Background and Motivation

1. Assessing Evapotranspiration, Recharge Rates and Shifts in Phreatophytic Water Source Jeremy E. Koonce1,2, Michael H. Young3, Dale Devitt4, ZhongboYu1, Amanda Wagner5, Lynn Fenstermaker6 1Department of Geoscience, University of Nevada Las Vegas, NV 2Division of Hydrologic Sciences, Desert Research Institute, Las Vegas, NV 3Bureau of Economic Geology, University of Texas at Austin, TX 4School of Life Sciences, University of Nevada Las Vegas, NV 5Water Resources Management Program, University of Nevada Las Vegas, NV 6Division of Earth and Ecosystem Sciences, Desert Research Institute, Las Vegas, NV • Background and Motivation • Spring • (23 March through 23 April 2011) • Summer • (28 May through 28 June 2011) T E • Soil water and temperature are important variables in energy and water balance studies, particularly to processes involved in evapotranspiration (ET), which provides a direct link between the balances and is crucial for closing the water budget. • With a large uncertainty in precipitation rates from interannual variability and increased demand for water resources, understanding these processes is critical when assessing the movement of mass and energy through the vadose zone. T E • Surface: Frozen/Moist • Infiltration: Low • Recharge: Low • Near Surface θ: Mod • Water Level: Mod / Low • ET: Low E / Low T T E T E 0 35 9 35 • April (2–8) precipitation event (20.07 mm) • May (28–30) precipitation event (28.19 mm) • Increased ET following precipitation events 30 30 8 2 4 7 25 25 6 20 20 6 Evapotranspiration (mm/day) Precipitation (mm/day) 5 8 15 15 10 10 4 10 I I 12 5 3 5 T – Transpiration E – Evaporation I - Infiltration R – Recharge – Water Table ---- Capillary Fringe I 14 2 0 0 16 -5 1 -5 I 0 -10 18 -10 • Diurnal temperature changes in the atmosphere • Lower temperatures during early Spring • Higher temperatures during early Summer • Goal and Questions Ambient Temp (Deg C) Ambient Temp (Deg C) R R R R • Overall goal of this research is to have a better understanding of the impact of interannual variability of shallow groundwater semi-arid systems. In doing so, we hope to answer the following questions: • Can we correctly estimate water flux in (infiltration/recharge) and out (evapotranspiration) of the vadose zone using soil temperature? • Do changes in temperature signals follow shifts in water sources for plants? • Although ET and recharge rates are primarily driven by atmospheric demand and water availability, to what extent does soil temperature change these rates? Fall Summer Spring Winter Focus of Study (Upper 200 cm) • 30 cm – Increase water content after precipitation events; decrease follows • 100 cm – Steady increase in water content in Spring; levels off in Summer • 200 cm – Constant water content (no recharge) 8 8 0.50 0.50 21 20 20 19 11 0.45 0.45 40 40 17 9 0.40 0.40 60 60 15 7 Water Content (m3/m3) Water Content (m3/m3) 0.35 0.35 13 5 80 80 0.30 0.30 NEVADA UTAH 11 3 0.25 0.25 9 100 100 Spring Valley 0.20 0.20 120 120 0.15 0.15 140 140 0.10 0.10 0.05 160 160 0.05 0.00 0.00 180 180 Ely, NV 16 200 200 • Experimental Location and Design • Diurnal variations in near surface due to atmospheric conditions; dampening effects deeper in the profile • Seasonal temperature changes through entire profile • FO DTS appears to pick up wetting fronts at the near surface • Small variations at 100 to 199 cm possibly due to poor insulation of equipment box 25 SV6 14 12 20 Hourly Averaged Soil Temp (Deg C) Depth (cm) Depth (cm) Hourly Averaged Soil Temp (Deg C) 10 8 15 6 Eddy Covariance & Meteorological Tower 4 Type T Thermocouples 10 2 Focus of Study (Upper 200 cm) 0 5 Specific Depths Soil Temp (Deg C) Specific Depths Soil Temp (Deg C) TDR / HDU 3/24 3/28 4/1 4/5 4/9 4/13 4/17 4/21 5/29 6/2 6/6 6/10 6/14 6/18 6/22 6/26 • Conclusions • Surface: Moist • Infiltration: Low • Recharge: Low • Near Surface θ: Mod • Water Level: Low • ET: Low E / Low T FO DTS Pole • Data provides interannual atmospheric and physical soil variability in Spring Valley, NV (early Spring and Summer, 2011) • Changes in soil temperature observed during this time period are due mostly to atmospheric conditions, but are also influenced by changes in water content (volumetric heat capacity) due to recharge and ET • Using high resolution FO DTS in the subsurface provides continuous temperature measurements both spatially and temporally allowing for a better understanding of the processes within the vadose zone, and subsequently a better understanding of the flux in and out of the system • In regard to phreatophyticwater sources, isotopes and sap flow measurements are being used to determine shifts of plant water from vadose and phreaticzones Groundwater Well w/ Pressure Transducer • Acknowledgments Funding for DTS supplies in Spring Valley, NV, provided by NSF Cooperative Support Agreement (EPS-0814372). Educational opportunities and funding for JK provided by NSF EPSCoR under Cooperative Support Agreement (EPS-0814372). Additional support from my academic committee (Dr.’s Young, Yu, Nicholl, Jiang, and Devitt), UNLV Geoscience, and Desert Research Institute (J. Healey, B. Lyles, and Dr.’s Berli, Jasoni, and Arnone).