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User Forum

All Topics | Topic "Estimate catchment precipitation"
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Author Message
Dumindu Jayasekera

Subject: Estimate catchment precipitation   
Posted: 9/9/2011 Viewed: 29645 times
Hi,

I am working in rural basin and would like to estimate precipitations for different catchments in the basin. But I don't have rainfall stations for couple of catchments in the basin. So, I thought of area interpolate the precipitation using Thiessen polygon method with available rainfall data.
My question is should I enter the areal precipitation values for catchments I created in the WEAP environment.? Also, I estimated the monthly areal mean values and used the WEAP interpolate function to estimate the daily values. Am I doing the correct thing here? If not please correct me someone who has good experience in WEAP.
Devaraj de Condappa

Subject: Re: Estimate catchment precipitation   
Posted: 9/9/2011 Viewed: 29627 times
Dear Dumindu,

1. You don't need to enter all the rainfall data in WEAP. You can use instead the ReadFromFile function in the Expression Builder where you ask WEAP to read the rainfall stored in a text file. Check the user guide to use ReadFromFile. If you associate the sub-catchments in WEAP with your Thiessen polygons (if ever it makes sense), you can ask WEAP to read in each sub-catchment the rainfall data of the corresponding polygon.

2. In my opinion, it's not a good idea to use WEAP's interpolate function to calculate daily rainfall from monthly rainfall as usually daily rainfall is not even in a month. I would see 2 options:
- you know how to disaggregate manually (outside of WEAP) monthly rainfall into daily rainfall, based on your knowledge of the region you are analysing; in this case disaggregate outside of WEAP and use the ReadFromFile function to read in WEAP;
- if you cannot disaggregate manually, then I would advise to model on a monthly time step, so that you don't create spurious rainfall data which could corrupt your modelling (you can choose any modelling time step in WEAP - daily, sub-monthly, monthly, sub-annually, annually).

Hope it helps.

Devaraj de CONDAPPA
Associate of the Stockholm Environment Institute – US Center
Puducherry
INDIA
Dumindu Jayasekera

Subject: Re: Estimate catchment precipitation   
Posted: 9/10/2011 Viewed: 29615 times
Thanks again for the answer.

I have seen the ReadFromFile function in WEAP and used to read from .csv files. But I do not know a way to associate WEAP with ArcGIS (ArcMap 9.3) to read from the corresponding polygons. In my case for example, I have a catchment (size about 4500 sq.km. ) which lies over different values of Thiessen polygons. Thats why I estimated the mean areal precipitation for that catchment (monthly time step) using ArcGIS and then used that mean monthly areal precipitation values to interpolate using the WEAP interpolate function.

Also, if you know how to read from ArcGIS polygon values directly from WEAP, I would like to know how to do it too. I think that will be more efficient.

In this exercise, I am using the monthly time step in WEAP. I have 17 years of data and when I drag a catchment then opne the precipitation data tab, it shows me precipitation (daily) on the heading of the graph. Because it does not plot the monthly values and it needs daily values. Thats why i used the WEAP interpolate function. Am I correct?

I appreciate your replies.
Devaraj de Condappa

Subject: Re: Estimate catchment precipitation   
Posted: 9/11/2011 Viewed: 29606 times
Dear Dumindu,

1. WEAP & ArcGIS. I'm sorry that I don't really know ArcGIS and I don't know if you can read in WEAP the database file associated to a shapefile (*.dbf file). To my knowledge you can only read a text file. If I understood correctly, you're using ArcGIS to distribute in the sub-catchments the spatial rainfall calculated with Thiessen polygons, for each monthly time step. If this is correct, I would prepare from this database file (*.dbf I guess) a text file containing the monthly rainfall for each sub-catchment. Please refer to the user guide for preparing the text file, basically each line of the file should be a month and each column can be a sub-catchment. Or you can create one file per sub-catchment, as you prefer.

If you have not generated with ArcGIS the rainfall per sub-catchment for each monthly time step, I would advise to do it manually, for instance in Excel. You can get from ArcGIS how the area of the Thiessen polygons are distributed in the set of sub-catchment and use this distribution to spread yourself the rainfall in each sub-catchment (that's how I would do it).

2. Daily rainfall. I'm surprised: if you have selected a monthly time step in WEAP (by going to General > Years and Time Steps, and selecting 12 for 'Time Steps per Year'), then the heading of the Precipitation graph in WEAP should show 'mm/month' (not 'mm/day'). If you select a monthly calculation time step as explained above, then WEAP is expecting monthly rainfall while reading the rainfall file. I think it's not a good idea to use WEAP's interpolate function to get daily rainfall from monthly, you should work with everything in monthly time step.

Hope it helps.

Devaraj de CONDAPPA
Associate of the Stockholm Environment Institute – US Center
Puducherry
INDIA
Dumindu Jayasekera

Subject: Re: Estimate catchment precipitation   
Posted: 9/11/2011 Viewed: 29600 times
Dear Dr. Devraj,

AS you explained I have used ArcGIS to estimate the areal mean monthly rainfall and then read the monthly values from the .dbf file created in ArcGIS. But in WEAP, even though I selected monthly (General-->Years and TimeSteps-->12 (monthly) it appears me precipitation (mm/day) not in mm/month. That's why I used the WEAP interpolate function to estimate the daily values. Other than that I have created the monthly values .csv file to read from in WEAP. Do you know how to correct this to appear to mm/month time step.?
Devaraj de Condappa

Subject: Re: Estimate catchment precipitation   
Posted: 9/11/2011 Viewed: 29596 times
Surprising... Even if you open any other tab and click back on Precipitation you see the unit in mm/day?

If yes, I have no explanation, sorry.

Jack: would you have an idea?

Devaraj de CONDAPPA
Associate of the Stockholm Environment Institute – US Center
Puducherry
INDIA
Dumindu Jayasekera

Subject: Re: Estimate catchment precipitation   
Posted: 9/12/2011 Viewed: 29587 times
Well...I found the reason. Under advanced tab if you select RainfallRunoff(FAO) the precipitation will give in monthly time step whereas if you select MABIA(FAO 56,Kc,daily) it will display precipitation in daily time step even though you have initialized under General, Year and Time step to show in monthly time step.
Thanks again for clarifying things.

Topic "Estimate catchment precipitation"