Wall material and capping effects on microlysimeter temperatures and evaporation

The microlysimeter (ML) is useful for measurements of evaporation from soil but questions persist regarding correct ML design. We studied the effects of length and wall material on evaporation and the effects of wall material and capping on ML temperatures. Cylindrical steel and plastic MLs of 10-, 20-, and 30-cm lengths and 8.8-cm o.d. were used in two field experiments on a bare Pima clay loam (fine-silty, mixed, thermic Typec Torrifluvent). Steel MLs significantly understimated 8-d cumulative evaporation compared with 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 with 0.6-cm-thick plastic disks caused accumulation of heat in the MLs. For plastic MLs, only the 20- and 30-cm lengths were long enough for continuous use for 9 d under our conditions. It was unclear if cumulative evaporation varied with length for steel MLs. We recommend that walls be constructed of material with low thermal conductivity and that end caps be designed to maximize thermal transfer between the soil inside and below the ML. A length of at least 30 cm is recommended if measurements are needed at the same location for several days