Agricultural Meteorology最新文献

This paper presents the characteristics of agroclimatic variables identified in part II of this study as being relevant to crop production potential. A soil-water balance simulation and agronomic data for selected locations in India were used to assist in this analysis. Based on these observations the successful cropping systems and crop species for similar soil types were also discussed. Clearly, the cropping pattern is not only influenced by the mean effective rainy period but also by its variability, as well as the variability in the times at which the sowing rains commence. However, the crop varieties that are suitable for these cropping patterns differ significantly. They are associated more with soil type, and the wet and dry spells within the effective rainy period.

Several recent crop stress and yield models depend on the difference between air and leaf temperature. When this difference is zero, the ‘equivalence point temperature’ is defined, if the crop is well-watered under sunny conditions. A simple energy budget equation is presented, which shows that the theoretical equivalence point temperature is quantitatively related to the absorbed radiation load, the vapor pressure deficit, the surface and aerodynamic resistance to water vapor transport, and the emissivity of the leaf. This implies that the theoretical equivalence point varies diurnally, rising in the morning, reaching a relative maximum near midday and falling in the afternoon and evening. At certain times and under certain conditions, the theoretical equivalence point may be physically impossible. The widely reported value of the equivalence point temperature, 33°C, is not supported by theory or empirical evidence.

To arrange the 190 locations in India, Senegal and Upper Volta into finite groups, numerical taxonomic techniques were adopted, using 11 agroclimatic variables derived from parts I, II and III of this study. The results obtained using the data matrix of 7 principal coordinates with Euclidean metric — unweighted pair group method using arithmetic averages (UPGMA) fusion strategy appear to be the most suitable to identify groups formed in India and West Africa. In general, this study separated the highly undependable locations with different effective rainy periods. The groups identified are not homogeneous in terms of dependability defining dry-seeding feasibility, at the lower end of the range, and water-logging hazards. Therefore, to overcome this problem, the groups were first arranged into 3 broad zones, namely arid, semi-arid and sub-humid according to a modified Thornthwaite approach, and the semi-arid locations were further divided into 5 zones based on the effective rainy period related to the cropping pattern. As the primary groupings refer to the most stable situation, they were again divided into 5 sub-zones each based on the level of dependability, which was characterized by the standard deviation of the commencement time of sowing rains. To account for some of the operational problems, such as water-logging, availability of field work days for cultural operations, etc., they were further divided into groups and sub-groups based on wet and dry spells and aridity index.

The derived zones and sub-zones of the semi-arid tropics are straight forward and easily memorised, and make sense agronomically. With the given classification structure it is easy to add new locations without further numerical analysis.

Rose plants in pails were weighed at the beginning and end of several nights to determine the rate of transpiration under a range of air temperatures. The data were reduced to obtain the leaf resistance (both sides of the leaf in parallel), its mean being 2400 s m⁻¹ ± 30%. Once the night-time leaf resistance is known, the transpiration rate can be calculated from measured values of the leaf area index, the leaf temperature and the ambient temperature and dew-point temperature. If the leaf temperature cannot be measured, it can be estimated from the inside temperature and dew-point temperature, and the outside temperature. A calculated example shows that there is agreement within a factor of two between an estimate based on the present model and the rate of night evapotranspiration in greenhouses, as reported in the literature.

A simple method based on rainfall (R) and potential evapotranspiration (PE) for deriving variables to classify the semi-arid tropics into relevant agronomically homogeneous zones is suggested. A term ‘available effective rainy period’ is introduced for this purpose. The available effective rainy period is defined as the number of consecutive weeks in which the 14-week moving average of R/PE is ⩾ 0.75, but for the initial week the value of R/PE is ⩾ 0.50. The preceding week is taken to be the week of commencement of the sowing rains. The method permits estimation of wet and dry spells during the available effective rainy period and an estimate of the likely percentage of crop-failure years. These selected parameters allow a more relevant and realistic assessment of the agroclimatic environment and agricultural production potential of a selected location or region.

This paper describes an experimental study of the radiation balance over a growing soybean (Glycine max (L.) Merrill) crop at a tropical agricultural experimental station, Jaboticabal (22°15′S 48°18′W), São Paulo, Brazil. Incoming shortwave radiation, S, net radiation, R, and the shortwave reflection coefficient, α, were measured over this crop from November 1976 to April 1977, and net longwave radiation, L, was deduced from the radiation balance equation. Hourly variation of the radiation components, S, αS, R and L, are presented for 4 nearly clear sky-days during the different growing crop periods. The diurnal variation of α is also presented for these 4 days. The mean daily albedo values for this vegetative surface were between 0.12 and 0.26, and inversely related to the vegetation height. A regression analysis of the radiation balance equation has been made, and the heating coefficient, β, and the longwave exchange coefficient, λ, obtained. The results of the study indicate that both β and λ are found to be descriptively helpful parameters in agricultural meteorology. Predicted regression equations for net radiation from shortwave radiation are presented for the different periods.

Two methods were developed by which daily net short-wave radiation $\text{(K∗)}$ can be evaluated from Bellani pyranometer readings. The first method involves a simple regression equation. The second method uses a physical approach taking into account the effect of the Bellani's geometry on its response to direct and diffuse radiation throughout the day. Both methods, when tested on experimental data, tended to underestimate the measured $\text{K∗}$, the regression approach exhibiting a higher variance of the error.

Eight agroclimatic variables related to crop production potential in the semi-arid tropics of India are identified. These are used to assess dry-seeding feasibility, water-logging hazard, risk in agricultural production, cropping patterns and their spatial distribution, using data from 80 locations in India. Regression analysis has been used to identify differences between locations at different scales, i.e., local differences caused by orography, regional differences associated with circulation patterns and continental differences associated with general circulation patterns. Using the data of 8 variables from 199 locations in India and two west African countries (Senegal and Upper Volta) three dissimilarity parameters were derived. The basic dissimilarity observed on a continental scale is that for the same amount of mean annual rainfall the growing season is longer in west Africa than in India. Thus, in west Africa the corresponding wet and dry spells within the available effective rainy period are quite different from India. This will have a significant influence on farming systems in general, and on the identification of adopted crop/cropping systems in particular.