BACKGROUND

Bhutan is small mountainous country located between latitudes 26° 45’ N to 28° 10’ N and longitudes 88°45’ E to 92°10’ E, bordered by China to the north and India to the south.

The variation of climate and its extremes are dependent on the altitude. In case of Bhutan, the vast elevation differences for small stretch (approximately elevation of 200m in south to 7000m in North within a distance of less than 175km) with steep terrain expose the country to different hydro-meteorological hazards. In addition to the recurrent flash floods, landslides, wind storms and river erosions, Bhutan is considered highly vulnerable to the threats of glacial lake outburst floods (GLOFs) because, most of the important infrastructure, agricultural land and over 70% of human settlements are located along the main drainage basins.

Climatology and Hydrological Features    

The Indian southwest monsoon, which sweeps in from the Bay of Bengal has strong influence on the climate of Bhutan with dry winter and wet summer monsoon, and the temporal variations or diversity in climate are correspondingly extreme. Thus, while southern Bhutan is generally hot and humid sub-tropical conditions, the high Himalayan Mountains in the northern borders of the country experience severe alpine climate, and are under perpetual ice & snow.

Bhutan’s climate does vary considerably between the valleys and within the valleys, depending on altitude levels. These features also make the rainfall differ considerably within relatively short distances due to rain shadow effects. From past records, the mean annual rainfall varies approximately from 2,500 to 5,500 mm in the southern foothills, 1,000 to 2,500 mm in the inner valleys, and 500 to 1,000 mm in the northern part of the country.

River System of Bhutan

Bhutan is landlocked country with abundant water resources, which are mostly in the form of rivers. There are quite a number of lakes located in remote and high altitude alpine areas of this country. These lakes are formed due to increased melting of glaciers and sudden outburst of these lakes would result in flash floods/GLOF with severe damages to the life and property downstream.

The major rivers of the country flow north to south, with their sources in the alpine zone and flowing right down to the tropical zone on the border with India. The rivers of Bhutan generally have steep gradients and narrow steep-sided valleys, which occasionally open up to give small areas of flat land for cultivation. They carry large volumes of flow and sediment during the monsoon season and significant snowmelt at the end of the dry season. Short rain-fed tributaries descend even more steeply from east or west to join these major rivers. Due to the steep longitudinal gradients and the high annual runoff, these rivers provide significant potential for hydropower development, with an estimated theoretical potential of 30,000 MW (PSMP, 2004).

The existences of distinct rainy and dry seasons are the main reasons for large seasonal variations in river flows. The rivers in Bhutan carry large volumes of flow and, often, also high sediment during the monsoon season. However, the flow is relatively low during the dry season due to the limited rainfall and limited existence of major groundwater reservoirs. Snowmelt from the high altitude alpine areas in the north contributes to the flow at the end of the dry season. Apart from the major north-south flowing rivers, Bhutan has a dense network of small perennial and rain-fed tributaries that flow down the steep slopes and side valleys, often as waterfalls, to join the major rivers.

Bhutan is broadly divided into four major river basins viz. Amochu, Wangchu, Puna-Tsangchu, and Manas River Basins. They all originate from the high altitude alpine snow cladded area in the north, and flows into the Brahmaputra River to the south in Indian plains.

The Amochu is trans-boundary river that origin in China and flows through the western districts of Ha and Samtse, before it finally drains into the plains of India via Phuntsholling. The Jaldhaka is small trans-boundary river that that originates from southeastern Sikkim in the eastern Himalayas. The river enters into Bhutan and is joined by Bindu Khola and other small tributaries under Sibsoo Dungkhag, Samtse.

The Wangchu consists of three major tributaries from the three valleys of Thimphu, Paro and Ha. They originate within Bhutan from the glaciers and snow-capped mountains in the north.  It flows south to the Indian plains through Chukha district.

The Punatsangchu (or Sankosh river) consists of two major tributaries, Phochu and Mochu, that originate from Gasa Dzongkhag. The two rivers join at Punakha Dzong to become Punatshangchu (Sankosh) that flows through Wangdue Phodrang, Tsirang and Sarpang dzongkhags, before flowing to the Indian Plains.

The Manas is the biggest river basin, which drains almost all the catchments of Central and Eastern Bhutan. It comprises of four major sub-basins, namely: Mangde Chu and Chamkhar Chu (which both originate close to Gangkhar Puensum), Kuri Chu (which originates from China), and Dangmechu (formed by joining two main tributaries- Kholongchu that originates from the northern-eastern part of Trashi Yangtse and Gongri that originates from China  and  flows  from  Aurunacha  Pradesh  in  India  to Bhutan). Kurichu and Gongri are transboundray Rivers that originates from Autonomous region of Tibet, China.

The three minor rivers that drains through the southeastern corner of the country are Nyera Ama Ri, Nonori Chu, and Jomo Chu (Dhansari).