Model “Agricultural Non-Point Source (AGNPS)” merupakan suatu model yang
dapat menganggar pemindahan tanah daripada suatu kawasan tadahan air, sama ada
bagi ribut sebenar atau ribut anggaran. Model untuk menganggar pemindahan tanah ini
didirikan berdasarkan konsep “Universal Soil Loss Equation (USLE)” di mana knosep
ini menganggarkan kadar pemindahan tanah tahunan berdasarkan bentuk hujan, jenis
tanah dan topografi. Penentuan kecerunan dan panjang kecerunan adalah berdasarkan
kecerunan dan kelas “aspect”. Dalam projek ini, lebih kurang 500 titik yang
mengandungi ketinggian tanah telah diambil dengan menggunakan “Global Positioning
System (GPS)”. Kesalahan atau kesisihan data telah ditentukan dengan menggunakan
“Root Mean Square Error (RMSE)” dan didapati bahawa keputusan adalah
memuaskan. Dengan menggabungkan titik ketinggian dan peta kontour, satu “Digital
Elevation Model (DEM)” akan dapat dihasilkan. Dengan menggunakan DEM pula,
beberapa parameter seperti arah pengaliran, kapasiti pengaliran, kawasan tadahan air
sebenar, kecerunan dan “LS factor” dapat dihasilkan. Arah pengaliran didapati
berdasarkan kecerunan yang paling curam manakala kapasiti pengaliran
membayangkan kandungan air hujan yang akan mengalir melalui setiap sel. Daripada
keputusan yang diperolehi, “LS factor” mempunyai nilai 0 – 111.
_______________________________________________________________________________________________________
Agricultural Non-Point Source (AGNPS) is a distributed parameter model that
predicts soil erosion and nutrient transport from agricultural watersheds for real or
hypothetical storms. Erosion modeling is built upon the Universal Soil Loss Equation
(USLE), which predicts the long term average annual rate of erosion on a field slope
based on rainfall pattern, soil type, topography, crop system and management practices.
The determination of slope and its corresponding slope length is identified by slope and
aspect classes. In the present study, elevations of about 500 well-distributed points
were taken by using Global Positioning System (GPS). Root Mean Square Error
(RMSE) of these elevations was determined by comparing the elevation interplolated
from the contours. By combining the elevation of points and the contour maps, a
Digital Elevation Model (DEM) was created by using Geographical Information
System (GIS). From the DEM, various parameters such as flow direction, flow
accumulation, watershed area, slope, aspect, hillshade and LS factor map was
produced. Flow direction is determined by finding the steepest descent from each cell
while flow accumulation represent the amount of rain that would flow through each
cell, assuming that all rain become runoff. The watershed areas are described as the
surface runoff as being the locus of points within an area where runoff produced inside
the parameter will move into a single watershed outlet. Slope, aspect and hillshade are
useful in determining the LS factor. A detailed analysis for determination of LS factor
is carried out using the AGNPS – GIS interface. Results shows that the values of LS
factor vary from 0 – 111.
Use of gps and gis for preparation of input maps for agnps model based on elevation / Wong Kok Foo