Fig. 7.

Rainfall and lithological effects on Hack’s parameters for different classes of basin shapes. a, b Maps of the six main rainfall classes¹⁶ and the four major geological units in Bhutan¹⁷. c–f Influence of rainfall (c, e) and lithology (d, f) on Hack’s exponent (c, d) and coefficient (e, f). Colours of dots indicate the GC class as shown in Fig. 1. Geological units are: Siwalik Hills (SH), Lesser Himalayas (LH), Greater Himalayas (GH) and Tethyan Sedimentary Series (TSS). Hack’s exponent shows dispersion of GC values within a given rainfall or lithology class but remains generally constant. Hack’s coefficient remains constant for a given GC class regardless of rainfall or lithology. The SH lithology class displays very strong dispersion associated with a scarce dataset (13 basins) and is discarded from our analysis