Interactions of topography and landuse in the runoff response from meso scale basins on the Brazilian craton.

摘要

Channel networks integrate the spatial and temporal variation of runoff from their adjacent hillslopes and convey that water downstream. At the outlet of a watershed, the discharge hydrograph results from a time-space integration of these lateral inputs along the channel network. In Rondonia, where the incised channels have simple morphology and the water travels downstream at a constant rate, the floodwave can be modeled without dispersion. The water balance on the landscape controls the lateral inputs to the channel network and ultimately the volume and timing of the seasonal flood. In this work, model-based comparisons are made of three meso-scale watersheds that represent variations in topography and landuse observed on the northern margin of the Brazilian craton, Rondonia, Brazil. These comparisons rely on the success of utilizing generalized plot and field observations representing large areas within a model framework to produce believable results. Maintaining a consistency between, interpretations and models applied at the point scale, and observations at larger scales, is the challenge of a process-based meso scale model.; A mechanistic hydrologic hillslope model is developed to predict the runoff response from hillslopes under forest and pasture landcover. The uncalibrated model is parameterized with field and laboratory measurements. Typical rainfall characteristics are applied to the model and estimates of the expected runoff volume and timing are calculated. Hydrologic insights gleaned from the hillslope scale model are incorporated in the HEC-HMS basin scale model. HEC-HMS is parameterized with values measured in the field, laboratory, or from representative studies. Hourly rainfall totals are derived from natural rainfall data and uniformly applied to the model. Comparisons are made between predicted and observed hydrographs, and between basins.; Results from the models suggest that deforestation increases the runoff from meso scale basins, and favors flow paths which results in a 2-fold increase in quick flow as compared to a fractional increase in delayed flow, and that the spatial and temporal representation of rainfall is critical to the basin hydrology. The models also suggest that the observed topographic variation plays an insignificant role in runoff generation as compared to deforestation.