Subject: Setting up values for Soil Moisture Method Posted: 4/27/2009 Viewed: 27949 times
I am in the process of setting up the WEAP model for a river basin in SA. My confusion begins with determining the parameters for the 2 'buckets' and how to differentiate their extents. Does the top bucket strictly represent, in reality, only the depth to which the roots go? therefore, the parameters for this bucket are the soil parameters to that depth? Does the 2nd bucket represent the layer of soil immediately following the top soil? and therefore its parameters belong to that soil group? what if, as is the case on ground, there are many layers of soil. Does that mean an average of the parameter values need be applied?
Any assistance will greatly be appreciated!
Thank you very much.
Devaraj de Condappa
Subject: Re: Setting up values for Soil Moisture Method Posted: 4/28/2009 Viewed: 27933 times
Answers to your questions may depend on your spatial and temporal scales.
What is the area of your basin? And your temporal calculation step?
Subject: Re: Setting up values for Soil Moisture Method Posted: 4/29/2009 Viewed: 27926 times
Many thanks for your email.
The basin area is approximately 55,000km2 and calculation is on a monthly time step. The basin has been subdivided into 80 smaller subcatchments. My interest is specifically on the surface water to meet future abstraction requirements in view of possible climate change. Therefore, groundwater is not a priority at this stage of the study.
Thanks very much for your help.
Devaraj de Condappa
Subject: Re: Setting up values for Soil Moisture Method Posted: 4/29/2009 Viewed: 27921 times
You better check in the manual of WEAP, but from memory, schematically the 1st bucket is something like the soil media (a media more or less homogeneous in porosity), which should be mainly unsaturated, and the 2nd bucket the less homogeneous media (e.g., gradually weathered rocks) which could be partly or totally saturated.
Do you have perennial streamflows / baseflow in your basin? If yes, then I think you should consider including the 2nd bucket which contributes to baseflow. If no and you do not care of groundwater at this stage, then you should not need the 2nd bucket : there's an option in WEAP to avoid this 2nd bucket and link all the vertical outflow of the 1st bucket to a sort of groundwater "blackhole".
The parameters of the 1st bucket have sense if you think locally (for instance if you view a pit, with its soils layers etc.). But in your case, your basin is 55,000 km² and you consider 80 sub-basins, that means that roughly the sub-basin have area of about 700 km². So you can imagine that at this scale the parameters of WEAP loose their physical meaning. They are kind of agglomerating all the processes happening on an area of 700 km².
I have been manipulating WEAP with even larger sub-basins (about 10,000 km²). So here's my advices:
* keep the physical meaning in the background, just to check that the values of buckets parameters you'll define are not aberrant, but I would advise not to fix values corresponding to very local observation (eg a soil pit) as, in my case, I coined sub-basins values very different (but plausible) to what could be observed very locally,
* pick-up one or some sub-basin(s) where you have nice observed streamflows and play with the parameters of the bucket, to understand their effects on the streamflow and possibly calibrate by reproducing observed streamflows,
* then upto you to a give set of parameters for all the sub-basins, to calibrate each sub-basin or do a mixture of both (some parameters are fixed while playing with others).
I hope that it helps, even if it's very general. I would advise you to experiment...
Subject: Re: Setting up values for Soil Moisture Method Posted: 4/30/2009 Viewed: 27915 times
Thank you very much for your suggestions, they were a great help. I agree with your approach. Now to see what kind of output I get!
Subject: Re: Setting up values for Soil Moisture Method Posted: 5/5/2009 Viewed: 27893 times
The top bucket is the store from which the plants can remove water. So, the Soil Water Capacity variable in WEAP is the amount of water that plants can access which is generally thought to be the difference between saturation and permanent wilting point over the depth of the root zone. While considering these values keep in mind that you should not necessarily expect text book values of conductivity and root zone thickness to always work correctly as those measurements were made at short time scales compared to the typical model time step.
The bottom bucket represents the slower hydrologic response in a basin. This is generally the source of base flows during the dry season. My experience has been that the modeler must adjust the capacity, conductivity, and flow direction of bucket 2 to appropriately model the low flows. IF the region of interest has deep alluvial soil you might expect to have a fairly large capacity and lower conductivity. In a shallow poorly developed soil there would be a shallow, high conductivity bucket 2. Always take care to assure there is no trend in the storage of bucket 2 over the period of simulation. For instance, if the capacity is large and the conductivity is low it is possible to have water accumulate in bucket 2.
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