Paired Watershed Study Research Articles

Past and future roles of paired watersheds: a North American inventory and anecdotes from the Caspar Creek Experimental Watersheds

Given the high costs of constructing, maintaining, monitoring, and sampling paired watersheds, it is prudent to ask “Are paired watershed studies still worth the effort?” We present a compilation of 90 North American paired watershed studies and use examples from the Caspar Creek Experimental Watersheds to contend that paired watershed studies are still worth the effort and will continue to remain relevant in an era of big data and short funding cycles. We offer three reasons to justify this assertion. First, paired watersheds allow for watershed-scale experiments that have produced insights into hydrologic processes, water quality, and nutrient cycling for over 100 years. Paired watersheds remain an important guide to inform best management practices for timber harvesting and other land-management concerns. Second, paired watersheds can produce long climate, streamflow, and water quality records because sites are frequently maintained over the course of multiple experiments or long post-treatment periods. Long-term datasets can reveal ecological surprises, such as changes in climate-streamflow relationships driven by slow successional processes. Having multiple watershed records helps identify the cause of these changes. Third, paired watersheds produce data that are ideal for developing and testing hydrologic models. Ultimately, the fate of paired watersheds is up to the scientific community and funding agencies. We hope that their importance continues to be recognized.

Effectiveness of side-inlet vegetated filter strips at trapping pesticides from agricultural runoff

Agriculture can be a contributor of pollutants, including pesticides and excess sediment, to aquatic environments. However, side-inlet vegetated filter strips (VFSs), which are planted around the upstream side of culverts draining agricultural fields, may provide reductions in pesticide and sediment losses from agricultural fields, and have the additional benefit of removing less land from production than traditional VFS. In this study, reductions of runoff, the soluble pesticide acetochlor, and total suspended solids were estimated using a paired watershed field study and coupled PRZM/VFSMOD modeling for two treatment watersheds with source to buffer area ratios (SBAR) of 80:1 (SI-A) and 481:1 (SI-B). Based on the paired watershed ANCOVA analysis, runoff and acetochlor load reductions were significant following the implementation of a VFS at SIA but not SI-B, indicating the potential for side-inlet VFS to reduce runoff and acetochlor load from a watershed with an area ratio of 80:1 but not a higher ratio of 481:1. VFSMOD simulations were consistent with the results of the paired watershed monitoring study, where simulated reductions of runoff, acetochlor loads, and TSS loads were substantially lower for SI-B than SI-A. VFSMOD simulations of SI-B with the SBAR ratio observed at SI-A (80:1) also show that VFSMOD can be used to capture variability in effectiveness of VFS based on multiple factors including SBAR. While this study focused on the effectiveness of side-inlet VFSs at the field scale, broader adoption of properly sized side-inlet VFSs could improve surface water quality at the watershed or larger scales. Additionally, modeling at the watershed scale could aid in locating, sizing, and assessing the impacts of side-inlet VFSs at this larger scale.

Effects of watershed-scale green infrastructure retrofits on urban stormwater quality: A paired watershed study to quantify nutrient and sediment removal

Urban stormwater represents a substantial source of nutrients and sediment to aquatic ecosystems. Green infrastructure (GI), including bioretention and permeable pavement, is an increasingly utilized method to treat stormwater pollutants. Using soil and plants as natural filters, these systems are effective at the site scale, but little evidence exists regarding their performance at the watershed-scale. Blueprint Columbus is an effort by the City of Columbus, Ohio, USA to retrofit GI to eliminate sanitary sewer overflows and remove 20% of total suspended solids (TSS) from existing developed areas. Changes in water quality resulting from the combined effects of many GI practices installed in 11.5 and 47.8 ha treatment watersheds were quantified using a paired-watershed approach. Based on water quality data collected by automated samplers over a 3.5-year period, significant reductions in particulate and dissolved nutrients as well as sediment were observed following the installation of GI in both treatment watersheds compared to the control. Total nitrogen (TN), phosphorus (TP), and TSS concentrations decreased by 13.7–24.1%, 20.9–47.4%, and 61.6–67.7%, respectively. Runoff attenuation by GI contributed to pollutant load reductions of 24.0–25.4% (TN), 27.8–32.6% (TP), and 59.5–78.3% (TSS). Orthophosphate concentrations and loads increased in the watershed with bioretention only but significantly decreased in the treatment watershed with bioretention and permeable pavement. Reductions in TSS concentration were similar (within a margin of 5%) to the percent of the watershed imperviousness treated by GI. Results demonstrate that GI was effective in reducing runoff event mean concentrations and loads at the watershed-scale.

Hydrologic impacts of sewershed-scale green infrastructure retrofits: Outcomes of a four-year paired watershed monitoring study

• Four-year monitoring hydrologic changes in four sewersheds within Columbus, OH USA. • Paired watershed approach to assess impacts of green infrastructure (GI) retrofits. • Reductions in peak flow rates, increases in lag-to-peak following GI implementation. • Additional improvements to infrastructure increased inflows, countering GI benefits. Cities across the world are implementing green infrastructure (GI) retrofits to manage stormwater, but limited research has been performed to quantify the hydrologic impact of these efforts at the watershed-scale. To fill this knowledge gap, this study aimed to monitor and evaluate the impact of GI stormwater control measures (SCMs) on sewershed-scale runoff hydrology across multiple treatment sewersheds with varying GI implementation. A paired watershed approach was applied in which a control (i.e., no GI), and three treatment sewersheds (208 bioretention cells and 8,400 m 2 of permeable pavement in total) were monitored from 2016 to 2019 in Columbus, Ohio, USA. Further infrastructure changes, such as lining sanitary sewer laterals to prevent infiltration and inflow of stormwater, were anticipated to counterbalance the hydrologic improvements provided by the GI retrofits by routing more stormwater to the storm sewers. Significant decreases in runoff depths and peak flow rates (35–62% and 40–58% respectively) and increases in lag-to-peak (6–64%) were observed in the treatment sewersheds following the installation of GI retrofits. Compared to the control sewershed, the treatment sewersheds had slight increases (1–3 mm) in runoff thresholds and lower runoff coefficients post-GI. Following additional infrastructure changes, increases in volume and rate of flows were observed, but hydrologic indicators did not significantly differ from pre-GI levels (i.e., no net impact of the overall project on runoff hydrology). These responses indicated that sewershed-scale GI implementation successfully mitigated peak flow rates; however, the additional infrastructure improvement projects appear to have neutralized volume reductions by routing additional stormwater to the GI. Results confirm the impacts of sewershed-scale GI retrofits; however, research investigating the optimization of GI retrofit location, climate change impacts, and GI design, construction, and maintenance to maximize benefits of distributed SCMs in urban areas should be further explored.

Stacked conservation practices reduce nitrogen loss: A paired watershed study

Combinations of best management practices (BMPs) are needed to achieve nutrient reduction goals in the Mississippi/Atchafalaya River Basin (MARB), but field results are crucial to encourage stacked adoption of BMPs. A paired catchment-scale study (2015–18) was done to assess the impact of (i) BMPs, (ii) precipitation patterns, and (iii) seasonality on nitrogen (N) export. Flow-weighted samples were collected and analyzed for total ammonia nitrogen (TAN), nitrate (NO3–N), and total nitrogen (TN). Catchments Low-BMP 11 and High-BMP 12 had 27.6% and 87.6% areal coverage of BMPs, respectively. No significant difference (p > 0.05) in TAN concentrations was found between Low-BMP 11 (0.023 mg L−1) and High-BMP 12 (0.020 mg L−1). However, NO3–N and TN concentrations were significantly higher (p < 0.05) at Low-BMP 11 (NO3–N: 26.0 mg L−1, TN: 28.7 mg L−1) than at High-BMP 12 (NO3–N: 8.8 mg L−1, TN: 9.2 mg L−1). Two precipitation factors that affected N export patterns were observed. First, N flushing could continue for several years after a drought as elevated NO3–N concentrations were observed in 2015 (i.e., two years after the 2011–2013 drought). Second, higher annual N export was observed when more precipitation occurred during the pre-planting or early-growing season versus later periods. For both catchments, the highest 50% of flows were responsible for majority of the NO3–N export. We estimated that 33–37%, 61–62%, and 82–85% of the NO3–N loads occurred in the 90th, 75th, and 50th flow percentiles, respectively. As demonstrated in High-BMP 12, stacked BMP application effectively lowered NO3–N and TN loads by 60.3% and 59.1%, respectively, relative to Low-BMP 11. Although 27.6% BMP coverage area in Low-BMP 11 was considered low for this study, this coverage area is higher than many other parts of the MARB. This research highlights the importance of joint efforts between landowners in a watershed to meet downstream water quality goals.

How the capacity of bedrock to collect dust and produce soil affects phosphorus bioavailability in the northern Appalachian Mountains of Pennsylvania

AbstractMore above‐ground biomass (kg m−2) grows in the northern Appalachian Mountains (USA) in forests on shale than on sandstone at all landscape positions other than ridgetops. This has been tentatively attributed to physical (rather than chemical) attributes of the substrates, such as elevation, particle size, and water capacity. However, shales have generally similar phosphorus (P) concentrations to sandstones and, in the Valley and Ridge province, they erode more quickly. This led us to hypothesize that faster replenishment of the lithogenic nutrient P in shale soils through erosion + soil production could instead control the differences in biomass. To test this, soils and foliage from 10 sites on shales and sandstones in the northern Appalachians from roughly the same elevation and aspect were analysed. We discovered that, when controlling for location, concentrations of bioavailable P in soils and P in foliage were higher and P resorbed from senescing red oak leaves was lower on slower‐eroding sandstone than on faster‐eroding shale. Lower resorption generally can be attributed to lower P limitation for trees. Further investigation of weathering and erosion on one of the sandstone–shale pairs within a larger, paired watershed study revealed that the differences in P concentrations in biomass and foliage between lithologies likely developed because sandstones act as ‘collectors’ that trap nutrients from residual and exogenous sources, while shales erode quickly and thus promote production of soil from bedrock that releases P to ecosystems. We concluded that the combined effects of differential rates of dust collection and erosion results in roughly equal biomass growing on sandstone and shale ridgetops. This work emphasizes the balance between a landscape's capacity to collect dust versus produce soil in controlling bioavailability of nutrients.

A Field-Based Experiment on the Influence of Stand Density Reduction on Watershed Processes at the Caspar Creek Experimental Watersheds in Northern California

Forests are integral to sustaining clean water resources and healthy watersheds. It is critical, therefore, that managers fully understand the potential impacts of their actions on myriad ecosystem services provided by forested watersheds. While forest hydrologists have long used paired-watershed experiments to elucidate the complex interactions between forest management and watershed biogeochemical and ecohydrological processes, there is still much to learn from these studies. Here, we present an overview of the process for designing a paired-watershed study using a large harvesting experiment at the Caspar Creek Experimental Watersheds in coastal California as an example. We detail many considerations when designing such an experiment and highlight the wide range of scientific investigations that are part of the larger experiment. Paired watershed studies are a great example of community engaged scholarship and offer the unique opportunity to work with land managers to solve applied problems while simultaneously discovering new fundamental knowledge about how watersheds function.

Fifty‐eight years and counting of watershed science at the Caspar Creek Experimental Watersheds in northern California

AbstractThe Caspar Creek Experimental Watersheds are the site of a long‐term paired watershed study in the northern Coast Ranges of California. The watersheds are predominately forested with coast redwood and Douglas‐fir. Old‐growth forest was logged between 1860 and 1904. Two harvesting experiments have been completed since then and a third experiment is currently underway. Caspar Creek data are split into three phases corresponding to three experiments: Phase 1 (1962–1985) reports on a selection harvest (1971–1973) and initial recovery in the South Fork watershed; Phase 2 (1985–2017) includes clearcut harvesting of ~50% of the North Fork watershed (1985–1992) and recovery; and Phase 3 (2017 onward) corresponds to a second selection harvest in the South Fork watershed with a range of subwatershed harvest intensities (2017–2019) and recovery. All three experiments included harvest‐related road‐building and relied primarily on measurements of streamflow and sediment delivery from both treated and reference watersheds. Major findings include modest increases in post‐harvest peak flows and cumulative flow volumes, post‐harvest low flows that initially increased and then decreased 12 to 15 years after harvesting, and the consequences of different yarding techniques and road design on sediment yields. Some of the data for Phase 1 and Phase 2 are available in a USDA Forest Service online archive. The archived data include precipitation, streamflow, suspended sediment concentrations, turbidity, accumulated weir pond sediment volumes, bedload transport rates, water stable isotope data, and geospatial data. Archiving activities are ongoing. Phase 3 data are currently being collected and will be archived after a post‐harvest monitoring period.

Hydrologic impacts of retrofitted low impact development in a commercial parking lot

This paired watershed study investigated the hydrologic impacts of low impact development (LID) stormwater control measures (SCMs) receiving runoff from a highly impervious, highly connected commercial parking area located in Reynoldsburg, Ohio, USA. The site was retrofitted with a bioretention cell and permeable pavement, treating 0.60 ha and 0.42 ha of the parking lot, respectively, following a 12-month monitoring period of the existing parking lot hydrology (i.e., calibration period). Monitoring continued for an additional 20 months after construction (i.e., treatment period). The bioretention cell significantly reduced runoff depths and peak flow rates from the catchment, reducing mean values by 83% and 86%, respectively. Subsurface infrastructure (i.e., a gravel trench enclosing an adjacent sewer pipe and a nearby curb drain) was suspected to provide preferential pathways which resulted in substantial exfiltration from the cell despite poorly infiltrating underlying soils. Conversely, no significant differences in catchment hydrology were observed following the installation of the permeable pavement. The median surface infiltration rate decreased by 96% during the study, indicating that surface clogging occurred in the upper layers of the permeable pavement. Clogging was attributed to concentrated sediment loading delivered from the impervious catchment, along with lack of routine maintenance and snow plowing activities. It was concluded the loading ratio (27.6:1) of the permeable pavement was much too high, and this level of run-on should be avoided in future retrofits. Despite this, the combined effects of the bioretention cell and the permeable pavement led to significant runoff mitigation from the parking lot, reducing both runoff depths (47%) and peak flow rates (56%). Results were similar to previous studies of LID implemented in commercial catchments with lower levels of directly connected impervious cover than those herein. This study demonstrates the effectiveness of SCMs in highly impervious, highly connected catchments and their ability to mitigate runoff in commercial settings when properly sized and maintained.

Understanding dissolved oxygen concentrations in a discontinuously perennial stream within a managed forest

The concentration of oxygen dissolved in water affects aquatic organisms. Unrestrained logging and site preparation can depress dissolved oxygen (DO) concentrations in adjacent streams by elevating temperatures (with removal of riparian shade), impounding channels (reduced reaeration rates), and introducing fresh, oxygen-depleting slash. One of the first paired watershed studies investigating the use of streamside management zones, the Alsea Watershed Study (AWS) in coastal Oregon (1958–1973), highlighted the potential to depress DO concentrations in streams adjacent to harvest units. This and other studies spurred development of Best Management Practices (BMPs) and forest practice rules to maintain, among other things, favorable DO concentrations in streams. A new study of the Alsea watersheds (2006–2015) found little signal of management effects on DO concentrations. It did find that a discontinuously perennial stream (where reaches go subsurface during low flows) can experience naturally low DO concentrations. Forest managers can avoid exacerbating low DO conditions using simple BMPs, but discontinuously perennial forest streams are predisposed to low DO concentrations.

Effects of construction-related land use change on streamflow and sediment yield

Observations from four small watersheds by the Reedy River in upstate South Carolina, USA, were used to evaluate the effects of urban development due to residential construction on streamflow and sediment yield, and to assess the effectiveness of Best Management Practices (BMPs). Paired watershed studies were used to quantify changes in flow magnitudes and sediment outputs at the watershed scale. A novel method based on the Revised Universal Soil Loss Equation was developed to quantify the contribution from each land use to watershed sediment yield.Area-normalized stormflows and peak flows in developed watersheds were 2–9 times greater than those from an undeveloped reference watershed. Sediment yield (SY) and event mean concentration (EMC) were 6 times greater in a developed watershed that had no ongoing construction. In actively developing watersheds, however, SY and EMC were 60–90 times greater compared to the reference. Sediment contribution factor (10−2 kg h MJ−1 mm−1), defined as SY per unit rainfall erosivity, for each land use with 95% confidence interval was: Forest = 4 ± 2, Pasture = 2 ± 2, Full Development = 18 ± 11, Active Development = 440 ± 120. These values can be used to predict long-term change in sediment yield due to a future land-use change. Significant increases in flow and sediment occurred despite the use of BMPs, so improvements to their implementation and/or proper maintenance may be necessary to ensure that their protective goals are met.

Nonstationary frequency pairing reveals a highly sensitive peak flow regime to harvesting across a wide range of return periods

Decades of pairing peak flows of forested and harvested watersheds by equal chronology when evaluating the effects of forest harvesting on peak flows in paired watershed studies lately came under criticisms; and suggestions have been made to abandon the practice and pair by equal frequency instead. However, the stationarity assumption imposed by conventional frequency analyses complicates the use of frequency pairing because peak flows contain change-point and trend-shifting nonstationarities caused by continuous harvesting and forest regrowth. Here we introduce a new nonstationary frequency pairing method for evaluating harvesting effects by allowing the parameters of peak flow frequency distributions to change in time using physically based covariates. The method’s demonstration on a paired watershed study in the snow environment of British Columbia, Canada, reveals how both small (return periods < 10 years) and large (return periods > 10 years) peak flows are highly sensitive to harvesting within the critical mid-elevation south-exposed slopes of the watershed. Such outcomes challenge an age-old deterministic wisdom, but are consistent with the emerging probabilistic understanding of how forests affect peak flows. Advantages of the new method include: (i) bypassing the need for the calibration equation traditionally used in paired watershed studies, thereby eliminating some associated sources of uncertainty; (ii) making use of longer peak flow records by explicitly accounting for the physical causes of the nonstationarities, and thus more explicit inferences about the effects of harvesting on the larger peak flows; and (iii) allowing for the estimation of harvesting effects on peak flows at different points in time during the disturbance history of a watershed, thus providing a direct evaluation of the hydrologic recovery. This study falls within the newly emerging field of “attribution science” which uses a combination of observations and models to identify separately the factors that contribute to extremes. It paves the way to a wider range of applications, not just in the wider hydrology but also other geosciences.

The Use of Low-Altitude UAV Imagery to Assess Western Juniper Density and Canopy Cover in Treated and Untreated Stands

Monitoring vegetation characteristics and ground cover is crucial to determine appropriate management techniques in western juniper (Juniperus occidentalis Hook.) ecosystems. Remote-sensing techniques have been used to study vegetation cover; yet, few studies have applied these techniques using unmanned aerial vehicles (UAV), specifically in areas of juniper woodlands. We used ground-based data in conjunction with low-altitude UAV imagery to assess vegetation and ground cover characteristics in a paired watershed study located in central Oregon, USA. The study was comprised of a treated watershed (most juniper removed) and an untreated watershed. Research objectives were to: (1) evaluate the density and canopy cover of western juniper in a treated (juniper removed) and an untreated watershed; and, (2) assess the effectiveness of using low altitude UAV-based imagery to measure juniper-sapling population density and canopy cover. Ground- based measurements were used to assess vegetation features in each watershed and as a means to verify analysis from aerial imagery. Visual imagery (red, green, and blue wavelengths) and multispectral imagery (red, green, blue, near-infrared, and red-edge wavelengths) were captured using a quadcopter-style UAV. Canopy cover in the untreated watershed was estimated using two different methods: vegetation indices and support vector machine classification. Supervised classification was used to assess juniper sapling density and vegetation cover in the treated watershed. Results showed that vegetation indices that incorporated near-infrared reflectance values estimated canopy cover within 0.7% to 4.1% of ground-based calculations. Canopy cover estimates at the untreated watershed using supervised classification were within 0.9% to 2.3% of ground-based results. Supervised classification applied to fall imagery using multispectral bands provided the best estimates of juniper sapling density compared to imagery taken in the summer or to using visual imagery. Study results suggest that low-altitude multispectral imagery obtained using small UAV can be effectively used to assess western juniper density and canopy cover.

Overstory–Understory Vegetation Cover and Soil Water Content Observations in Western Juniper Woodlands: A Paired Watershed Study in Central Oregon, USA

The effects of western juniper (Juniperus occidentalis) control on understory vegetation and soil water content were studied at the watershed-scale. Seasonal differences in topsoil (12 cm) water content, as affected by vegetation structure and soil texture, were evaluated in a 96-ha untreated watershed and in a 116-ha watershed where 90% juniper was removed in 2005. A watershed-scale characterization of vegetation canopy cover and soil texture was completed to determine some of the potential driving factors influencing topsoil water content fluctuations throughout dry and wet seasons for approximately one year (2014–2015). We found greater perennial grass, annual grass, and shrub cover in the treated watershed. Forb cover was no different between watersheds, and as expected, tree canopy cover was greater in the untreated watershed. Results also show that on average, topsoil water content was 1% to 3% greater in the treated watershed. The exception was during one of the wettest months (March) evaluated, when soil water content in the untreated watershed exceeded that of the treated by &lt;2%. It was noted that soil water content levels that accumulated in areas near valley bottoms and streams were greater in the treated watershed than in the untreated toward the end of the study in late spring. This is consistent with results obtained from a more recent study where we documented an increase in subsurface flow residence time in the treated watershed. Overall, even though average soil water content differences between watersheds were not starkly different, the fact that more herbaceous vegetation and shrub cover were found in the treated watershed led us to conclude that the long-term effects of juniper removal on soil water content redistribution throughout the landscape may be beneficial towards restoring important ecohydrologic connections in these semiarid ecosystems of central Oregon.

Water sustainability and watershed storage

The paired watershed approach is the most popular tool for quantifying the effects of forest watershed management on water sustainability. But this approach does not often address the critical factor of water stored in the landscape. Future work needs to quantify storage in paired watershed studies to inform sustainable water management.

Bison and Cattle Grazing Impacts on Baseflow Suspended Sediment Concentrations within Grassland Streams

Prior to European colonization and the introduction of cattle, bison were the prominent grazing ungulates throughout North American grasslands. Yet, relative zoogeomorphic impacts between bison and cattle on grassland streams remain largely unknown. Utilizing a paired watershed study design, we compared baseflow suspended sediment concentrations across ten watersheds and four grazing treatments (ungrazed, bison-grazed, moderately stocked cattle-grazed, and intensively stocked cattle-grazed) in the Flint Hills subregion of the Great Plains. Additionally, we determined whether periods of increased thermal stress led to higher sediment concentrations within each treatment. Water samples were analyzed for total suspended solids (TSS, mg/L), nonvolatile suspended solids (NVSS, mg/L), and percentage organic matter (POM, percent). Intensively stocked cattle-grazed treatments produced significantly higher TSS and NVSS concentrations relative to ungrazed (TSS p = 0.012, NVSS p < 0.01) and bison-grazed treatments (TSS p = 0.082, NVSS p < 0.01). Moderately stocked cattle-grazed treatments contained significantly higher NVSS concentrations relative to bison-grazed treatments (p = 0.057). Bison-grazed and ungrazed treatments contained similar sediment concentrations (TSS and NVSS p > 0.10). Additionally, intensively and moderately stocked cattle-grazed treatments showed a significant increase in sediment concentrations with increasing temperature (p = 0.024 and p = 0.08, respectively), whereas bison-grazed and ungrazed treatments did not (p > 0.10). At the subdaily timescale, the highest sediment concentrations within cattle-grazed treatments and the greatest difference in sediment concentrations between cattle-grazed and ungrazed treatments coincided with the hottest daily temperatures, further highlighting that cattle-grazing impacts are influenced by thermal conditions.

A multicatchment analysis of headwater and downstream temperature effects from contemporary forest harvesting

AbstractStream temperature is a key physical water‐quality parameter, controlling many biological, chemical, and physical processes in aquatic ecosystems. Maintenance of cool stream temperatures during summer is critical for high‐quality aquatic habitat. As such, transmission of warm water from small, nonfish‐bearing headwater streams after forest harvesting could cause warming in downstream fish‐bearing stream reaches with negative consequences. In this study, we evaluate (a) the effects of contemporary forest management practices on stream temperature in small, headwater streams, (b) the transmission of thermal signals from headwater reaches after harvesting to downstream fish‐bearing reaches, and (c) the relative role of lithology and forest management practices in influencing differential thermal responses in both the headwater and downstream reaches. We measured summer stream temperatures both preharvest and postharvest at 29 sites—12 upstream sites (4 reference, 8 harvested) and 17 downstream sites (5 reference, 12 harvested)—across 3 paired watershed studies in western Oregon. The 7‐day moving average of daily maximum stream temperature (T7DAYMAX) was greater during the postharvest period relative to the preharvest period at 7 of the 8 harvested upstream sites. Although theT7DAYMAXwas generally warmer in the downstream direction at most of the stream reaches during both the preharvest and postharvest period, there was no evidence for additional downstream warming related to the harvesting activity. Rather, theT7DAYMAXcooled rapidly as stream water flowed into forested reaches ~370–1,420 m downstream of harvested areas. Finally, the magnitude of effects of contemporary forest management practices on stream temperature increased with the proportion of catchment underlain by more resistant lithology at both the headwater and downstream sites, reducing the potential for the cooling influence of groundwater.

An Improved Representation of Vegetative Filter Strips in SWAT

Abstract. Vegetative filter strips (VFS) are popular conservation practices installed at the edges of agricultural fields to reduce losses of pollutants from agricultural areas to receiving waterbodies. The recent interest in using VFS areas as multifunctional landscapes necessitates an improved depiction of VFS in simulation models. This study is aimed to enhance the physical representation of VFS in the Soil and Water Assessment Tool (SWAT) to improve the representation of ecohydrologic processes and land management practices in VFS areas. The proposed enhancement enables routing of water, sediment, and nutrients from the source area through the VFS area and makes the infiltrated water and nutrients available for filter crop uptake. The improvements are implemented in SWAT by modifying input files through Matlab scripts and by changing SWAT subroutines to enable routing. The model improvements are tested with three paired watershed studies with and without edge-of-field VFS in central Iowa. The improved model estimated 46% runoff reduction, 91% sediment reduction, 83% total phosphorus reduction, and 54% nitrate reduction with VFS, and these estimates closely matched the measured VFS reductions. The improved model was able to represent increased infiltration, soil moisture, and denitrification in the VFS area, along with accurately capturing crop yields in the source and VFS areas. Overall, the results indicate improved physical representation of VFS in SWAT. Keywords: Conservation measures, Flexible buffer strips, Multifunctional buffers, SWAT model, Vegetative filter strips.

Calibration of paired watersheds: Utility of moving sums in presence of externalities

AbstractHistorically, paired watershed studies have been used to quantify the hydrological effects of land use and management practices by concurrently monitoring 2 similar watersheds during calibration (pretreatment) and post‐treatment periods. This study characterizes seasonal water table and flow response to rainfall during the calibration period and tests a change detection technique of moving sums of recursive residuals (MOSUM) to select calibration periods for each control–treatment watershed pair when the regression coefficients for daily water table elevation were most stable to minimize regression model uncertainty. The control and treatment watersheds were 1 watershed of 3–4‐year‐old intensely managed loblolly pine (Pinus taeda L.) with natural understory, 1 watershed of 3–4‐year‐old loblolly pine intercropped with switchgrass (Panicum virgatum), 1 watershed of 14–15‐year‐old thinned loblolly pine with natural understory (control), and 1 watershed of switchgrass only. The study period spanned from 2009 to 2012. Silvicultural operational practices during this period acted as external factors, potentially shifting hydrologic calibration relationships between control and treatment watersheds. MOSUM results indicated significant changes in regression parameters due to silvicultural operations and were used to identify stable relationships for water table elevation. None of the calibration relationships developed using this method were significantly different from the classical calibration relationship based on published historical data. We attribute that to the similarity of historical and 2010–2012 leaf area index on control and treatment watersheds as moderated by the emergent vegetation. Although the MOSUM approach does not eliminate the need for true calibration data or replace the classic paired watershed approach, our results show that it may be an effective alternative approach when true data are unavailable, as it minimizes the impacts of external disturbances other than the treatment of interest.

Improving model prediction reliability through enhanced representation of wetland soil processes and constrained model auto calibration – A paired watershed study

Process based, distributed watershed models possess a large number of parameters that are not directly measured in field and need to be calibrated, in most cases through matching modeled in-stream fluxes with monitored data. Recently, concern has been raised regarding the reliability of this common calibration practice, because models that are deemed to be adequately calibrated based on commonly used metrics (e.g., Nash Sutcliffe efficiency) may not realistically represent intra-watershed responses or fluxes. Such shortcomings stem from the use of an evaluation criteria that only concerns the global in-stream responses of the model without investigating intra-watershed responses. In this study, we introduce a modification to the Soil and Water Assessment Tool (SWAT) model, and a new calibration technique that collectively reduce the chance of misrepresenting intra-watershed responses. The SWAT model was modified to better represent NO3 cycling in soils with various degrees of water holding capacity. The new calibration tool has the capacity to calibrate paired watersheds simultaneously within a single framework. It was found that when both proposed methodologies were applied jointly to two paired watersheds on the Delmarva Peninsula, the performance of the models as judged based on conventional metrics suffered, however, the intra-watershed responses (e.g., mass of NO3 lost to denitrification) in the two models automatically converged to realistic sums. This approach also demonstrates the capacity to spatially distinguish areas of high denitrification potential, an ability that has implications for improved management of prior converted wetlands under crop production and for identifying prominent areas for wetland restoration.