Agricultural Meteorology最新文献

Daily run-of-wind (km) was recorded at a control station in the open and at a distance of $\text{4 H}$ (where $\text{H}$ is barrier height) leeward of a medium porous type of hedge during the spring and autumn periods between 1979 and 1981. Daily relative winds (RW) and the run-of-wind differences between the open and sheltered positions (DU) were derived from these measurements. RW at 4 $\text{H}$ was found to be primarily a function of the incident wind angle and independent of the mean open wind speed within the range 1–12 ms⁻¹. The RW at the fixed position was, therefore, suggested as a useful single index of the combined influence of wind direction and that of the barrier itself. For a particular RW value the daily run-of-wind difference between open and sheltered positions was found to vary positively with the mean daily wind speed.

A simple model for growth of energy forests is constructed, in which the dominant variables for growth are air temperature and radiation. Processes included in the model are: photosynthesis; light penetration in the canopy; respiration; allocation between above- and below-ground parts; and allocation between stems and leaves. Other factors influencing growth are assumed to be close to optimal, e.g., water-supply and nutrition.

The model is tested on willow, using actual meteorological data and seasonal field measurements of growth of yearly coppied shoots in established stands. Good agreement between simulated growth and field measurements was obtained for two sites with different climates and plant densities expressed in stools m⁻².

For one-year-old shoots in 1981, the model predicted a maximal LAI of $\text{∼ 7}$ and an above-ground biomass production of $\text{∼ 14}$ tonnes ha⁻¹ for the southern part of Sweden (latitudes 55–60°N), and LAI and production of $\text{∼ 4}$ and 8.5, respectively, for the north-eastern coastal area (latitudes 63–66°N).

In West Africa, three populations of millet were grown to assess how growth was related to population and water supply. Stomatal resistance was measured with a porometer 3 times a day on 14 days, boundary-layer resistance was estimated on the same days using blotting-paper replicas for leaves, and wet- and dry-bulb thermocouples were used to determine concentration differences of water vapour. Changes in the mean rate of transpiration estimated from these quantities were strongly correlated with changes of green leaf area during the season. Seasonal changes of stomatal resistance were much less significant in determining the seasonal trend of transpiration rates. Estimates of water loss by transpiration agreed well with measurements of soil-water extraction obtained with a neutron moisture meter.

Daily rainfall records for two sites in Northern Syria are compared by fitting probability and frequency models to the observations. A model is interpolated for the main experimental site of the International Centre for Agricultural Research in the Dry Areas (ICARDA) at Tel Hadya, a site which is intermediate between the weather stations and for which long term measurements were not available. Various rainfall statistics relating to crop improvement programmes are calculated from the model and the rainfall patterns of the last three years are placed in a long term perspective.

Wind chill can be calculated in two ways from the estimate of sensible (non-evaporative) heat loss (H_n) that a sheep experiences as a result of its exposure to the weather variables of air temperature (T_a), wind speed (V_a), sunshine, cloud and rain. By one method, that part of the heat loss that is due to wind (H_v) is calculated; H_v varies with the fleece depth, which provides the animal with the largest part of its thermal insulation. The second method leads to an estimate of the fall in temperature under conditions of no wind (ΔT_v) that would produce the same value of H_n that occurs under the actual conditions; ΔT_v is influenced to only a small degree by fleece depth.

H_v at Aberdeen (Scotland) constituted 25–30% of the annual H_n in 1973. H_v persists at a high level in the summer due to the dissipation of solar heat; in the winter, H_v is associated with the enhancement of the cooling effect of low temperatures. The estimation of ΔT_v from temperature and wind alone is compared with its estimation from the combination of all factors. ΔT_v per knot of meteorological wind speed (measured at 10 m height) is ∼ 1 K when T_{a = 10}°C, with an inverse variation of ∼ 30% for 10 K.

The effect of wind can be estimated as the accumulation of heat loss during periods when heat loss exceeds 55 W m⁻², the rate that is expected at the critical air temperature. If the wind speed to which sheep were exposed in 1973 at Aberdeen had been halved, with temperature and other conditions unchanged, the year's integral of (H_{n − 55}) would have fallen from 107 to 36 MJ m⁻² for sheep with a fleece depth of 50 mm. This provides some measure of the value that can be attached to a wind break.

Plant temperature of alfalfa (Medicago sativa L.) grown with different amounts of irrigation water has not been reported. The objective of this experiment was to determine if progressive differences in canopy temperature existed among plots of alfalfa (ev. Cody) subjected to 7 graded watering treatments. Irrigation water (0, 2.5, 5.1, 7.6, 10.2, 12.7, 15.2 cm) was added after each of three harvests in 1980 and 1981. Extremes in weather between the summers of 1980 and 1981 enabled comparison of data from a stressed season (1980) with those from a non-stressed season (1981). Throughout the growth period in both years, canopy temperatures, leaf area, stem dry weight, leaf dry weight and total dry weight were determined. Canopy temperature was measured with an infrared thermometer. The relationship between canopy-minus-air temperature (T_c — T_a) versus vapor-pressure deficit (VPD) was determined on well watered alfalfa for 1980 and 1981.

Differences in canopy temperature, leaf area index, leaf dry weight, stem dry weight, and total dry weight, due to treatments, were evident in the dry year (1980), but not in the wet year (1981). In the dry year, the irrigated plots generally had cooler canopy temperatures and higher dry weights than the dry land plots, but differences due to the level of water added were not apparent. In both the dry year and the wet year, (T_c — T_a) was inversely related to VPD. Also, in both years, and for all treatments, leaf dry weight was about equal to stem dry weight.

MORECS is an acronym for the Meteorological Office rainfall and evaporation calculation system. In its operational form it uses daily meteorological data to produce weekly estimates of evapotranspiration, soil moisture deficit (SMD) and hydrologically effective rainfall for each square of a 40 × 40 km grid superimposed upon Great Britain. Grid square estimates of meteorological data are found using interpolation methods. A modified version of the Penman—Monteith equation is used to calculate evapotranspiration; a two-reservoir model is used to simulate the extraction of water in the SMD calculations.

The accuracy of SMD estimation by MORECS was investigated, for grassland, by comparing the SMDs with field-measurements made using neutron probes. MORECS was run retrospectively using meteorological data from stations close to the soil moisture measurement sites, to produce point SMD estimates. The effects of the interpolation methods were thus circumvented.

The principal finding was that there was a definite bias in the model towards SMD overestimation in nearly all years except those with very dry summers when underestimation occurred. The reasons for this are explored and areas where improvements might be made are outlined.