Rain-free Days Research Articles

Estimating one-time-of-day meteorological data from standard daily data as inputs to thermal remote sensing based energy balance models

Air temperature, atmospheric water vapor (e.g. relative humidity or vapor pressure), solar radiation and wind speed are required inputs when remotely sensed thermal observations are used with resistance energy balance models (REBM) to estimate specific time-of-day components of the energy balance at the time of remotely sensed data acquisition. Times of interest include the afternoon, when Advanced Very High Resolution Radiometer (AVHRR) data are acquired, and mid-morning, when LANDSAT Thematic Mapper (TM) data are acquired. Using data from sites in Australia and China, it has been shown how: (1) air temperature ( T a) at these times can be estimated effectively using the model of Parton and Logan [Parton, W.J., Logan, J.A., 1981. Agric. For. Meteorol. 23, 205–216]. This model only uses daily maximum ( T x) and minimum ( T n) air temperatures as inputs; (2) to calculate relative humidity ( h), dew point has often been equated to T n. An improvement due to Kimball et al.[ Kimball, J.S., Running, S.W., Nemani, R.R., 1997. Agric. For. Meteorol. 85, 87–98] reduces the bias in the consequent estimates of h, but the root mean squared deviation (RMSD) about the mean does not reduce. Here, the estimate is improved by converting h to vapor pressure ( e a) and linearly interpolating the e a between the times of sunrise for consecutive days; (3) shortwave solar radiation ( R s) can be effectively estimated by modifying the model of Hungerford et al. [Hungerford, R.D., Nemani, R.R., Running, S.W., Coughlin, J.C., 1989. INT-414, USDA Forest Service] and locally calibrating the Bristow and Campbell [Bristow, K.L., Campbell, G.S., 1984. Agric. For. Meteorol. 31, 159–166] model which estimates the daily total atmospheric transmittance as a function of a rain modulated difference between T x and T n. On clear, rain free days, when good quality remotely sensed data are most likely be acquired, the estimates of R s using both models were within 13% of observations; (4) for wind speed it was found that the sensitivity to using default assumptions such as locally set average was greater for potential evapotranspiration ( λE p) than for actual evapotranspiration ( λE a), which affects the resulting moisture availability (the ratio of λE a to λE p). The variations in λE a and net radiation ( R n) estimates, introduced by applying these interpolation methods to provide specific time-of-day meteorological inputs to the REBM, were examined for a site in China. When estimating λE a at AVHRR times on days when no rain fell, the RMSD/Mean of measurement was 27% when all meteorological values were estimated and remained 27% when all values were known. Estimating R n for the same times and conditions the RMSD/Mean of measurement was 15% when all meteorological values were estimated. This value reduced to 9% when all values were known. The results of this investigation provide support for the approach of estimating meteorological variables from daily air temperature extremes and daily rainfall at the times when clear remotely sensed data are acquired.

Effects of altering water temperature on leaf wetness in paddy rice crops

Leaf wetness on rain-free days is caused by dew formation and guttation. The effect of paddy water temperature on these two processes in rice crops was investigated by a field experiment in the tropics and a phytotron experiment, respectively. A micrometeorological model was used to analyze the dew formation data. The experimental data indicated an optimum paddy water temperature for rice leaf guttation of about 30°C. The simulation analysis results showed that dew amount increases with water temperature and is highly sensitive to the stratification condition above the canopy. Under a stable stratification condition, the dew amount increased about 4 times faster with water temperature than under neutral conditions. However, when the water temperature was high enough to generate an unstable stratification condition, the effect of water temperature on dew formation in the rice canopy almost disappeared. The critical water temperature for onset of dew depends not only on the stratification condition, but also on the nocturnal net radiation and vapour pressure deficit. A comparison of the measurements of guttation water with the total amount of water on leaves in the middle of the rice canopy showed that the guttation water amount was at least half of it. Therefore, guttation, as a contributor to leaf wetness in paddy rice crops, is of similar importance to dew formation.

519 The Negative Impact of Sevin XLR Plus Thinner Use on Apple Fruit Quality under Certain Environmental Conditions

The use of Sevin XLR Plus in chemical thinning applications at or shortly after petal fall in 1998 resulted in serious negative fruit quality effect, especially on `Red Delicious'. This negative fruit quality impact was worse in certain Henderson Co., N.C., orchards. Fruit diameter was reduced by 9%, fruit weight was reduced by 22%, misshapen fruit increased by 30%, eliminator fruit (<2.25″) increased by 102%, severely stunted fruit (<2.0″) increased by 317%. Surface russetting increased by 160% and seed numbers per fruit dropped by 27%. These are “never before seen” fruit quality problems for North Caronlina, which appear to be related to the unusual environmental conditions between full bloom and fruit thinning. Comparing the 30-year weather records, Apr. 1998 was the wettest April on record with twice the average rainfall. There were only two rain free days between bloom and petal fall, and only 9 of the 30 days following bloom were rain-free. Daily lows were at or above while daily highs were below the 30-year average, resulting in an extremely small daily temperature fluctuation. The departure from normal for average daily temperature was highly negative from partial petal fall until 2 weeks post petal fall. Daily relative humidity values for daily minimum, average and maximum were consistently and substantially above 30-year average values. It appears that these environmental extremes with Sevin XLR Plus use has an impact on fruit quality. In one `Golden Delicious' orchard where Sevin formulations could be compared side by side, fruit russetting jumped from 5.3% with 50 WP to 36.2% with the XLR Plus formulation.

Dispersal of Passalora personata conidia from groundnut by wind and rain

In experiments where groundnut plants with sporulating late leaf spot lesions were exposed to increasing ‘gusty’ and ‘steady’ wind speeds, conidia were more effectively removed by gusty winds than by steady winds of the same mean wind speed. Rain drops dislodged conidia more efficiently than wint, with large drops dispersing conidia more effectively than small ones. No conidia were caught in still air. After most of the conidia had been removed by exposure to high wind speeds, rain was found to remove additional conidia. When plants were exposed to continuous wind or rain, most of the conidia were dispersed during the first few minutes, and the concentration of conidia then declined exponentially with time. From field observations on rain-free days, airborne concentrations of conidia tended to increase as the wind speed increased during the day and with a further increase in wind speed, concentrations decreased. On rainy days, the onset of the first shower coincided with a phenomenal increase in the airborne concentrations of conidia.

Water use by Eucalyptus tereticornis stands of differing density in southern India.

We studied water use by Eucalyptus tereticornis Sm. in two plantations, differing in tree density (1800 stems ha(-1) at Site I and 1090 stems ha(-1) at Site II), in different years. At both sites, stomatal conductance, predawn and midday water potentials and microclimate were measured and used to estimate hourly transpiration by the Penman-Monteith equation. Growth in girth was also measured. Stomatal conductance was closely correlated with atmospheric vapor pressure deficit (D); however, stomata did not close completely even at high D ( approximately 5.0 kPa). Midday leaf water potentials did not fall below -2.0 MPa during any part of the year at either site. Predawn leaf water potentials were greater than -0.25 MPa during the postmonsoon period, but declined to -0.7 MPa at Site I during the premonsoon period. Transpiration estimates ranged from 0.6 to 1.2 mm h(-1) at Site I and from 0.2 to 0.6 mm h(-1) at Site II. The extrapolated transpiration values for the rain-free days of the year were 1563 mm and 853 mm for Sites I and II, respectively. Growth in girth was negligible during the premonsoon period. Photosynthesis was not affected by the minor water stress that developed during the premonsoon period.

Evaluation of Field Screening Techniques for Resistance to Sorghum Grain Molds

Three methods of promoting grain mold development (overhead sprinkler irrigation on rain-free days, inoculation of panicles with mold-causing fungi, and bagging of panicles) for field screening of sorghum lines for resistance to grain molds were evaluated for two seasons (1982 and 1983) using genotypes susceptible and resistant to grain mold. Threshed grain mold ratings (TGMRs) of these genotypes in overhead sprinkled plots were not significantly different from ratings in unsprinkled plots when rainfall was abundant and frequent from flowering through grain maturity to harvest. In a season of low and infrequent rainfall, however, TGMRs were higher in sprinkled plots. Grain germination was significantly lower in sprinkled than in unsprinkled plots in the season of frequent rainfall. There were no significant differences in TGMRs between inoculated and/or bagged treatments and noninoculated and nonbagged treatment for susceptible genotypes in years of either frequent or infrequent rainfall. Inoculation and bagging increased moldiness in resistant genotypes but not to the threshold level of susceptibility. Screening nursery results (1983–1985) showed that mold resistance screening without inoculation and bagging of panicles appears feasible if overhead sprinkler irrigation is used from flowering to harvest.

Evapotranspiration from pasture: a comparison of lysimeter and Bowen ratio measurements with Priestley-Taylor estimates

Abstract The objectives of this study were to compare evapotranspiration (ET) data obtained using 2 independent measurement techniques, and to compare measured ET with ET estimates obtained using a commonly-applied formula. The measurements were made in a mixed-pasture paddock near Palmerston North. The soil — Manawatu fine sandy loam — contained adequate plant-available water during the study. One set of ET measurements was made using a weighing lysimeter with surface area 2 m2 and a soil depth of 1 m. The lysimeter had a suction-operated drainage system at its base. The other set of ET data was obtained using the Bowen ratio-energy balance technique, with duplicate sets of Bowen ratio apparatus, net radiometers, and soil heat flux sensors. Daily ET estimates were made with the PriestleyTaylor formula, using measured net radiation and air temperature values. The daily ET for wellwatered full-cover pasture measured by the lysimeter on rain-free days was in good agreement with the ET measured using the Bow...

The use of weather forecasts to improve hay-making reliability

An index of weather forecast reliability was derived to assist farmers in making hay. The chances of field drying hay in four days or less were shown to be improved when the first one or more days after cutting were known to have no rain. The critical reliability levels which make two-, three- or four-day forecasts better criteria for scheduling hay cuts than one-day forecasts were demonstrated. This index could promote the optimum use of weather forecasts in scheduling hay cuts. However to apply the index, the actual reliability (chances of success) of forecasting sequences of rain-free days must be known.

Studies on pollen allergy in Delhi. Diurnal periodicity of common allergenic pollen.

The volumetric sampling of the air has been carried out with the Suction Impaction Sampler devised by Tilak & Kulkarni (1970). The pollen counts have been expressed as number per cubic metre volume of the air. Based on the percentage of pollen peaks, diurnal patterns of the total count of all the types, and six dominant allergens have been determined. For these allergens, daily fluctuations have also been recorded. The influence of meteorological factors such as temperature, wind, rainfall, relative humidity and sunshine hours/day on the variation patterns in the concentration of atmospheric pollen has been discussed. It has been shown that 25-30 degrees C temperature, 5-15 km/h wind speed, 60-70% RH, bright sunny and rain-free days are favourable for high pollen catch. These weather conditions might be conducive for the high discharge, dispersal, and deposition of air-borne pollen. Too low or too high temperature, very high wind speed, high humidity, and cloudy and rainy days hinder high pollen catch.