Figure 1.  Location of Chao Phraya River estuary, Thailand.

The Chao Phraya River (13.58°-15.67° N, 100.10°-101.00° E) is 375 km long, and flows from Nakonsawan Province to the Gulf of Thailand in Samut Prakarn Province.  The river basin is about 19,390 km².  The population living in the basin is approximately 8 million.  The depth of the river ranges from 5 to 20 m and the width ranges from 200 to 1,200 m.  The river traverses several large cities and the major agricultural region of the country, hence this river receives large amounts of wastes along its path.  Pollutants discharged into the river are from point and non-point sources.   Major point sources of pollutants to the Chao Phraya River basin include domestic and industrial waste discharges as well as some agricultural point sources such as pig, duck, fish and other farms.  Only two urban centres, Bangkok and Uthai Thani have operating sewage treatment systems.  Although some industrial sites have some form of wastewater treatment systems, most tend to dispose of raw effluent to the nearest waterways, usually a street drain or smaller stream.   Non-point sources include agricultural areas (paddy fields), and orchards, which constitute the main land uses in the basin.

 The river is dammed and used to irrigate the agricultural land, so the discharge is strongly regulated.  Water is released from the Chao Phraya Dam to the lower basin is for the purposes of salinity control, irrigation, navigation, industry and domestic consumption.   However, the discharge hydrographic of the Chao Phraya River still exhibits a periodic variation with a cycle of one year.  The hydrological cycle starts in April when the discharge is typically at its minimum.  From May to August the discharge gradually increases, while from August to October the increase is more rapid, peaking in October.  The discharge then decreases fairly rapidly during November and December, with the rate of decrease then slowing until minimum flow conditions are again experienced in April.  During the low flow periods from January to April the discharge typically ranges from 50 to 200 m³ sec^-1.  Tidal intrusion extends to Angthong (175 km) during low stream flow conditions and to about 75 km upstream during high stream flow conditions.  At a freshwater discharge rate of 4,000 m³ sec^-1, common during the wet season, tidal fluctuations are not observed upstream of Pak Kret.  However, salinity effects from the sea are only noticeable downstream of Nontaburi, about 60 km from the river mouth.

 The estuarine section of the Chao Phraya (approximately 70 km from the river mouth) can be divided into two portions, the upper estuary and the lower estuary.   Physical dimensions of the estuary are shown in Table 6.11.  For the purposes of the budgetary analysis, the estuary is assumed to be a well-mixed system.  Organic loads into the estuary were estimated using data from previous studies (Pollution Control Department 1997).  Total N and P waste loads were obtained by conversion of BOD loading using the stoichiometric relationships of C:N:P ratios in organic waste materials (San Diego-McGlone, Smith and Nicholas 1999).

 Table 1.  Physical dimensions of the Chao Phraya River estuary.

Water and salt balance

Figures 2 and 3 present the water and salt budgets for the Chao Phraya River estuary in the dry (April) and wet (August) seasons of 1996 respectively.  The calculations assumed an upstream river inflow salinity of zero psu.  River runoff is based on the average daily measurements of the flow of all the diverting water gates, as recorded by the Royal Irrigation Department.   River runoff (V_Q) is estimated to be 10x10⁶ m³ day^-1 in the dry season, and 22x10⁶ m³ day^-1 in the wet season.  Groundwater flow is assumed to be zero.  Rainfall (V_P) is estimated to be 60x10³ m³ day^-1 in dry season and 220x10³ m³ day^-1 in the wet season.  Evaporation (V_E) is estimated to be 130x10³ m³ day^-1 in the dry season and 100x10³ m³ day^-1 in the wet season.  However, net precipitation and evaporation are regarded as insignificant to the water budget when compared with the river runoff.  Although the volume of waste is small relative to river freshwater input, dissolved P and N are highly concentrated in this waste discharge so that nutrient flux from this source is significant to the nutrient budget. 

Following the LOICZ Biogeochemical Modelling Guidelines (Gordon et al. 1996), the water exchange time of the upper estuary is calculated to be 10 days in the dry season and 6 days in the wet season.  For the lower estuary, the exchange time is shorter, two days in the dry season and one day in the wet season.  The water exchange time for the entire estuary is five days in the dry season and four days in the wet season.

Figure 2.  Water and salt budgets for Chao Phraya River estuary in the dry season.  Volume in 10⁶ m³, water fluxes in 10⁶ m³ day^-1, salt fluxes in 10⁶ psu-m³ day^-1 and salinity in psu.

Figure 3.  Water and salt budgets for Chao Phraya River estuary in the wet season.  Volume in 10⁶ m³, water fluxes in 10⁶ m³ day^-1, salt fluxes in 10⁶ psu-m³ day^-1 and salinity in psu.

Budgets of nonconservative materials

DIP balance

Figures 4 and 6 display the dissolved inorganic phosphorus (DIP) budget, and Figures 5 and 7 show the dissolved organic phosphorus budget (DOP) for the Chao Phraya River estuary in the dry and wet seasons.   The wet season budgets have a greater magnitude of nutrient fluxes, with much larger quantities of nutrients carried in with river runoff than in the dry season.

Figure 4.  Dissolved inorganic phophorus budget for Chao Phraya River estuary in the dry season.  Fluxes in 10³ mol day^-1 and concentrations in mmol m^-3.

Figure 5.  Dissolved organic phophorus budget for Chao Phraya River estuary in the dry season.  Fluxes in 10³ mol day^-1 and concentrations in mmol m^-3.

Figure 6.  Dissolved inorganic phophorus budget for Chao Phraya River estuary in the wet season.  Fluxes in 10³ mol day^-1 and concentrations in mmol m^-3.

Figure 7.  Dissolved organic phophorus budget for Chao Phraya River estuary in the wet season.  Fluxes in 10³ mol day^-1 and concentrations in mmol m^-3.

The upper estuary appears to be a sink and the lower estuary a source for both DIP and DOP in all seasons.   However, the entire system in general seems to be a net sink of dissolved phosphorus (DIP + DOP) in the dry season and a net source in the wet season. 

N balance

A similar balance is estimated for DIN and DON (Figures 8 to 11): the upper estuary is a net sink and the lower estuary is net source for DIN and DON in both the dry and wet seasons.  The whole system behaves as a net source of dissolved nitrogen except in the dry season for DON which makes the system a sink of dissolved nitrogen in the dry season.

Figure 8.  Dissolved inorganic nitrogen budget for Chao Phraya River estuary in the dry season.  Fluxes in 10³ mol day^-1 and concentrations in mmol m^-3.

Figure 9.  Dissolved organic nitrogen budget for Chao Phraya River estuary in the dry season.  Fluxes in 10³ mol day^-1 and concentrations in mmol m^-3.

Figure 10.  Dissolved inorganic nitrogen budget for Chao Phraya River estuary in the wet season.  Fluxes in 10³ mol day^-1 and concentrations in mmol m^-3.

Figure11.  Dissolved organic nitrogen budget for Chao Phraya River estuary in the wet season.  Fluxes in 10³ mol day^-1 and concentrations in mmol m^-3.

Stoichiometric calculations of aspects of net system metabolism

Table 2 shows the results for stoichiometric analysis of the nonconservative nutrients, but the values are extremely high for biotic processes thus cannot be interpreted for system metabolism.  The system seems overwhelmed with the huge nutrient loads, and “slight” errors in these loads may be introducing unacceptable errors in the nonconservative flux estimates.   

Table 2.  Stoichiometric calculations of aspect of net system metabolism for Chao Phraya River estuary.

	April 1996 (dry) mmol m-2 day-1			August 1996 (wet) mmol m-2 day-1
	Upper estuary	Lower estuary	Whole estuary	Upper estuary	Lower estuary	Whole estuary
(p-r)1	+195	-280	+14	+276	-2,237	-681
(nfix-denit)1	+11	+24	+16	+10	-122	-50
(p-r)2	+83	-119	+6	+117	-950	-289
(nfix-denit)2	-31	+47	+7	-33	+76	+8
1 based on C:P of 106:1;  and N:P of 16:1 (plankton-dominated system) 2 based on C:P of 45:1; and N:P of  7:1 (terrestrial organic detritus dominated system)

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Last Updated 21 May 2006 by DPS