Subject: hydrology: sensitive parameters Posted: 10/16/2007 Viewed: 42013 times
hydrology: sensitive parameters Anne Chaponniere achaponniere email@example.com Hi, I'm a new WEAP user. I need to apply WEAP on the Volta basin (West Africa) and I would like to check the results of the rainfall-runoff subroutine first. So far, the computed runoff is very far from the observed flows at different locations. But a number of parameters are just "best guesses" since I don't have access to the exact values. I would thus like to tune the model and therefore I would like to know what are the more sensitive parameters in the different runoff/rainfall routines? Thanks.
Dr. Mohammad Rayej
Subject: hydrology: sensitive parameters Posted: 10/16/2007 Viewed: 41997 times
hydrology: sensitive parameters Mohammad Rayej rayej firstname.lastname@example.org I am assuming your using the rainfall-runoff (two-bucket soil moisture method) in WEAP to simulate runoff in your basin. If that is the case, LAI (Leaf Area Index) representing vegetation canopy, would be your best bet to control runoff. You can use a very small value of LAI (say; 0.1) to get large value of runoff. If you use LAI=0., this gives the largest runoff quantity for the given rainfall; you basically transform all of your rainfall to runoff; i.e. nothing will be stored in the root zone. At LAI=0., if you are not getting enough runoff, there may be something wrong with your rainfall data !!.
On the other hand, if you use a large LAI (say 40.), that would reduce your runoff drastically to almost no runoff. Of course, all these depend on your root zone soil moisture content (z1). At a given LAI (say 5), low z1 (drier soil) would not produce as much runoff as in higher z1 (wetter soil) which makes sense because in the 2-bucket method formulation; Runoff =Precip x z1 ^LAI.
In your e-mail, you did not specify if your getting too much runoff or not enough !!
Good luck !
************************************************************** Mohammad Rayej, Ph.D., P.E. Senior Engineer, W.R. California Department of Water Resources Statewide Water Planning Branch 901 P Street, 2nd Floor Sacramento, CA 94236 (916)-653-8017 email@example.com
Subject: Re: hydrology: sensitive parameters Posted: 10/17/2007 Viewed: 41992 times
Re: hydrology: sensitive parameters Isaac Asamoah azoris2000 firstname.lastname@example.orgHi, Which of the catchment simulation methods are you employing? If FAO crop Requirement Approach is being used then effective precipitation, Pe, is the percentage of rainfall available for evapotranspiration. The higher the value of Pe, the lower the runoff. So look at it again and also if the climate data not correct, you may have problems.
On the other hand, if Soil Moisture Method is being used, then surface runoff is very much dependent on Leaf Area Index, LAI, of land cover. Lower LAI values lead to more surface runoff and vice versa. Also look at Z1 values. Higher z1 means the soil is relatively wet and will produce much more runoff than than lower Z1 (drier soil).
ISAAC ASAMOAH WATER RESOURCES ENGINEER WATER RESOURCES COMMISSION, GHANA, P.O. BOX CT 5630, CANTONMENTS, ACCRA, GHANA
Subject: RE: hydrology: sensitive parameters Posted: 10/17/2007 Viewed: 41989 times
RE: hydrology: sensitive parameters Spyros Michas smichas email@example.comA comment irrelevant to Mohammad's analysis of the WEAP model:
If you are still referring to the serious time shift between rainfall and runoff of the Volta basin that you mentioned in previous messages, LAI will most probably not help you. I suppose you do not want to "lose" your rainfall (big LAI), you just want to store it and have it as runoff later. I do not think you can properly model this if the time steps you are using are smaller than the timescale of the runoff delay. If you still fail after trying LAI sensitivity, you could try a bigger timestep (a rough approach) or better yet, a different rainfall-runoff model, that can handle some hydraulic routing.