Submitted to: Agronomy Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/29/2006
Publication Date: 2/2/2007
Citation: Timlin, D.J., Fleisher, D.H., Kim, S., Reddy, V., Baker, J.T. 2007. Evapotranspiration measurement in controlled environment chambers: a comparison between time domain reflectometry and accumulation of condensate from cooling coils. Agronomy Journal. 99:166-173. Interpretive Summary: Sunlit, controlled environment growth chambers with soil bins are important research tools for quantifying photosynthesis and water use in agricultural crops. Water use (evapotranspiration, ET) by plants in these growth chambers is measured by collecting water that condenses on the cooling coils in the air handler. This only provides information on total ET, however and not on soil water content or status. We developed an automated method to measure soil water contents over time in the growth chambers as an alternative method to obtain ET rates and to monitor soil water content. Daily ET rates from water contents compared favorably with ET rates from the condensate system. The correspondence was better for the treatments with higher amounts of irrigation and ET. We determined that differences between ET values determined using the two methods were partly due to the orientation of the water content measuring devices. There were advantages to both methods. The condensate method provided better short term (15 minute to hourly) measurements of ET and responded rapidly to solar radiation. The soil water content method provided better long term (daily to seasonal) estimates of ET and was useful for estimating irrigation requirements based on amount of water in the soil and depth of root activity. The results of this study will be useful to scientists and agricultural managers who need to measure actual water use by plants.
Technical Abstract: Sunlit controlled environment chambers are an important research tool to quantify canopy level photosynthetic processes under controlled temperatures and carbon dioxide. The measurement of water fluxes from canopy surfaces is carried out by measuring condensate draining from cooling coils in a constant humidity environment. This provides a direct measure of evapotranspiration (ET). However, in growth chambers with soil bins, this does not give information on soil water status or root activity. We instrumented the soil bins of daylit growth chambers with an automated Time Domain Reflectometry (TDR) system. The soil water contents were monitored once an hour at five vertical depths with three measurement locations per depth. Data from an irrigation x carbon dioxide study on potato (Solanum tuberosum L.)were used for this study. The treatments were two levels of CO2 (740 and 370 micromol mol-1) and three irrigation treatments, a well watered control and 25 and 75% of control. The correspondence between daily ET rates for the two systems was good. Maximum daily ET rates were near 6.1 to 7.1 mm cm-2 d-1 (7 to 8 L d-1 on a chamber basis) and differences were on the order to 0.89 to 1.8 mm cm-2 d-1 (1 to 2 L d-1). The higher the daily ET rate, the closer the daily values from the two methods were. The correspondence between hourly measurements of ET measured from the condensate system and calculated from TDR water contents was poor due to instrument and soil variability. A significant source of error was vertical variation in water content in the soil between horizontally placed TDR probes, especially during irrigation events. Evapotranspiration data from TDR water content measurements were much more robust for calculation of water use over a period of time. Data from the condensate system were most useful for quantification of diurnal transpiration rates and were better correlated with radiation.