Analysing the sensitivity of two variogram models for the characterisation of the spatial pattern of depth in rivers.

Monica Rivas Casado 1, Sue White 1, Pat Bellamy 1, Mike Dunbar 2, Douglas Booker 2, Ian Maddock 3
1 Cranfield University at Silsoe, Institute of Water and Environment
Barton Road, Silsoe, Bedfordshire, MK45 4DT, UK
2 Centre for Ecology and Hydrology, CEH Wallingford
Maclean Building, Benson Lane, Crowmarsh Gifford, Wallingford, Oxfordshire, OX10 8BB, UK
3 Department of Applied Sciences, Geograpy & Archeology
University of Worcester, Henwick Grove, Worcester, WR 6AJ, UK.

Depth data are measured in river channels to obtain a three dimensional representation of the habitat available for the species inhabiting the system. Data can be interpolated through the application of geostatistical techniques to obtain information of those areas that have not been sampled. This interpolation requires the fitting of a model to an experimental variogram that will then be used to predict values of depth at non measured locations. Before proceeding to calculate the experimental variogram the following parameters need to be decided on: lag distance (or step), azimuth tolerance and  maximum distance of analysis. This poster summarises the results obtained for a sensitivity  analysis of the calculation of experimental variograsm to the three parameters noted above, and demonstrates the effect on the fitting of two models; the spherical and the exponential. The analysis has been developed for depth data sets collected at eighteen river sites. Results show that it is necessary to be aware of the effect of choosing values of the three parameters when estimating the experimental variogram and hence the model. Models can give distorted  values of range, sill and nugget for variograms calculated for specific combinations of  minimum lag distance and maximum distance. Spherical and exponential variogram models can  introduce considerable differences in terms of spatial structure for the same river sites.

Keywords: spatial pattern, geostatistical analysis, kriging, river depth

In: Caetano, M. and Painho, M. (eds). Proceedings of the 7th International Symposium on Spatial Accuracy Assessment in Natural Resources and Environmental Sciences, 5 – 7 July 2006, Lisboa, Instituto Geográfico Português

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