Water yield model inputs: evapotranspiration data current and future, biophysical table, etc

  • Hello All, 
  • This is a series of emails with Rich and Perrine about water yield model and the discussion is still active...any comments is welcome:
  • Hello,
    I have a quick question:
    I'm a really new InVEST user and I'm trying to develop inputs for "Water yield" model to be applied in both current map (2013) and in four future scenarios maps (year 2040) and then compare their results. I already have evapotranspiration data (1950-2000) from cgiar (http://www.cgiar-csi.org/data/global-aridity-and-pet-database) and precipitation data as well to be used in current conditions 2013. However, I'm wondering if I have to create rasters for future evapotranspiration and precipitation to be able to calculate future water yield or how that works?  

    Sorry for email you directly but it seems no body answer my questions in the discussion list,
    Thank you very much for any comment,
  • RichRich September 22
    Hi Danka, sorry, I am not familiar at all with the science side of this model.  I've emailed Perrine, our hydrologist who should be able to get back to you on the forum.
  • PerrinePerrine September 22
    Hi Danka, 

    The "potential" evapotranspiration data from CGIAR is the same as "reference". There is often a confusion between these terms in the literature: the way we use them at NatCap is consistent with the definition from the FAO 56 document, where reference is the evaporative demand of well-irrigated grass, and "potential" evapotranspiration is the evaporative demand of another plant (i.e. reference evapotranspiration times a crop coefficient).
    So the short answer is that you can use the CGIAR data for the InVEST model.
    For future climate, I don't believe the CGIAR dataset provides reference ET. You can compute it based on projections of monthly temperature (e.g. WorldClim) and an approximation of ET as a function of min and max temperature (see User's guide).
  • DankaDanka September 29
    Hello Perrine,
    Ok, I can use CGIAR in InVEST. However, do you think I can use CGIAR data for current conditions in 2013 EVEN THOUGHT this data is an average from 1950 to 2000?. In the user guide it says "Climate data (total precipitation and potential evapotranspiration) should also match the date of the land use map". I'm a little concern about the dates.
    Thank you so much for your response Perrine,
  • PerrinePerrine October 11
    Hi Danka, 

    You are right about the date mismatch, it's definitely something to take it into consideration.
    Fortunately, reference ET is less variable than precipitation,so using time series from a different period won't influence the model as much as it may for precipitation for example.

    You could use the CGIAR data and perform a simple sensitivity analysis: e.g. increasing reference ET by 5% (you can estimate what this would mean for temperature increase with the formula for reference ET in the user's guide), and quantifying the difference it makes to model outputs.

    Otherwise, you can use some temperature time series for, say, the 2000-2010 period and use the formula for reference ET in the user's guide to translate temperature into reference ET (you can interpolate data spatially if your temperature data are given for points in space, i.e. from weather stations).


  • DankaDanka 9:45AM
    Hello Perrine,

    Ok, I already have the CGIAR data (1950- 2000) which I'll use for the current conditions. Now, I have few more questions that I hope you can help me with:

    A) To develop the future evapotranspiration for 2040 I have data on projections of monthly temperature from worldclim (average years 2041-2060) AND precipitation from Government (year 2040). However, I have three questions here:
    1) It is OK to use data from these different sources and years?

    2) Is this the equation you mention above (not sure since you didn't mention precipitation or extraterrestrial radiation in the last emails):
    ETo=0.00013 x 0.408 x RA x (Tav +17) x (TD - 0.0123P)0.76 

    3) If the equation above is the one that I have to use, the problem is that I cannot find data on future extraterrestrial radiation (RA). Do you know a source for future RA and if I cannot find it what other equation can I used?

    B) Biophysical table include information basically about Kc and root depth per each LULC. Since I have to evaluate current and future conditions, do you think that I have to generate two tables for current and future conditions? 

    C) Biophysical table I'm trying to calculate Kc for urban and roads classes. In the user guide says "built
    areas can be set to f*0.1 +(1-f)*0.6 where f is the fraction of impervious cover in the area...". Since my 
    research area is in the Amazon, towns are pretty rural (mostly pervious areas), there is not information or papers about this percentage of impervious areas in this region....how do you think should I calculate this %?. Could be done based on my knowledge on the regions, like 20% of  impervious cover?
    I really appreciate you comments, they are helping me a lot!



  • Hello all,

    I have another question about the Kc values. I'm working with the Kc_calculator (http://data.naturalcapitalproject.org/invest-data/Kc_calculator.xlsx). In this table there are four sheets: Kc_calculator, references, FAO tables, and crops_inches_day.  I'm a little confused about the Kc values in the Kc_calculator and FAO tables sheets.
    In the Kc_calculator sheet, column P shows the Kc coefficient for different LULC. For example: Kc for corn=0.672 (based on 12 Kc coefficients and monthly average PET) BUT in FAO_Tables, column L shows Kc for corn=0.83 (based on 3 Kc coefficients and the length of crop development stages). 
    The questions are: 1) why these Kc are different for corn? and 2) These two sheets (Kc_calculator and FAO tables) are showing two different ways to calculate Kc?

    I'm sorry for all of these questions but the user guide and the Kc calculator table are not that clear to me, my bad. 
    Thank you very much for any help,

  • Somebody??
  • RichRich Administrator, NatCap Staff
    Hi @danka, sorry you're not getting a response here.  Perrine is the only expert on staff who can help with Kc issues (unless you want me to guess :p ) and she's overwhelmed with some other NatCap issues right now.  I know she cares and will get back to you as soon as she can.

  • PerrinePerrine Moderator, NatCap Staff
    Hi Danka, 

    Relative to your post about scenarios:
    A1) As mentioned, it's best to use data from the same time periods but you it's often the case that we miss data for one or the other period, so using 2040 and 2041-2060 sounds good.
    A2) Yes the equation you wrote is the one I was thinking of  for ET_0
    A3) Tables on extraterrestrial radiation can be found here: http://www.fao.org/docrep/x0490e/x0490e0j.htm
    And the whole chapter might be a good read: http://www.fao.org/docrep/x0490e/x0490e07.htm#solar radiation

    B) If your future vegetation properties do not change, you can use the same biophysical table (the scenario will only reflect the change in climate, assuming that vegetation properties like crop coefficient do not change)

    C) Yes, if you don't have the exact % of impervious areas, you can use regional estimates.

    Relative to your last post on the Kc_Calculator:
    1) Kc values are different because they come from a different method
    2) The method in Kc_calculator is slightly more sophisticated in the sense that it uses a weighted average of crop stages values (where the weights correspond to the ET_0 values per month): you can see the formula in row O9 of the Kc_calculator sheet. This approach seems logical given that monthly Kc values will apply to monthly ET_0 values, but the simpler average method (in the FAO table sheet) is not unreasonable either.

    I hope this helps!

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