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The magnitude and mechanisms of nitrous oxide (NO) release from rivers and streams are actively debated. The complex interactions of hydrodynamic and biogeochemical controls on emissions of this important greenhouse gas preclude prediction of when and where NO emissions will be significant. We present observations from column and large-scale flume experiments supporting an integrative model of NO emissions from stream sediments. Our results show a distinct, replicable, pattern of nitrous oxide generation and consumption dictated by subsurface (hyporheic) residence times and biological nitrogen reduction rates. Within this model, NO emission from stream sediments requires subsurface residence times (and microbially mediated reduction rates) be sufficiently long (and fast reacting) to produce NO by nitrate reduction but also sufficiently short (or slow reacting) to limit NO conversion to dinitrogen gas. Most subsurface exchange will not result in NO emissions; only specific, intermediate, residence times (reaction rates) will both produce and release NO to the stream. We also confirm previous observations that elevated nitrate and declining organic carbon reactivity increase NO production, highlighting the importance of associated reaction rates in controlling NO accumulation. Combined, these observations help constrain when NO release will occur, providing a predictive link between stream geomorphology, hydrodynamics, and NO emissions.

Advances in topobathymetric LiDARs could enable rapid surveys at sub‐meter resolution over entire stream networks. This is the first step to improving our knowledge of riverine systems, both their morphology and role in ecosystems. The Experimental Advanced Airborne Research LiDAR B (EAARL‐B) system is one such topobathymetric sensor, capable of mapping both terrestrial and aquatic systems. Whereas the original EAARL was developed to survey littoral areas, the new version, EAARL‐B, was also designed for riverine systems but has yet to be tested. Thus, we evaluated the ability of EAARL‐B to map bathymetry and floodplain topography at sub‐meter resolution in a mid‐size gravel‐bed river. We coupled the EAARL‐B survey with highly accurate field surveys (0.03 m vertical accuracy and approximately 0.6 by 0.6 m resolution) of three morphologically distinct reaches, approximately 200 m long 15 m wide, of the Lemhi River (Idaho, USA). Both point‐to‐point and raster‐to‐raster comparisons between ground and EAARL‐B surveyed elevations show that differences (ground minus EAARL‐B surveyed elevations) over the entire submerged topography are small (root mean square error, RMSE, and median absolute error, M, of 0.11 m), and large differences (RMSE, between 0.15 and 0.38 m and similar M) are mainly present in areas with abrupt elevation changes and covered by dense overhanging vegetation. RMSEs are as low as 0.03 m over paved smooth surfaces, 0.07 m in submerged, gradually varying topography, and as large as 0.24 m along banks with and without dense, tall vegetation. EAARL‐B performance is chiefly limited by point density in areas with strong elevation gradients and by LiDAR footprint size (0.2 m) in areas with topographic features of similar size as the LiDAR footprint. © 2018 John Wiley & Sons, Ltd.

Dissolved oxygen (DO) concentrations and consumption rates are primary indicators of heterotrophic respiration and redox conditions in the hyporheic zone (HZ). Due to the complexity of hyporheic flow and interactions between hyporheic hydraulics and the biogeochemical processes, a detailed, mechanistic, and predictive understanding of the biogeochemical activity in the HZ has not yet been developed. Previous studies of microbial activity in the HZ have treated the metabolic DO consumption rate constant (KDO) as a temporally fixed and spatially homogeneous property that is determined primarily by the concentration of bioavailable carbon. These studies have generally treated bioactivity as temporally steady state, failing to capture the temporal dynamics of a changeable system. We demonstrate that hyporheic hydraulics controls rate constants in a hyporheic system that is relatively abundant in bioavailable carbon, such that KDO is a linear function of the local downwelling flux. We further demonstrate that, for triangular dunes, the downwelling velocities are lognormally distributed, as are the KDO values. By comparing measured and modeled DO profiles, we demonstrate that treating KDO as a function of the downwelling flux yields a significant improvement in the accuracy of predicted DO profiles. Additionally, our results demonstrate the temporal effect of carbon consumption on microbial respiration rates.

A "quality-based" measure of housing affordability problems employing the cost of housing just meeting adequacy standards is proposed as an improvement over the conventional "high" rent-to-income criterion. Based on "Annual Housing Survey" data, affordability difficulties grew between 1975 and 1983 by either measure. The conventional measure, however, overestimated the extent of quality-based affordability difficulty for renters by 20% in 1975 and 24% in 1983 based upon Section 8 housing quality standards. In addition, 35% of rental households with an affordability problem by the conventional measure did not have an affordability problem by the quality-based measure, while 19 to 23% of rental households found to have an affordability problem by the quality-based measure were not so classified using the conventional measure. Copyright American Real Estate and Urban Economics Association.