SWAT use of WATR land use classificaiton

[Amber Ignatius]

Greetings, SWAT-users!

I am seeking clarification about how SWAT interprets and processes the different land cover classifications, specifically open water (WATR).  I am interested in whether improving the open-water land cover input data may improve model capabilities. For example, how would inclusion of numerous small water bodies within the land cover data change the model process?

I know there are mechanisms in SWAT to add reservoirs/ponds (.res and .pnd files, etc.), however, those methods seem inefficient as my study area includes hundreds of small water bodies (.5-10 hectare ponds/reservoirs), many are off-stream, and storage capacity information is largely unavailable.

I have looked in the "SWAT Input/Output Documentation 2012" (http://swat.tamu.edu/media/69296/SWAT-IO-Documentation-2012.pdf) and found excellent information about land cover characteristics, water balance, water cycling, depressional storage, and how different plant type land covers are processed. However, I haven't located specific information about how the model processes open-water vs. other land cover types.  Is there a way to predict/describe model input changes due to increased open water?

I hope this makes sense. Please let me know if you need additional clarification.  Thank you, SWAT-community!!

~Amber Ignatius

[Jim Almendinger]

The last time I looked, WATR land-use types did not contribute any water, nutrients, or sediment -- they were essentially ignored in the calculations, except that evaporation was allowed (I'm not entirely sure of the reasoning).  I believe all the HRUs based on other land-use types are virtually expended a bit in calculating yields (of water, nutrient, & sediment), so that the total area of the subbasin is accounted for, as if the WATR type wasn't there.  For "small" (how small?  I don't know....) areas of WATR this doesn't seem to cause problems. 

However, if you've deemed WATR as a significant component of the landscape (>5%?  >10% I'm guessing), then you should account for these areas as an aggregated Pond or Wetland.  Then SWAT will explicitly allow for their areas, reducing the areas of land-based HRUs accordingly (I'm pretty sure this is true).  Especially if these depressions intercept much of your subbasins' runoff, then they will have a tremendous influence on nonpoint-source pollutant transport and processing (how much of the eroded sediment and nutrients actually make it to the stream?). 

Yes -- it's difficult to make good guesses about drainage areas, principle and emergency volumes, etc.  But assuming the depressions have no influence is a very large (and poor) assumption -- unless of course, the area of WATR is "small."  And you can't really treat the WATR types as a typical HRU, because then they can't receive inputs from other HRUs, which is what they commonly do.  A Pond or Wetland in SWAT is a tool or feature that allows it to process pollutants from the landscape before they enter the Reach (stream). 

-- Jim

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From: "Amber Ignatius" <arign...@gmail.com>
To: swat...@googlegroups.com
Sent: Monday, October 20, 2014 10:54:46 AM
Subject: [SWAT-user:5063] SWAT use of WATR land use classificaiton

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

[Amber Ignatius]

Thank you for the quick and thoughtful reply!

My study area has only .5% water when using the NLCD landcover but the percentage nearly triples to ~1.4% water when I include additional open-water Lake/Pond features from the NHD. While this is still a relatively small fraction of the total area, I wonder whether it isn't hydrologically significant.

I had a quick follow-up question.  I understand "WATR land-use types...were essentially ignored in the calculations, except that evaporation was allowed".  Do you know how evaporation calculations from WATR differ from evaporation/transpiration calculations for other land covers?

Finally, per your suggestion, I do plan on experimenting with treating the open water as an "aggregated Pond or Wetland". Currently, I am only modeling stream flow and not pollutant transport. However, I believe these ponds aggregately intercept a great deal of the sub-basin runoff and am very curious about their cumulative impact on flow timing as well as pollutant transport/processing.

Thanks again for your help!!

~Amber

[Janne Helin]

I don't think it would be too difficult to develop a GIS routine for aggregating the water cells from the landuse grid to autofill the pond surface areas for subbasins. The area of the ponds could be used as a lower bound for the drainage area fraction too, although a better estimate could be done with watershed tools. I am not clear about how to implement the division between reservoirs and ponds. For example I have 400 ponds/lakes in my watershed of 1680km2. I guess the stream network could be used for defining what is getting the inflow from the stream and hence what would be classified as reservoir instead of a pond.

-Janne

[Jim Almendinger]

I think the WATR evaporation is based on the PET times a coefficient, which is either 0.6 or 0.7 (I forget which).  If you search the source code for WATR you could be able to find out exactly. 

-- Jim

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From: "Amber Ignatius" <arign...@gmail.com>
To: swat...@googlegroups.com

Cc: arign...@gmail.com
Sent: Tuesday, October 21, 2014 7:47:16 PM
Subject: Re: [SWAT-user:5066] SWAT use of WATR land use classificaiton

[Amber Ignatius]

Firstly, thank you Jim and Janne for your helpful responses from a few months back!!  Jim, you were correct that the WATR ET calculations are based on PET times a coefficient. The source code for water_hru.txt states: etday = .7 * pet_day

Just as a reminder, I am interested in evaporation changes running SWAT with and without small water bodies included.

I am trying to run calibration/validation iterations in SWAT CUP for 2 models: 1)default NLCD landcover and default .pnd files, 2)modified landcover and modified .pnd files to include small water bodies.  I will then compare "output.std" files to see changes in ET and PET "average annual basin values".

Janne:  I have pursued your suggestion to use .pnd files as a way to cumulatively include my study area's 2000+ small ponds within SWAT.  I used ArcHydro tools to calculate the small waterbody catchment area, estimate volume, etc. However, I am having a bit of difficulty ensuring that the .pnd files are running properly within SWAT CUP.

An example .pnd file for one of my headwater HRUs is pasted below (my study area has 11 total HRUs). Just for reference, this particular HRU is ~24,000 hectares in size and contains ~280 ponds ranging from .02-11 hectares. As a pond is located on the main stem of the river downstream from the HRU,100% of the HRU area drains into ponds.  From your experience, does this input look correct?  My flow simulations do not seem to change whether I use this .pnd file or simply leave the default.  I am particularly unsure about what to enter for FLOD values.  Thank you!!

 .Pnd file Subbasin: 1 1/12/2015 12:00:00 AM ArcSWAT 2012.10_2.15

Pond inputs:

           1.000    | PND_FR : Fraction of subbasin area that drains into ponds. The value for PND_FR should be between 0.0 and 1.0. If PND_FR = 1.0, the pond is at the outlet of the subbasin on the main channel

         241.575    | PND_PSA: Surface area of ponds when filled to principal spillway [ha]

         440.224    | PND_PVOL: Volume of water stored in ponds when filled to the principal spillway [104 m3]

         241.575    | PND_ESA: Surface area of ponds when filled to emergency spillway [ha]

         942.497    | PND_EVOL: Volume of water stored in ponds when filled to the emergency spillway [104 m3]

         440.224    | PND_VOL: Initial volume of water in ponds [104 m3]

           0.000    | PND_SED: Initial sediment concentration in pond water [mg/l]

           0.000    | PND_NSED: Normal sediment concentration in pond water [mg/l]

           0.000    | PND_K: Hydraulic conductivity through bottom of ponds [mm/hr].

               0    | IFLOD1: Beginning month of non-flood season

               0    | IFLOD2: Ending month of non-flood season

           0.000    | NDTARG: Number of days needed to reach target storage from current pond storage

          10.000    | PSETLP1: Phosphorus settling rate in pond for months IPND1 through IPND2 [m/year]

          10.000    | PSETLP2: Phosphorus settling rate in pond for months other than IPND1-IPND2 [m/year]

           5.500    | NSETLP1: Initial dissolved oxygen concentration in the reach [mg O2/l]

           5.500    | NSETLP2: Initial dissolved oxygen concentration in the reach [mg O2/l]

           1.000    | CHLAP: Chlorophyll a production coefficient for ponds [ ] 

           1.000    | SECCIP: Water clarity coefficient for ponds [m]

           0.000    | PND_NO3: Initial concentration of NO3-N in pond [mg N/l]

           0.000    | PND_SOLP: Initial concentration of soluble P in pond [mg P/L]

           0.000    | PND_ORGN: Initial concentration of organic N in pond [mg N/l]

           0.000    | PND_ORGP: Initial concentration of organic P in pond [mg P/l]

           0.000    | PND_D50: Median particle diameter of sediment [um]

               1    | IPND1: Beginning month of mid-year nutrient settling "season"

               1    | IPND2: Ending month of mid-year nutrient settling "season"

Thank you so much for any feedback!!!!  I am so grateful for this user group!

~Amber

[dipak...@atree.org]

 I am trying to use .pnd file and feeding various related parameters into my SWAT model (QSWAT version: 1.3), as there are several number of ponds in my study area. In QSWAT, PND_PSA paramter is not allowing the value more than 20, similarly PND_VOL is not allowing the value more than 100. In this conversation, I found these values are entered quite large (241.575    | PND_PSA, and 440.224    | PND_PVOL, as mentioned by Amber  ). Is is problem associated with QSWAT or any other things needed to be done. I need your suggestions. Thanks for such a nice discusion in SWAT-user group.