Hi Andrew,
In our drainage paper mentioned below, an equation was developed to
estimate SWCON from DUL (drained upper limit soil water content). This
equation was built on the basis of Darcy's law and Brooks and Corey
equation. SWCON for many soils from more than 10 different countries agreed
very well with that equation. The C in the drainage equation mentioned
below (SWTi = SWCi-1 - F*C (SWCi-1 - DUL)) is SWCON itself, however, F is
a new coefficient added to the equation to accounts for the incoming flow
(water moving from one layer to another). As you know, we would expect that
the change of soil water of a layer due drainage to be less if this soil
layer is receiving some water from an upper soil layer. All of this is
mentioned and discussed in our drainage paper.
Regarding the soil layer thickness: In CERES water balance, the soil layer
thinness does not matter because the whole soil profile has a single value
of SWCON and the CERES drainage equation (SWTi = SWCi-1 - SWCON (SWCi-1 -
DUL)) does not account for the incoming soil water flow (does not have F,
the other coefficient). IF [SWTi = SWCi-1 - F*C (SWCi-1 - DUL)] is used
(as in SALUS water balance), the soil layer thickness does matter. I would
say that, in general, the thinner the layers and the more uniform (layers
of equal thickness) is the more accurate. The soil layer thickness in SALUS
grows linearly (Layer thickness = 2* layer number -1 cm). The errors that
results from using these layer thicknesses is minimal.
I will send you copies of the papers mentioned below.
Ayman
PS: Please note that one term was missing in the evaporation and drainage
equations. They should be as follows:
Evaporation equation is: SWTi = SWCi-1 - C (SWCi-1 - SWad)
Drainage equation is: SWTi = SWCi-1 - F*C (SWCi-1 - DUL)
At 03:45 PM 9/3/2001 +0100, you wrote:
>Hi,
>
>I have a general question (or two!) for the list on the above subject:
>
>In the abscence of field measurements, how is SWCON determined in the
>DSSAT drainage equation? Ritchie and Crum (1989) list SWCON for
>generalised Permeability classes. Is there something more specific? It
>seems to me that the equation will give different results for different
>soil layer thicknesses (the cascade taking longer for thinner layers). Is
>this effect accounted for explicitly anywhere?
>
>I also have a specific question for Ayman Suleiman:
>
>F*C in your drainage equation seem equivalent to SWCON. How are your
>constants determined? Could you possibly send me some relevant papers of
>yours? It would be greately appreciated.
>
>Regards,
>
>Andrew
>
>On Mon, 27 Aug 2001, Ayman Suleiman wrote:
>
>> Hello Mark
>>
>> Overall, in SALUS water balance, the governing equation for upward soil
water
>> movement due to evaporation is
>>
>> SWTi = C (SWCi-1 - SWad)
>>
>> where
>> SWTi : is today's soil water content
>> SWTi-1 : is previous day soil water content
>> SWad : is air dry soil water content, and
>> C : is a conductance coefficient depends on the soil layer depth
>>
>> and the governing equation for downward soil water movement due to vertical
>> drainage is
>>
>> SWTi = F*C (SWCi-1 - DUL)
>>
>> where
>> SWTi : is today's soil water content
>> SWTi-1 : is previous day soil water content
>> DUL: is drained upper limit soil water content
>> C : is a conductance coefficient depends on DUL, and
>> F: is a coefficient that depends on both saturated hydraulic conductivity
>> (Ksat) and incoming water flow.
>>
>> I have developed a functional lateral downslope drainage model that is
>> based on
>> the vertical drainage model mentioned above. This lateral model has not
>> incorporated in SALUS water balance because the SALUS water balance is
>> one-dimensional model. However, incorporating this lateral model with SALUS
>> water balance can be done if needed.
>>
>> The following papers explain some details of the soil water movement
used in
>> SALUS. I will be happy to provide you or any colleague with copies.
>>
>> Suleiman, A.A., and J.T. Ritchie. 2001. Modeling soil water redistribution
>> under second stage evaporation. Soil Sci. Soc. Am. J (In review).
>>
>> Suleiman, A.A., and J.T. Ritchie. 2001. A simple model to estimate daily
>> vertical drainage. Soil Sci. Soc. Am. J (In review).
>>
>> Suleiman, A.A., and J.T. Ritchie. 2001. Functional lateral downslope
drainage
>> model. Water Resou. Research. J. (In review)
>>
>> Suleiman, A. A., and J. T. Ritchie. 2001. Estimating saturated hydraulic
>> conductivity from soil porosity. Trans. ASAE. 44(2):235-239.
>>
>> Ritchie, J.T., Gerakis, A., Suleiman, A. 1999. Simple model to estimate
>> field-measured soil water limits. Trans. Am. Soc. Agri. Eng. 42:1609-1614.
>>
>>
>> Good Luck
>>
>> Ayman
>>
>>
>>
>> Ayman Suleiman, PhD
>> Research Associate
>> Civil and Environmental Engineering Dept.
>> 7B DANA Engineering Building
>> Bucknell University
>> Lewisburg, PA 17837
>> Voice: (570) 577-1692
>> Fax : (570) 577-3415
>>
>
>________________________________
>Andrew Challinor,
>CGAM,
>Dept. of Meteorology,
>University of Reading,
>Reading RG66BB
>U.K.
>
>http://www.met.rdg.ac.uk/~ajc
>
>Tel (44) (0)118 9875123 ext 4281
>Fax (44) (0)118 9318316
>--------------------------------
>
Ayman Suleiman, PhD
Research Associate
Civil and Environmental Engineering Dept.
7B DANA Engineering Building
Bucknell University
Lewisburg, PA 17837
Voice: (570) 577-1692
Fax : (570) 577-3415
|
