Consequences of Simplification Versus Complexity in Estimating Unsaturated Travel Times in a Multi Layered Soil.

An appraisal of unsaturated zone conservative vertical travel times (Tt_v) offers a more realistic timescale for expected water quality improvements in response to agricultural mitigation measures. Using actual soil profile data for such estimations is avoided as the standardised laboratory techniques involved are time consuming and laborious (e.g. water retention curve (WRC) data using 0-15 bar pressure plates). Instead estimates of Tt_v are often made assuming saturated conditions, using generic values of saturated hydraulic conductivity (k_s) and effective porosity (n_e). Such an assumption may lead to misleading estimates of Tt_v and false assertions with respect to meeting water quality targets. This study estimated Tt_v in six layered soil (Ballymacart, Waterford, South Ireland) to 1.25 m depth using different levels of complexity: 1) generic k_s and n_e values and effective Darcian assumptions were used, 2) modelled k_s and n_e using the RETC model (input data were soil texture, bulk density and WRC data from pressure plates and centrifuge method (takes 2 days) but still under saturated conditions 3) modelled effective rainfall (ER) values utilising weather data from 1978 to 2007 with modelled and generic n_e and 4) modelled soil physical parameters using RETC and Hydrus 1D models under unsaturated conditions. Results showed that the saturated Tt_v estimate to 1.25 m depth 8.8 days using generic values (0.010 and 7% for immature till). The saturated equivalent using modelled k_s (0.026 m day^-1), gradient 1 and n_e (40%) was 19.2 days. Using modelled ER of 0.75 m, Tt_v of 243 and 42 days, were estimated, respectively. Inputting all available data into Hydrus 1D using unsaturated conditions, Tt_v was 467 days. Overall results using actual unsaturated conditions were comparable utilising WTC input data from both pressure plate (taking months) and the centrifuge (taking days) setups. The time and accuracy achieved should encourage others to follow this methodology.