八代平野のイグサ田（32° 33.9′N, 130° 38.4′E, 標高1 m）において，1999～2000年（2000年期）と2000～2001年（2001年期）の移植期（12月）から収穫期（7月）まで熱収支ボーエン比法により蒸発散量を測定した。蒸発散量（E）の季節変化は，移植期から収穫期にかけて気温と日射量の増加にともない増加する傾向を示した。生育期間中のEの平均値は2000年期，2001年期でそれぞれ2.02, 2.16 mm day-1で，積算値はそれぞれ491, 499 mmであった。イグサ田の蒸発散量について，他の植生に対する相対的な値と有効な推定方法を検討するため，Eの日平均値を基準蒸発散量E0およびポテンシャル蒸発量EPと比較した。E/E0の季節変化は0.46から1.00へとイグサの生育にともない増加する傾向を示した。同様にE/EPの季節変化も0.42から0.84へと増加する傾向を示した。この結果をもとに，E/E0とE/EPをイグサの植物面積指数（PAI）の関数で表すことができた。この関数を用いて推定した蒸発散量のRMSEは推定の時間スケールによって異なり，1日平均の場合で0.5 mm day-1，5日平均の場合で0.2 mm day-1，10日平均の場合で0.1 mm day-1であった。また，E/E0とE/EPはPAI以外にも，イグサの蒸散量の変化を通じて土壌水分，風速および飽差の影響を受けたと推察された。しかしながら，これらの環境要因のみから2000年期と2001年期との間にみられたE/E0およびE/EPの値の違いを説明することは困難であった。2000年期と2001年期ではイグサの茎色が大きく異なっていたことから，イグサの生理的活性の違いがE/E0とE/EPの値に影響を与えたことが示唆された。
コンクリートマルチが地温および土壌水分におよぼす効果について，中国河北省南皮県の中国科学院石家庄農業現代化研究所南皮農業生態実験ステーションにおいてナツメ（Ziziphus jujube Mill.）畑とライシメータを対象にマルチ区と裸地区の比較観測を行った。観測内容は地表面熱収支，表面温度，地中温度および土壌水分の垂直分布である。ライシメータにおける比較試験によれば1998年9月の土壌蒸発量は裸地区の1.74 mm day-1に対しマルチ区では僅か0.04 mm day-1であった。1998年12月から1999年2月にかけての土壌凍結/融解期におけるナツメ畑の表層10 cmの土壌水分はマルチ区では0.030 g g-1増加したのに対し，裸地区では0.032 g g-1減少した。9月の熱収支観測によれば，マルチ区における日中の純放射量は裸地区より少なく，深さ5 cmの土壌水分も少ないにもかかわらず，地中熱伝達量はマルチ区の方が裸地区より多かった。ライシメータ区におけるコンクリートマルチの表面温度は夏季には57℃に達し，裸地地表面温度より約10℃高かった。しかしその傾向はナツメ畑では緩和された。ライシメータ区における深さ5 cmと10 cmの夜間地温は夏季，冬季を通じて裸地区よりおよそ2℃ほど高温であった。
Similarity law is tested over a forest in a complex terrain. For the wind from the valley zero-plane displacement showed reasonable value, about 0.7 of the canopy height. However, for the wind from the ridge, it sometimes became negative. The non-dimensional profiles of wind speed and temperature for the wind from the valley indicated a similar shape as observed over flat terrains; however, for the wind from the ridge it did not obey the law. Wind profiles inside the canopy showed a secondary peak near the forest floor for the wind from the ridge, but this peak was not clear when wind was from the valley. These characteristics including the variance of the turbulence are examined by using the 2nd-order turbulence closure model for the horizontally homogeneous field. The results indicate that the flow from the valley in this terrain is similar to that for homogenous flat forests.
This study investigated the potential of thermal remote sensing for estimating ecosystem surface CO2 flux. Ecosystem surface CO2 flux was measured by the eddy covariance method for more than three years, in conjunction with thermal and optical remote sensing measurements as well as micrometeorological, soil, and plant measurements. The soil was Andisol (Hydric Hapludands), a humic volcanic ash soil, which is the major cultivated soil for upland crops in Japan. The soil surface CO2 flux under bare soil conditions was best correlated with the remotely sensed surface temperature, while air temperature was less well correlated and soil temperature and soil water content were poorly correlated. The relationship was well expressed by an exponential Q10 function (r2=0.70, RMSE=0.095). The value of Q10 and the threshold temperature at which the CO2 flux approached zero were estimated to be 1.31 and 10.0°C, respectively. Results suggested that the soil surface temperature had the dominant effect on the microbial respiration as well as on the physical processes determining the CO2 gas transfer at the soil-atmosphere interface. Remotely sensed surface temperature will provide useful information for investigation of CO2 transfer processes near the soil surface, as well as for quantitative assessment of ecosystem surface CO2 flux.
The study investigated diurnal and seasonal dynamics of evapotranspiration (ET) and transpiration (Tr) in a soybean canopy, as well as the relationships among ET, Tr, solar radiation and remotely sensed spectral reflectance. The eddy covariance method (ECM) and stem heat balance method (SHBM) were used for independent measurement of ET and Tr, respectively. Micrometeorological, soil, and spectral reflectance data were acquired for the entire growing season. The in-stantaneous values of canopy-Tr estimated by SHBM and ET by ECM were well synchronized with each other, and both were strongly affected by the solar radiation. The daily values canopy-Tr increased rapidly with increasing leaf area index (LAI), and got closer to the ET even at a low value of LAI such as 1.5—2. The daily values of ET were moderately correlated with global solar radiation (Rs), and more closely with the potential evapotranspiration (ETp) estimated by the “radiation method.” This fact supported the effectiveness of the simple radiation method in estimation of evapotranspiration. The ratio of Tr/ET as well as the ratio of ground heat flux (G) to Rs (G/Rs) was closely related to LAI, and LAI was a key variable in determining the energy partitioning to soil and vegetation. It was clearly shown that a remotely sensed vegetation index such as SAVI (soil adjusted vegetation index) was effective for estimating LAI, and further useful for directly estimating energy partitioning to soil and vegetation. The G and Tr/ET were both well estimated by the vegetation index. It was concluded that the combination of a simple radiation method with remotely sensed information can provide useful information on energy partitioning and Tr/ET in vegetation canopies.
Parameters used for a three-layered soil model are determined for the purpose of understanding the soil water content and heat balance on the Loess Plateau, China. The soil types on the Loess Plateau are divided into heavy loam, medium loam, light loam, sandy loam and sand. The soil water potential and the surface albedo are parameterized as the function of soil water content. The diffusion distance of water vapor that decides the evaporation efficiency can be simulated by using the saturated volumetric soil water content, field capacity of soil water content, wilting point and maximum value of the diffusion distance, even if the soil types are different. The model calculation is verified by comparing the observed results. The calculated results of the soil water content and evaporation coincide with the values observed by gravimetric sampling and lysimeter. According to the results observed by the column experiment, the model calculation represents the qualitative difference of evaporation due to the soil texture. Seasonal variation of the soil water content is simulated by using the actual meteorological data for the Loess Plateau. The soil on the Loess Plateau was generally superior from the viewpoint of the maintenance ability of moisture. It is found that the fractional extractable water calculated by the soil water content can be an index to classify the present state of vegetation cover such as forest, shrub, grassland or desert.
Water temperature (Tw) is known to have an effect on the developmental rate (DVR) of paddy rice. However, Tw is not employed as input data in most developmental models. Moreover, DVR is often modeled as a function of air temperature (Ta) alone for northern Japan. The purpose of this study is to determine the reason why accurate prediction using models without consideration of Tw is possible and to determine whether the models are valid even for cool years. The results of the first experiment showed that soil temperature (Ts) affects DVR (Ts at a depth of 1 cm was measured instead of Tw in this study). In the second experiment, pot containers with equally raised rice seedlings were buried in nine paddy fields in the three prefectures of Aomori, Iwate and Miyagi. Rice growth, Ta and Ts in all of the fields were monitored. The following results were obtained. (1) A correlation exists between Ta and Ts. (2) Parameters for the five developmental models were determined. Accuracy of the simple EAAT-model, in which only air temperature is employed, is as good as that of other models. This is because the correlation between Ta and Ts is incorporated in the EAAT-model and Ts is implicitly estimated from Ta. (3) The correlations between Ta and Ts in cool years with short periods of sunshine and in other years are not the same. This difference caused larger error in prediction in cool years such as 1993. In conclusion, the EAAT-model can be used for northern Japan for years with normal weather conditions but not cool years with short periods of sunshine. Some method for adjusting the prediction by an existing model should be developed.