A new leaf area algorithm was added in SWAT and used for maximum seasonal leaf area index (LAI) calculation (Equations (1) and (2)), which was useful for simulating tree growth prior to maturity (Guo et al., 2015; Guo et al., 2018). A new tree leaf area parameter, TreeD, was added in the plant database, to describe how LAI increases to the maximum potential LAI (BLAI) with varying densities. An algorithm used for calculating dropping leaves weight was added to estimate leaf dropping as a user defined fraction of annual accumulated tree biomass instead of total aboveground biomass (Equation (3)) (Guo et al., 2015; Guo et al., 2018).
LAIcurrentyear = LAIcurrentyear-1 × 10RateTree (1)
RateTree = log10(currentyear/CLAIYR) × TreeD (2)
Where CLAIYR is number of years until maximum LAI is attained (for any species), and TreeD is a tree parameter defining how LAI increases up to BLAI.
FALF = BIO_LEFT * TreeBioIni (3)
Where FALF is weight of dropping leaves, TreeBioIni is annual accumulated tree biomass, and BIO_LEAF is a user defined fraction in plant.dat.
The modified SWAT code and executable file are available for downloading below. The modified code was written in FORTRAN. The modified code and SWAT executable file are 3.14MB and 3.98MB, respectively. No special hardware is required for this executable file. Please contact Bernard Engel (engelb@purdue.edu) or Tian Guo (guopurdue@gmail.com), if you have any questions about it.