Uncertainties in leaching risk assessments due to field averaged transfer function parameters

The transfer function model is widely used to estimate solute transport patterns at the field scale. Leaching risk assessments with the transfer function model may be influenced by spatial variability in toe net applied water (NAW) distribution, but few researchers have investigated this possibility. The objective of this study was to evaluate the impact of spatial variability in the NAW distribution on leaching risk assessment and identification of leaching risk categories at the field scale. Bromide concentration profiles, irrigation depths, bulk densities, and soil moisture contents were measured in 40 plots across a 57-ha potato (Solanum tuberosum L.) farm, along with field-scale evapotranspiration estimates, to estimate solute transport parameters. Values for field- and plot-scale means and standard deviations for the NAW distribution were estimated using the stochastic convective lognormal transfer function (CLT) model. The probability of NO₃^- leaching below a depth of 2 m was then estimated using field-averaged versus plot-scale estimates for the mean and standard deviation of the NAW distribution. For 30-cm NAW, NO₃^- leaching risks estimated with the CLT model and plot-scale means and standard deviations were very high in 0.4 ha of the field, high in 1.8 ha, moderate in 8.7 ha, low in 23.0 ha, and none in 23.1 ha. In contrast, when field-scale average estimates of NAW were used, there was a low risk of NO₃^- leaching for the entire field. Thus, when estimating leaching risks using the CLT, information about spatial variability of the NAW distribution is important.