The effects of warm water during vegetative growth on cold tolerance for sterility induced by low temperatures at the booting stage were investigated in a field experiment. In an experiment in Aomori Pref. and Miyagi Pref., four rice cultivars were grown under two water temperature conditions (control and warmer than control by 2.2 or 3.7°C) during vegetative growth over a short period of time (21 days after transplanting for 21-27 days), and tested for their cold tolerance. Warm water promoted the heading date up to four days and it did not affect the number of spikelets. Cool-water treatment induced spikelet sterility in the range of 84-95% in Aomori Pref. and 54-90% in Miyagi Pref., while warm water applied during vegetative growth decreased the spikelet sterility of all cultivars and in both locations by 2-11 percentage points in Aomori Pref. and by 2-25 percentage points in Miyagi Pref. The results indicate that applying warm water during vegetative growth over short periods of time can improve cold tolerance in rice in the field.

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Yield losses caused by low temperatures at the booting stage are a critical factor for producing rice in northern Japan. There is little information available on the spatial distribution of temperature at the booting stage that takes into account rice growth stages. The present study analyzed the spatial distribution of air temperatures not only at (1) the critical stage of the booting stage, but also at the two other stages that affect cold tolerance of (2) the early reproductive growth stage and (3) the late vegetative growth stage to produce a risk map for cold damage to rice in the Tohoku region of Japan. Regions that have lower temperatures at the booting stage are located to the north and east sides of Tohoku. Regression analysis using residual with temperature data at the booting stage and other two stages, respectively, highlighted the regions that carry a higher risk for suffering cold damage due to low temperatures at the booting stage; lower temperatures at the early reproductive growth stage and also at the late vegetative growth stage under temperatures identical to the booting stage. To reduce the risk for cold damage, manipulation of the cropping schedule is a powerful option. Delaying the heading date by one week can increase the critical temperature at the booting stage and also at the other two stages by up to 1°C.

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Climate change often causes unexpected problems for agricultural production. In the Tokachi region of Hokkaido, northern Japan, soil-frost depth has been decreasing since the late 1980’s, facilitating the winter survival of small potato (Solanum tuberosum L.) tubers that remained unharvested and emerge as weeds in the following cropping season. Soil frost control by snow plowing (yukiwari), which removes snow cover and allows deep soil frost to kill the potato tubers, is spreading as a practical countermeasure. However, the technique has largely been empirically based, and the level of expertise varies among farmers. We present the final limit for yukiwari based on a numerical model of soil temperature for Tokachi and other potato-producing regions in Hokkaido, with annual fluctuations taken into account. An average of 60800 unharvested potato tubers was present per hectare, mostly distributed in the top soil to a depth of 0.15 m. As the survival probability of potato tubers was almost zero where the maximum soil-frost depth reached deeper than 0.30 m, the objective frost depth was set to 0.30 m. In the Tokachi region, soil-frost depth reached 0.30 m by yukiwari in most areas except Hiroo on the coast in the southernmost area. The final limit of yukiwari in a 30-year return period was estimated to be around 20 February in mountainous areas in the north and from late January to early February in other areas. For other potato-producing zones in Hokkaido, soil froze to 0.30 m deep by yukiwari in the inland areas from the Pacific Ocean, the Sea of Okhotsk, and the Sea of Japan, but soil frost was not as deep in the southwestern part of Hokkaido. The numerical model for estimating soil temperature facilitates the decision on the work schedule of the practice of yukiwari in order that the damage from volunteer potatoes may be minimized.

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In order to precisely evaluate the snowmelt energy at our flux observation site located in an apple orchard and thereby contribute to reliable flux observation, we derived an equation on the basis of the energy budget theory for predicting the daily snowmelt energy at the snow surface. For practical use at farmers’ fruit orchards, we also derived some simpler prediction equations in which the number of meteorological elements used as explanatory variables was restricted. The optimum values of the parameters for these equations were then determined by using the data for the snow weight, monitored near the flux observation site. The accuracy of these equations was also examined.
Using a theoretical equation in which five meteorological elements (daily integrated solar radiation, daily wind run, daily mean snow albedo, air temperature, and specific humidity deficit) and daily integrated clear-sky solar radiation were used as explanatory variables, we could predict the daily snowmelt energy with a root mean square error (RMSE) of 0.84 MJ•m^-2. The values of the bulk transfer coefficients for sensible and latent heat, estimated on the basis of the optimum values of the parameters for the prediction equation, were slightly smaller than but fairly agreed with those in previous studies. The prediction accuracy hardly decreased in the case of simpler equations in which wind run, specific humidity deficit, and clear-sky radiation were not used as explanatory variables. However, the accuracy decreased considerably when solar radiation or snow albedo was excluded. When the daily snowmelt energy was predicted on the basis of only the mean air temperature, RMSE increased up to 1.34 MJ•m^-2. For farmers’ fruit orchards, it seemed desirable to predict the daily snowmelt energy on the basis of the integrated solar radiation in addition to the mean air temperature.

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The concept of developmental index (DVI) is used to estimate crop phenology from climatic conditions. The DVI model comprises a summation of polynomial expressions describing the developmental rates (DVR) that are individually functions of climatic parameters such as daily mean air temperature and photoperiod. Using the defined polynomial DVR functions, the DVI model can be solved using linear algebra by finding the linear least squares solutions. This method is unique and has high reliability. Using the results of field experiments, both the DVR of rice plants for period from emergence to heading and the DVR of cabbage from planting to harvest were estimated. These results are consistent with cultivation guides. These findings can be used to develop ways to analyze phenological data based on climatic parameters.

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The effect of soil-water content θ in dust-source regions on the Asian dust event was examined using the AMSR-E soil-water product from 2003 to 2011. Soil water content θ ranging from 100 to 150 (g/cm³×10³) has the highest correlation with Asian dust events in dust-source regions and over Japan. The NDVI-θ relationship was proposed to determine the effect of surface conditions on the Asian dust event. Actual relationships in 2005 (2006), when the Asian dust events were less frequent (more frequent), were far from the assumption. It can be considered that there are some issues in using the AMSR-E soil-water product when the Normalized Difference Vegetation Index (NDVI) is high because of difficulties in monitoring the soil-water content itself independent of the vegetation structure.

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We used a portable spectroradiometer to measure solar photon flux densities in the spectral regions of 400-500 nm (B), 500-600 nm (G), 600-700 nm (R), and 700-800 nm (FR) throughout one year at Ayabe city, Kyoto (135°16′E, 35°17′N). The relationships between sun elevation and the respective ratios of the R, G, and B components to photosynthetic photon flux density (PPFD: 400-700 nm) were relatively constant when the sun elevation was higher than 20°. Under these conditions, the G/PPFD ratio was almost invariant at 0.35-0.36. The R/PPFD and B/PPFD ratios averaged 0.35 and 0.30, respectively, and varied within a wider range of ±0.05 that depended on the weather. The relationship between sun elevation and the R/FR ratio was relatively constant when the sun elevation was higher than 15°, under which conditions it was almost always within the range of 1.0-1.2 regardless of the weather.

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The density correction, which is widely used in eddy covariance calculations to compensate for density fluctuations arising from heat and water vapor transfer in order to determine the surface (ecosystem) flux of a gaseous constituent such as CO₂, was revisited based on its historical background and recent discussions. Firstly, the derivations previously published were categorized into three by their prerequisites: (1) mean vertical wind (the conventional Webb-Pearman-Leuning derivation), (2) expansion/compression of air parcel, and (3) the mass balance and continuity of both the target constituent and dry air. All the categories of derivation implicitly or explicitly assume no sink/source of dry air at the surface and arrive at the same equation for the correction, which relates an eddy covariance flux from mass or molar density measurements to the corresponding mixing ratio flux. Secondly, we examined the underlying assumptions or concepts used in those derivations, especially mean vertical wind to compensate for the turbulent flux of dry air. Thirdly, practical aspects of the density correction were considered for both open- and closed-path eddy covariance, and critical problems such as sensor heating of an open-path IRGA were also discussed. Our conclusions were that the density correction is theoretically sound as long as there is no sink/source of dry air at the surface but difficult to apply correctly because, in disagreement with its theory, different sensors with different time constants, which are usually positioned separately, are used in actual eddy covariance systems to calculate the correction terms. Closed-path systems are advantageous over open-path systems under such a condition that the surface flux is expected to be an order smaller in magnitude than the correction terms, due to their ability to point-by-point convert density measurements to the mixing ratios using the temperature, pressure and water vapor density that are simultaneously measured in the gas analyzer.

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