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United States Department of Agriculture

Agricultural Research Service

Title: Wall Material and Capping Effects on Microlysimeter Temperatures and Evaporation

Authors
item Evett S R,
item Warrick A W, - UNIV. OF ARIZONA
item Matthias A D, - UNIV. OF ARIZONA

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 5, 1994
Publication Date: N/A

Interpretive Summary: Microlysimeters (MLs) are tubes inserted into the soil, removed with the soil inside intact, and then capped at their bottoms. They are replaced in holes in the soil such that the surface of the soil in the tube, the top of the tube, and the surrounding soil surface are all at the same elevation. They are periodically removed and weighed in order to measure evaporation. Various problems have been associated with the use of microlysimeters including lack of knowledge of proper construction materials and adequate lengths. We investigated the effects on performance of metal versus plastic tubes, length of the tubes, and capping of the tube bottom. We showed that metal tubes conduct heat away from the surface and thus may reduce the energy available to cause evaporation. Therefore, metal tubes should be avoided since evaporation measurements made with metal tubes may be less than what occurs in the field. Thermally nonconductive end caps, such as plastic, caused a heat build up in the tube and therefore may cause inaccurate evaporation measurements. We recommend that microlysimeter tubes be made of a thermally nonconductive material such as plastic, while end caps should be made of a thermally conductive material such as stainless steel or aluminum. We found that 1 foot long tubes were long enough for measurements up to 9 days after an irrigation.

Technical Abstract: Microlysimeters (MLs) have proved useful for measurements of evaporation from soil but questions persist regarding appropriate microlysimeter design. Cylindrical steel and plastic MLs of 10, 20, and 30 cm lengths and 8.5 cm diameter were used in 2 field plot experiments on a bare clay loam. Steel MLs significantly underestimated cumulative evaporation compared to plastic MLs for 20-cm lengths. Steel MLs conducted heat more easily and their surfaces were significantly cooler during the day and warmer at night than either plastic MLs or the adjacent field soil. Capping the bottoms of MLs with 0.6 cm thick plastic disks caused accumulation of heat in the MLs and may have resulted in overestimation of evaporation. For plastic MLs only the 20 and 30 cm lengths appeared to be long enough for continuous use over 9 days under our conditions. It was not clear if cumulative evaporation varied with length for steel MLs. It is recommended that ML walls be constructed of materials with low thermal conductivity and that end caps be made of material with high thermal conductivity.

Last Modified: 7/24/2014
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