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A so-called selective inverted sink (SIS) was validated in frost protection of a 20-ha almond orchard. Daily counts of flower buds were made for two branches of some selected almond trees in every plot to determine frost damage percentage. Temperatures increased due to the SIS system, but there was an average gradient of temperature decrease of about 0.4°C per 100 m with distance from the SIS. The minimum air temperature increased from 0.5 to 2.8°C, with the highest increase closest to the SIS. The percent of frost-damaged flower buds of almond relative to the control plot with distance to the SIS system had its maximum gradient (8% per 100 m ) in 100–200-m distance from the SIS, but this gradient decreased to a minimum (4% per 100 m) in 500–700-m distance from the SIS. The ANOVA and Duncan’s multiple-range test of air temperature and frost damage data confirm that the significant influence zone of this local SIS was about 500 m.

Central Alborz is one of the important agricultural regions of Iran. Occurrence of the first frost in fall and the last frost in spring causes damage to the crops in this region every year. Information about the probable dates of frost occurrence helps farmers in preventing or reducing the damages caused by frost. Six stations, with 34 years of daily minimum temperature data, were selected at various elevations. Dates of frost occurrences in three degrees of severity (mild, moderate, and severe) and frost-free periods were obtained for each year. Appropriate distributions were determined for each frost series (dates of frosts and frost-free periods) on the basis of relevant statistical methods. It was found that the Pearson type III distribution was most appropriate. Probability distribution was constructed for each frost series. Frost occurrences up to a given date and maximum lengths of frost-free periods, with their probabilities, were determined. Correlations between the dates of frost occurrence and elevation, and between the maximum lengths of frost-free periods and elevation were identified in some probability levels. Our results show a positive correlation between the frequency of frost and elevation, and a significant negative correlation between the duration of frost-free period and elevation. Frost is more frequent in higher areas. The shortest and the longest frost-free periods occur on top of high mountains and on low altitude areas, respectively

Six seasons of experiments in Machakos, Kenya, revealed that above about 150 mm of rainfall, maize yields per row in alley cropped "replacement" agroforestry (AF) plots, of Cassia siamea Lam. and maize (Zea mays, cv. Katumani Composite B), may be expected to exceed those in the control (sole maize) plots. Such yields were insufficient to compensate for the area "lost" to the hedgerows. Below about 150 mm the control plots may be expected to perform better. This result was due to competition for water. Greater association of the fine roots of Cassia and maize was observed in the middle of the alleys than near the hedgerows. Photosynthetic consequences of shading were insignificant relative to other factors. In the alleys, reductions of soil temperature due to shade in the western and eastern maize rows were higher than in the middle row. Soil moisture extraction was higher in the AF than in the control plots. In the AF plots, moisture extraction was greater under the central maize rows than under those nearest the Cassia. Yield patterns followed such soil temperature and soil moisture patterns. Maize transpiration and photosynthetic rates were significantly higher in the control than in the AF plots during a below-average rainy season but not during above-average rainy seasons. It is concluded that alley cropping under semi-arid conditions should be approached differently from the system worked on. It must at least provide strong physical protection of crops and/or soils and have a strong economic incentive to be of interest to the farmers.

Alley cropping of Cassia siamea and maize was studied in semi-arid Kenya for soil fertility improvement. Katumani composite maize was planted except in the short rains of 1988 (SR88) when a hybrid variety was sown. Therefore the grain yield per row increased differently in the alley cropped maize (CM). Sole maize (SM) and CM yields were higher in SR88 than in the long rains of 1988 (LR88) by 62% and 38%, while yields from the same treatments in LR89 were only 21% and 45% of those in SR88. These differences in relative maize yields are attributed to differences between the two maize varieties in competition under nutrient stress conditions.

SUMMARY Theory and practice of a sensitive infrared surface thermometer are dealt with. Several problems arising in the field use of this simple instrument are discussed and explained in detail. It is concluded that with the indicated precautions the instrument is operating at optimum reliability and accuracy. However, further increase in sensitivity will not yield higher accuracy.

The use of temperature measurements above, below and within plant stands in physiological and ecological studies is briefly reviewed and the choice of sensor and protection from radiation effects in measuring temperatures within a maize crop are discussed. The horizontal homogeneity of temperature was investigated. Measurements in 1973 and 1974 showed the greatest horizontal temperature difference at 1.8 m above ground level due to an increase in leaf area density just above the height where the crop became closed and air flow was reduced; at lower levels temperature differences increased as turbulence was reduced, and differences in soil surface conditions influenced temperature measurement. The need for a large number of measuring points in the lower parts of the canopy is emphasized. (Abstract retrieved from CAB Abstracts by CABI’s permission)

A systematic analysis is presented on the influences of the pressure dependence of the psychrometric “constant” and the inherent pressure dependence of the wind function in expressions commonly used for calculations of evaporation from open water and crops. It is shown that the pressure dependence of the wind function has to be incorporated in the Penman equations in exceptional cases only. However, the results show that the pressure dependences of the wind function in Dalton-like evaporation expressions have to be different from those in use.

As the testing of infrared thermometers under actual field conditions has been reported to be indispensable, a calibration plate has been constructed for such purposes. A simple way of measuring surface temperatures by contact is shown to yield accurate results for comparison. When the emission coefficient of the calibration plate is known, its temperature can be used to investigate the performance of infrared thermometers in field use.

Choice, improvement and field use of a differential thermocouple psychrometer are described. Special attention is paid to improved design features and considerations regarding calibration. The instrument is specially suitable for within crop measurements.

This paper reports on the methods smallholders employ to shade soil, seedlings, plants and crops to protect them from environmental stresses and to improve their yield capacity. Natural and artificial shading are discussed, both in the form of open shades and mulches, as well as other means of covering. Consequences of shading and shade reduction are reviewed from the point of view of modifying terms of the energy balance of surfaces. Other consequences of shading techniques and other manipulations providing shade as a side effect are considered. Some specific examples collected from Tanzanian practice are described as evidence of today’s use of shading methods in traditional farming. It is concluded that we can learn a lot from small farmers in this respect but that the science of micrometeorology could assist in improving efficiency and labour requirements of traditionally employed techniques, as well as in their wider dissemination in the farmer community.

Results are presented of a comparison of measured and calculated evaporation rates of the Piche evaporimeter under indoor and outdoor (within a meteorological screen) conditions. In both cases, application of mass transfer formulae in use for horizontal (turbulent) flow to the evaporating blotting paper of the instrument yield very good results under pure forced convection conditions. For mixed convection regimes, comparisons using either pure free (combined heat and mass transfer) or pure forced convection equations give as expected too low calculated values. Reasons for such differences with measured values are reviewed. Our forced convection results confirm that main stream turbulence is only of influence on mass transfer to a zero incidence flow in combination with pressure gradient (bluff body) effects, which under our conditions appear to be absent around the Piche surfaces. The same results prove absence of any influence of the particular temperature distribution over the blotting paper on the mass transfer. The understanding and importance of these conclusions in relation to the use of the Piche evaporimeter as a simple integrating mass transfer meter under actual farming conditions are discussed. The importance to obtain such mass transfer data is explained in the introduction.

It is shown from literature research that constants in the statistical relationship between total global solar radiation and sunshine duration are now well established for East Africa. Taking into account the differences in data collection errors with other simple methods of solar radiation determination, we show Campbell-Stokes data to be suitable for checking, calibrating or temporarily replacing these other methods where no calibrated pyranometers are available in the tropics. An example is given for Gunn-Bellanis.

Because of the importance in some field studies of above crop liquid precipitation measurements, in this article an interpretation and an error assessment of above maize precipitation measurements are given. With Snowdon rain gauges from 20 to 40 cm above the maize, errors seem not to be higher than with a 40 cm Snowdon gauge above grass, as long as the wind field is again adjusted to new roughness conditions and turbulence is not enhanced otherwise. Differences found between gauges correctly and incorrectly exposed were relatively small due to the fact that the maize growing season falls clearly within the period of lowest influence of wind on rain gauge catch in the Netherlands. The use of comparative measurements between a 40 cm Snowdon gauge above grass and Snowdon ground level gauges installed following latest W. M. O.-recommendations appeared to be indispensable for ultimate error assessment.

Two recent proposals for the generalization of the Penman equation based on completely different calibration attempts appear to yield a strong basis for such a generalization, if combined. Consequences of the combination and its use are discussed in the light of each of the original proposals.

The uncertainties in average relationships between paint cloudiness and solar energy flux density can largely be accounted for through scatter in the overestimation of cloud shades by point cloudiness. This is demonstrated using monthly averaged data from Tanzanian stations. The scatter is caused by differences in cloud type and cloud distribution in different seasons and regions under conditions of equal sunshine duration and global solar radiation flux density.

The use of physics in agricultural research is discussed with emphasis on environmental physics. An attempt is made to classify physical aspects of the radiation environment, the aerial environment...

It is shown that a recently suggested use of a pressure dependence of the psychrometric “constant” is no improvement on existing practice. A plea is made for a through experimental and theoretical investigation into the pressure dependence of γ, given the facts that we do still rely completely on empirical values at normal pressure and that our lack of understanding of processes involved is still high.

Simple infrared field thermometers appear to suffer from small internal heat flows which influence the sensor when the instrument is used outdoors. Further research into higher sensitivity of the sensor must be accompanied by increased attention to heat insulation of the instrument and sensor.