Mechanisms, timing, and rates of arid region mountain front recharge

Abstract

Summary Arid region mountain front recharge (MFR) for two catchments (17.5 and 25.6 km 2 ) in the Silver Island Mountains, Utah, USA was evaluated by an analysis of catchment precipitation, mountain front and playa infiltration rates, aquifer parameters, and groundwater level responses in 59 shallow monitoring wells located at the distal end of the catchments. The 3–7 km long mountain front surfaces transition from alluvial fan deposits to a thin alluvial covering on lacustrine pediment surfaces. Precipitation and groundwater level data were collected from October 2005 to October 2010, which represents five recharge cycles. Average annual precipitation for all rain gauges during the 2005–2010 water years was 14.9 cm. Monitoring well water levels increased 91–97% of the time in response to storm events during times of general monitoring well water level increases and 29–38% of the time during times of general monitoring well water level declines. A surprising 81.7% of total catchment precipitation resulted in positive water level responses. Precipitation events as small as 2 mm resulted in positive water level responses and initial responses occurred within hours of some precipitation events. Water level responses that occur within hours of the larger precipitation events are attributed to focused groundwater recharge in near field braided stream channels. The time lag between the end of precipitation events and the maximum water level responses, which may be several days, is attributed to far field recharge. Direct MFR was calculated using a modified version of the water table fluctuation method. For the two catchments the average calculated direct MFR was 2.7–3.2% using PRISM precipitation data, and 3.7–4.5% using rain gauge data.