Climatic and ecological controls of equilibrium drainage density, relief, and channel concavity in dry lands

Drainage density has long been observed to vary among climates, a relationship often attributable to differences in the erosivity of runoff and resistivity of vegetation. There is also evidence, though much less, that relief and channel concavity also vary with climate. The biophysical chain of events that connect climate to these topographic expressions, however, deserves greater attention. Using a numerical landscape evolution model, we examine how a gradient of mean annual precipitation is expressed in the topography of low-to medium-relief, sediment-mantled, water-limited ecosystems. We find equilibrium landscapes have the lowest drainage density, greatest relief, and lowest concavity at intermediate levels of rainfall. This climatic threshold represents the transition from vegetation-dominated sediment flux in drier climates to runoff-dominated flux with more precipitation. In drier climates, marginal increases in precipitation manifest themselves primarily as increases in vegetation while runoff is relatively constant; this acts to suppress sediment transport and decreases the drainage density. In wetter environments, marginal increases in precipitation lead to increases primarily in runoff while vegetation cover remains relatively constant; this results in increased sediment transport and increases the drainage density. The location of the transition depends at least in part on plant community structure and composition. The modeling study illustrates the complexities inherent in biogeomorphic systems but also that a simplified conceptual model of landscape evolution may indeed be sufficient to understand the large-scale patterns. The study also illustrates the opportunities offered by approaching questions of landform development from an ecohydrological perspective.