Agricultural Meteorology最新文献

A method for analyzing radiation interception by an individual tree is presented. The use of a whole set of expensive radiation instruments is eliminated, the work-intensive field experiments are reduced to half a day for a large tree (9 m), and neither clear sky conditions nor diffuse radiation data are necessary. It is assumed that for a given azimuth and solar elevation the shading caused by a tree can be estimated photographically by taking a slide in the opposite direction. From slides taken of the whole tree in different directions and from different distances (i.e., different view angles), gap frequencies for certain heights are measured. At each height, gap frequency decreases significantly with increasing solar elevation. By calculating the mean path length within the crown (s) corresponding with each gap frequency (g), a highly significant exponential relationship between both is obtained [g = exp(−0.42s)]. With this equation and by knowing the tree dimensions calculated from the slides, direct and diffuse radiation interception of the whole tree can be estimated. Total leaf area and leaf area density are also calculated. These values agree very well with direct measurements on similar cut trees.

New experimental evidence is presented to resolve a long-standing controversy relative to the existence or non-existence of evaporation control by stomata in situations where the water supply to plant roots is non-limiting. It is shown that both sides of the argument were correct in certain respects, but that the claim for significant stomatal control was the one most representative of reality in the broadest perspective.

Two experiments on the measurement of leaf wetness are reported. From the results of a previous experiment, a thin (0.28 mm), double sided circuit board sensor was constructed (sensor C). It was compared to the two sensors used in the original experiment, a cotton cloth on a grid network of two independent fine wires (sensor A) and a thick single board painted with gray latex paint (sensor B). As in the initial experiment, sensor A was found to be the superior sensor, while sensor C gave better results than sensor B. Honeydew, the sugary excrement of sucking insects, caused spurious results from sensors B and C, while sensor A appeared to be unaffected. In the second experiment, leaf wetness was measured with the same three sensors in two plots; a plot treated with an insecticide and a non-treated plot. Sensor A was the superior sensor based on the greatest duration of leaf wetness and comparison with visual observations. The hygroscopic nature of the honeydew increased the duration of leaf wetness between the plots by ∼25% as measured by sensor A.

The effect of atmospheric vapor pressure deficit (VPD) on canopy-air temperature differnce (δT) of corn (Zea mays L.) is simulated using a soil—plant—atmosphere model together with weather data for several clear-sky days. Considering empirical dependence of stomatal resistance of corn on the leaf water potential and insolation, the model finds an analytic solution for the leaf water potential which satisfies Monteith's and van den Honert's equations for transpiration. The simulated leaf water potentials are compared with observations. For a large portion of the daylight period a near-linear relationship between δT and VPD is found at various soil water potentials. Among weather variables, the air VPD appears to be the most important factor affecting δT for unstressed canopies. With decreasing soil water potential the sensitivity of δT to wind speed increases. The effect of differing rooting densities on δT is also studied. As the rooting density increases the δT values decrease, indicating that δTv. VPD relationship could change with crop age and the soil strength conditions affecting the root growth.

Wilson et al. proposed a simple micrometeorological method of estimating the rate of gaseous mass transfer to the atmosphere from a small circular plot which required measurements of time-average species-concentration and horizontal windspeed at a single height. This method has been applied here to estimate the rate of volatilization of ammonia from a 25 m radius plot treated with urea fertilizer. The emission rates thus obtained agreed satisfactorily with estimates based on a mass balance which employed concentration and windspeed measurements at five levels.

A set of photosynthetic responses of a Scots pine (Pinus sylvestris L.) shoot to light was derived from the shoot geometry and the photosynthetic response of a single needle. Computations showed that the shape of the photosynthesis light-curves varies substantially depending on the direction of radiation relative to the shoot position. Differences in the initial and maximum rates of photosynthesis were due to changes in the effective projection area and the irradiated fraction of the shoot, respectively.

Two cultivars of wheat, contrasting in their drought resistance characteristics, were grown under irrigated and non-irrigated conditions in the field for two seasons. The water-use efficiency (WUE) ranged from 0.5 to 13.8 g dry matter kg⁻¹ water used depending upon the growth stage, cultivar and water stress. The average WUE for the season was between 3.34 and 4.70 g dry matter kg⁻¹ water used. Regression analysis indicated that WUE was a function of dry matter produced and had no direct statistical relationship with the weather parameter. The WUE of plants having the C₃ system of photosynthesis, like wheat, need not be poorer than the plants with the C₄ system if comparisons are made of plants growing in their respective preferred ecological niches. Various possible reasons for the low values of WUE reported for C₃ plants are discussed.