Freeze and Banner (1970) studied groundwater recharge processes at three sites in the Spirit Lake Basin of southeastern Saskatchewan, Canada. Instruments, installed at several depths, included soil-water tensiometers, electrical resistance blocks for monitoring soil-moisture content, and piezometers in the saturated zone. A rain gage was also installed at each site. Laboratory measurements were conducted to determine moisture-retention curves, sat­urated hydraulic conductivity, bulk density, and other physical properties. Freeze and Banner (1970) did not apply the water-table fluctuation method to estimate recharge at their sites, but they present sufficient information to allow application of the method for a few discrete rainfall events.

Figure 6A shows rainfall and depth to water table for site A for 25 days during July and August 1966. Figure 6B shows precipitation and water-level rise at site B for parts of June and July 1967. The water-retention curves for Yorkton sandy loam (site A) and Meota sand (site B) are displayed in Figure 6C. The curve for the sandy loam was determined with field data, whereas that for the sand is based on laboratory data.

Problem 6.1A

For site A, estimate values of specific yield, S_y by using Equation (6.5). Describe how S_y varies as a function of depth to the water table (H_d). Compare estimates of S_y with those for generic soils with similar texture (Table 6.4). Apply the water-table fluctuation method (Equation 6.2) to estimate the total amount of recharge. Explain how the recession curve was approximated.

Problem 6.1B

For site B, estimate values of S_y by using Equation (6.5). The magnitude of the saturated hydraulic conductivity for the Meota sand was measured at 1.67x10^-6 m/s. Apply the Nachabe (2002) method (Equation 6.8) to estimate S_y for site B. This will require estimating the parameters of the Brooks-Corey water-retention curves (see Problem 5.1). How do calculated values of S_y compare with those in Table 6.4? Apply the water-table fluctuation method (Equation 6.2) to estimate the total amount of recharge. Discuss the differences in water-table reponses and recessions for the two events. How do the estimates of recharge compare with measured precipitation? What could contribute to the more rapid rise in water table at site B?

Figure 6A

Figure 6B

Figure 6C