Agricultural Meteorology

Changes in evaporation from small dishes, and in temperature and vapour pressure of the air, were observed in the Sudan Gezira, where large cotton fields are interspersed amongst uncropped dry fallows. Lateral movement of energy resulted in high evaporation rates near the leading edges of irrigated fields and in a progressive decrease, of about 30%, in evaporative demand within the leading 60 m of a cotton field. Evaporation rates increased as the wind blew across dry, uncropped fields lying downwind of the cotton. Water losses from evaporimeters, with similar exposures, decreased at the rate of nearly 2% per km over a transect of 17 km (about 10 miles). The lateral variations in evaporation rates were due to changes in temperature, vapour pressure deficit, and wind speed along the transect. Advection was found to be less apparent in temperate conditions in England than in the hot arid climate of the Sudan.

Air-flow studies were conducted in a 4.85 ha vineyard. Mean horizontal wind-speed profiles were obtained with cup anemometers. Propeller anemometers measured longitudinal, lateral, and vertical velocities above the rows as a function of height, wind speed, and wind direction.A constant momentum-flux zone above the vineyard, within the measured heights of 227 to 641 cm, was not found. The semilog horizontal wind-speed profiles, obtained over the vineyard, fitted two straight lines, representing an inner and outer zone within the boundary layer. Z0 was nearly constant.The number of uninterrupted updrafts of digitized propeller anemometer records was approximately equal to the number of uninterrupted downdrafts at each height, but the duration of these uninterrupted values was different. The total number of updrafts and downdrafts were, in general, not equal.Spectra from the four vertical anemometers showed that the total variance increased with height, as contributions to the low frequency portions of the spectra increased.

The chlorophylls and carotenoids extracted from plant leaves have been introduced as a colouring pigment into an epoxy resin. The photo-decomposition of the pigment is thereby retarded and the material can be used as an optical filter for absorbing photosynthetically-active radiation. Experiment has shown that the clear sky value of the ratio of photosynthetically-active radiation (as defined by the filter used here) to total short-wave radiation is 0.39. This ratio is also a good long-term average for cloudy skies, but on individual cloudy days the ratio may range from 0.22 to 0.52. Experiment has indicated also that typical single leaves may transmit less than 3% of the radiation defined here as photosynthetically-active.

The measurements were made to provide a basis for discussion of the definition of “photosynthetically active radiation”. The action spectrum, absorptance and spectral quantum yield of CO2 uptake were measured, for leaves of 22 species of crop plant, over the wavelength range 350 to 750 nm. The following factors were varied: species, variety, age of leaf, growth conditions (field or growth chamber), test conditions such as temperature, CO2 concentration, flux of monochromatic radiation, flux of supplementary white radiation, orientation of leaf (adaxial or abaxial surface exposed). For all species and conditions the quantum yield curve had 2 broad maxima, centered at 620 and 440 nm, with a shoulder at 670 nm. The average height of the blue peak was 70% of that of the red peak. The shortwave cutoff wavelength and the height of the blue peak varied slightly with the growth conditions and with the direction of illumination, but for the practical purpose of defining “photosynthetically active radiation” the differences are probably insignificant. The action spectrum for photosynthesis in wheat, obtained by Hoover in 1937, could be duplicated only with abnormally pale leaves.

Humicap∗∗ relative-humidity probes and a microcomputer system were integrated to provide an unattended, continuous monitor of tick microclimates. Design and operation of the integrated unit are explained and discussed. Use of the system to collect tick microclimate RH and temperature data from dissimilar range vegetation habitats is evaluated by comparison of vapor pressure deficits. Statistically significant (P< 0.01) differences were observed between the tick microclimates and air/buffelgrass interface and mesoclimate. Microclimate vapor pressure deficits in mixed brush-buffelgrass communities or mesquite-buffelgrass communities were significantly different (P< 0.01) from open buffelgrass communities. Statistical and biological differences in activity of unfed adult Amblyomma maculatum (Koch) ticks are interpreted on the basis of microclimate vapor pressure deficits and temperature.

Data from seven different crops, some of which were grown in three different locations and in three different years, with a variety of planting dates, seeding rates, and irrigation regimes have been combined to develop a single broad relationship between yield, as normalized for solar radiation reception over the crop's vegetative growth stage, and the summation of stress-degree-days accumulated over the crop's reproductive growth stage. Some reasons for expecting a common relationship of this type are presented, along with suggestions for future modifications that may be required as data from still other crops and climates are acquired.

The water-budget approach to estimating evapotranspiration was tested against energy-budget calculations to determine its potential accuracy. Measurements of soil moisture change were made using both neutron attenuation and gravimetric techniques for one-day and four-day periods. Hydraulic head gradients were measured routinely in the field and moisture characteristics and capillary conductivities were calculated from both field and laboratory data. The energy-budget calculation of evapotranspiration was derived from measurements of net radiation, soil heat flow and Bowen ratios. The measurements were made in a corn field and the experimental period spanned 25 days in July, 1969.An error analysis of the water-budget approach and the experimental results lead to the same conclusion. The water-budget cannot accurately estimate evapotranspiration on a daily basis due to unavoidable large potential errors. Accuracy arises with an increasing time span between soil moisture measurements. If actual evapotranspiration is high, there is no precipitation and six sites are averaged a reasonable accuracy can be achieved for four-day periods. If only one measurement site is used or if evapotranspiration is low or if precipitation occurs, the necessary time lapse between measurements increases substantially.Moisture characteristics and capillary conductivities varied greatly from site to site and from soil horizon to soil horizon. For the entire measurement period the water lost by deep seepage was less than 10% of that lost by evapotranspiration.

An integrator compatible with analog input type digital data loggers is described. The device uses the voltage-to-frequency integration technique, and a digital counter and digital-to-analog converter to provide an accurate analog output of the time integral. The integrator is designed so that it can be automatically reset by the data logger during each data scan following recording of the integral value.

A numerical method is described by which the instantaneous evaporation rate from bare soil, regardless of its wetness, can be estimated from standard weather data and the physical characteristics of the soil profile. It is used to calculate potential evaporation from a surface energy balance without the commonly used approximations. The results show that the soil heat flux and the soil surface emittance may vary enough with soil water content so as to make the concept of potential evaporation ambiguous. From a practical viewpoint, however, these differences in the evaporation rate from a wet surface are not large enough to invalidate the simpler combination or energy balance formulas, in which the surface albedo and roughness are the only non-climatic parameters.The method is particularly useful in simulating the evaporation process from drying surfaces. The results support the existence of separate potential and falling rate stages of evaporation, but not that of a third, constant rate stage. A rapid increase of the diurnal amplitude of the surface temperature is the clearest indicator of the transition between the two stages.

Results of eddy correlation measurements of exchange coefficients for sensible heat (KH) and water vapor (KW) made over alfalfa under conditions of regional sensible heat advection are presented. KH was found to be generally greater than KW under moderate to strong advection. The inequality of KH and KW under advective conditions was inferred by Blad and Rosenberg (1974) and has been demonstrated with other micrometeorological methods by Verma et al. (1978). The inequality of KH and KW is particularly significant with respect to the Bowen ratio-energy balance method of evapotranspiration estimation, which is based on the assumption that KH = KW. Use of this assumption results in consistent underestimation of evapotranspiration under advective conditions.

Results of evapotranspiration research on the energy balance of corn and sorghum plants and their respective leaf and air temperatures are presented. The difference between crop canopy and air temperatures provides the gradient for the flow of sensible heat toward or away from a vegetal surface. Sensible heat flux toward the canopy is an important source of energy for evapotranspiration.The energy balance of corn (Zea mays L. cv. Prairie Valley) and of sorghum (Sorghum bicolor L. cv. SG-40 GBR) was determined during the 1978 growing season. Actual evapotranspiration (ET, determined using weighing lysimeters), net radiation (Rn), and soil heat flux (S) were measured. Other data collected were soil water content (0), stomatal resistance (Rs), leaf-water potential (ψL), and leaf temperatures (TAandTL).Generally, ET of well-watered crops with full cover nearly equalled Rn. But on days when TA exceeded 33°C, sensible heat flux was toward the cooler canopy, resulting in ET greater than Rn. Ratios of ET/(Rn—S) were as great as 1.23 and 1.44 for corn and sorghum, respectively. When more than 35% of available soil moisture had been depleted, the leaves lost turgor, stomatal resistance increased, and leaf temperatures rose above air temperatures because of reduced transpiration rate. Leaf—air temperature difference appeared to be a useful indicator of plant stress.

The aeromechanical behavior of three Douglas-fir is described, using wind shear force as the input variable and tree deflection as the output variable. Variance spectra maxima of wind-produced tree deflection were always higher in frequency than the incident shear force. A compliance transfer relation developed from these variables indicated a marked tendency for vibrations to build up as turbulent energy in the wind increased. This analytical form is proposed as a method for examining the involvement of mechanical and aerodynamic factors coupling the tree to the wind.

Na, Cl, SO4, and K were determined in near-ground aerosols on a traverse normal to the coast up to a distance of 6 km from the shore at Caesarea, Israel. Statistically significant power or exponential functions can both express the rate of decrease in concentration with distance from the shore. The larger droplets which were deposited mainly up to 1,500 m were essentially of sea water composition with regard to the major ions. The remaining smaller particles had a relatively larger excess of SO4.The stormier wind regime during the winter season produced a larger amount of sea spray which was not carried so far inland as the dry season spray. The behaviour of SO4 differs from that of Cl and Na. Local topography has a considerable influence on the distribution and deposition of the airborne salts. The frequency distribution of concentrations at any one station tends to be lognormal.

A quantitative scale of growth (Corn Leaf Number Index or C.L.N.I.) based upon leaf emergence rates was developed from observations of corn (Zea mays L.) in controlled environment chambers and tested in the field. C.L.N.I. measurements accurate enough to quantify growth on a daily basis can be made using a handheld ruler. Seasonal growth (G) as measured using the C.L.N.I. was found to be an approximately linear function of time. The slope () between any two growth measurements represents the growth rate. A series of growth chamber experiments demonstrated that the rates are linearly dependent upon diurnal temperature range and photoperiod, and are related to mean temperature by a third order polynomial. The regression model formulated from these relations was correlated with growth in the field but predicted too slow a growth. The difference between observed and calculated growth is attributed to the square-wave temperature pattern used in the controlled environment chamber and differences between thermometer shelter and canopy/soil temperatures.A model that did not incorporate photoperiod was found to produce results virtually identical to those from the complete model. This suggests that growth rates are only weakly influenced by differences in daylength.

A saturation deficit index (S.D.I.) is defined for the purpose of assessing the regional weather-induced mortality of T. erytreae in southern Africa. The S.D.I. is based on a regression curve which reliably depicts the combined effects of temperature and humidity on the mortality of eggs and first instar nymphs in the field. Series of S.D.I. values are computed for eight representative weather stations, for periods ranging from three to twelve years. Consideration of the accumulated effect, the duration and also the seasonal distribution of near lethal S.D.I. values, clearly explains the known geographic distribution of T. erytreae. Despite some incompleteness of data, 30-year series of annual accumulated values of the S.D.I. in a defined lethal range are estimated for two areas. These values throw light on past outbreaks of the insect itself and of greening, the virus disease transmitted by T. erytreae.

The methods of Blaney-Criddle and Thornthwaite were used to estimate evapo-transpiration and the estimates were compared with similar estimates provided by the Doorenbos and Pruitt method as well as with the measured values of Class A pan evaporation. It was concluded that Thornthwaite underestimates under the arid conditions. Blaney-Criddle method, though developed in arid areas and can provide good estimates of the mean values in an arid area, has an insufficient sensitivity to the inter-annual variation of the evaporative demand of the atmosphere. The development of a vast irrigation project in a previously barren arid land allowed the examination of the effect of the release of latent heat in the estimates of evapotranspiration.

The adoption of a farming system is successful only if it is ecologically viable and economically profitable. Standard climatic classifications are of little use in delineating ecological limits of various crops. Several procedures are cited to illustrate the complexity of the problem. Crop yields are determined by both the photosynthetic process and the phenological development of a crop and can be related to the energy budget and water balance. Several formulas for estimating maximum crop yields are discussed. In general, crop response to fertilizer application is reduced in areas of low climatic yield potential.

Information useful in agricultural planning may be obtained from rainfall data and the fitting of models provides a powerful method of analysing such data. The models used are briefly described, examples being given for data from Hyderabad, India. Results of interest may be obtained from the model using recurrence relations or simulation. The two techniques are compared by producing results on dry spells, the start of the rains and water balances.

The famous East Africa Groundnut (Peanut) Scheme of the late 1940's is cited as an example of a costly miscalculation of land use that might have been prevented by climatic assessment. At the site of this scheme, in what is now central Tanzania, water is the most restrictive factor determining natural environment. Adequate soil moisture, both near the surface and in the deeper root zone, is essential for groundnut production. Soil moisture data derived for Dodoma for 30 years show that in the period mid-January to mid-April the probability that the topsoil will remain dry for at least 16 consecutive days is 57%, and drops only to 37% for 21 or more consecutive days — even though this is the “rainy season”. The analysis further indicates that the elimination of the most serious periods of moisture shortage would require irrigation, at least once a year, in about 75% of the years. Moisture supplies are optimum for December through April in only about 3% of the years in both Dodoma and Kongwa.For comparison, the same type of climatological analysis for Tifton, Georgia and Norfolk, Virginia, in the major peanut producing areas of the eastern coastal plains region of the U.S.A., indicates that 30 years produced no occurrences of dry topsoil for as long as 16 consecutive days. Also, the climate there is so suitable for growing peanuts that irrigation in only 10% of the years would practically eliminate all except the minor periods of water shortage hazard. The condition in Tifton is slightly less favorable than in Norfolk. However, even there, optimum moisture conditions were not high during the critical months.

A specialized weather service tailored to the needs of agriculture in nine areas of the United States is being provided by the Weather Bureau. The four-phase program of (a) data collection, (b) forecasting, (c) research and (d) communications is carried on by the Weather Bureau with the cooperation of agriculture-related business, agricultural experiment stations and the Agricultural Extension Service. The nine areas of the country involved in the specialized weather service program, shown in Fig. 1, are: (1) New Jersey. (2) An area including northern Virginia, northeastern West Virginia, western and central Maryland and south-central Pennsylvania. (3) South Carolina. (4) Tri-State area which includes southern Georgia, southeast Alabama and northwest Florida. (S) Mid-South which includes northern Mississippi, western Tennessee, Arkansas, Louisiana delta, and the bootheel of Missouri. (6) Lower Rio Grande Valley of Texas. (7) Western Lower Michigan. (8) Southern Idaho. (9) Oregon.

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.

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.

Coffee accounts for one half of the total exports and more than 70% of the total agricultural exports of Colombia. Agroclimatic analyses are being made as part of a program to develop the potential for other products so as to decrease the national economy's heavy dependence on a single crop. Tea is a crop of interest. A computer-statistical procedure of paired comparison and regression-correlation techniques were used to compare climatic data from 54 locations in Colombia with data from each of 11 tropical tea growing regions of the world. Five Colombian locations were found to be climatically analogous with the tea growing region in the Cameron Highlands of Malaysia.

A new water balance method for agroclimatological purposes is presented, using: (a) evapotranspiration determined by Penman's method (1948) (adapted to Brazilian climatic conditions), multiplied by a crop factor as function of the stage of development of the crop; (b) the 80% probability value of monthly rainfall (dependable precipitation) according to the incomplete gamma function; and (c) rate of reduction of evapotranspiration below the potential caused by low soil moisture restrictions according to Thornthwaite-Mather (1957) concept.Water deficiencies and surplus for the wheat-soybean rotation crop were determined and mapped for the Great South Region of Brazil, as an exemplification for the new method.

Agricultural services are considered from management and economic perspectives. Services are identified and yield forecasts used to initiate debate about the data requirement needed to provide such service. From a wider perspective, it seems that the meteorological information component is small and instrumental precision is unnecessary. Some economic and policy issues are raised for discussion.