TROPICS Vol. 14 (2) Issued Feburary 28, 2005

Mapping of climatic data in Northeast Thailand: Rainfall

1) 2) 1) 3) 1) Eiji NAWATA , Yoshikatsu NAGATA , Arimichi SASAKI , Kenji IWAMA and Tetsuo SAKURATANI

1) Graduate School of Agriculture, Kyoto University, Kyoto, 606-8502, Japan 2) Graduate School for Creative Cities, Osaka City University, Osaka, 558-8585, Japan 3) School of Environmental Science, The University of Shiga Prefecture, Hikone, Shiga, 522-8533, Japan

ABSTRACT Interpolation of rainfall databases of Northeast Thailand of daily measurements at rain stations over 20 years were mapped with GIS tools. Various maps clearly visualized the characteristics of rainfall in this area. Annual and mid-rainy season rainfall showed a descending trend from the northeastern area to the southwestern area, probably due to the southwest monsoon and mountains in and between Central and Northeast Thailand. Yearly variation in annual rainfall in this area was not particularly high, suggesting that its general image as a drought-prone area may be caused by the undulating topography and predominance of sandy soils in this area, which contribute to its vulnerability to flooding and drought. Both mean rainfall amount per rainy day and the number of rainy days in the rainy season showed large regional and yearly variation, but mean rainfall per rainy day was correlated to annual rainfall, whereas the number of rainy days was not strongly correlated to annual rainfall. Large regional variation in the number of rainy days in the rainy season suggests large regional variations in agricultural productivity in this area. The fact that areas near many of the provincial capitals have a high number of rainy days in the rainy season with small yearly variation may indicate relatively high and stable agricultural productivity in such areas that probably supported the establishment of those cities in early days. The duration, onset and end of the rainy season showed small yearly variation, suggesting that these parameters are not solely responsible for the unstable and erratic rainfall in Northeast Thailand.

Key words: agricultural production, GIS, monsoon, vegetation

INTRODUCTION

Natural ecosystems and agricultural production systems are significantly affected by climate, such as air temperature, radiation, rainfall, wind direction and speed, to name a few. Among these, the influence of rainfall is very large in both natural and agro-ecosystems. Generally, rainfall in the tropics is known to be unstable in both amount and distribution, especially under tropical savanna and monsoon climates (Jackson, 1977). Regional variation is also large, and it is not uncommon that even places nearby each other show very different rainfall characteristics (Yanagisawa and Nawata, 1996). Thus, the influence of rainfall characteristics on natural vegetation may appear in a rather complicated in tropical savanna and monsoon areas. Agricultural production is naturally unsteady under such unstable and erratic rainfall conditions, unless irrigation systems are supplied. Northeast Thailand is notorious for its low agricultural productivity. Here agriculture is mainly practiced under rain-fed conditions. The predominance of sandy soils with low water and nutrient holding capacities and low fertility, along with unstable and erratic rainfall, is considered to cause low agricultural productivity (Fukui, 1996). The MAPNET (Modeling agricultural productivity in Northeast Thailand) project was implemented in order to evaluate land productivity appropriately as a basis for the sustainable development of agriculture in Northeast Thailand, using a simulation model that has been described in detail elsewhere (Kono, 2001). In the process of implementing this project, we constructed climatic databases using the measured data at the meteorological stations and interpolation, for imputing climatic data into the model for the estimation of crop yields. These climatic databases could be mapped with GIS tools, and are useful in analyzing various characteristics of climatic factors in this area. In our previous study, we mapped the databases of air temperature and solar radiation in Northeast Thailand over the past 20 years, and discussed general characteristics of these climatic factors and their implications for natural ecosystems and agriculture in this area (Nawata et al., 2005). Although a clear descending trend in annual rainfall from the northeastern to the southwestern area is known to exist in Northeast Thailand (Takagaki, 1987), detailed analysis of the characteristics of rainfall in this area and their influences on vegetation and agriculture has not yet been conducted. In this study, we mapped the databases of rainfall and analyzed the relationship between rainfall characteristics and natural vegetation and agricultural production. 192 Eiji NAWATA, Yoshikatsu NAGATA, Arimichi SASAKI, Kenji IWAMA and Tetsuo SAKURATANI

MATERIALS AND METHODS

Just like the data on air temperature and sunshine duration described in our previous work (Nawata et al., 2005), rainfall data were collected daily at the Meteorological Department in . There are approximately 300 rain-measuring stations in Northeast Thailand, among which 16 stations measured not only rainfall but also other climatic factors, under the direct control of the Meteorological Department. Among the approximately 300 rain stations, some started measurements in 1951, most of which are directly controlled by the Meteorological Department. However, most of the stations started the measurements only relatively recently. In addition, at some stations the measurements were not always reliable, with lack of data for some periods and inconsistent data. For the construction of a rainfall database over the past 20 years (from 1979 to 1998), 104 rain stations were selected. For the selection of reliable stations, figures showing the accumulated rain amount for each station with five neighboring stations for each year were drawn and the stations showing an extremely different accumulated rainfall pattern from neighboring stations were excluded from the analysis. Fig. 1 shows an example of the figures used to choose stations. In this case, although Station 407002 had less rainfall, the pattern was similar to that of the other stations. Thus, considering the results in the other years this station was accepted. This analysis was carried out for all stations, and 104 stations were retained for their high reliability and sufficient data. The location of the selected stations is shown in Fig. 2, along with the topography and province names of Northeast Thailand.(Fig.1,Fig.2) As with the air temperature and solar radiation study, the whole area of Northeast Thailand was divided into about 6,000 grids, each with the size of 3 minutes (about 5 km square). In grids with rain stations, daily rainfall data measured at the stations were used as daily rainfall of the grid. In grids without rain stations, daily rainfall was interpolated from the measured values of the nearest three stations. The average of the measured values of the nearest three stations, weighted

2000

1500 407009 407501 407002 407006 407005 1000 407008 Accumulated rainf all (mm) Accumulated

500

0 -Jul -Jan -Jun -Feb -Sep -Oct -Dec 1 -Apr -Aug -Mar -Nov 1 1 -May 1 1 1 1 1 1 1 1 Date 1

Fig. 1. Accumulated annual rainfall atPhibun Mangsahan station (407009) and the nearest 5 stations (407501, 407002, 407005, 407006 and 407008) in 1980. Mapping rainfall in Northeast Thailand 193

The River 1

2 5 3 4 6 7 8 9 10 11 13 14 The 12 19 The Mun RRiveriver 15 16 17 18

Fig. 2. Topography and provinces of Northeast Thailand. Elevation > 250 m ASL ○ : Rain stations Province : 1. Nong Khai, 2. Loei, 3. Nong Bua Lamphu, 4. Udon Thani, 5. Sakhon Nakhon, 6. Nakhon Panom, 7. Khon Kaen, 8. , 9. Muk Dahan, 10. Chaiyaphum, 11. Maha Sarakam, 12. , 13. , 14, , 15. Nakhon Rachasima, 16. Buri Ram, 17. Surin, 18. Si Saket and 19. Ubon Rachathani. by the reciprocal of the distance between the grid and the station, was used as the daily rainfall data of the grid. Database construction was carried out using VBA of Microsoft Excel (Microsoft Co. Ltd.), as was done in air temperatures and solar radiation study. Based on the constructed database for daily rainfall, the annual rainfall, average rainfall amount per rainy day and the number of rainy days with more than 0.1 mm of rain in the rainy season were calculated. In addition, the onset and end of the rainy season were determined according to the criterion of Stern et al. (1982) after modification. The criterion of the onset of the rainy season was rainfall of at least 15 mm for 2 continuous days, but if more than 14 non-rain days followed within one month after the onset, the determined onset date was rejected and another date was sought. If it did not rain for 15 days after September 20, the final date of rainfall was defined as the end of rainy season. September 20, as the date defining the end of the rainy season, was chosen because conspicuous dry spells (non-rain period during the rainy season) of longer than 15 days, appear in some years before that date. Coefficient of variation was also calculated for each parameter. Obtained databases on the above parameters were then mapped using VBA of Microsoft Access (Microsoft Co. Ltd.).

RESULTS

Validation of interpolation method Validation of the interpolated data was conducted by comparing the measured values of 16 stations under the direct control of Meteorological Department, with the estimated values from the measured data of near stations from 1989 to 1998. Similar to the air temperature and solar radiation study, the measured values of the nearest station and averages of the measured values of three nearest stations weighted by the reciprocal of the distance between the stations were compared. In addition, considering general characteristics of rainfall in the tropical savannas (e.g. strong intensity for a short period within a limited range), averages of the measured values of three nearest stations weighted by the reciprocal of the square of the distance between the stations were also used for the comparison. When the measured values of the nearest stations (average distance; 19.6 km) were directly used as the estimated values, the accuracy of the interpolation was very low, as shown by the root mean square error (RMSE) and the coefficient value (r) (Table 1). Averages of measured values of three nearest stations weighted by the reciprocal of the distance between stations improved the accuracy significantly, but those weighted by the reciprocal of the square of the distance between stations did not improve the accuracy of weighted three-station 194 Eiji NAWATA, Yoshikatsu NAGATA, Arimichi SASAKI, Kenji IWAMA and Tetsuo SAKURATANI

Table 1. The comparison of interpolation methods to estimate daily rainfall Interpolation Methods Station Name Nearest Station* Nearest 3 Stations* Adjusted 3 Stations*** RMSE (mm) ** r*** RMSE (mm) ** r*** RMSE (mm) ** r*** Nong Khai 12.31 0.16 9.47 0.65 10.27 0.57 Loei 9.60 0.49 9.26 0.54 9.06 0.57 Udon Thani 14.12 ns 10.88 0.45 10.93 0.44 Sakon Nakhon 16.05 ns 12.82 0.53 13.31 0.47 Nakohn Phanom 16.93 0.51 14.14 0.69 14.78 0.66 Khon Kaen 11.77 0.37 10.43 0.57 10.37 0.57 Mukdahan 15.17 ns 11.66 0.55 11.84 0.53 Kosum Phisai 10.15 0.57 8.71 0.71 8.77 0.70 Chaiyaphum 10.41 0.43 9.11 0.61 9.73 0.54 Roi Et 13.95 ns 10.40 0.44 10.53 0.42 15.96 ns 11.61 0.54 11.77 0.52 Nakhon Ratchasima 9.79 ns 8.35 0.51 8.36 0.50 Chok Chai 10.58 ns 8.59 0.52 8.57 0.53 Surin 13.05 ns 9.69 0.64 10.05 0.61 Tha Tum 12.64 0.37 10.63 0.63 11.43 0.54 Nang Rong 10.59 ns 8.87 0.47 9.04 0.44

Average 12.69 0.42 10.29 0.57 10.55 0.54

* Nearest Station: Direct use of the measured value at the nearest station Nearest 3 Stations: Averaged value of nearest 3 stations weighted by the reciprocal of the distance Adjusted 3 Stations: Averaged value of nearest 3 stations weighted by the square of the reciprocal of the distance ** Root mean square error *** all “r” values were highly significant

average. Thus, we adopted averages of measured values of three nearest stations weighted by the reciprocal of the distance between stations.

Maps of rainfall characteristics Fig. 3 shows the average annual rainfall for 20 years in Northeast Thailand. A descending trend from the northeastern region, more than 2,000 mm, to the southwestern region, less than 1,000 mm, was observed, but in the southeastern parts (most of Ubon Ratchatani Province and part of ) annual rainfall was also high. Yearly rainfall variation, indicated by the coefficient of variance (CV), ranged from less than 10 to more than 20 percent (Fig. 4). Regional variation in CV was large, but specific relationships between the amount of annual rainfall and yearly variation were not found.(Fig.3,Fig.4) Fig. 5 shows the average monthly rainfall over 20 years in this area. During the rainy season, from early to mid April to late October or early November, especially in June, July and August, monthly rainfall showed trends similar to those of annual rainfall, i.e. a descending trend from the northeastern to the southwestern parts. In this period of the year, monthly rainfall in the northeastern corner of this area exceeded 350 mm. In September, regional variation in monthly rainfall became smaller, and in most regions, monthly rainfall ranged from 150 to 300 mm. In this month, low annual rainfall regions near the southwestern corner, monthly rainfall was most in the year. In October, monthly rainfall decreased to a level comparable to the beginning of the rainy season, April, although the northeastern parts still showed more monthly rainfall than the other parts. In November, usually the last month of the rainy season and the first month of the dry season, monthly rainfall was less than 40 mm in most of Northeast Thailand, and that of the southern regions was slightly more than the northern regions. In December and January almost no rain was observed, but in February, which is regarded as the middle of the dry season, there was some rainfall. In March, monthly rainfall increased, even though Northeast Thai people do not Mapping rainfall in Northeast Thailand 195

Fig. 3. Average annual rainfall over 20 Fig. 4. CV of annual rainfall over 20 years in years in Northeast Thailand Northeast Thailand : Provincial capital, lines in the : Provincial capital, lines in the map: map: provincial boundaries provincial boundaries

Fig. 5. Average monthly rainfall over 20 years in Northeast Thailand : Provincial capital, lines in the map: provincial boundaries define this month as the start of the rainy season.(Fig.5) Fig. 6 shows the CV of monthly rainfall over the analyzed 20 years. Yearly variation in monthly rainfall was relatively large but much smaller in the rainy season than in the dry season. Regional variation in yearly variation during the rainy season was small, but drier regions, in the southwest, showed higher yearly variation in monthly rainfall, especially in June, July and August. In the first half of the dry season, from November to January, yearly variation in monthly rainfall was very large, whereas in the latter half of the dry season, yearly variation in monthly rainfall was much smaller than that of the early dry season, but much larger than that of the rainy season. During the rainy season, regional difference of CV of monthly rainfall was generally small, although in May the center of the northern parts and the southeastern corner showed relatively 196 Eiji NAWATA, Yoshikatsu NAGATA, Arimichi SASAKI, Kenji IWAMA and Tetsuo SAKURATANI

Fig. 6. CV of monthly rainfall over 20 years in Northeast Thailand : Provincial capital, lines in the map: provincial boundaries large yearly variation. The beginning and end of the rainy season, April and October, respectively, showed medium yearly variation between the rainy and dry seasons.(Fig.6) Fig. 7 shows that the average mean rainfall per rainy day during the rainy season over the analyzed 20 years has large regional variation. Mean rainfall per rainy day ranged from less than 10 mm to more than 20 mm per day. Several areas, such as the northeastern corner, southwestern part of the Phu Phan Mountains, the northern part of Buri Ram Province (southwest of this area), the southern part of Si Saket Province (southeast of this area) and the eastern half of Ubon Rachatani Province (the southeastern corner of this area) showed very high mean rainfall per rainy day, whereas (the northwestern corner), areas surrounding Khon Kaen city and the southwestern parts covering the southeast of Nakorn Rachasima Province and southwest of showed very low mean rainfall per rainy day. Fig. 8 shows yearly variation in mean rainfall per rainy day in Northeast Thailand. Generally, yearly variation for this parameter was large, and ranged between less than 10 and more than 30 percent. The areas with small yearly variation in this parameter were scattered over Northeast Thailand, including the western part of Loei Province (the northwestern corner), northeastern parts, areas near the Phu Phan Mountains, the central part of Ubon Rachatani Province (the southeastern corner) and the southeast of Nakorn Rachasima Province (the southwestern corner). In contrast, areas of high fluctuation of mean rainfall per day were found in eastern and southeastern areas, the southwest of (the central part of this area) and the south of Si Saket Province.(Fig.7,Fig.8) Fig. 9 shows the number of rainy days in the rainy season over the 20 years. Large regional variation was found, with the range from less than 60 days to more than 110 days. The areas with abundant rainy days in the rainy season were found in the northeastern and southeastern parts, and areas near many of the province capitals showed high values for this parameter. The areas with few rainy days in the rainy season were scattered in the central parts of this area and some were found in the southwestern and southeastern parts. Fig. 10 shows the CV of the number of rainy days in the rainy season. This parameter also shows large regional variation. Small yearly variation in this parameter was observed along the Mekong River and other scattered areas, whereas large yearly variation was found in some of the central and southwestern parts of this area. There is a trend of areas with abundant rainy days in the rainy season having small yearly variation and vice versa. Mapping rainfall in Northeast Thailand 197

Fig. 7. Average rainfall amount per rainy day Fig. 8. CV of mean rainfall per day over 20 over 20 years in Northeast Thailand years in Northeast Thailand : Provincial capital, lines in the map: : Provincial capital, lines in the map: provincial boundaries provincial boundaries

Fig. 9. Average number of rainy day in the rainy Fig. 10. CV of the number of rainy day in the season over 20 years in Northeast rainy season over 20 years in Thailand Northeast Thailand : Provincial capital, lines in the map: : Provincial capital, lines in the provincial boundaries map: provincial boundaries

Fig. 11. Average duration of the rainy Fig. 12. CV of the duration of the rainy season season over 20 years in Northeast over 20 years in Northeast Thailand Thailand : Provincial capital, lines in the : Provincial capital, lines in the map: provincial boundaries map: provincial boundaries 198 Eiji NAWATA, Yoshikatsu NAGATA, Arimichi SASAKI, Kenji IWAMA and Tetsuo SAKURATANI

Fig. 13. Average onset of rainy day in the Fig. 14. CV of the onset of the rainy season rainy season over 20years in over 20 years in Northeast Thailand Northeast Thailand : Provincial capital, lines in the : Provincial capital, lines in the map: provincial boundaries map: provincial boundaries

Fig. 15. Average end of the rainy season over Fig. 16. CV of the end of the rainy season over 20 years in Northeast Thailand 20 years in Northeast Thailand : Provincial capital, lines in the : Provincial capital, lines in the map: provincial boundaries map: provincial boundaries

Areas near provincial capitals generally showed small yearly variation in rainy days in the rainy season.(Fig.9,Fig.10) Fig. 11 shows the average duration of the rainy season over the analyzed 20 years with large regional variation, from less than 180 days to more than 210 days. In most of Northeast Thailand, the duration of the rainy season was around 190 days, but there is a trend that the areas with many rainy days in the rainy season (Fig. 9) had a longer rainy season. In north of Amnat Charoen Province and southwest of Si Saket Province, the rainy season was particularly short. Fig. 12 shows the yearly variation in the duration of the rainy season. Regional variation in this parameter was not large, ranging mostly from 7 to 13 percent, although the southeastern corner showed very small yearly variation, whereas most of the central portion of this area, eastern central and southern central parts showed relatively large variation in the rainy season duration.(Fig.11,Fig.12) Fig. 13 shows the average onset of the rainy season. The regional variation in this parameter was small in Northeast Thailand, and the average start time of the rainy season ranged from less than 100 DOY (days of the year, April 10th in a non-leap year) to more than 110 DOY (April 20th in a non-leap year). There was a general trend that the rainy season started earlier in northern half of this area than southern half. Yearly variation in the onset of the rainy season was also not very large in most of this area (Fig. 14), with some exceptions such as some portions of Chaiyaphum, Khon Kaen and Si Saket Provinces.(Fig.13,Fig.14) Fig. 15 shows the average end of the rainy season. Just like the onset, the regional variation in the termination time of the rainy season was not very large in Northeast Thailand, ranging from less than 290 DOY (October 17th in a non-leap year) Mapping rainfall in Northeast Thailand 199 to more than 310 days (November 6th in a non-leap year), but larger than that of the onset. Generally, the rainy season ended earlier in northern half than in southern half in this area, except Loei Province, in which the end of the rainy season was later. The yearly variation in this parameter was small with small regional variation (Fig. 16).(Fig.15,Fig.16)

DISCUSSION

As with our previous work on air temperature and solar radiation (Nawata et al., 2005), mapping of the rainfall database clearly visualized the climate characteristics in Northeast Thailand. As described previously, a clear descending trend from the northeastern to the southeastern parts is known in this area (Takagaki, 1987), and the 20 year average of annual rainfall (Fig. 3) and average monthly rainfall during the period from June to August, the middle of the rainy season (Fig. 5), also supported this trend. In addition to this trend, the areas along the Mekong River, including the southeastern part, showed high annual and mid-rainy season rainfall. This may be because of the mountains in Laos, which lie just to the north, northeast and east of areas along the Mekong River in Northeast Thailand. The humid southwest monsoon area facing the high mountains in Laos may provide a lot of rain in the downwind areas along the Mekong River during the rainy season. In contrast, the southwestern part of Northeast Thailand, where average annual rainfall was less than 1,000 mm over the past 20 years, is situated in the northeast of the mountains between Central and Northeast Thailand. Areas downwind from here are humid southwest monsoon areas providing considerable amounts of rain. However, the dry wind that comes over the mountains provides little amount of rain to the southwestern part of Northeast Thailand in the rainy season. Yearly variation in annual rainfall in this area ranged from less than 10 to more than 20 percent (Fig. 4), and is comparable to that of Central (approximately 18 percent over the same period, average of 20 rain stations calculated from the data obtained at the Meteorological Department) and North Thailand (approximately 19 percent over the same period, average of 20 rain stations calculated as above), and not much different from that of temperate monsoon areas (e.g. approximately 18-20 percent in various places in Japan). Thus, yearly variation in annual rainfall in this area is not particularly large. The general impression that yearly variation in rainfall is large in this area (Fukui, 1996) seems to be exaggerated, probably due to drought- and flood-prone characteristics caused by topography and soil traits. No correlation was found between yearly variation and annual rainfall. In addition, there were no specific trends in regional variation in CV of annual rainfall, although large regional variation was found, highlighting the difficulty in predicting rainfall in this area. In September typical trends of rainfall in the rainy season shown above almost disappeared, and most regions in Northeast Thailand had a considerable amount of rainfall with little regional variation (Fig. 6). Towards the end of the rainy season, the southwest monsoon, which may lead to the descending trend of rainfall from the northeastern to the southwestern parts, is weakened and monthly rainfall becomes more level across the whole area (Kanae et al., 2002). In the dry season, monthly rainfall was zero or very small, but in February and March, towards the end of the dry season, small but significant rainfall was observed. In both natural and agricultural ecosystems, this amount of rain may have a role in mitigating water stress in this period. Especially for the cultivation of upland field crops, such as cassava and sugarcane, this small amount of rainfall may be important, because in Northeast Thailand these crops are usually planted at the end of the rainy season and tolerate severe conditions during the dry season as young seedlings. Mean rainfall amount per rainy day showed large regional and yearly variation (Figs. 7 and 8), and generally was correlated to annual rainfall, but it had larger and more complicated regional variation than annual rainfall. It is not clear what mechanisms are responsible for such complicated regional variation in mean rainfall per rainy day in this area, although the differences in topography may play a role. The number of rainy days in the rainy season (Fig. 9) showed interesting trends. This parameter also showed very large regional variation, but it was not strongly correlated to annual rainfall. In the least annual rainfall regions in this area, the southwestern parts, some areas had a lot of rainy days, whereas some humid regions, such as the southern part of Nakorn Panom province, had a small number of rainy days in the rainy season. For agricultural production, the number of rainy days is more important than mean rainfall amount per rainy day or annual rainfall. Well-distributed rain may reduce the occurrence of drought. Dry spells, which are one of the biggest factors reducing agricultural productivity and stability in Northeast Thailand (Fukui, 1993), may occur with lower frequency in areas with abundant rainy days in the rainy season. In this area, the southwestern part is characterized by low annual rainfall, as stated previously, but agricultural productivity is not always low, partly because of the large number of rainy days in the rainy season along with the predominance of soils 200 Eiji NAWATA, Yoshikatsu NAGATA, Arimichi SASAKI, Kenji IWAMA and Tetsuo SAKURATANI with relatively high clay content, which is exceptional in Northeast Thailand. In addition to this fact, it is interesting to note that areas near many of the provincial capitals had many rainy days in the rainy season with relatively small yearly variation (Figs. 9 and 10). This may not be a coincidence. As most of the provincial capitals in Northeast Thailand are not big cities, the heat-island phenomenon, which sometimes causes more rain in cities than surrounding areas (Thielen & Garian, 1997), may not occur. The provincial capitals generally were established a long time ago. If the present observed rainfall trend has been unchanging since long ago, the areas near the present provincial capitals may have shown higher agricultural productivity and stability than surrounding areas, which may have contributed to the establishment of the city in early days. The reason why this phenomenon exists is not clear, but the subtle difference of topography and general wind direction in the rainy season may be related. The duration of the rainy season showed relatively large regional variation with small yearly variation (Figs. 11 and 12), and was not strongly correlated to annual rainfall, but correlated to the number of rainy days in the rainy season. The regions with abundant rainy days generally showed long duration of rainy season and vice versa. In the regions with long rainy seasons, the rainy season usually started earlier and terminated later; likewise those areas with short rainy seasons the season started later and terminated earlier (Figs. 13 and 15). The northeastern corner of this area is an exception to this general trend: both onset and end of the rainy season were earlier, probably because of higher sensitivity to the change of monsoon wind direction due to its close proximity to influential mountains. Yearly variation in both the onset and end of the rainy season was small in Northeast Thailand (Figs. 14 and 16), contributing to small yearly variation in the duration of the rainy season. Rather stable onset and end of the rainy season, and resultant stable duration of the rainy season indicate that these parameters may not be responsible for the low and unstable agricultural production in Northeast Thailand.

Fig. 17. Vegetation of Northeast Thailand adapted from the vegetation map of the Royal Thai Survey Department.

Fig. 17 shows the natural vegetation of this area. Large agricultural areas prevalent in the central undulating plains are considered to be formerly dry dipterocarp forests, because remaining vegetation there is mostly dipterocarps. Considering the variation in rainfall characteristics in the rainy season there, the determinant factors for the natural vegetation may be the existence of a long dry season, relatively uniform air temperature and solar radiation characteristics and predominance of soils with low water retention. In mountainous areas in Loei province (the northwestern corner), Phu Phan mountains and southeast of Ubon Rachatani province (the southeastern corner), moist upper mixed deciduous forests are dominant. In these areas, although annual rainfall varies, the number of rainy days in the rainy season is consistently high. Humid conditions generated by abundant rainy days may be partly responsible for the predominance of this type of vegetation. In Mapping rainfall in Northeast Thailand 201 contrast, in mountainous areas in the southwestern corner and southern edge of this area, dry evergreen forests are dominant. There is not much difference in air temperature and solar radiation characteristics from those in mountainous areas in the northwestern corner. Thus, in this area, the determinant factors for the natural vegetation may be the drier conditions. As illustrated above, mapping rainfall characteristics seems very useful, providing prolific data in terms of time and space on a map. Although accuracy is not very high at present (Table 1), these maps may provide plenty of useful information. For example, areas with abundant rainy days in the rainy season have a possibility to be agriculturally more intensified with small risks, because of the stability of rainfall patterns. Future increases in the number of rain stations, improved reliability of each station and utilization of remote sensing data, including various satellite data (Doraiswamy et al., 2000; Thielen et al., 2000) may improve the accuracy and increase the reliability and usefulness of these maps.

ACKNOWLEDGEMENTS The present research was supported by a Grant-in-Aids for Scientific Research (No. 09NP0901, 09460152 and 13306026) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. We would like to express our gratitude to the Meteorological Department of Thailand for providing us with various climatic data, and the National Research Council of Thailand for allowing us to carry out this research work in Thailand.

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