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Modeling Phosphorus Transport via  Surface Runoff in Songkhla Lake Basin,  Thailand Kitipan  Kitbamroong G.Padmanabhan  Penjai Sompongchaiyakul  

Songkhla Lake Basin

A Lake In Distress Current and Future State  Macrophyte blooms lasting for several months have been 

 

observed in the middle of the lake. Increasing use of agricultural chemicals in the catchments and consequent loads of phosphorus discharging into the lake over the coming decades is of concern. Could devastate the health and local economy of some 1.5 million people living in this region. Major economic activities in the basin and the lake include rubber plantation, paddy rice farming, fruit tree orchards, fishery, aquaculture and husbandry. A healthy lake is critical to the local economy.

Research Objective The comprehensive goal of this research is to model the non-point source loading of phosphorus to the Songkhla Lake from the surrounding drainage area and to develop a decision support system to assist water quality management of the lake. Focus of this presentation is the modeling part.

Model Framework SFA

Input Pre-Processing


Model Post-Processing

Mgt Tools

Model Development Model Selection Criteria Can the model handle predominantly agricultural land use?  Do we have the required input data available?  Does the model have GIS interface? 

Selected Model  Annualized Agricultural Non-Point Source Pollution Model (AnnAGNPS)

Model Development  AnnAGNPS is a cell-based spatially

distributed model designed to track non point source pollutant transport via surface runoff in agricultural watersheds.  It is capable of tracking sediments and nutrients such as phosphates and nitrates.

Input Preparation  The study used data for the year 2004.  The data was assembled into a GIS database.

Topographic data used is in the Digital Elevation Model (DEM) format with 30mx30m resolution.  Crop, fertilizer and management data on field, schedule and operation were collected from field survey and from farmers, fertilizer resellers, cooperatives, and farmers.  The phosphorus data available for transport was estimated using Substance Flux Analysis (SFA).  Non-crop data was obtained from field surveys and literature reviews.

Model Calibration   The AnnAGNPS model was calibrated for surface

runoff by varying the CN parameter for the cells.  The sediment yield estimation was improved by varying the cropping factor (C) in the USLE and the hydrologic shape factor.  The other soil erosion coefficient such as soil erodibility factor (K), practice factor (P), surface condition constant, and soil texture for a particular cell or subcell were assigned according to the field observation or analyses based on the observed data or suitably taken from the literature.

Scenarios modeled Scenario  Description No. 1 Change in  fertilizer usage

Strategy Change in fertilizer application rate by  2. +10% 3. +50%

4.  ­10% 5.

2 3


Types of  Replacement of  8­24­24 and 13­13­21 by 15­15­15  fertilizer applied  Change in type of  Replacement of horticultural crops by rubber crop (N­P­K) crops 

Results ď Ž One third of the phosphorus contribution occurred from U-Tapao

and Eastern Coast Sub Basin 4 sub-watershed, followed by Klong Pa Payom & Thanae, Phru Poh & Rattaphum subwatershed, with approximately equal relative contributions of 15% each respectively ď Ž The amount of P was not excessively high. However, P transport was of concern because Chlorophyll a was found to be high in the upper and middle of the Songkhla Lake.


Results  High fertilizer application rate could be found in horticultural crops to

achieve high yield. Low fertilizer application rate could be found in field crops, rubber and palm plantations  A potentially favorable scenario for phosphorus loading reduction appears to be the one of adopting rubber cultivation instead of horticultural crops. Point

2004  load  P (mg/kg  dry soil)





1.1 28.52

1.2 37.61

1.3 1.4 2 3 21.26 15.65 15.03 14.43



102.1 124.92

60.31 36.96 31.69 27.17







55.96 42.82





35.65 22.52 19.64 17.12



439.52 528.26 244.15 140.5 116.5 96.64

Annual Loading (tons/year)


 18,796   23,164   11,372   7,129   6,245   5,491 

156.26 195.99 100.09 65.97 58.96 52.69 42.3 41.79



Conclusions  Modeling was accomplished by delineating the basin into 8

major sub-watersheds. Paucity of data is a major problem particularly for applying data-intensive distributed parameter models. Efforts to build a good data base for the models need to be continued.  U-Tapao and Eastern Coast Sub Basin 4 sub-watershed contributed one third of the phosphorous loading to the lake. Resources could be proportionately allocated to those subwatersheds for phosphorus loading reduction programs.  The scenario results for phosphorus reduction from this study indicates a potentially favorable scenario of adopting rubber cultivation instead of horticultural crops.

Acknowledgements The authors acknowledge  ­ The National Research Center for Environmental and  Hazardous Waste Management, Chulalongkorn University,  Bangkok, Thailand for sponsoring this research ­ The Department of Civil Engineering, Faculty of  Environmental Management, Prince of Songkhla  University, Songkhla, Thailand for providing laboratory  facilities ­  The Department of Civil Engineering, North Dakota  State University, USA for hosting the first author as a  visiting scholar during the early stages of the study. 

Modeling Phosphorus transport via Surface Runoff