Pakphanang River
Suphaphorn Rakkhiaw
Study area description
The Pakphanang River (8.37°N, 100.18°E) is in the south of Thailand (Figure 1). The river is about 150 km long, to the mouth at Pakphanang Harbour. The upper reach has a steep slope, while the lower reach has moderate slope and water level is at mean sea level. In the dry season (January to May), there is seawater intrusion into the Pakphanang River. In very dry years, seawater will intrude into the Pakphanang River up to Amphoe (district) Chian Yai. The Pakphanang River has a depth of 7-10 m in the upper reach (40-90 km from the ocean). The upper river reach has DO higher than 5 mg L-1 because the steep slope results in high flow velocity. In the lower reach (0-40 km from the ocean), DO decreases due to wastewater from various sources discharging into the Pakphanang River such as from Amphoe Cha-uat (16 km), and from non-point source (rural communities, agricultural area and pig farms) at 24 km (Pollution Control Department 1998).
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Figure 1. The Pakphanang River flows into the southern end of Pakphanang Harbour.
The budget presented here in based on data collected between November 1996 (representing the wet season) and April 1997 (representing the dry season) (Predalumpaburt et al., 1999.) In the wet season, total size of system (Pakphanang River estuary) is 8x106 m2, the mean depth about 6m, volume 47x106 m3, rainfall 22 mm day-1, evaporation 3 mm day-1 and runoff 2x106 m3 day-1. In the dry season, the system covers 8x106 m2, the mean depth is about 6 m, the volume 47x106 m3, rainfall 9 mm day-1, evaporation 5 mm day-1 and runoff 1x106 m3 day-1. The fluxes of water, salt, DIP and DIN were calculated using a simple box model following Gordon et al. (1996).
Figures 2 and 3 summarise the water and salt budgets for Pakphanang River for the wet and dry seasons respectively. In the wet season, the residual water flow (VR) out of this system was about 2x106 m3 day-1. The salinity of freshwater inflow was assumed as 0 psu. Oceanic salinity was estimated as 12.0 psu and average system salinity was 4.0 psu. Wet season water exchange time was 12 days. In the dry season, the residual flow (VR) out of system about 1x106 m3 day-1. Oceanic salinity and average system salinity were about 21.0 psu and 11.0 psu, respectively. Dry season water exchange time was calculated to be 16 days.
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Figure 2. Water and salt budgets for the Pakphanang River in the wet season (November 1996). Volume in 106 m3, water fluxes in 106 m3 day-1, salt fluxes in 106 psu-m3 day-1 and salinity in psu.
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Figure 3. Water and salt budgets for the Pakphanang River in the dry season (April 1997). 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 5 summarise the dissolved inorganic phosphorus (DIP) budgets for Pakphanang River for the wet and dry seasons respectively. The system was a net phosphorus source for both seasons: DDIP = +1,000 mol day-1 or +0.1 mmol m-2 day-1 for the wet season, and DDIP = +3,000 mol day-1 or +0.4 mmol m-2 day-1 for the dry season. The positive DDIP values apparently indicate that there was net release of DIP within the system. This is probably not a correct conclusion because there is an undefined amount of waste discharge into the system. In order to estimate the nonconservative fluxes it will be necessary to establish an estimate of this waste load.
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Figure 4. Dissolved inorganic phosphorus budget for the Pakphanang River in the wet season (November 1996). Fluxes in 103 mole day-1 and concentrations in mmol m-3.
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DIN balance
Figures 6 and 7 summarise the dissolved inorganic nitrogen (DIN) budgets for Pakphanang River for the wet and dry seasons respectively. There was a positive nonconservative flux of DIN within this system for both seasons: DDIN = +33,000 mol d-1 (+4 mmol m-2 day-1) for the wet season and DDIN = +37,000 mol d-1 (+5 mmol m-2 day-1) for the dry season. However, the result is not conclusive because of the absence of waste load.
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Figure 6. Dissolved inorganic nitrogen budget for the Pakphanang River in the wet season (November 1996). Fluxes in 103 mole day-1 and concentrations in mmol m-3.
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Figure 7. Dissolved inorganic nitrogen budget for the Pakphanang River in the dry season (April 1997). Fluxes in 103 mole day-1 and concentrations in mmol m-3.
Stoichiometric calculation of aspects of net system metabolism
It is not possible to assess the nonconservative fluxes and their stoichiometry at this time until waste load is considered in the budget.
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Last Updated 21 May 2006 by DPS