PATTERN OF SEASONAL WATER-USE OF ASIAN PEARS DETERMINED BY LYSIMETERSAND THE HEAT-PULSE TECHNIQUE

Seasonal water use data are presented for 4-year-old Pyrus serotina Re hder cv. Hosui growing in drainage lysimeters and trained onto a Tatur a trellis. Weekly water use (WU) was calculated using the mass balance approach. For 8 consecutive weeks during late summer, instantaneous W U was also measured by the compensation heat-pulse technique for measu ring sap flow. Although good agreement was found between the two metho ds for 4 weeks after probe installation, discrepancies increased after this time. Water use was highest in early to mid-January in New Zeala nd, averaging almost-equal-to 8 liters/tree per day, or 2 liters.m-2 c anopy surface area/day. Total water use over the growing season was 10 70 liters/tree, or 245 liters-M-2 canopy surface area. The correlation coefficient between weekly WU and evaporation from a nearby Class A p an was 0.81 for the season. Weekly crop coefficients thus calculated f or the well-watered trees ranged from 0.15 to 0.55 and 0.20 to 0.83 wh en calculated using canopy surface area and projected ground area, res pectively. Low values were due to low values of canopy leaf area early in the season. Withholding irrigation during three periods resulted i n a gradual decline in water use. Water-stressed trees had a lower pre dawn water potential than fully irrigated trees. This pattern was foll owed by a more-rapid decline during the morning, and a slower recovery during late afternoon and early evening. Midday leaf water potential never fell below -2.5 MPa.