BIOM in output.hru: timeseries or monthly values?

[Willem Vervoort]

Hi all,

we have been debating the interpretation of the value of BIOM in the output.hru file and we hope someone (maybe from the group in Texas) can help us a bit further.

The manual on page 454 (SWAT INPUT/OUPUT File Dociumentation) says:

BIOM:  Biomass. Total biomass, i.e. aboveground and roots at the end of the time period reported as dry weight. Daily biomass is reported in kg ha-1, monthly in tons ha-1 and yearly in tons ha-

1

The question is about "at the end of the time period". Does the mean (for monthly data) that the February value = January value + whatever grew in February? Or does it mean just the biomass that grew in February. We would like to summarise to seasonal values, so don't know if we take March for the end of a season or the sum of Jan, Feb, March

Thanks for your help

Willem

[Jim Almendinger]

An ecologist would interpret "biomass" as "standing crop," i.e., the dry mass of vegetation (above and below ground) existing at a point in time, as tons/ha (or some other per area measurement).  "Productivity" would be biomass created per unit time (per unit area).  Biomass (standing crop) for March, for example, would be the value on the last day of the month, namely the same as for 31 March.  Biomass at the end of the (calendar) year would be the biomass on 31 December.  This wouldn't be very meaningful in most cases... 

 

So it doesn't look like SWAT is interpreting biomass that way, since SWAT differentiates units between a daily (kg/ha) versus monthly & annual (t/ha) values.  So yes, I'd like to see some help from the folks in Texas, too!  (the answer may be in the Theory manual -- I didn't review this). 

Thanks,

-- Jim

---

James E. Almendinger, PhD Director, St. Croix Watershed Research Station 16910 152nd St N, Marine on St. Croix, MN  55047 e: jalmen...@smm.org o: (651) 433-5953 x 11

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[Willem Vervoort]

Aah, SWAT theory manual, good point Jim. 

I think that suggests that it is actually biomass/time period, so just for the month February. This would be my interpretation of page 318 in the Theory manual:

The total biomass on a given day, d , is calculated as:

bio = sum_over_i(delta_bio_i) (eq 5:2.1.3)

where bio is the total plant biomass on a given day (kg ha-1), and delta_bio_i is the increase in total plant biomass on day i (kg/ha). 

Of course that is on a daily basis, but I would think the same applies to monthly.

Maybe Georgie or Nancy can confirm?

Willem

[Willem Vervoort]

Hi Jim and all,

I had a reread of Jim's message and a rethink, but I think my last interpretation is incorrect. Jim is saying that the biomass is the biomass as "exists" at a single point in time, so in this case it is the time series of biomass, rather than the time series of biomass time period increments. In other words, the value in time is the current total biomass rather than the increase in biomass in the time step. 

Georgie, can someone from the SWAT team please confirm if we interpret this right. The equation suggests per time step? But that would make a lot of the output very high, which would make Jim's interpretation more logical.

Regards

Willem

[Willem Vervoort]

Or is there a difference between annual crops and perennials, because there is no harvest, as seems to be suggested in section 5:2.1.1.2 p 320

Regards

Willem

[Aisha Javed]

Hi everyone, 

I have an update to this. It seems like to go from daily to monthly, SWAT uses the last day (28 in February and 30/31 depending on the month). For instance, to get the biomass for Jan. 2002, SWAT will use the value on 2002-01-31. Additonally, to go from daily to annual scale, SWAT uses the biomass from the day before the plant dormancy period (you can get this information from the output.mgt file).  In my case, day 352 marks the initiation of the dormancy period, so I would have to take the biomass from day 351 as my biomass for that year. 

Another finding is that the daily biomass in the output.hru file seems to be printed in tonnes/ha and not kg/ha as is reported in the manual. 

Kind regards, 

Aisha

[Hector Fajardo]

Hello guys, 

I think that in the output.hru for a daily simulation, the biomass (BIOM) is in Tons/ha and yield (YLD) is kg/ha. Contrary to what is indicated int the manual. 

Can someone confirm this? 

Best,

Hector