Temperature hazards to horticultural crops in Canterbury

Abstract

Air temperature is a major environmental factor affecting crop production. This project is concerned with three aspects of temperature micro-climate important to horticultural crop production in this region; (i) climatological analyses of long-term air temperature and temperature related data, (ii) nocturnal micro-climate during radiation inversions/freezes, and (iii) modification by a wind-machine of micro-climate over low-growing crops during radiation freezes. All experimental work was carried out at Lincoln College except for short-term micro-climate recordings in an apple orchard at Belfast, North Christchurch.

A statistical procedure for analysing temperature-related (including air temperature) climatological series is outlined, and analyses of air temperature series, freeze series, chilling series and growing degree days series are presented. These series were found to be random and normally distributed. Normal probability tables were developed for each variable and the significance of these to horticultural production is discussed briefly.

Freezes in this region are of the radiation type with screen minima usually higher than 24°F. About 95% of associated winds are from the north, and about 80% of winds are less than 3 mph at 15ft. Average inversion strength over exposed sites (30ft-1ft inversion) and over deciduous orchards before leaf development (50ft-5ft inversion) is 5-6°F. The inversion temperature profile over exposed sites is continuous, but over tree orchards there is a distinct discontinuity in the profile within the tree zone, particularly when leaves expand in late spring.

Several characteristics of nocturnal micro-climate during freezes or potential freezes were examined. Nocturnal atmospheric stability studies based on Richardson Numbers indicated that 4 mph (at 15ft) is the critical wind speed for stable radiation inversion formation; thus radiation freezes are unlikely when winds are greater than 4 mph. A power law was found to adequately describe the wind speed vertical profile during radiation inversions. Characteristics of radiation freezes such as freeze-duration and maximum freeze levels can be estimated accurately from temperature data.

The Brunt, Angstrom, Geiger, Swinbank, and Idso and Jackson empirical formulae for estimating atmospheric radiation from clear skies were examined for clear night data. Most accurate of these was the Brunt formula which gave estimates of atmospheric radiation within approximately 2% of measured values for the range likely to be encountered during radiation inversions/freezes. The temperature dependent Swinbank and Idso and Jackson formulae underestimate atmospheric radiation during strong inversions.

Meteorological conditions in this region are suitable for wind machines. However, this method of freeze protection is not likely to be economic here due to high costs of installation and the relatively low risk of spring freeze damage to commercially important crops, mainly tree and berry fruits. Results of field trials suggest that wind-machines-are not particularly suitable for protecting very low-growing crops such as strawberry, essentially because of the marked influence of even light winds on the magnitude and location of the protection pattern.