|Evett, Steven - Steve|
Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/2/1997
Publication Date: N/A
Interpretive Summary: Weighing lysimeters are required for accurate evapotranspiration measurements needed to calibrate evapotranspiration models, but lysimeter design requires skilled personnel and lysimeter construction is costly. To reduce both the design effort and the construction cost, the authors developed a simplified lysimeter design and tested it with a weighing lysimeter constructed at Bushland, Texas, USA and two lysimeters constructed at Ismailia, Egypt. At Bushland, the soil tank contains a soil monolith and at Ismailia, the soil tanks contain reconstructed soil. The lysimeters utilize shop-assembled, steel soil and enclosure tanks that can be hauled to the construction site and placed on a simple concrete pad foundation. A factory assembled load-cell scale requires only a suitable foundation and wiring to the data logger for complete installation. Field construction is focused on excavating the pit for the lysimeter, constructing the concrete foundation, and filling the soil tank either wit a monolithic or reconstructed soil. By using a proven design adaptable to a wide range of lysimeter operations, weighing lysimeters can be constructed with minimal design and greatly reduce cost. For example, the simplified lysimeter design was used with only small modifications for a weighing lysimeter installed at Paraipaba, Brazil by the University of Tennessee and the Empresa Brasileira de Pesquisa Agropecuaria.
Technical Abstract: A simplified weighing lysimeter applicable to both monolithic and reconstructed soils was developed and tested in two lysimeters installed at Ismailia, Egypt and one installed at Bushland, Texas, USA. A monolithic lysimeter was used to measure reference grass evapotranspiration (ET) at Bushland because the dense subsoil and calcic horizon of the clay loam soil lcannot be reconstructed in a soil tank. The desert sand at Ismailia allowed the use of reconstructed soils for measuring reference ET of alfalfa and ET of field crops. The main lysimeter components are the concrete foundation, deck scale, soil tank and enclosure consisting of a base, a tank and a top. The steel soil tank and enclosure are shop-fabricated, and the scale is commercially available. Field construction consists mainly of excavation, collection of the soil monolith, if needed, and installation of the concrete foundation. Field calibration of the Bushland lysimeter over a 214-mm ET range resulted in a standard error of 0.1 mm and r**2 = 0.9999. Similar calibration of one of the Ismailia lysimeters over an 87-mm ET range resulted in a standard error of 0.02 mm and r**2 = 0.9999. In initial tests, the Kimberly Penman equation overestimated grass reference ET, and the Penman-Monteith equation slightly underestimated grass reference ET for the Bushland environment. Hourly grass ET measured with the Bushland lysimeter agreed closely with hourly grass ET calculated by the 1963 Penman equation.