This function calculates the permeability of the top soil
     
    
    calc_permeability(A_CLAY_MI, A_SAND_MI, A_SILT_MI, A_SOM_LOI)
 
    
    Arguments
- A_CLAY_MI
- (numeric) The clay content of the soil (%) 
- A_SAND_MI
- (numeric) The sand content of the soil (%) 
- A_SILT_MI
- (numeric) The silt content of the soil (%) 
- A_SOM_LOI
- (numeric) The organic matter content of the soil (%)