THE POTENTIAL OF DISTANCE-LIMITED TOPEX IN THE PREDICTION OF SITE WINDINESS

Topex, an index of topographic exposure, has been an important compone nt of regression models that predict site windiness. However, field-ba sed topex can overestimate the shelter of distant hills and does not d iscriminate between hilltop and plateau sites. By using digital terrai n models, it has become possible to calculate alternative formulations of topex with limits to the distance within which the skyline is soug ht, and with declinations (negative values) permitted. These alternati ves have been tested using a dataset of tatter flags in upland Britain . A series of multiple regressions was performed with explanatory vari ables grid, elevation and modified topex, expressed as total topex and as sinusoidal terms to represent aspect and valley shape and directio n. A distance limit of 0.5 km proved superior to thresholds of 0.25, 0 .75, 1.0, 2.0, 3.0 and 10.0 km, and to field topex. Each term except a spect showed a peak in variance ratio at the 0.5 km distance. Although the coefficients varied little between regressions, the topex compone nt of estimated exposure for hilltop sites can vary substantially with the choice of threshold. The use of a distance threshold can lead to improved estimates for these sites. There was no overall improvement i n prediction of site windiness when compared with a previous regressio n and recalculation of the windiness index of upland Britain currently used in prediction of windthrow and in ecological site classification is not obviously required. However, the use of a distance limit would be worth while in future studies using topographic indices because of the apparent improvement in site discrimination and because distance- limited topex may be more easily calculated.