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Posted from Conversations with Greg Hancock & Ken Evans, regarding using CAESAR for overland flow/soil erosion prediction.
About tuning CAESAR. First some context, It’s a catchment model concieved for green temperate UK catchments, where slope processes are somewhat more benign than in arid/semi-arid environments, so it has more focus on the channel processes - with the slope processes being creep and landslips. If there is no creep or slips then the only way material will travel from slope to channel is if there is enough overland flow for it to get there.
So, I think there are two ways you can increase the input from the slopes (which I think is probably why CAESAR may be under-predicting compared to some other models e.g. RUSLE).
Firstly, the creep parameter could be tweaked up to factor in some overland flow terms. Creep is simply Slope*Timestep*Constant
In the code this is slope * time(in years) * constant(the constant inputted) / DX(grid cell size). I suspect this could be tweaked to represent a different slope term quite easily (e.g. add an exponential exponent etc..).
Secondly, Increase the overland flow. In CAESAR there is one input paramter (in the 'numerical' tab) that acts as a threshold, above which the model calculates a flow depth, thus overland flow, thus fluvial erosion. Its in the 'Min Q for depth calc' box. Above this discharge a grid cell will calculate a flow depth. Now, it’s a bit of a rule of thumb job here as what to use. But I take the view that if you have a 10m wide cell - and have 1 cumec flowing over it at 1ms-1, then you have 10cm of water depth - enough to move things. However, 0.01 cumucs, 0.1ms-1 and you have 1cm.. A bit more of a tougher call. In practice, you can run the model and when a storm/flood comes along, if Min Q is set lower you can see that more of the catchment wets up. This will mean that (probably) more material from the slopes will be swept into the channels - but the downside is that the model will run more slowly... Because more wet cells = more calcualtions = slower CAESAR. |
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Some comments from a email conversation with Katharine, T
Hi Tom,
Thanks for the reply. I have a few more questions/confirmations...!
1. Is root just set to 7.07 as a default value in the variables? The
actual value of 'root' is calculated from the cell size in the header
file when the ascii loads up?
Yup - just a default value.
2. Does dividing by root instead of cell size throughout means that the
diagonal cells get less water/less creep taking place than the
up,down,left,right cells?
Yes - because the distance is longer as they are diagonal. Its so the
slopes are properly calcualted.
3. Within slide_gs, are the temp/diff/amount variables used to switch
sediment/soil/water from one cell to another?
Yes - slide_gs ensures that grainsizes from one cell are transferred to
another when there is a mass movement (soil creep, soil erosion or
landslide). The name of the function represents this - slide and gs for
grainsize.
4. From what you were saying about area[x,y] , does this mean that this
is the contributing area above a cell ie. area[x,y] are the cells
contributing to moving sediment and water into any one cell?
Not exactly - they are the number of cells that drain into that cell.
the contributing area.
5. The last part of the soil erosion rate equation, you said was slope
length - does this just mean the cells contributing directly 'in a
straight line' above the cell in question ie. [x,y+1], [x,y+2], [x,y+3],
[x,y-1] etc.? rather than an area (such as area[x,y]) whereby the cells
such as [x-1,y] and [x+1,y] etc. can contribute to slope length?
Its very hard to calculate the exact slope length - as this varies very
rapidly from one part of the catchment to another - how do you determine
which top of a slope you measure to! So the easy way around this is to
use the square root of the drainage area, hence the
Math.Pow(area[x,y],0.5) line... which returns the square root of
area[x,y].
Sorry for all the questions, just trying to get the code clear in my own
mind!
Thanks
Katharine
Subject: RE: Soil erosion rate
Hi Katharine,
Time is the time in years...
area[x,y] is the drainage area (in cells) above a point. The last part
of the equation here returns slope length (simmilar to square root of
the drainage area)
Tom
Sent: 08 February 2008 15:25
To: Thomas J Coulthard
Subject: Soil erosion rate
Hi Tom,
I've got 2 questions regarding the soil erosion rate parameter within
CAESAR (from the line below)
temp = ((elev[x, y] - elev[x, y - 1]) / DX) * SOIL_RATE * time / DX *
Math.Pow(area[x,y]*DX*DX,0.5);
1) What is 'time' and why is it set to 0.028 in particular?
2) What is area[x,y]? Our thoughts are that it might be if certain cells
are wet/dry OR is there water amount attached to each cell for this
part? Is that right? |
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