WATER QUALITY MODEL OUTPUT UNCERTAINTY AS AFFECTED BY SPATIAL RESOLUTION OF INPUT DATA1

ABSTRACT: Resolution of the input GIS data used to parameterize distributed‐parameter hydrologic/water quality models may affect uncertainty in model outputs and impact the subsequent application of model results in watershed management. In this study we evaluated the impact of varying spatial resolutions of DEM, land use, and soil data (30 × 30 m, 100 × 100 m, 150 × 150 m, 200 × 200 m, 300 × 300 m, 500 × 500 m, and 1,000 × 1,000 m) on the uncertainty of SWAT predicted flow, sediment, NO₃‐N, and TP transport. Inputs included measured hydrologic, meteorological, and watershed characteristics as well as water quality data from the Moores Creek watershed in Washington County, Arkansas. The SWAT model output was most affected by input DEM data resolution. A coarser DEM data resolution resulted in decreased representation of watershed area and slope and increased slope length. Distribution of pasture, forest, and urban areas within the watershed was significantly affected at coarser resolution of land use and resulted in significant uncertainty in predicted sediment, NO₃‐N, and TP output. Soils data resolution had no significant effect on flow and NO₃‐N predictions; however, sediment was overpredicted by 26 percent, and TP was underpredicted by 26 percent at 1,000 m resolution. This may be due to change in relative distribution of various hydrologic soils groups (HSGs) in the watershed. Minimum resolution for input GIS data to achieve less than 10 percent model output error depended upon the output variable of interest. For flow, sediment, NO₃‐N, and TP predictions, minimum DEM data resolution should range from 30 to 300 m, whereas minimum land use and soils data resolution should range from 300 to 500 m.     

The impact of sewage sludge exposure on the reproduction of the sand goby, Pomatoschistus minutus

Adult sand gobies were exposed to 0.1% sewage sludge for 19 weeks prior to the end of spawning. Exposure to sewage sludge had a significant effect on male mortality rates but no significant effects on the gonadosomatic index of males or females. There were no major effects of sludge exposure on testes androgen content or on testes release of androgens after in vitro gonadotrophin stimulation. Fecundity and the number of larvae produced were not significantly affected by the sludge exposure. There was a tendency for eggs and larvae from sludge-exposed females to have a higher mortality rate. At a population level, however, there was a major reduction in the number of eggs and larvae produced in the sludge-exposed population which reflected a failure of some females to spawn. Of the larvae produced, 60-70% from sludge-exposed parents were lighter and had a larger yolk-sac volume compared to larvae from non-exposed parents which may have indicated impaired yolk utilisation.