AbstractThornthwaite’s climatic division system based on potential evapotranspiration (PE) and climatic water

balance is well known. In this study, a climatic division of the rice-producing district in the Chubu region of Japan is proposed based on Thornthwaite’s system. The PE and climatic water balance were calculated using 1-km mesh units on mesh climate charts derived from data acquired by the AMeDAS system. From published sources, data was gathered on the rice yields of all the municipalities in the Chubu region from 1979 to 2001, and related to the PE and climatic water balance data calculated using Thornthwaite’s method. It emerged that the rice yields were optimal for an August mean PE of 140 mm and enhanced in dry conditions. Moreover, to make the climatic division, a cluster analysis was performed for each municipality using PE, the moisture index (degree of humidity derived from a water balance calculation), and rice yield data. Consequently, the Chubu region was divided into 5 categories: Wetland, Nagano basin, Mountain, Hokuriku plain, and Tokai plain. This division expresses the basic climatic features that are important to rice production. Our climatic division differs from others in that our division is based both on climatic elements, which are an origin, and on rice yields, which are agricultural outputs.

Key words: Chubu region, Climatic division, Climatic water balance, Rice, Thornthwaite’s climate classification.

Climatic water balance and climatic division of rice producing districts in the Chubu region, Japan

Masana YOKOYA*, and Takayoshi AOYAMA***Shimonoseki Junior College, Shimonoseki, Yamaguchi, 750–8508, Japan

**Department of Geography, Senshu University, Kawasaki, Kanagawa, 214–8580, Japan

Received; March 23, 2009.

Accepted; August 1, 2009.

1. Introduction

Thornthwaite proposed his now well known climate classification system based on potential evapotrans-piration (PE) and climatic water balance in 1948 (Thornthwaite, 1948). The Thornthwaite climate classification system incorporates two viewpoints: the climatic thermal state and the degree of humidity. PE plays the central role and is defined as the hypothetical evapotranspiration that would occur from the surface if soil moisture conditions were sufficient. In addition, he used PE as a comprehensive index indicating the thermal state of the climate. He also proposed the water budget model by comparing the PE with precipitation and derived the climatic water balance from the same; using this balance as an index of humidity. The Thornthwaite climate classification system differs from previous systems that were based on the vegetation

distributions and is referred to as a rational climatic classification (Yazawa, 1989).

If the Thornthwaite climate classification system, with its continental scalings, is applied to Japan, only 2 or 3 divisions result. However, the concept of the climatic classification system and the idea of climatic divisions remain applicable to Japan.

PE and climatic water balance are important, essential climatic elements (Fukui, 1957). However, the relationship between crop productivity and them has not received sufficient attention (Aoyama et al., 1998), and climatic divisions based on the same have never been proposed.

To address this issue, the results of our research concerning the relationship between these climatic elements and rice productivity are presented in the current document. In addition, the possibility of climatic division in the rice-producing districts based on these climatic elements is also considered.

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2. Method

2.1 Study areaThe Chubu region of Japan, with its many rice-grow-

ing areas, contains many variations in topography and climate. For the sake of convenience, 11 prefectures (Niigata, Toyama, Ishikawa, Fukui, Yamanashi, Na-gano, Gifu, Aichi, Shizuoka, Mie, and Shiga) were selected as our study area. This area contains plains, mountainous regions, and basins, as well as eminent rice-growing areas (Fig. 1).2.2 Calculation of PE and the climatic water bal-

ance In Thornthwaite’s climate classification system, PE

is calculated from the accumulated temperature and monthly mean-temperature, making it an index of temperature conditions. Furthermore, in the formula for the surface energy balance, the ratio of latent heat flux will increase under conditions to make PE, making it an index of net radiation.

In this study, the term ‘climatic water balance’ is used to refer to the balance between the inflow of water from precipitation and the outflow of the same by evapotranspiration calculated using the water budget model. In essence, the climatic water balance

Table 1. Results of calculations of monthly PE and climatic water balance using Thornthwaite’s method. This example shows the results for an area within Nagoya city in 2001. In this table, T: monthly mean tempera-ture, I: heat index, Unadj PE: unadjusted potential evapotranspiration, PE: potential evapotranspiration, P: precipitation, P–PE: precipitation minus the potential evapotranspiration, AcWL: accumulated potential water loss (accumulated sum of the negative P–PE values), ST: soil water storage, ∆ST: change in soil moisture, AE: actual evapotranspiration, D: moisture deficit, S: moisture surplus, and MI: moisture index (ST+S–D, proposed by authors).

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year

T(℃) 4.2 5.9 8.8 14.8 20.1 23.6 28.6 27.8 23.9 18.7 11.9 7.2

I 0.77 1.29 2.35 5.17 8.22 10.41 14.02 13.43 10.68 7.37 3.72 1.74 79.17

Unadj PE (mm) 0.2 0.3 0.6 1.6 2.8 3.7 5.1 4.9 3.8 2.5 1.1 0.5

PE (mm) 5 8 19 52 102 134 188 171 117 73 28 13 910

P (mm) 146 48 76 33 145 194 29 377 213 176 51 30 1518

P–PE (mm) 141 40 57 –19 43 60 –159 206 96 103 23 17 608

AcWL (mm) 0 0 0 –19 0 0 –159 0 0 0 0 0

ST (mm) 200 200 200 182 200 200 89 200 200 200 200 200

∆ST (mm) 0 0 0 –18 18 0 –111 111 0 0 0 0

AE (mm) 5 8 19 51 102 134 140 171 117 73 28 13 861

D (mm) 0 0 0 1 0 0 48 0 0 0 0 0 49

S (mm) 141 40 57 0 25 60 0 95 96 103 23 17 657

MI (mm) 341 240 257 181 225 260 41 295 296 303 223 217

Fig. 1. Topography around the study area. Several volcanic ranges running through the Chubu region bestow great variety on the topographical features of the region.

1000m�

100km

500m�

200m�

0�200m

�Nagano

�Kanazawa

�Matsumoto

�Niigata

�Nagoya

�Takayama

�Owase

�Otsu �Shizuoka

�Hamamatsu

�Toyama

�Fukui

�Kofu

�Karuizawa

�Minamiizu

�Nakatsugawa

136�E

138�E

35�N

37�N

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refers to the degree of humidity as determined from climatological considerations.

PE and climatic water balance are calculated using the computation tables in Thornthwaite and Mather (1957). These consist of empirical coefficients that reveal the PE and water balance from temperature and precipitation data. 1-km mesh climate charts were prepared to serve as the raw data for our analysis. These were derived from the data acquired by the automated meteorological data acquisition system (AMeDAS), and include the daily mean temperature and daily precipitation estimated within 1-km mesh units (Seino, 1993). They were provided by the National Institute for Agro-Environmental Science. A 23-year period, from 1979 to 2001, was analyzed and the daily data processed to obtain monthly averages, which were then used to calculate the monthly PE and climatic water balance for 1-km mesh units. Because climatic water balance calculations must be executed sequentially, as an initial value for our analysis, the soil moisture content for January 1979 of 200 mm was used.

The following example demonstrates the 2001 water budget calculation for areas around Nagoya, which is based on the computation table in Thornthwaite and Mather (1957) (Table 1). In this calculation, the thermal state is expressed by the amount of PE, and the humidity by the soil water storage (ST) and the water surplus (S) and deficit (D) (Table 1). The moisture index (MI) is therefore defined to express the degree of humidity as

MI ST S D= + - (1)

In this way, the monthly average values for PE and MI are calculated for a 23 year period.2.3 Relationship between climatic water balance

and rice yieldsA total of 23 years of rice yield data from 757

municipalities were collected in the study area using various statistical notes that were published by the Office of Statistics and the Survey Division in the Ministry of Agriculture and Forestry. Furthermore, the yield per unit of land area in every municipality was also calculated for the same period. In calculating the climatic elements, only mesh units in which rice fields existed were averaged for each municipality. The digital national land numerical information (land use mesh data from the Ministry of Land, Infrastructure, and Transport, Japan, 1997) was used to relate the yield

data to the calculated climatic elements (PE and MI); the yield data was found to correspond to the climatic elements, as explained below.2.4 Climatic division

Since the Thornthwaite climate classification system incorporates both the thermal state and the degree of humidity, the same strategy was applied to the climatic division of the rice-growing areas.

Since monthly averages were calculated for the climatic elements, and August has the biggest influence on rice yields (Murata, 1964), in the analysis, each month’s target was set to the August value, which is the heading and ripening period in the Chubu region.

A cluster analysis was performed to make an objective climatic division, using three parameters: the 23-year mean August PE, the 23-year mean August MI, and the rice yield per unit land area for each municipality. The mean of the data for the municipalities was normalized to have a 0 mean and 1 standard deviation.

3. Results and Discussion

3.1 Distribution of climatic elements and yieldsFig. 2 shows the distribution of the 23-year mean

August PE. The data show that mean PE in August exceeded 120 mm in the plain and basin areas. Conversely, few mountainous areas showed a mean PE exceeding 100 mm in August. This result suggests that the differences in PE are attributable to variable

50 75 100 125 150 175 �mm�

Fig. 2. Distribution of 23-year mean August PE. Amounts of PE are different depending on altitudes. PE exceeds 150 mm in plain areas.

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altitude rather than latitude.Fig. 3 shows the distribution of the 23-year mean

August MI. According to these data, the mean MI values are less than 0 mm in parts of the plain and basin areas, indicating that in August, PE normally exceeds precipitation in these areas. The annual cli-matic water balances indicate that the annual rainfall exceeds annual PE in all areas in the Chubu region. However, when the climatic water balance is calculated on a monthly basis, the month when PE exceeds

precipitation can be determined.Fig. 4 shows the distribution of the 23-year mean

rice yields in each municipality. Only meshes where paddy fields exist are indicated. The yields were high in the basin area of Nagano prefecture and the plain area of Niigata prefecture.3.2 Relationship between climatic water balance

and rice yieldFig. 5 shows the relationships between the 23-year

mean August PE and the 23-year mean rice yields for each of the 757 municipalities. The data indicate a maximum rice yield at a PE of approximately 140 mm, which is equivalent to 23℃ when converting it into temperature. It is well known that an optimum temperature exists for rice growing. Murata (1964) investigated the relationship between rice yields and temperature in 46 prefectures in Japan and reported an optimum mean temperature of 21.2℃ for the period from August through September. In some regions, rice is cultivated at temperatures below the optimal temperature, and above it in others.

Fig. 6 shows that the relationship between the 23-year mean August MI and the 23-year mean rice yield for each municipality is relatively linear. The data indicate that the highest rice yields occur for the lowest MI values, meaning that the mean rice yields increase under dry conditions.

Thus, the rice yields in this region can be explained simply by considering the thermal state and the degree of humidity. It is known that increased rice yields are directly proportional to the magnitude of irradiance

100 150 200 250 300 350 400 �mm�

Fig. 3. Distribution of 23-year mean August MI. The plain and basin areas show small amounts of MI.

350 400 450 500 550 600 650 �kg�10a-1�

Fig. 4. Distribution of 23-year mean yields (per unit land area). Only the meshes where paddy fields exist were indicated.

200

250

300

350

400

450

500

550

600

650

700

100 120 140 160 180

Potential evapotranspiration (mm)

gk( sdleiY

a011-)

0.36 140 +671

Fig. 5. Relationship between 23-year mean August PE and 23-year mean yields. The formula was obtained by the least square method for the data set of maximum values of PE at 100-mm intervals. The maximum yield is obtained at a PE of 140 mm.

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and that the optimal temperature will contribute to maximize the yields (Sugihara and Hanyu, 1980).3.3 Climatic division

Fig. 7 shows the dendrogram obtained from our cluster analysis using Word’s method with Euclidean distances. The distance between groups changes sig-nificantly when the number of groups changes from 5 to 6, indicating that these regions can be divided into at least 5 climatic divisions. The geographical continuity of the divided groups is also important; i.e. municipalities that belong to the same climate group should exhibit regional continuity. The regional continuity of the divided groups is maintained when the number of groups is less than 5.

It should be possible to explain the differences in rice yields between the groups by relying only on the thermal state and the degree of humidity. When the municipalities are divided into so many groups, however, it is difficult to explain the small differences in rice yields between groups by relying on comprehen-sive climatic elements alone. As explained below, it is confirmed that when the number of groups is smaller than 5, the differences in rice yields between groups

can be explained by considering only the thermal state and humidity.

From these results, the conclusion that the study area can be divided into at least 5 climatic groups was made.

Fig. 8 shows the geographic distribution of the

200

250

300

350

400

450

500

550

600

650

700

100 200 300 400 500 600

Moisture Index (mm)

gk( sdleiY

a0 11-)

0.88 +812

Fig. 6. Relationship between 23-year mean August MI and 23-year mean yields. As the MI values decrease, the yields increase. Thus, greater yields are obtained under dry conditions.

Table 2.　Average values of climatic elements, yields, and altitudes in each divided type.Hokuriku Plain

DivisionTokai Plain

DivisionNagano Basin

DivisionMountain Division

Wetland Division

Total

Potential evapotranspiration (mm)Moisture Index (mm)Yield (kg・10a–1)Precipitation (mm)Temperature (℃)Altitude (m)

14718450115325.3127

15818244915926.643

13118655313922.9701

14225245520724.6267

13232441426323.1561

14422047418124.8284

ToyamaKanazawa

OtsuKofu

Fukui

Niigata

Hamamatsu

Nagoya

Matsumoto

Nagano

Shizuoka

Karuizawa

300 250 200 150 100 50 0

Distance

Clu

ster

Den

drog

ram

Owase

Nakatsugawa

Minamiizu

Fig. 7. Result of cluster analysis. A dotted line is added to the figure for convenience. The distance between groups changes significantly when the number of groups changes from 5 to 6.

M. Yokoya et al. :Climatic water balance and climatic division of rice producing districts

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proposed 5 climatic divisions. The boundary lines between these areas are for descriptive purposes only. The 5 divisions are named Hokuriku plain, Tokai plain, Nagano basin, Mountain, and Wetland.

Table 2 shows the mean values for the climatic ele-ments and the rice yields calculated for each group.

The Hokuriku plain division is distributed in the plain areas of the Hokuriku district, which is in the northern part of the Chubu region. This division exhibits relatively high PE and a low MI. Plains in the Hokuriku district are cooler than those in the Tokai district (in the southern part of the Chubu region) and warmer than basin areas with precipitation smaller than the mountainous region.

The Tokai plain division is located in the plain area of the southern part of the Chubu region and exhibits relatively high PE and a low MI. On the Pacific side of Central Japan including Tokai plain, the summers are hot and humid and influenced by the seasonal winds from the southeast. In this region, high temperatures hamper efforts to grow rice.

The Nagano basin division is located in the basin area of Nagano prefecture, which is at a relatively high

altitude (above 500 m). This division produces the maximum rice yield. This climate is characterized by optimum PE and a low MI. The factors characterizing this climate type are high amounts of solar radiation and low humidity.

The Mountain division is distributed throughout the low altitude mountainous areas of the Chubu region. This climate type is characterized by high MI, meaning it receives excessive precipitation and insufficient solar radiation. As a result, the rice yield for this climate type is relatively small.

Finally, the Wetland division exhibits low PE and a high MI, leading to small rice yields. This climate type is scattered over the relatively high-altitude mountain-ous regions and their southern parts. It includes both the coldest area and those with the heaviest rainfall in the Chubu region. In these regions, high amounts of precipitation hamper rice cultivation.

Thus, the relative rice yields for each division are well explained by considering the thermal state and humidity.3.4 Comparison with other climatic divisions

Our climatic divisions are similar to several cli-matic divisions that are well known in Japan (Yoshino, 2003). Furthermore, the geographic distribution of our climatic divisions is similar to the distribution of the agro-climatic index (the climatic productivity index of paddy rice) proposed by Hanyu and Sugihara (1981). The difference between our climatic division and theirs is that ours is based both on climatic elements, which are at the origin of agricultural productivity, and on rice yields, which are a result of agriculture.

4. Conclusion

The concept of climatic division proposed by Thorn-thwaite was applied to the rice-producing areas of the Chubu region in Japan. A comprehensive relationship between rice yields and the climatic elements of PE and water balance was determined.

Considering the results of our cluster analysis, the rice-producing areas of the Chubu region were separated into 5 climatic divisions. The rice yields for each division are well explained by considering the thermal state and the humidity.

Our climatic division system differs from others because the difference in rice yields for each climatic division can be explained by considering the differ-ences in the climatic elements.

Fig. 8. Distribution of municipalities in the pro-posed 5 divisions. Boundary lines are added for descriptive purposes, and the 5 divisions are referred to as the Hokuriku plain, Tokai plain, Nagano basin, Mountain, and Wetland.

Hokuriku Plain Division

Tokai Plain Division

Nagano Basin Division

Mountain Division

Wetland Division

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Acknowledgements

We would like to thank the anonymous referees for their helpful comments. In addition, the authors would like to the thank members of Senshu University for their assistance with the research.

References

Aoyama, T., Hasegawa, N., and Yanase, S., 1998: Climatic environments of fruit growing regions in Tohoku district. Q. J. Geogr., 50, 139–153.

Fukui, E., 1957: The climate of Japan according to the new Thornthwaite classification. Geogr. Rep. Tokyo Metropol. Univ., 1, 103–112.

Hanyu, J., and Sugihara, Y., 1981: Studies on the evaluation of climatic productivity of paddy rice. II. Local variations in the climatic productivity index. J. Agrc. Meteorol., 36, 257–261.

Murata, Y., 1964: On the influence of solar radiation and air temperature upon the local differences in the productivity of paddy rice in Japan. Jpn. J. Crop

Sci., 33, 59–63.Seino, H., 1993: An estimation of meteorological

elements using GIS and AMeDAS data. J. Agric.Meteorol., 48, 379–383.

Sugihara, Y., and Hanyu, J., 1980: Studies on the evalu-ation of climatic productivity of paddy rice. I. An attempt for the evaluation of climatic productivity. J. Agrc. Meteorol., 36, 71–79.

Thornthwaite, C. W., 1948: An approach toward a rational classification of climate. Geogr. Rev., 38, 55–94.

Thornthwaite, C. W., and Mather, J. R., 1957: Instruc-tions and tables for computing potential evapo-transpiration and the water balance. Publications in Climatology (Drexel Institute of Technology), 10, 185–311.

Yazawa, D., 1989: Treatise on climatic regions. (Kiko Chiiki Ronko). Kokon Shoin, Tokyo, pp 198–199.

Yoshino, M., 2003: Climatic division based on a bio-climatology. Glob. Environ. Res., 8, 121–135.

中部地方の気候学的水収支と稲作地帯区分

横家将納 *・青山高義 **

要 約

ソーンスウェイトは可能蒸発散量の概念とそれに基づく気候区分の案出者として知られるが，本研究ではこのソーンスウェイトの方法と気候区分の概念に則り，中部地方の稲作地帯の気候区分を行った。可能蒸発散量と水収支の計算をメッシュ化アメダスデータを用いて 1 kmメッシュ単位で行った。また 1979年から 2001年にかけての中部地方のすべての市町村の稲の反収のデータを作物統計などから収集した。反収と可能蒸発散量などの気候要素との関係を調べたところ，稲の収量は 8月の平均可能蒸発散量がおよそ 140 mm前後で，乾燥の程度が進むほど多収となることがわかった。また，8月の平均可能蒸発散量，8月の水分指数 (乾湿の程度を示す )及び

平均反収のデータを用いて，中部地方の全ての市町村をクラスター分析にかけたところ，少なくとも中部地方の稲作地域は５つの特徴ある気候地域に分けられることがわかった。そしてそれらを北陸平野型，東海平野型，長野盆地型，山地型，湿地型と名づけた。これらの区分は稲作に大きく関わる基本的な気候の特徴を現していると考えられる。我々の気候区分は成因であるところの気候要素と，結果であるところの収量の両方に依拠して行われているので，他の気候区分とも若干異なる分布の形態を示した。キーワード： 稲 ,気候学的水収支 ,気候区分 ,ソーンス

ウェイト ,中部地方

*下関短期大学

**専修大学文学部環境地理学科

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