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All Topics | Topic "What does the bottom bucket in MABIA do and why is the maximum depletion the AWC?"
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Author Message
Mr. Michael Mutz

Subject: What does the bottom bucket in MABIA do and why is the maximum depletion the AWC?   
Posted: 3/28/2018 Viewed: 62 times
I have two questions regarding the MABIA method.

1. According to the WEAP User Guide, MABIA uses two buckets, one of which is the root zone (including the soil surface bucket for the evaporation component), the height of the 2nd bucket is the remainder of the total soil thickness minus the root zone. The WEAP User Guide states:

"We strongly recommend using the two bucket method because it will give more realistic
results. (The one bucket method is included for backward compatibility with datasets that were
created in older versions of WEAP, before the two bucket method was added.)"

Neither from that nor any other source I could find information on what the 2nd bucket is there for and how it is implemented. From the description it seems to be a simple buffer layer between bucket 1 and groundwater that transports water only vertically and only downwards if it overflows and is characterised by an AWC. If the model is run multiple years, the bucket 2 will eventually fill up no matter what it's configuration is and all water that comes in from bucket 1 will be passed through to the groundwater. This way it should only delay the flow to groundwater until a certain point and forever retain the water that it needs to fill up.

I'm not sure if this is what the 2nd bucket does, but in what way is the 2 bucket method superior to the 1 bucket method? How could I even choose the 1 bucket method in a current WEAP version?

2. Why is the maximum bucket depletion equal to AWC=FC-WP? I understood that this is a dimensionless volume percentage and soil property that is scaled with the actual bucket depth to arrive at the TAW in case of the root zone, so the actual bucket height. Why is the maximum depletion not equal to TAW? If it is equal to FC-WP, the actual bucket depth does not play a role, also the unit is not correct since it is no length then.

Kind regards,

Michael






Mr. Chuck Young

Subject: Re: What does the bottom bucket in MABIA do and why is the maximum depletion the AWC?   
Posted: 4/19/2018 Viewed: 54 times
Hi Michael,

The main purpose of the 2nd bucket is to maintain mass balance. In the MABIA module the root zone depth can increase during the growing season. If a 2nd bucket is not employed, an assumption about the soil moisture status of the newly occupied portion of the root zone must be made, potentially violating mass balance.

The 2nd bucket will not always be at field capacity. For instance, an irrigated crop may have a maximum root zone depth of 1 meter. During the growing season bucket 1 slowly increases in depth, occupying more and more of the profile occupied by bucket 2. At the end of the season most or all of bucket 2 is occupied by roots and dry due to root water uptake. Then, depending on the amount of rain that occurs during the off-season, bucket 2 may remain quite dry until it is occupied by the roots and irrigated.


AWC is expressed in % moisture content. TAW is simply equal to AWC*RootZoneDepth.

Regards,

Chuck


Topic "What does the bottom bucket in MABIA do and why is the maximum depletion the AWC?"