Assessing groundwater recharge with two unsaturated zone modeling technologies

Two different unsaturated zone modeling technologies representing two main classes of unsaturated zone models (water-balance and Richards equation-based) were used to assess groundwater recharge for a hypothetical homogeneous profile and three real profiles of the Waterloo Moraine with deep groundwater level. The average rate assessed with the HELP technology for Waterloo Moraine is 8.3 cm /year for low-permeability silt till and silt areas, and 18.7 cm /year for areas built from sandy and gravely deposits. Assessments of the average annual recharge obtained with HELP and HELP/VS2DT technologies are very close, with the maximum difference equal to 12%. Comparison with results obtained independently with the tritium profile method was in favor of the HELP technology, which predicted the annual recharge with 4% error. The effectiveness of the two modeling technologies was assessed. The HELP technology, because of the solution method and interface capabilities, provides a quick and effective means for assessing groundwater recharge. By contrast, the HELP/VS2DT technology appears to be very sensitive to the profile settings and much harder to manipulate. Under conditions of multi-layer profile with the layers of contrast lithologies, the VS2DT model becomes nearly inapplicable. It is contended that the HELP technology has all the necessary qualities to become a core of the computational technology for assessing groundwater recharge rates.