Temporally Variable Stream Width and Surface Area Distributions in a Headwater Catchment

Headwater stream networks expand and contract in response to event-driven and seasonal catchment wetness conditions. This dynamic behavior drives variability in the width, length, and surface area of streams, important parameters for constraining a range of ecological and biogeochemical processes, such as atmospheric gas exchange. While the longitudinal expansion and contraction of streams has been studied for some time, variability in stream widths remains poorly understood. Recent studies have found that stream widths at average baseflow conditions follow a log-normal frequency distribution across diverse physiographies. To examine how the distribution of widths varies with flow conditions, we surveyed stream widths 12 times across a 48.4-ha research watershed, located in the Duke Forest in central North Carolina, USA. Here, we show that as runoff increased from the 37th to 99th percentiles of flow, flowing streams widened across the network (“lateral expansion”) and streamflow simultaneously extended upstream to reactivate dry channels (“longitudinal expansion”). In general, as runoff increased, the marginal increase in stream surface area was equally divided between longitudinal and lateral expansion. Even so, the median stream width widens on average with increasing runoff, suggesting that longitudinal and lateral expansion affect the distribution of stream width differently. We find that the form of the relationship between stream width and runoff is a power law, which can be used to refine models for surface area estimation

The On-orbit Calibrations for the Fermi Large Area Telescope

The Large Area Telescope (LAT) on--board the Fermi Gamma ray Space Telescope began its on--orbit operations on June 23, 2008. Calibrations, defined in a generic sense, correspond to synchronization of trigger signals, optimization of delays for latching data, determination of detector thresholds, gains and responses, evaluation of the perimeter of the South Atlantic Anomaly (SAA), measurements of live time, of absolute time, and internal and spacecraft boresight alignments. Here we describe on orbit calibration results obtained using known astrophysical sources, galactic cosmic rays, and charge injection into the front-end electronics of each detector. Instrument response functions will be described in a separate publication. This paper demonstrates the stability of calibrations and describes minor changes observed since launch. These results have been used to calibrate the LAT datasets to be publicly released in August 2009.Comment: 60 pages, 34 figures, submitted to Astroparticle Physic

The future of hydrology: An evolving science for a changing world

Human activities exert global-scale impacts on our environment with significant implications for freshwater-driven services and hazards for humans and nature. Our approach to the science of hydrology needs to significantly change so that we can understand and predict these implications. Such an adjustment is a necessary prerequisite for the development of sustainable water resource management strategies and to achieve long-term water security for people and the environment. Hydrology requires a paradigm shift in which predictions of system behavior that are beyond the range of previously observed variability or that result from significant alterations of physical (structural) system characteristics become the new norm. To achieve this shift, hydrologists must become both synthesists, observing and analyzing the system as a holistic entity, and analysts, understanding the functioning of individual system components, while operating firmly within a well-designed hypothesis testing framework. Cross-disciplinary integration must become a primary characteristic of hydrologic research, catalyzing new research and nurturing new educational models. The test of our quantitative understanding across atmosphere, hydrosphere, lithosphere, biosphere, and anthroposphere will necessarily lie in new approaches to benchmark our ability to predict the regional hydrologic and connected implications of environmental change. To address these challenges and to serve as a catalyst to bring about the necessary changes to hydrologic science, we call for a long-term initiative to address the regional implications of environmental change.Water ManagementCivil Engineering and Geoscience