Progress in scientific studies of groundwater in the hydrologic cycle in Canada, 2003-2007

Canadian Water Resources Journal, Summer, 2009 by Masaki Hayashi, Garth van der Kamp

Since it is difficult to quantify the recharge rate from direct measurement of water flux, natural tracers are commonly used to estimate recharge. For example, Hinton (2003) used chlorofluorocarbon to estimate recharge rates through thick sandy sediments in the Sandilands Provincial Forest in Manitoba. The recharge rates ranged from 50 to 230 mm [yr.sup.-1], where mean annual precipitation is 550 mm [yr.sup.-1]. Van der Kamp et al. (2003) described a method for determining the soil water balance at a scale of hectares by precise measurements of pore pressure changes within thick aquitards, induced by the changes of mechanical load corresponding to changes of total moisture. Such methods provide localized estimates of infiltration and recharge, but they may not represent the average recharge over a large area. To overcome this problem, stream baseflow is commonly used to estimate watershed-average recharge rates assuming that baseflow represents the long-term average rate of groundwater discharge, which is equal to recharge. Neff et al. (2005) applied this method to estimate spatial distribution of groundwater recharge to shallow (< 30 m) aquifers for the Great Lakes basins. Although baseflow appears to be a well defined concept, quantitative determinations of baseflow using streamflow records depend on the use of appropriate methods for separating baseflow from the total flow during flow peaks due to precipitation or snowmelt. Piggott et al. (2005) introduced a method of calculating baseflow and described its application to streams in southern Ontario.

In the late 1990s, groundwater hydrologists started using simple soil water balance models to estimate spatially distributed recharge flux by means of geographical information systems (GIS), and use it to specify transient boundary conditions for threedimensional groundwater flow models (e.g., Jyrkama et al., 2002). "Ibis approach has become increasingly popular in the past five years as it provides a useful tool to examine the sensitivity of groundwater to climate change scenarios. For example, a hydrologic model linked to a GIS was used to calculate recharge flux from the climate change scenarios downscaled from global climate model runs to local conditions of Grand Forks and Abbotsford, British Colombia (Scibek and Allen, 2006a; 2006b). Using the calculated recharge flux with a three-dimensional groundwater flow model (MODFLOW), they showed that under the predicted climate change scenarios, recharge to an unconfined floodplain aquifer will increase in spring and summer, but such an increase will have minor effects on the aquifer because it is strongly controlled by the riveraquifer interaction through permeable sediments (Scibek et al., 2007). A similar approach was used to simulate the changes in surface runoff, evaporation, and groundwater recharge under several different climate scenarios of increasing precipitation and temperature in the Grand River watershed in southern Ontario (Jyrkama and Sykes, 2007).