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A "Micro-Lysimeter" Method for Determining Evaporation from Bare Soil: Description and Laboratory Evaluation¹

ABSTRACT

A new ("micro-lysimeter") method for estimating evaporation from soil consists of pushing a thin-walled cylinder 76 mm in diameter into field soil, removing the soil-filled cylinder from the field, closing the bottom to make it water-tight, determining the mass of the micro-lysimeter, replacing it in the field with its top surface even with the surrounding soil, leaving it exposed to environmental conditions for a period of time (typically 1 d), and redetermining its mass. Evaporation loss from the micro-lysimeter is the difference between the two masses.

The objective of this study is to quantify the systematic deviations of such micro-lysimeters from reality in order (i) to determine how long a given micro-lysimeter can be used before its soil must be discarded (and a new sample taken if further readings are desired), and (ii) to correct for systematic deviations. The deviations are quantified by comparing short micro-lysimeters with effectively "infinitely long" ones. The deviation is influenced by the time that the micro-lysimeter is exposed to environmental conditions and by the length of the soil sample. For a silty clay loam soil under evaporativity conditions ranging from 2 to 9 mm/d, the measurement error for micro-lysimeters 70 mm in length is found to be < 0.5 mm for 1 or 2 d, depending on whether the initial soil condition is "wet" or "dry," respectively (0.26 or 0.13 g of H₂O per gram of dry soil in the top 20 mm).

Hence, for many applications the method is valid for 1 or 2 d. Correction equations for deviations of up to 0.5 mm are given. The method requires little equipment and makes possible evaporation measurement under some conditions (for example, at large numbers of sites or in areas of partial crop shading) where micrometeorological, water balance, and traditional lysimetric methods are unpractical or impossible.

¹ Contribution from the Dep. of Agronomy, Univ. of Illinois, Urbana, IL 61801. Supported in part by project no. 15-0356 of the Illinois Agric. Exp. Stn.

² Associate Professor and Technical Assistant, respectively.

Received for publication July 27, 1981. Accepted for publication February 26, 1982.

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