PERFORMANCE OF DRAINAGE LYSIMETERS FOR THE EVALUATION OF WATER-USE BY ASIAN PEARS

The design of a type of drainage lysimeter, as tested with trees of Pyrus serotina Rehder var. culta Rehder 'Hosui' is described. All lysimeter operations and monitoring of irrigation and drainage volumes were managed by a "multi-tasking" controller/datalogger. It was possible to apply different irrigation levels to each of three sets of four random lysimeters. Evapotranspiration (ET) was calculated using a conservation of water equation, with differences between irrigation inputs and drainage outputs corrected for changes in soil-water content. ET ranged between 3.3 and 10.7 liters/tree per day in well-watered, noncropped trees in late Summer and Fall 1990. These rates correspond to ET of 0.13 to 0.43 liter.cm-2.day-1 and 0.96 to 3.10 liters.m-2.day-1 on trunk cross-sectional area and canopy area bases, respectively. The correlation coefficient between ET and Class A pan evaporation was > 0.9 during this period. Weekly crop coefficients for the well-watered trees averaged 0.52 when calculated on a canopy-area basis. When irrigation was withheld for 18 days, the crop coefficient declined to 0.38. There were no differences in ET between trees growing in the two soil profiles, despite significant differences in soil water distribution.