Unsaturated Time Lag: Managing the Expectations of Policymakers Using Numerical Models.

Vertical time lag (t_u) of nutrients through the unsaturated zone may impair the ability of some agricultural watersheds to attain water quality improvements within expected timescales. Consequently, appraisal of t_u can create realistic timescales for mitigation measures to take effect. Numerical models, (e.g. Hydrus 1D) allow estimates of t_u based on soil characteristic data. This study presents an assessment of meteorological resolution and soil hydraulic parameter complexity on t_u estimates using this model. A conservative tracer was simulated through twelve soil profiles using hourly and daily meteorological records. For first breakthrough of the tracer at the base of the profile, the difference between daily and hourly timesteps never exceeded 0.01 yrs, suggesting that a daily resolution is sufficient. However, when total exit of the solute is of interest, an hourly timestep is recommended, as the difference in t_uranged from 0.10 to 0.42 yrs.

Simulations were then conducted based upon (a) textural class, (b) particle size distribution, (c) the full soil water characteristic curve (SWCC) and (d) a partial SWCC. Using generic data (methods (a) and (b)) underestimated t_u by an average of 0.65 and 0.49 yrs respectively, compared to using the full SWCC. Excluding the arduous -15 bar pressure step (d), typically had a minor effect on t_u compared to the full SWCC (av. 0.08 yrs). These results should inform model users of the optimum input data requirements for calculating t_u.