SPRINGTIME SOIL TEMPERATURES IN LYSIMETERS IN CENTRAL TEXAS

Weighing lysimeters are often used as a standard for evaporation measurements and are typically constructed with steel walls, a material with a thermal conductivity approximately 40 times that of soil. This highly conductive material can increase the rate of vertical energy transfer in the lysimeter and alter lysimeter soil temperatures. The objective of this study was to quantify effects of the vertical energy transfer by lysimeter walls on soil temperatures in lysimeters with surface areas of 0.175, 0.75, and 3.0 m2. Hourly average soil temperatures were measured at 0.1 and 0.4 m from 7 through 29 May 1985 in a field and at various positions in the lysimeters at the Blackland Research Center, Temple, Texas. The diurnal amplitude of hourly temperatures was smaller in the field than in lysimeters, was inversely related to lysimeter surface area, and decreased with distance away from the lysimeter wall. Mean daily temperatures in the lysimeters were up to 3-degrees-C greater than those in the field. Temperature differences were greater for smaller lysimeters. Diurnal temperature differences between the field and lysimeters were primarily caused by energy incident upon the exposed steel at the surface which was conducted into the soil by the wall. Temperature differences could impact early-season root and shoot growth and thus affect representativeness of lysimeter evaporation measurements. Lysimeters constructed with steel walls should be designed to minimize the exposed area of steel at the surface.