The accuracy of evaporation measurements from small lysimeters

Abstract

Weighing lysimeters are generally considered the standard for evaporation ( E) measurements. They are usually installed as a single unit, so it is difficult to use them to determine treatment effects on E. The use of numerous, small lysimeters ( < 1 m 2 surface area) is one means of solving this problem, but small lysimeters may have design properties that could affect the accuracy of E measurements. The objective of this research was to evaluate the accuracy of sorghum and wheat E measurements from small lysimeters. Large, medium, and small lysimeters, with surface areas of 3.0, 0.75, and 0.18 m 2, respectively, were used. The single large lysimeter provided the standard to which seasonal and 5 to 29 day E values from four medium and four small lysimeters were compared. Seasonal sorghum E totals for the medium and small lysimeters were significantly different from the large lysimeter value ( α = 0.05, two-tailed), presumably because leaf area index (LAI) differed among lysimeters. When sorghum E values were adjusted to account for LAI differences, totals were not significantly different. When the means of daily E values from the medium or small lysimeters, calculated from the 5 to 29 day periods, were compared, using linear regression, to the value from the large lysimeter the slope was significantly different from 1.0. Adjusted for LAI differences, slopes were not significantly different from 1.0. For wheat, neither the mean seasonal E of the medium or small lysimeters was significantly different from the large lysimeter value, nor were the slopes different from 1.0. No consistent effect of lysimeter size on accuracy was found for either crop. Properly used, small lysimeters provide a technique for seasonal E measurements. Based upon the results from this study and previous work, statistical differences between treatments in seasonal E (using four small lysimeters, area <1 m 2) would not be demonstrable if the difference was less than 52 mm.