Topography

Most of Mae Hong Son Province area is a high mountain complex and still a richness natural forest. The forest area is approximately 11,104.4464 square kilometers, accounting for 87.5658 percent of the provincial area. There are mountains along the north-south parallel. There are two important mountain ranges. The Dan Laos range is at the northern end of the province, is border between Thailand and the Republic of the Union of Myanmar (Myanmar). The Thanon Thong Chai range, consists of three mountain subranges, the western part of the Thanon Thong Chai range is Thailand and the Republic of the Union of Myanmar (Myanmar) border, the middle part of the Thanon Thong Chai range is between the Yuam river and the Mae Chaem river and The eastern part of the Thanon Thong Chai range is a border between Mae Hong Son province and Chiang Mai province. The highest peak is Mae Ya Peak, located on The eastern part of the Thanon Thong Chai range in Pai district. It is about 2,005 meters above sea level. Small plains are found on all all district, mostly in Mae Sariang district, Khun Yuam district and Pai district. Overall area  is only 338.26 square kilometers.

 

Water Resource

Mae Hong Son province is located in 2 river basins. About 12,643.52 square kilometers, or 99.10 percents of the province, is in the Salawin River Basin and about 123.75 square kilometers, or 0.97 percent of the province area, is in the Ping River Basin.

Salawin River Basin

The Salawin River Basin located in the north of Thailand. The total area of the basin is 19,103.50 square kilometers. It covers areas in 4 provinces, most of which are in Mae Hong Son province, some areas in Tak province and Chiang Mai province and tiny areas in Kamphaeng Phet province. The topography of the basin is mainly steep mountain range, with a lateral line from the Himalayas. It is very different from the estuary, which is about 200 meters above sea level, to Mae Ya Peak, which is 2,005 meters above sea level. This complex terrain cause disparate sub-basins, such as watershed characteristics and flow direction. The main rivers are Pai river, Moei river and Yuam river. Pai river originates in the mountains of Dan Lao Range. Moei river originates in the mountains boundary between Thailand and Myanmar.  Yuam river originates in the mountains of Dan Lao Range, Khun Yuam District, The tributaries of the Yuam river are Pon river and Mae La Luang river. 

The Salawin River Basin is divided into 4 sub-basin groups / 17 river sub-basins, as follows.

  1. Pai River sub-basin group consists of 6 sub-basins, namely, Mae Nam Pai Ton Bon, Huai Mae Sa, Nam Khong, Nam Mae Pai Ton Lang, Nam Mae lamat and Nam Mae Sarin.
  2. Yuam River sub-basin group consists of 6 sub-basins, namely, Mae Nam Yuam Ton Bon, Nam Mae La Luang, Mae Nam Yuam Ton Bon, Nam Mae Sariang, Nam Mae Rit and Nam Mae Ngao.
  3. Salawin River sub-basin group consists of 2 sub-basins, which are Mae Nam Salawin Ton Bon and Nam Mae Ngae.
  4. Moei River sub-basin group consists of 3 sub-basins, including Mae Nam Moei Ton Bon, Huai Mae Lamao and Mae Nam Moei Ton Lang.

Mae Hong Son Province has an area of 12,643.52 square kilometers in the Salween Basin, accounting to 99.10 percents of the province area, 66.18 percents of the Salawin River Basin's area. Mae Hong Son Province is in the three sub-basin groups, as follows.

  1. Pai River sub-basin group located in the upper part of the Salawin basin, in Mae Hong Son province and Chiang Mai province. The total area of the sub-basin group is 6,233.08 square kilometers. Most of the area is in Mae Hong Son province, about 5,101.64 square kilometers with natural features of high mountains and small steep river valleys in Pai district and Mueang Mae Hong Son district. The Pai River originates from Li thi mountain and Chang mountain, which is border between Thailand and the Republic of the Union of Myanmar (Myanmar), in the district of Pai. The river flows first in a north-south direction and then in an east-west direction, through Mae Hong Son province and across the Thai-Myanmar border at Nam Phiang Din village, Pha Bong sub-district, Muang Mae Hong Son district. The river tributes the Salawin River in the Republic of the Union of Myanmar (Myanmar), about 60 square kilometers from the border. Total length of the river is approximately 230 kilometers, about 180 kilometers in Thailand. The major tributaries of the Pai river such as Huai Mae Sa, Nam Khong, Nam Mae Sa-nga, Nam Mae Surin, Nam Mae Samat, etc.
  2. Yuam River sub-basin group located in the middle east of the Salawin basin, in Mae Hong Son province and Tak province. The total area of the sub-basin is 6,084.61 square kilometers. Most of the area is in Mae Hong Son province, about 5,649.35 square kilometers. the Yuam River originates from Phatu Wiang mountain and Nam Pong mountain, in Khun Yuam district. The river flows from north to south, through Khun Yuam distirct, Mae La Noi district and Mae Sariang district. The river tributes Nam Mae Ngao river at Sop Nago village. Then the river flow in a east-west direct and tributes Moei river. The landscape is slender in the north-south direction. The east mountain is a barrier between Yuam river sub-basin group and Mae Chaem river sub-basin of the Ping River basin. The river's total length is about 240 kilometers. The major tributaries of the Yuam river are Nam Mae La Ka , Num Mae La Luang, Nam Mae Sariang, Nam Mae Rit and Nam Mae Ngao.
  3. Salawin River sub-basin group located in the middle west of the Salawin basin. It has an area of 1,892.57 square kilometers. The landscape is slender in the north-south direction, with a mountains natural area. It is located in Mae Sariang district and Sop Moei disctict, Mae Hong Son province. The main tributary of the Salawin river is Ngao river, flows from north to south.

(2) Ping River Basin

The Ping River Basin is located in the north of Thailand. The total area of the river is 34,536.83 square kilometers. It covers the area of 9 provinces, mostly in Chiang Mai province, Lamphun province, Tak province and Kamphaeng Phet province, some areas in Nakhon Sawan province and small areas in Mae Hong Son province, Sukhothai province, Lampang province and Chiang Rai province. Mae Ping River originates in the Phe Pan Nam range in Chiang Dao district,  Chiang Mai Province. The river flows in a north-south direction, through the area of Chiang Mai province, Lamphun province, Tak province. It joins the Wang River at Tak province, then flows through Kamphaeng Phet province and joins the Nan River at Nakhon Sawan province. The major tributaries are Mae Taeng river, Mae Ngat river, Mae Kuang river, Mae Khan river, Mae Chaem river, and also Wang River, which is the largest main branch.

The Ping River Basin is divided into 20 sub-basins, namely Mae Nam Ping Ton Bon, Nam Mae Ngat, Mae Nam Mae Taeng, Mae Nam Ping Suan Ti Song, Nam Mae Rim, Nam Mae Kuang, Nam Mae Ngan, Nam Mae Li, Nam Mae Klang, Mae Nam Ping Suan Ti Sam, Nam Mae Cham Ton Bon, Nam Mae Cham Ton Lang, Nam Mae Hat, Nam Mae Tuen, Mae Nam Ping Suan Ti Si, Huai Mae To, Khlong Wang Chao, Khlong Mae Raka, Khlong Suan Mak and Mae Nam Ping Ton Lang.

Mae Hong Son Province's areas are 123.75 square kilometers in the Ping River Basin, or 0.97 percents of the province's area and 0.36 percents of the Ping River Basin's area. Mae Hong Son province is in 5 sub-basins, namely Mae Nam Mae Taeng, Nam Mae Rim, Nam Mae Ngan, Nam Mae Cham Ton Bon and Nam Mae Cham Ton Lang.

 

Watershed Classification

Most areas of Mae Hong Son province located in the Watershed Classification class 1A : WSC1A (Protected or Conservation forest and headwater source, no activities allowed) And the Watershed Classification class 2 : WSC2  (commercial forests, some useful activities allowed such as mining will be allowed within legal boundaries) have an area of 11,543.93 square kilometers, accounting for 90.32 percent of the province's area.

Watershed Classification class 1

The hypothetical characteristics of Watershed Classification class 1 can be described as protected or conservation forest and headwater source. This class is divided into 2 subclasses. The Watershed Classification class 1A (WSC 1A) are protected forest areas and include the headwaters of rivers. The areas are usually at high elevations and have very steep slopes and should remain in permanent forest cover. The Watershed Classification class 1B (WSC 1B), are those areas which have similar physical and environmental features to WSC 1A, however portion of these areas have already been cleared for agricultural use of occupation. These areas may be fallowed or cultivated; they therefore require special soil conservation protection measures. Where possible, they should be replanted to forest or maintain in permanent agro-forestry.

Watershed Classification class 2

The hypothetical characteristics of Watershed Classification class 2 can be described as commercial forest. These areas are designed for protection and/or commercial forests where mining and logging will be allowed within legal boundaries, usually at high elevations with steep to very steep slopes. Landforms usually result in less erosion than WSC 1A and WSC 1B. The areas may be used for grazing or crop production if accompanied by appropriate soil protection measures.

 

Climate

Mae Hong Son province possesses tropical climate, distinctively divided into 3 seasons.

  • The hot season is from the middle of February to the middle of May. The weather is hot and extremely hot in some areas. Tropical storms occur in the form of thunderstorms, strong gust and hail. Convection and the confluence of two different air streams, cold and warm, are the main factors of thunderstorms.
  • The rainy season is from the middle of May to the middle of October. Influence by the southwest monsoon which starts in May brings a stream of warm moist air from the Indian Ocean towards Thailand causing abundant rain over the country.
  • The cool season is from the middle of October to the middle of February. Influenced by the northeast monsoon which starts in October brings the cold and dry air from the anticyclone in China mainland over major parts of Thailand. the weather is very cold.

 

Meteorological statistics: Since the beginning of statistics (1951)

Temperature statistics: The maximum temperature is 44.6 degrees Celsius on April 28, 2559, and the lowest temperature is 3.9 degrees Celsius. On December 25, 1999, the highest average temperature is 35.62 degrees Celsius and the minimum average temperature is 17.98 degrees Celsius.

Relative humidity The average relative humidity is 96.99% and the minimum relative humidity is 20%.

The annual rainfall of 1,064.9 millimeters was 24 hours, 130.4 millimeters, on April 23, 1968, and the number of rainy days was 130 days.