Impacts of watershed hydrology on long-term landscape evolution

Numerous studies have examined the impacts of geomorphology on the hydrologic processes of river basins, but much less attention has been given to the opposite problem: the effects of hydrologic processes on the evolution of river basin topography. Fluvial erosion processes are driven by water discharge on the land surface, which depends on precipitation rates, soil moisture conditions, and groundwater discharge. Much evidence indicates that these hydrologic processes have influences on landscape evolution. This dissertation focuses on obtaining a deeper understanding about how watershed hydrology affects the long-term evolution of a fluvial geomorphic system.; The dissertation is a collection of three papers. The first paper investigates the implications of the Geomorphic Effective Event (GEE) in landscape evolution. The behavior of the GEE is examined when a generic stream power model with a threshold is used to describe either the detachment or transport of sediment by flowing water. The results suggest that the return period of the GEE depends primarily on the threshold value when the exponent on discharge is less than two. Otherwise, it depends primarily on the exponent. The GEE usually cannot be substituted for the probability density function of discharge because it produces a different long-term erosion rate. Furthermore, the return period of the GEE can vary spatially in a basin.; The second paper presents a detailed hydrologic model, which is imbedded into a widely-used landscape evolution model, and evaluates the role of groundwater dynamics in long-term drainage basin evolution. In the hydrologic model, precipitation is generated by a stochastic process, and the precipitation is partitioned between surface runoff and groundwater recharge using a specified infiltration capacity. Groundwater flow is simulated by a dynamic two-dimensional Dupuit equation for an unconfined aquifer with an irregular underlying impervious layer. The model is applied to the WE-38 basin, an experimental watershed in Pennsylvania, because 60-80% of the discharge from that basin is derived from groundwater and substantial hydrologic and geomorphic information is available for model calibration. The results of this modeling exercise indicate that the basin can be divided into three zones with distinct streamflow generating characteristics. Over long periods of time, scenarios in which groundwater discharge is large tend to modify the topography in a way that promotes groundwater discharge.; Finally, in the third paper the combined hydrologic/geomorphic model is used to investigate the sensitivity of two specific watershed characteristics---the slope-area relationship and hypsometric curve---to the streamflow generation mechanisms. The comparison between simulated results and observations shows that the steady state groundwater-discharge topography has a more complex slope-area relationship and a lower hypsometric curve than the steady state Horton-runoff topography. The sensitivity analysis suggests that groundwater discharge produces an abrupt hillslope-valley transition only if the erosion threshold is close to zero and the exponent on discharge is below one. The hypsometric curve is highly dependent on the stage of development of the river basin. However, the hypsometry in a groundwater-discharge basin appears to be much more sensitive to the erosion threshold than the hypsometry in a Horton-runoff basin.