The percentage hypsometric curve (area-altitude curve) relates horizontal cross-sectional area of a drainage basin to relative elevation above basin mouth. By use of dimensionless parameters, curves can be described and compared irrespective of true scale. Curves show distinctive differences both in sinuosity of form and in proportionate area below the curve, here termed the hypsometric integral. A simple three-variable function provides a satisfactory series of model curves to which most natural hypsometric curves can be fitted. The hypsometric curve can be equated to a mean ground-slope curve if length of contour belt is taken into account.
Stages of youth, maturity, and old age in regions of homogeneous rock give a distinctive series of hypsometric forms, but mature and old stages give identical curves unless monadnock masses are present. It is therefore proposed that this terminology be replaced by one consisting of an inequilibrium stage, an equilibrium stage, and a monadnock phase.
Detailed morphometric analysis of basins in five sample areas in the equilibrium stage show distinctive, though small, differences in hypsometric integrals and curve forms. In general, drainage basin height, slope steepness, stream channel gradient, and drainage density show a good negative correlation with mean integrals. Lithologic and structural differences between areas or recent minor uplifts may account for certain curve differences. Regions of strong horizontal structural benching give a modified series of hypsometric curves.
Practical applications of hypsometric analysis are foreseen in hydrology, soil erosion and sedimentation studies, and military science.