The R1 index has three inputs: a.) hypsometric curve, b.) long profile of the trunk stream, and c.) hypsometry of the drainage network. It is based on the understanding that uplift produces an erosion wave that propagates through a basin’s river network beginning at its outlet and proceeding to its headwaters. The drainage network responds (basin changes size) over a certain amount of time as the wave rolls through. This change is recorded in the morphology of the three input factors listed above. The slope (S_r) of the function for R₁ should be steep for a brief period of time during initial uplift, then diminish over time. The index does not constrain age of relative uplift (rock uplift or base level fall), but R₁ has been shown to correlate with basin size (A). The index is designed to minimize influence of bedrock lithology.

R₁ = f(lnA)

Refs:
Demoulin (2010)

More to come…