Paper No. 3
Presentation Time: 9:00 AM-6:00 PM
HYDROLOGIC RESPONSE OF AVERY BROOK , WEST WHATELY, MASSACHUSETTS TO EVAPOTRANSPIRATION DURING SUMMER LOW FLOW EVENTS
NEWTON, Robert M., Department of Geosciences, Smith College, Northampton, MA 01063 and BROADWATER, Katherine L., Dept of Geosciences, Smith College, Northampton, MA 01063, rnewton@smith.edu
Evapotranspiration (ET) creates a daily flow oscillation in Avery Brook during summer baseflow periods. Avery Brook is a second order stream draining a 756 hectare forested headwater catchment (235m relief) located in western Massachusetts near the town of West Whately. Most of the watershed lies within protected lands of the Conway State Forest and land owned by the City of Northampton for the protection of its municipal water supply that is fed mainly by Avery Brook. The gage station is equipped with a Campbell Scientific CR10X data logger that reports 10-minute average stage values based on 1 second measurements. Specific conductance, water temperature and air temperature are also measured at the same frequency. Stream discharge is estimated from stage measurements based on a stage-discharge relationship built from discharge measurements determined using the velocity area method. Precipitation is measured by a heated tipping bucket rain gauge located at the Smith College Macleish Field Station located less than a kilometer from the gage. During the 2010-11 water year the discharge of Avery Brook varied from less than 1cfs (0.03cms) during July and August to over 750cfs (24cms) during Tropical Storm Irene when over 155mm of rain fell in less than 24 hours. Total annual runoff was 1,520mm compared to 1,782mm of precipitation. However, analysis of rain gauge data shows that rainfall is strongly dependent on elevation so measurements near the gage underestimate total watershed precipitation.
Stream discharge displays a daily oscillation of approximately 0.12cfs (0.003cms) during summer baseflow periods that is driven by the daily ET cycle. Discharge decreases during the day and partially recovers at night. Stream specific conductance also shows a daily oscillation of 5 to 10 µS/cm that is offset such that the peak occurs just after minimum flow. During the day as the ET rate exceeds the rate of groundwater recharge, discharge decreases and specific conductance increases due to evaporative concentration. Specific conductance then decreases in response to recharge as the specific conductance of the shallow groundwater is less than the surface water. These oscillations occur during periods when the ET rate exceeds the rate of groundwater recharge and are mainly limited to the months of July and August.
