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Description of the Ping river
and its catchment
(for other information about waterways and management, click here)
The Mae Ping river is a 740 km long river in Northern Thailand, flowing South to turn into the Chao Phraya river in Nakhon Sawan. The Ping takes its source 190 km upstream of Chiang Mai in Doi Thuai, a mountain near the Burmese border that also serves as the dividing range for Salween (Myanmar) and Mekong (Laos) rivers.
Figure 1. Simplified map of continental South-East Asia with the major catchment of each river. The Chao Phraya catchment technically include the 5 sub-catchment around it, presented here independtly.
As the Ping flows down, it is joined by several tributaries such as Mae Taeng, Mae Rim, Mae Ngat, Mae Kuang, Mae Wang, Mae Tha and Mae Li along with Mae Chaem when it reached Doi Tao Lake. The whole Ping catchment is 34000 km2, the size of a small European country, Taiwan, Maryland, etc. and passes through important towns such as Chiang Mai, Lamphun, Tak, Kamphaeng Phet and Nakhon Sawan before flowing south as the Chao Phraya to Bangkok and the sea.
Figure 2. Map of the Upper Ping river catchment with all main tributaries sub-catchments.
The Ping is divided into a lower Ping, downstream of Bhumibol Dam, and the upper Ping upstream of Doi Tao Lake. The latter is the northern extension of the water reservoir hold by Bhumibol Dam and can be considered as one single lake. In the upper Ping, the headwaters are mostly in steep gravel valleys with significant slope eroding into the rock basement. It changes north of Mae Rim district when the Ping, joined with the Mae Taeng and Mae Ngat, arrives in a flat plain 25 km wide by 140 km long called the Chiang Mai – Lamphun Basin. In that zone, the Ping was naturally a braided river due to the lower slope present (360 to 280 m) but has been chanellized into a single river more than a century ago. The basin is a tectonic opening creating a subsidence that has accumulated 1 to 2 km of sediments. The Ping river flows through it depositing sediments and eroding previously formed river terraces (see geology).
The average flow of the Ping river is 0.5 m/s (40-50 km/day) with faster flower rates in the hilly northern areas than in the basin. In volumes, it is around 25 m3/s yearly average and vary between 1 and 60 m3/s as base flow for the dry and wet season respectively. Peak flow during major rainfall, often reach around 400 m3/s, mostly in August or September. The total annual runoff of the Ping river in Doi Tao Lake is 7000 Mm3.
Figure 3. Topographic map of the Upper Ping catchment. The Chiang Mai - Lamphun basin is clearly visible in yellow .
Aside from the main river and its tributaries, the Ping catchment also has 3 important reservoirs. The Bhumibol and Doi Tao Lake, connected by the Ping gorge lake, represents a volume of 12462 Mm3, around twice the annual output of the Ping river, held back by the Bhumibol Dam build in 1962 near Tak. Its role is essential in controlling flood waters reaching Bangkok and providing irrigation for the Central Plains all year long. Upstream of Chiang Mai, the Mae Kuang reservoir (263 Mm3) and Mae Ngat reservoir (265 Mm3) play a significant role in maintaining base flow during the dry season, irrigation of the basin and flood management. In addition to these three reservoirs, there are numerous smaller artificial lakes with around 50 with volumes between 0.05 and 20 Mm3 representing a combined volume around 200 Mm3.
Outside the Chiang Mai-Lamphun Basin, the Ping and its tributaries passed through narrow floodplains often covered with rice paddies. On the edge of the main basin, rivers are eroding into old weathered terraces while in the basin, the Ping and its tributaries are channels 20 to 130 meters wide on a sandy riverbed with levees 3 to 4 meters above it and 0.5 to 1 meters higher than the surrounding floodplain. The Ping river floodplain itself is around 3 kilometers wide, covered of rice paddies with soils made of silty clay and sand sheets of past river activity in the last thousand years forming natural levees on which villages were found before the extensive urbanization of the last few decades.
Figure 4. Water level variation between 2000 and 2013 in the Ping river and some tributaries
Figure 5. Simplified hydrographic system of the Upper Ping with available river stage stations used to monitor river levels.
P.1 (Chiang Mai - Nawarat Bridge) and P.67 (Mae Ping after Mae Ngat & Mae Taeng confluence) are the most well-known.
The Ping has characteristically a brown colour which is a normal and natural thing at low latitudes. The intense weathering caused by tropical climate release significant amount of sediments in waterways and while European or North American river will reach 0.05 g/l (less than human hair in a liter of water) during floods, the Ping river has 0.5 mg/l has a standard concentration in the rainy season. It gets a lot higher during floods, reaching values as high as 10 g/l, (one teaspoon of mud per liter of water). In normal conditions, while turbidity is average in the uppermost Ping, it increases considerably after Mae Rim river confluence, peaks in the city and decrease afterwards. While it might seem that the Ping is “polluted” by these sediment, the flora and fauna is adapted to these murky but natural conditions.
Despite the high sediment content, the water quality of the Ping river is relatively good in the uppermost part of the catchment where it is surrounded by mostly deciduous forests. Orchards, small scale agriculture, industry and mining only minimally increase the pollution until Ping waters arrive in the urban area of Chiang Mai where pollution significantly increase. Part of the reason to the increase in pollution is the limited (around 50%) role that the centralized waste water system of Chiang Mai plays, relying on private septic tank for the release of effluents and an unknown volume of grey water releases. As a result, canals in Chiang Mai are among the most polluted waterways in the area. Even the Mae Kha canal, despite being considerably improved in the last couple of decades, still has insufficient flow rate through its water treatment plan to manage the normal flow (0.7 m3/s) of the canal. As a result, both reaches of the Ping in the city are significantly contaminated with pharmaceuticals (diabetes drugs, pain killers, anti-histamines) tracers and industrial waste (sugar substitutes, caffeine, surfactants, detergents,…) as well as more standard contaminants such as F, Cl, nitrates, sulfates that particularly increase once the Mae Kha flow into the Ping. Concentrations are however in an acceptable range and below detection limits for heavy metals. High concentrations in Na, Ca, Mg, Si, S and K are of natural origin and a product of chemical dissolution of eroded rocks.
Aside from these contaminants, the water quality index (similar to the AQI but for water) ranges from 0 (very poor) to 100 (very good) and gives value for the Ping around 70. Variations are around 60 at Mahidol Bridge and as low as 41 where highway [3029] crosses the Ping. Biologically, the Ping is considered relatively clean in the uppermost part (oligotrophic) and moderately clean (mesotrophic) for most of it with minimal anthropogenic eutrophisation in some locations causing algal blooms and high biological activity.