Evaluation of Modeling Bias and Uncertainty Using Different Field Capacity Estimates in the Apex Model.

The Conservation Effects Assessment Project (CEAP) uses the Agricultural Policy Environmental

Extender (APEX) simulation model to assess the effects and benefits of Farm Bill conservation

programs on U.S. croplands. The APEX model uses the 'field capacity' (FC) concept to represent

soil water conditions. Recommendations differ world-wide on how to represent FC of the soil in

a 'bucket' type water movement model like APEX is. In the U.S., FC is usually approximated by a

laboratory measured or estimated water retention value at -33 kPa pressure head. However, there

are continued reports that there is no single pressure head that can be reliably used to

approximate FC for all soil textures. We tested using (1) APEX's built in estimation functions;

(2) a k-nearest neighbor based estimation proposed by Nemes et al., 2009 and (3) a field

measurement based correction factor to -33 kPa estimates to parameterize APEX. We subsequently

ran the APEX model to characterize the impact of parameterization on the bias and uncertainty

of modeled outputs using simulations of nutrient and water management and crop growth under

different weather conditions. Regression tree methodology and commonly used statistical

measures were used to delineate groups of soil and weather conditions for which a significant

impact by different parameterization scenarios can be expected. While the general estimation

uncertainty was less affected, substantial differences were found in the range of model

estimates when the different FC estimates were used. A large proportion of these findings could

be explained by systematic differences between FC estimates that were identified earlier. Care

should be taken when parameterizing the hydrology component of models in large scale studies,

in which the use of measured soil hydraulic properties is not a feasible option.