A Humped Clay Production Model.

Traditional approaches of modeling clay production in soils have not yielded widely applicable results. Past attempts have focused on either specific locations or idealized landscapes or require a high degree of parameterization. We assumed a humped function to predict rates of clay production. As clay accumulates in the soil profile, pore spaces become restricted and water holding capacity is increased facilitating greater clay production; however, once a threshold amount of clay is produced, clay production slows as water flow through the profile becomes restricted and the supply of weatherable minerals declines. As such, we suggest that clay production is a function of the amount of clay present in the profile. We used a synthesis of chronosequence studies (n=126) to calculate rates of clay production and to test the humped clay production model and validate the approach. The model contained two fitted parameters, 1. a maximum clay production rate (C₀, kg clay m^-2 yr^-2) and 2. a threshold clay amount (α, kg clay m^-2). We empirically determined C₀ values using a measure of matter and energy fluxing into the soil system, called effective energy and mass transfer, and the minimum age of the soil chronosequence with an r²=0.75. We fitted the model to the calculated clay rates to determine an optimum α value of 600 kg clay m^-2. We validated the model on approximately 28% of the profiles, and effectively predicted clay production rates with an r²=0.81. Future steps will include incorporating the humped clay production model with a numerical landscape evolution model to include the influence of erosion on clay production rates. This approach is universally applicable with a low degree of parameterization and a limited amount of required data to make the approach operational.