TEMPORAL VARIABILITY OF TRANSPIRATION-INDUCED GROUNDWATER RECHARGE IN A SEMI-ARID RIPARIAN WETLAND

In semi-arid climates the primary fluxes of water to and from water table aquifers are groundwater recharge and transpiration by phreatophytic plants. During periods of drought there is limited recharge from precipitation, and phreatophytic plants draw heavily on the water table aquifer. Riparian zones are unique in semi-arid settings because perennial streams provide constant groundwater recharge, even during periods with limited recharge from precipitation. Transpiration by riparian vegetation draws down the water table during the day and the stream replenishes the groundwater at night, which provides vegetation a consistent supply of water in an otherwise water-poor system. The daily balance of transpiration and induced groundwater recharge causes readily observable diurnal water table fluctuations that can be used to quantify daily groundwater recharge and transpiration.

I monitored water table elevations every 20 minutes from July 13th to October 9th, 2005 in two wells in the riparian zone of Red Canyon Creek, a second-order stream in semi-arid Wyoming. I recorded water table fluctuations of 3.2 cm daily in mid-July. The fluctuations slowly decrease in amplitude throughout the study. On days with precipitation of < 1 mm (73 of 88 days), the water table peaks between 8:00 and 9:00 AM, following recharge by the stream at night, and is at a minimum between 6:00 and 7:00 PM, following drawdown by transpiration during the day. Water table elevations rise by rates of up to 5 cm/day during periods of maximum transpiration-induced recharge and drop to < 0.1 cm/day when the vegetation has senesced. Transpiration rates are about 3 mm/day in mid-July and less than 0.5 mm/day in early October, which is consistent with independent estimates of transpiration from radiation-based models.