C-factor of MUSLE
MFT
Before use SWAT in an agricultural catchment in Norway I would like to
make a previous evaluation of erosion in the same way SWAT does, it
is, using the MUSLE equation. The objective is to see how accurate the
estimations of sediment yield (sed) are.
Just starting I found some problems for calculating the daily cover
and management factor (C) (formula 4:1.1.10 and .11 in the page 230 of
the manual):
4:1.1.10 CUSLE = exp([ln(0.8) - ln(rsdsurf)] * exp[-0.00115 *
rsdsurf] + ln[CUSLE,mn])
4:1.1.11 CUSLE,mn = 1.463 ln[CUSLE,aa] + 0,1034
1. According to the manual, C depends basically on above-ground
biomass, residue on the soil surface (rsdsurf) and the minimum C
factor for the plant (CUSLE,mn). In the formula there is not a
component that provides information about the above-ground biomass.
The component rsdsurf only accounts for residue on the soil surface.
How this formula takes into account the canopy?
2. The mentioned formula takes the "minimum C factor for the
plant" (CUSLE,mn) from another formula (formula 4:1.1.11, p. 230). In
it we are required to calculate the natural logarithm of the annual C
value (CUSLE,aa), which should be between 0 and 1, obtaining a
negative value for CUSLE,mn if CUSLE,aa is smaller than 0.932 (very
likely). In the first formula we should calculate the natural
logarithm of this negative number, which is not possible. What am I
doing wrong?.
3. These formulas refer to the citation (Arnold and Williams, 1995). I
have consulted that document and the formula is slightly different.
C=exp[(-0.2231-CVM) exp(-0.00115 CV)+CVM]
Where CV is the sum of above ground biomass and crop residue (kg/ha)
and CVM the minimum value of CUSLE. CVM is calculated in the same way
as in the SWAT manual. The difference with the SWAT's definition is
that the natural logarithm has disappeared from CVM. What is the
reason of this change?. Besides, CVM does take into account the above-
ground biomass while rsdsurf don't.
4. Since the residue over the ground is not accumulated indefinitely,
how this formula takes into account for the daily calculation of C the
decay of the residue over the ground?
5. Since the C value is dependent on the distribution of the rain
intensity along the year, how this formula takes into account
precipitation patterns of different climates?
6. Final question: where can I find C annual values for Norway?. In
the original manual of the USLE (Wischmeier, W.H. and D.D. Smith.
1978. Predicting rainfall erosion losses: a guide to conservation
planning. Agriculture Handbook 282. USDA-ARS) it is possible to obtain
only values for U.S. climatic conditions.
Thanks in advance.
Florencia Hastings
Did you find an answer for this? It´s been a while since you asked but I´m also wondering how C_USLE is calculated in SWAT from CUSLE,mn.
In Uruguay, values of factor C of the Universal Soil Loss Equation (USLE/RUSLE) are reported in a range of 0.006 to 0.09, these values should correspond to the annual average CUSLE,aa. As you posted,if calculate CUSLE,mn = 1.463 ln[CUSLE,aa] + 0,1034 (eq. 4:1.1.11 of 2009 theoretical manual) I obtain a negative value. So, how should I calculate CUSLE,mn from CUSLE,aa?
About your first point, on page. 256 of SWAT theoretical manual states that soil residue is more effective than canopy cover. So, the formula 4:1.1.10 does not take into account the canopy cover to calculate soil erosion.
Thanks in advance,
Florencia
Florencia Hastings
Did you find an answer to this? It´s been a while since you asked but I´m also wondering how C_USLE is calculated in SWAT from CUSLE,mn.
In Uruguay, values of factor C of the Universal Soil Loss Equation (USLE/RUSLE) are reported in a range of 0.006 to 0.09, these values should correspond to the annual average CUSLE,aa. As you posted,if calculate CUSLE,mn = 1.463 ln[CUSLE,aa] + 0,1034 (eq. 4:1.1.11 of 2009 theoretical manual) I obtain a negative value. So, how should I calculate CUSLE,mn from CUSLE,aa?
Thanks in advance,
Florencia
