I am now using the major ion chemistry of HYDRUS to simulate the sodic soil development due to brackish groundwater. For the cation exchange, I only concern Ca2+ and Na+ ( I assume Ca2+ can represent all other cations ). I wonder when defining the reaction parameters, how to define K(Ca/Mg) and K(Ca/K), since I only concern of the exchange between Ca and Na and I can not define them as 0 (then HYDRUS will given an error)? Actually when I define the solution composition before, I also only consider Ca2+ and Na+ for cations and with equal amount of Cl- for satisfying charge balance. So I wonder If I just give a whatever value for K(Ca/Mg) and K(Ca/K), does it will have some influence on my final results? If possible could you also recommend some representative value of Gapon constants for normal soil like loam, sand and clay in HYDRUS? (I know the calculation of Gapon constants in HUDRUS-1D is based on activity but not concentration)
Thanks in advance and look forward to your reply!
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