GROUNDWATER TRANSIT TIME DISTRIBUTIONS IN WATERSHEDS WITH RECHARGE CONTROLLED OR TOPOGRAPHY CONTROLLED WATER TABLES

The groundwater transit time distribution (TTD) for a watershed may be represented by a lumped parameter model that only uses a few average parameters and that may or may not require calibration to field data. Three of the most common lumped parameter TTDs discussed in the literature are defined as: 1) exponential, 2) gamma, and 3) dispersion models. We demonstrate that the appropriate form of the TTD is dependent on the nature of the groundwater flow regime. In aquifers with recharge controlled water tables and Dupuit-Forchheimer flow, the TTD is generally exponential and fully defined by only three (spatially averaged) parameters: recharge, porosity, and saturated thickness. This TTD is independent of aquifer hydraulic conductivity and stream network characteristics and does not require calibration to field data. However, the presence of weak sinks (e.g. stream headwaters, small capacity residential wells) cause deviations in the TTD from the exponential distribution, although it still often provides a good approximation. For the case of weak sinks we found that the TTD can be improved by replacing the exponential distribution for shorter transit times by the gamma distribution. However, this gamma distribution contains a fitting parameter that must either be estimated or be calibrated for. In aquifers with topography controlled water tables (e.g. Toth’s model of regional flow), the TTD can be best represented by the dispersion lumped parameter model. This model, however, requires extensive calibration to field data and, unlike the exponential or gamma case, depends on the aquifer hydraulic conductivity. This need for model calibration hampers application of the dispersion model to large ecoregions with many watersheds.