Can weighing lysimeter ET represent surrounding field ET well enough to test flux station measurements of daily and sub-daily ET?

Authors: Evett, Steven - Steve; Schwartz, Robert; Howell, Terry; Baumhardt, Roland - Louis; Copeland, Karen

Submitted to: Advances in Water Resources
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/29/2012
Publication Date: 11/29/2012
Citation: Evett, S.R., Schwartz, R.C., Howell, T.A., Baumhardt, R.L., Copeland, K.S. 2012. Can weighing lysimeter ET represent surrounding field ET well enough to test flux station measurements of daily and sub-daily ET?. Advances in Water Resources. 50:79-90. http://dx.doi.org/10.1016/j.advwatres.2012.07.023.

Interpretive Summary: Determination of the water use of field crops or natural vegetation is necessary for effective water management in agricultural (irrigation) and natural landscapes and watersheds. A weighing lysimeter provides accurate direct measurements of water use, but is difficult or impossible to use in many situations because it is expensive, makes a localized measurement, and requires a deep soil profile. A weighing lysimeter consists of a deep container of soil resting on a scale in an underground housing with the top of the container flush with the field soil surface. One alternative soil water balance method of determining crop water use depends on accurate measurements of soil water content to depths well below the plant roots in any situation. These measurements are difficult and expensive to make and again consist of localized measurements. However, for the same cost, many more soil water content measurements than lysimeter measurements may be made. The average crop water use in a field may be well represented if sufficient soil water measurements are made. Finally, several other alternative methods of estimating crop water use depend on equipment that measures air humidity, temperature, sunlight and sometimes wind speed. These alternative methods are representative of larger surface areas than are lysimeters or soil water measuerments, but the methods require calibration and testing to determine if they accurately represent field crop water use. Weighing lysimeters are often taken to be the standard against which alternative measurements are tested and calibrated. Weighing lysimeters themselves may be unrepresentative of the field if vegetation grows differently on the lysimeter. We used a network of soil water content measurements to determine crop water use at several points in two fields and compared the mean field crop water use, so determined against that measured using two weighing lysimeters. One lysimeter was found to be representative of mean field crop water use. The difference between water used determined by the other lysimeter and the mean field crop water use was also determined, so that the lysimeter measurements could be adjusted for comparison with and testing of alternative methods.

Technical Abstract: Weighing lysimeters and neutron probes are two tools used to determine the change in soil water storage that is needed to solve for evapotranspiration (ET) using the soil water balance equation. Errors in the soil water balance due to errors in determination of precipitation and irrigation are common to both, but a weighing lysimeter encloses a soil volume and so entails zero error in horizontal or vertical soil water flux, whereas the soil volume pertinent to the neutron probe method is unconfined, allowing soil water fluxes into and out of that volume. We compared irrigated cotton ET determined using two large (3 x 3 x 2.4-m deep) weighing lysimeters and eight neutron probe soil water profiles, during the Bushland, Texas Evapotranspiration and Remote Sensing Experiment 2008 (BEAREX08). The objective was to determine if lysimeter-based ET fluxes were representative of those from the fields in which the lysimeters were centered, designated northeast and southeast. Fluxes of ET from the southeast lysimeter were representative of those from the field throughout the season and can be used with reasonable certainty for comparisons of ET fluxes and energy balance closure derived from Bowen ratio and eddy covariance measurements whose footprints lie in the southeast field. Fluxes of ET from the northeast lysimeter were larger than those from the northeast field during the period of rapid vegetative growth, up to and somewhat after peak plant height was achieved, that is from DOY 196-269. This was due to plants on the lysimeter having greater height and width than those in the field. Comparisons of ET fluxes from eddy covariance and Bowen ratio systems to fluxes from the northeast lysimeter, should take into account the fact that northeast lysimeter ET fluxes were approximately 12 to 18% greater than ET fluxes from the northeast field. Comparison of zero flux plane and simple soil water balance methods of calculating ET from the field soil water balance showed them to be equivalent in this case. However, the depth of the zero flux plane should be determined by flux calculations in addition to the assumption that this depth is equal to that at which the hydraulic gradient apparently reverses.