALL)- Delta(SW_END)  - Delta(DA_ST)        - Delta(SA_ST)

14.495 + 0.045- (0.000-0.000) - (26.059-11.564)- (1298.954-1299.230) - (319.267-319.560) = 0.614

In this case the resulting value is quite close to zero, which corresponds to the inputs (PRECIP + IRR) of that month. A month without rain might not be the best example, so here's another:

LULC  HRU  MON PRECIPmm SNOFALLmm SNOMELTmm IRRmm ETmm   SW_ENDmm SA_STmm DA_STmm  WYLDmm

OAK   1283 3   22.100   0.000     0.000     0.000 18.161 7.344    355.300 1361.176 0.674

OAK   1283 4   216.500  0.000     0.000     0.000 97.828 27.060   367.134 1389.866 55.579

ET     + WYLD  - Delta(SNOFALL) - Delta(SW_END) - Delta(DA_ST)        - Delta(SA_ST)

18.161 + 55.579- (0.000-0.000)  - (7.344-27.060)- (1364.176-1389.866) - (355.300-367.137) = 130.983

In this case, 130.983 mm is much lower than the total input to the HRU that month, that is 216.5 mm. I.e., we've lost 85.517 mm of water, not a negligible amount. So there are evidently other elements in the WB equation that escape my understanding.

Cheers,

    Santiago

[Jim Almendinger]

Santiago --

Not a problem.  We've all had similar frustrations!  The best users are the ones who try to understand mechanisms, which can be confusing and frustrating. I think water balance is a good example of this -- it's always interesting, confusing (to me), and important.  

If we define our "system" as an HRU (with a "box" around it), then

Flux_in = Flux_out + change in storage 

where a flux is a mass transfer across the boundary of the box, and change in storage is calculated for each sub-component of the box.  Transfers of water from one sub-component to another, within the box, are not fluxes in or out. 

What are the sub-components, or compartments?

-- Soil 

-- Vadose zone (not directly represented in SWAT)

-- Shallow Aquifer

-- Deep Aquifer

I think each HRU keeps its own accounting of water in its own part of the Shallow and Deep Aquifers -- like each HRU has its own mini-shallow-aquifer and mini-deep-aquifer, separate from those of the other HRUs.  This is a little conceptually weird, but it seems to work in aggregate -- it's mostly just of a way to simplify the computations.  

Note that Irrigation is not a flux in, or out, of our HRU box -- it is a redistribution of water from one compartment (shallow or deep aquifer) to another (soil) within the box.  The part of irrigated water that leave the system will be included in the ET and WYLD values.  

Flux_in:

This would be PRECIP, which from what I read in the manual, includes SNOFALL, which is just the part of PRECIP falling as snow, and SNOMELT is the part of SNOFALL that melts during the time period.  So if you wanted the amount of liquid water entering the box (HRU) during the time period, it would be --

PRECIP - SNOFALL + SNOMELT

Flux_out:

ET + WYLD

Change in storage:

-- Delta SW_END = SW_ENDt - SW_ENDt-1

-- (no output for Vadose zone)

-- Delta SA_ST = SA_STt - SA_STt-1

-- Delta DA_ST = DA_STt - DA_STt-1

(I see you did your delta values as t-1 minus t, so your signs were correct for your calculations)

So that's my take on how to calculate the liquid water balance for an HRU.  There is that one compartment, the vadose zone between the bottom of the soil and the top of the shallow aquifer, that is unaccounted for, and for which there may be a lag.  

Finally -- I have not checked any of this with any of my own data -- this is just from looking at the .hru output fields and conceptualizing what I think should be true.  It is entirely possible I've missed an important flux or compartment...  

I hope this helps more than confuses!

-- Jim

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[Jim Almendinger]

In your example near the bottom -- isn't your ET for month 4 = 97.8?  That would make your loss term = 210.65 or so, which is pretty close to your 216.5 mm of PRECIP.  (But I don't know where the remaining 6 mm is -- perhaps in the vadose zone?)

-- Jim

-- 

[Santiago Begueria Portugues]

Hi Jim, you are right I took the wrong ET! So the correct numbers are:

LULC  HRU  MON PRECIPmm SNOFALLmm SNOMELTmm IRRmm ETmm   SW_ENDmm SA_STmm DA_STmm  WYLDmm

OAK   1283 3   22.100   0.000     0.000     0.000 18.161 7.344    355.300 1361.176 0.674

OAK   1283 4   216.500  0.000     0.000     0.000 97.828 27.060   367.134 1389.866 55.579

ET     + WYLD  - Delta(SNOFALL) - Delta(SW_END) - Delta(DA_ST)        - Delta(SA_ST)

97.828 + 55.579- (0.000-0.000)  - (7.344-27.060)- (1364.176-1389.866) - (355.300-367.137) = 210.65

in which case we are loosing 'just' about 6 mm of water (210.65-216.500) or 2.7%. That's a pretty good water balance compared to what I am normally getting. Allow me some time to make this computation on a wide number of HRUs and months and I will tell you the outcome.

Santiago

[Jim Almendinger]

I think the annual water balance looks pretty good, unless I'm missing something.  A difference of +/- a few percent seems reasonable, as presumably storage in the system is flexible enough to handle that amount each year.  Although -- you'd think then, that changes in soil moisture and shallow aquifer would essentially equal that change in storage to balance the equation, and it seems these quantities are too small in the output.  The unknown amount in the vadose zone may be part of it, but who knows how much.  

   So -- I can't explain all of it, but at least (as we say in Minnesota), it could be worse.  

-- Jim

On Apr 1, 2013, at 10:42 AM, Noel wrote:

Have a look at the attached spreadsheet. I computed sub-basin level water budget for one of my modeled basins for the period 1950-2008. The sheets "monthly_watbal" and "annual_watbal" are monthly and annual water balance respectively.

I compute water balance as follows (no irrigation and snowfall in the basin),

   Precipitation = ET + WYLD + Deep aquifer recharge  (for both monthly and annual)

Here I consider (root zone + vadoze zone + shallow aquifer) as the model domain. Any water that leaves or enters this zone is a flux.

In the monthly case, the water balance will not close, but it ought to in the annual case. It looks like that way, though some years have large closing errors (range +/- 2%).

Any comments, suggestions feedback welcome!

Noel

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<SWAT_OUTPUT_COMPARISON_v1.xlsx>

[Noel]

A correction to my earlier post. The water balance should read as follows,

Precip = ET + Water yield + Change in soil storage + Percolation - GW_Q.

This can be further refined by accounting for change in shallow aquifer storage and REVAP. But, the above formulation, I think, is sufficient to check basin water balance.

I have attached the updated spreadsheet for a sub-basin in my study area.

Noel

[MPfa1983]

Have you thought about the groundwater delay? There must be also some water in the "virtual" storage between soil and shallow aquifer!

[sbegueria]

Hi all,

I am re-opening this thread because I had a chance to spend some time on the HRU water balance in SWAT in the last two days and came to some conclusions. It seems that the HRU water balance is not as intuitive as I first thought. For example, the flow through vadose zone is tricky because it is not explicitly accounted for in SWAT (as far as I understand it). Since I was unable to get good results (i.e., a water balance that balances out), I finally inspected the source code to find what is really going on. There is a watbal.f routing that makes precisely so (computing the HRU water balance), so it was not a difficult task. If you want to read the whole story, I have written a couple posts on my site (I hope I can put external links here):

http://santiago.begueria.es/2013/10/unraveling-the-hru-water-balance-in-swat-i/

http://santiago.begueria.es/2013/10/unraveling-the-hru-water-balance-in-swat-ii/

Not done definitive calculations, will let you know.

Best regards, 

Santiago

[sbegueria]

End of the story:

I read one year of data for one HRU from the output.hru in R and applied the water balance equation described in the linked posts. This is basically the following:

t = 2:12

tt = t-1

DStorage = c(NA, SNOFALL[t] - SNOMELT[t] + 

SW_END[t] - SW_END[tt] + SA_ST[t] -

SA_ST[tt] + DA_ST[t] - DA_ST[tt])

Flows = PRECIP + IRR + GW_RCHG -

ET - REVAP - (SURQ - TLOSS) -

LATQ - GW_Q - PERC

BAL = Flows - DStorage

The result:

   DStorage   Flows    BAL

1        NA  65.756     NA

2    22.953  23.520  0.567

3   -11.305 -12.256 -0.951

4   117.181 121.787  4.606

5    95.580  94.697 -0.883

6   -61.553 -65.376 -3.823

7   -44.491 -45.365 -0.874

8   -41.160 -41.339 -0.179

9    24.627  27.013  2.386

10   95.923  99.808  3.885

11  111.954 111.102 -0.852

12   75.269  73.153 -2.116

As it can be seen, the mass balance error (BAL) is one to two orders of magnitude smaller than the flows or the variation in storage. Please, note that I only had SWAT2005 output files at hand, so the balance for the snowpack is only approximate (since sublimation is not taken into account) and also QTILE is missing. Also, LATQ_CNT is not reported on SWAT2005, so I used LATQ instead. This, plus the absence of the lagged terms surf_bs and bss, may be the cause of the mass error.

Cheers,

Santiago

[Jim Almendinger]

Santiago --

Thanks very much for this -- it remains a confusing topic, but this should help guide users in how to interpret the SWAT output. 

My guess is that the small difference between changes in storage (DStorage) and Flows is somewhere buried in the vadose zone and perhaps related to the GW_DELAY parameter, which delays the time between percolation from the bottom of the soil and its inclusion in the GW_RCHG term. 

Nice blog posts, which should be retained if we ever get a wiki page started for a more organized way to explain SWAT usage. 

-- Jim

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Subject: [SWAT-user:4265] Re: HRU level water balance

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Dr. James E. Almendinger

St. Croix Watershed Research Station
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16910 152nd St N

Marine on St. Croix, MN  55047

tel: 651-433-5953 ext 19

[lei qiao]

By the way, why you add GW_RCHG in the flow equation. I think this term is not one part of inflow. Also, do you calculate on daily or monthly? if you did calculation on monthly, some storage terms need to be carefully treated, since they are just stored as the quantities of the last day rather than the average of the whole month. 

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Lei Qiao, Ph.D. 

Postdoctoral Research Fellow

 

Dept. of Natural Resource Ecology & Management
Oklahoma State University
008C Agricultural Hall, Stillwater, OK 74078

[Jim Almendinger]

Lei --

For the "storage boxes" listed in DStorage, the list of "Flows" tries to tally all the inflows and outflows.  So I think GW_RCHG is there because it is an "inflow" to the shallow and deep aquifers. Note that it is mostly balanced by PERC, which is the outflow from the soil layer.  You might be able to just skip both terms, but it might add noise to the result. 

What's the difference between SURQ_GEN and SURQ_CNT?  Is it simply that lagged runoff is subtracted from SURQ_GEN to obtain SURQ_CNT? 

-- Jim

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Cc: "sbegueria" <sant...@begueria.es>, "SWAT-user" <swat...@googlegroups.com>
Sent: Wednesday, October 2, 2013 11:56:57 AM
Subject: Re: [SWAT-user:4265] Re: HRU level water balance

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[sbegueria]

Dear Lei,

Please I note that I did not add anything myself. The equations on my post are a direct translation of what is on the SWAT source code (refer to the blog posts for more details).

As for the inclusion of GW_RCHG as an inflow, I too was shocked when I first saw it. The way I used to understand it, GW_RCHG was just an internal movement of water and hence it did not affect the HRU water balance. But, as Jim suggested, they needed to put it there because it is an inflow to the shallow / deep aquifers. Think of it this way: SWAT only considers four water storages at the HRU level: the snowpack, the soil profile (root zone), the shallow aquifer and the deep aquifer. All fluxes between these storages are internal, and hence should not affect the water balance. But, when water abandons the soil profile due to seepage, it takes some time moving through the vadose zone until it eventually reaches the aquifers. Water in the vadose zone is not considered explicitly in the SWAT water balance, hence it is considered as a (temporary) sink and source of water. It is what the SWAT manual calls a 'lagged term'. There are other lagged terms in SWAT: the surface and lateral Q flow are also lagged, i.e. there is a time span between the generation of runoff or lateral flow and its contribution to the stream flow. These terms are considered internally in the daily water balance of the HRUs, but they are not reported on the output.hru file so they cannot be taken into account unless modifying the code (what is pretty easy, by the way).

About the storage terms, you are right when you say that they represent the storage amount on the last day of each month. But that works perfectly for the water balance equation, since you can subtract the amount at the end of month j from the amount at the end of month j-1, thus obtaining the net variation of storage during the month. Precisely what we want to know!

Sntg

[sbegueria]

Hi Jim,

For the "storage boxes" listed in DStorage, the list of "Flows" tries to tally all the inflows and outflows.  So I think GW_RCHG is there because it is an "inflow" to the shallow and deep aquifers. Note that it is mostly balanced by PERC, which is the outflow from the soil layer.  You might be able to just skip both terms, but it might add noise to the result.

I think you're right. Since water stored within the vadose zone is not explicitly accounted for in SWAT, the water balance needs to consider PERC as an outflow and GW_RCHG as an inflow to the HRU storages. This is a bit odd from a conceptual point of view, but that's how it's been programmed.

 

What's the difference between SURQ_GEN and SURQ_CNT?  Is it simply that lagged runoff is subtracted from SURQ_GEN to obtain SURQ_CNT?

Yes, I believe you're right again. SURQ_GEN includes all the surface runoff generated in the HRU, while SURQ_CNT refers to the contribution of surface runoff to the stream flow. The difference between both, as far as I know, is just the lagged runoff (the amount of runoff water that is 'on its way' to be contributed to the channel).

Sntg

[Noel Aloysius]

So,

(a) the water balance for the soil zone at the end of the day

       P - PERC = ET + SUR_Q + LATQ + change_SW

(b) for the aquifer

      GW_RCHG = GWQ + DA_RCHG + change_SA_ST    ( this is ~PERC )

Am I right?

Noel Aloysius

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[lei qiao]

Hi Santiago,

Sorry, I made it wrong. You are right. the storage on the last day of month is exactly required for the water balance equation. If we want to compare with other source of measurement like the GRACE total water storage, we should be careful with the SWAT output, because in this case we need monthly average. If we consider the internal inflow and outflow process, yes, it will be hard to get the water balanced as you said the lag effects. You mentioned there are 4 type of storage, which remind me that SWAT take the recharge to deep aquifer as a disappearing term, It seems this term is missing in your equation if the GW_RCHG is recharge into the shallow aquifer. Another question is about the interception, does it affect the water balance? Thanks.

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[Prosper]

Hello SWAT everyone, I followed your conversation and I thought you may help me

I had a problem on how to estimate the recharge using arcSWAT2012,

In the file SWAToutput.mdb I am getting the average recharge of 12918.16 mm per year (from HRU file)

At the same time in output.std I am getting the recharge as following:

DEEP AQ RECHARGE = 53.55 mm

TOTAL AQ RECHARGE =  823.34 mm

However the average precipitation and recharge in the area of study has been reported to be 1600mm per year and 165 mm respectively. Obviously this recharge of 12918.16 is out of the range because the precipitation is the only input we have in the area.

I am also getting the snow whereas in that area there is no snow.

I calibrated the model using the river discharge.

Any suggestion on how I may be wrong in estimating the recharge?

How to obtain the same results from output.std and from HRU?

Any clarification will be highly appreciated. 

[Cva Shrestha]

Mr Prosper

If any development in your last section of email
i mean from hru to std
please let me know

Thanks
shiva

[Cva Shrestha]

How about diff between perc and GW_rchrg

[Willem Vervoort]

Hi all,

this is a question that has already been asked quite a few times. Is someone confident enough to define a standard answer? Is this not part of the FAQ and common questions on the SWAT website? I can't find the documents anymore since they have rearranged the SWAT website. But this question has been answered quite few times, maybe search for Jim Almendinger's answers in the google group, I am convinced he has answered it a few times

Willem

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