Ground-Based Thermography of Fluvial Systems at Low and High Discharge Reveals Potential Complex Thermal Heterogeneity Driven by Flow Variation and Bioroughness

Temperature is a primary physical and biogeochemical variable in aquatic systems. Field-based measurement of temperature at discrete sampling points has revealed temperature variability in fluvial systems, but traditional techniques do not readily allow for synoptic sampling schemes that can address temperature-related questions with broad, yet detailed, coverage. We present results of thermal infrared imaging at different stream discharge (base flow and peak flood) conditions using a handheld IR camera. Remotely sensed temperatures compare well with those measured with a digital thermometer. The thermal images show that periphyton, wood, and sandbars induce significant thermal heterogeneity during low stages. Moreover, the images indicate temperature variability within the periphyton community and within the partially submerged bars. The thermal heterogeneity was diminished during flood inundation, when the areas of more slowly moving water to the side of the stream differed in their temperature. The results have consequences for thermally sensitive hydroecological processes and implications for models of those processes, especially those that assume an effective stream temperature

Seasonal energy and water balance of a \u3ci\u3ePhragmites australis\u3c/i\u3e-dominated wetland in the Republican River basin of south-central Nebraska (USA)

energy and water balance, especially in dry climatic regions, can have a significant impact on local water availability and, therefore, water resource management. The purpose of this study is to quantify the energy and water balance of a riparian wetland in a subhumid region of the central US, as well as the role of seasonal climate variability and vegetation phenology. The site is located in the Republican River basin in south-central Nebraska, where decreases in streamflow have been observed in recent decades. In an effort to reduce consumptive water use from evapotranspiration (ET), and thereby reclaim surface water, invasive species such as Phragmites australis have been removed throughout the riparian corridor of the river basin. In this study, we used energy/water balance monitoring stations, a Large Aperture Scintillometer (LAS), and numerous water and soil temperature probes to determine the energy and water balance during the 2009 growing season (April 11−October 3). Sensible heat flux was measured using the LAS, while ET was calculated as a residual of the energy balance (i.e., net radiation minus sensible heat flux and heat storage rates in the canopy, water, and soil). Rigorous quality control and uncertainty analyses were performed, and comparisons were also made with ET rates calculated via the simpler Priestley–Taylor method. Results of the energy budget analysis indicate that the average ET rate for the wetland during the growing season was 4.4 mm day−1, with a maximum daily rate of 8.2 mm day−1 (occurring on June 29). Precipitation during the same 176-day period averaged 2.7 mm day−1. Net radiation and vegetation phenology were found to be the two largest drivers of seasonal variability in ET. Sensible heat flux was significantly larger than latent heat flux early in the season, when standing vegetation in the wetland was still dry and brown. By late May and early June, however, Bowen ratios had declined well below 0.5 in response to greener and more abundant vegetation, higher transpiration rates, and reduced sensible heat flux. Heat storage rates in the wetland were dominated by changes in water temperature (as compared to soil or canopy heat storage) and comprised a significant portion of the hourly energy balance. On daily mean timescales, changes in the rate of heat storage corresponded to ~13% of the variability in net radiation, while for the season-long average, the heat storage term was found to be essentially negligible. The Priestley–Taylor equation provided a reasonable estimate of ET during the height of the growing season but significantly overestimated ET during the beginning of the season (since it could not account for large sensible heat fluxes from the dry vegetation). Analysis of the wetland water balance showed seasonal variations in water level that were similar to changes in cumulative water inputs (i.e., precipitation minus ET). Portions of the season when the two curves had differing rates of change indicated periods of net water influx or outflux from other sources (primarily groundwater)

Seasonal Variation in Floodplain Biogeochemical Processing in a Restored Headwater Stream

Stream and river restoration activities have recently begun to emphasize the enhancement of biogeochemical processing within river networks through the restoration of river-floodplain connectivity. It is generally accepted that this practice removes pollutants such as nitrogen and phosphorus because the increased contact time of nutrient-rich floodwaters with reactive floodplain sediments. Our study examines this assumption in the floodplain of a recently restored, low-order stream through five seasonal experiments. During each experiment, a floodplain slough was artificially inundated for 3 h. Both the net flux of dissolved nutrients and nitrogen uptake rate were measured during each experiment. The slough was typically a source of dissolved phosphorus and dissolved organic matter, a sink of NO₃^–, and variable source/sink of ammonium. NO₃^– uptake rates were relatively high when compared to riverine uptake, especially during the spring and summer experiments. However, when scaled up to the entire 1 km restoration reach with a simple inundation model, less than 0.5–1.5% of the annual NO₃^– load would be removed because of the short duration of river-floodplain connectivity. These results suggest that restoring river-floodplain connectivity is not necessarily an appropriate best management practice for nutrient removal in low-order streams with legacy soil nutrients from past agricultural landuse

<i>Chaoborus</i> spp. Transport CH₄ from the Sediments to the Surface Waters of a Eutrophic Reservoir, But Their Contribution to Water Column CH₄ Concentrations and Diffusive Efflux Is Minor

<i>Chaoborus</i> spp. (midge larvae) live in the anoxic sediments and hypolimnia of freshwater lakes and reservoirs during the day and migrate to the surface waters at night to feed on plankton. It has recently been proposed that <i>Chaoborus</i> take up methane (CH₄) from the sediments in their tracheal gas sacs, use this acquired buoyancy to ascend into the surface waters, and then release the CH₄, thereby serving as a CH₄ “pump” to the atmosphere. We tested this hypothesis using diel surveys and seasonal monitoring, as well as incubations of <i>Chaoborus</i> to measure CH₄ transport in their gas sacs at different depths and times in a eutrophic reservoir. We found that <i>Chaoborus</i> transported CH₄ from the hypolimnion to the lower epilimnion at dusk, but the overall rate of CH₄ transport was minor, and incubations revealed substantial variability in CH₄ transport over space and time. We calculated that <i>Chaoborus</i> transport ∼0.1 mmol CH₄ m^–2 yr^–1 to the epilimnion in our study reservoir, a very low proportion (<1%) of total CH₄ diffusive flux during the summer stratified period. Our data further indicate that CH₄ transport by <i>Chaoborus</i> is sensitive to water column mixing, <i>Chaoborus</i> density, and <i>Chaoborus</i> species identity