Identification of Non-Point Source Pollution Risk Using GIS and Remote sensing Techniques
AGNPS Model Functionality
The AGNPS model operates on cell by cell basis and accordingly, the area was divided into 40 acre cells. Model parameters for the cells were entered into the model through the integration of GIS wearer each parameter was derived and mapped individually. Since there were no recording type rain gauges located within the study area, it was not possible to calculate the energy intensity values. Therefore, the simulations were made for some selected rainstorms ranging form 0.9 to 8 inches.
The model results were obtained for the sedimens and pollutants in terms of the production of COD, nitrogen and phosphorus and the catchment was zoned based on the magnitude of the pollutants generated by each and every land parcels. According to the results of four inch, two hour rain storm, four zones based on the sediments production capability and three zones based on the nitrogen and phosphorus production capability were identified. The identified zones are presented in Figure 02 and figure 03, respectively.
Figure 3 Identified zones based on nutrient production
The selected rainstorm provides estimates of pollutant loads only for that particular rain storm . the simulation was extended for the 171 rainly days found in the hydrological year 1991/92. the rainfall events which are greater that 0.8 inches were taken to calculate the annual pollutant loads of he catchment. These rain storms were divided into eight categories maintaining thresholds at 0.8, 1.1, 1.4, 1.6, 1.7, 2.0, 2.4 and 2.75 inches. The simulated results for each and every rainstorm category were multiplied by the number of storm events belonging to each category to calculate the total annual sediment production .
The calculated total sediment production for the hydrological year 1991/1192 was about a tons/hec. The Environmental and forest consecration division of Mahaweli Authority has obtained a value of 0.6 tons per hectare for the sediment yield in the hydrological year 1991/1992. although the predicted sediment yield is almost six times higher than the measured value, the results are acceptable due to number of reasons. The measurements have been conducted during duty periods nad the rock boulders across the Nilambe river from pools in different locations which can trap sediment during medium and low river dischagers. Further, the measurements have confined ot suspended sediments only and neglected the bed load component. The possibility of having high sediment yields as predicted by the model is confirmed by Fleddeermann and watusawithanan (1993) at Moolgame which is located within the study area. According to the result, plots without sloping Agricultural Land Technology (SAIT) have shown a soil loss of 1.4 tons per hectare during Maha season.
SALT as a management Practice
SALT is one of the widely used Best Management Practices (BMPs) in Upper Mahaweli Catchment Area which include Nilambe sub catchment. In this technology, soil erosion in sloping agricultural lands is minimized by cultivating the crops along the contours and tar4apping the sediment by hedgerows. In this study, SALT conditions were simulated at the critical locations to predict the sediment and nutrient production in order to identify the capability to BMOs in no-point source pollution control. The application of SALT has shown a considerable improvement in controlling land degradation and pollution in the aspects sediment, nutrient and COD production. Accordingly, the areas belonging to the critical pollutant production category have reduced to nearly 32 hectares in terms of both sediment and nutrient production. The spatial demarcation of sediment and nutrient production zones after applying SALT are presented in Figure 04 and Figure 05.
Figure 4 Identified zones based on sediment production
Figure 3 Identified zones based on nutrient production afterapplying SALT
Conclusions
The AGNPS model provides acceptable estimates of on-site soil erosion, sediment yield, production of nitrogen, phosphorous and chemical oxygen demand. Te comprehensive nature of the model output gives the capability of assessing quality of sediment and runoff at any point of catchment and also at the catchment outlet. This facilititates zoning and identifying critical and non-critical location. Further, it is concluded that the model is well suited for comparison
among defferent land parcels in terms of the amount of pollutants appropriate technology for
managing the land while reducing sediment and nutrient imagery in the identification of land
use and cover dynamics and simulate the model for the
changes in pollutant production accoding to the changes of land use and land cover.
Identification of the more suitable grid size instead of the used 40 acre grid size is also
important to obtain more realistic modelling results in addition to the detailed investigation
of the pollutants within a single cell.
References
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Dwivedi, R.S. & Rao, B.R.M. (1992). The selection of best Landsat TM band combination
for delineating salt-affected soil, Int. J. of Remote Sensing, Vol. 13 No. 11 2051-2056.
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Fleddermann A. & WJarusawithana, T.C. (1993). Soil erosion studies in Moolgama, Upper
Mahaweli Environment & Forest Conservation Division, MASL..
