Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 8/1/2013
Publication Date: 11/26/2014
Citation: Evett, S.R. 2014. Long-term lysimeter data on evapotranspiration. Meeting Abstract [abstract] Workshop on Investigation Hydrological Fluxes Using Long Term Lysimeter Date, Nov. 25-26, 2013, Juelich, Germany.skp
Technical Abstract: Long term crop evapotranspiration (ET) data measured using large weighing lysimeters have only been gathered in a few places in the world, yet are of great importance for ground truthing of many models of plant water use, mesoscale climate, remote sensing estimation of ET, climate change and climate change effects on crop yield and water use, and global climate itself. In the United States, the four large weighing lysimeters at Bushland, Texas, (35 degrees, 11 min N, 102 degrees, 06 min W, 1170 m elevation above MSL) are among the most accurate and longest running facilities still in use. Their long term use has involved addressing several research questions, including: (i) What is the crop water use and water use efficiency of the major crops of the U.S. Southern High Plains as affected by irrigation amount (deficit or full), dryland cultivation, and other management?; (ii) How does the advective environment (hot, dry, windy conditions) affect the crop and soil energy and water balances?; (iii) What role does evaporative loss of water from the soil play in overall crop water use?; (iv) How does the advective environment and high solar insolation affect canopy surface resistance?; (v) How do wind speed, temperature and humidity affect losses of water from spray irrigation and evaporation from leaves?; and many other important questions. Because they measure crop water use by mass balance, weighing lysimeters are the only direct measurement of ET. Field soil water balance is nearly as direct, but depends on more assumptions and on the calibration accuracy of the soil water sensor used to determine change in soil water storage in the control volume used to calculate ET by soil water balance. ET flux estimation methods, such as Bowen ratio and eddy covariance techniques depend on more calibrations and on conceptual models of water flux in the internal boundary layer above a crop, including the assumption that an internal boundary layer exists at all times. Testing of Bowen ratio and eddy covariance systems is best done by comparison with weighing lysimeter data, provided that the weighing lysimeter data can be shown to be representative of the surrounding field. The same proviso applies to tests of ET models based on satellite and aerial thermal imaging. Quality control of weighing lysimeter data has many aspects, including comparison of field and lysimeter soil water balances. Maintaining consistent and rigorous quality control over many years is a difficult task requiring a dedicated team and the willingness to accept that not all data are acceptable.