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Journal
of Environmental Hydrology ISSN 1058-3912 |
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Electronic
Journal of the International Association for Environmental Hydrology
JEH Volume 12 (2004), Paper 22 Posted December 28, 2004 EFFECTS OF CELL SIZE ON AGNPS INPUTS AND PREDICTIONS Kyle R.
Mankin1 ABSTRACT Distributed-parameter watershed models require division of a watershed into homogeneous areal units. The size of these units influences both model inputs and model accuracy. This study evaluated the response of the Agricultural Non-Point Source (AGNPS 5.0) model to different cell sizes. Red Rock Creek watershed, covering 135 km2 of south-central Kansas, was modeled at four cell-size resolutions (260, 65, 16, and 4 ha), and 24-hr storm events with return periods of 0.05, 0.5, 2, 20, and 200 years were applied. Remotely sensed Landsat-5 TM images were used to obtain land-cover data, and soil and topographic data were extracted from GIS layers using an AGNPS-ARC/INFO interface. Runoff depth decreased with increasing cell size. However, sediment and nutrient yields decreased with increasing cell size from 4 ha to 16 ha and then increased with further increases in cell size. This was a result of two primary factors: flow-path length and slope estimates. Slopes were systematically underestimated for larger cell sizes by the AGNPS-GIS interface, resulting in decreases in overland erosion as cell size increased. Flow-path lengths were calculated internally by AGNPS and generally decreased with increasing cell size, causing decreases in channel erosion but increases in delivery ratios. The net effect was a local minimum at 16 ha for sediment yield. However, comparisons with measured stream flow and sediment yield indicated that cell size selection induced only a small model response compared to natural variability. We recommend cell size between (slope-length)2 and the cell size that produces maximum modeled slope (in this study,0.5 to 4 ha). However, the large variability in measured runoff and sediment yields might make cell-size selection less important than other model factors, such as antecedent moisture condition. Reference: Mankin, K.R., S.J. Bhuyan and J.K. Koelliker; Effects of Cell Size on AGNPS Inputs and Predictions, Journal of Environmental Hydrology, Vol. 12, Paper 22, December 2004 CONTACT: Kyle R. Mankin E-mail:kmankin@ksu.edu |
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