A comparison of the spatial distribution of vadose zone water in forested and agricultural floodplains a century after harvest

The role of soils in the regulation of hazards and extreme events

The frequency and intensity of natural hazards and extreme events has increased throughout the last century, resulting in adverse socioeconomic and ecological impacts worldwide. Key factors driving this increase include climate change, the growing world population, anthropogenic activities and ecosystem degradation. One ecologically focused approach that has shown potential towards the mitigation of these hazard events is the concept of nature's contributions to people (or NCP), which focuses on enhancing the material and non-material benefits of an ecosystem to reduce hazard vulnerability and enhance overall human well-being. Soils, in particular, have been identified as a key ecosystem component that may offer critical hazard regulating functionality. Thus, this review investigates the modulating role of soils in the regulation of natural hazards and extreme events, with a focus on floods, droughts, landslides and sand/dust storms, within the context of NCP. This article is part of the theme issue 'The role of soils in delivering Nature's Contributions to People'.

A Case-Study Application of the Experimental Watershed Study Design to Advance Adaptive Management of Contemporary Watersheds

Land managers are often inadequately informed to make management decisions in contemporary watersheds, in which sources of impairment are simultaneously shifting due to the combined influences of land use change, rapid ongoing human population growth, and changing environmental conditions. There is, thus, a great need for effective collaborative adaptive management (CAM; or derivatives) efforts utilizing an accepted methodological approach that provides data needed to properly identify and address past, present, and future sources of impairment. The experimental watershed study design holds great promise for meeting such needs and facilitating an effective collaborative and adaptive management process. To advance understanding of natural and anthropogenic influences on sources of impairment, and to demonstrate the approach in a contemporary watershed, a nested-scale experimental watershed study design was implemented in a representative, contemporary, mixed-use watershed located in Midwestern USA. Results identify challenges associated with CAM, and how the experimental watershed approach can help to objectively elucidate causal factors, target critical source areas, and provide the science-based information needed to make informed management decisions. Results show urban/suburban development and agriculture are primary drivers of alterations to watershed hydrology, streamflow regimes, transport of multiple water quality constituents, and stream physical habitat. However, several natural processes and watershed characteristics, such as surficial geology and stream system evolution, are likely compounding observed water quality impairment and aquatic habitat degradation. Given the varied and complicated set of factors contributing to such issues in the study watershed and other contemporary watersheds, watershed restoration is likely subject to physical limitations and should be conceptualized in the context of achievable goals/objectives. Overall, results demonstrate the immense, globally transferrable value of the experimental watershed approach and coupled CAM process to address contemporary water resource management challenges.

Quantifying relationships between urban land use and flow frequency of small Missouri streams

Flow frequency is an important hydrologic statistic to consider in environmental flows assessment. However, there is a paucity of focused interdisciplinary hydrologic assessments that quantify human development influence on flow frequency of small streams (drainage area < 282 km²). Relationships between urban land use and land cover (LULC) and flow frequency were assessed for general trends at current gauged watersheds (n = 32) of Missouri, USA. Urban land use - flow frequency relationships changed from linear in developed areas with <50% total impervious surfaces (i.e. low density urban areas), to non-linear in developed areas with >50% total impervious surfaces (i.e. high density urban areas). Urban land use influence on flow frequency was not detected in events below median flow (0.02510 < R² > 0.03356; n = 32). Conversely, urban land use - flow frequency relationships were relatively strong above median flow (0.55500 < R² > 0.78703; n = 32). Further, explained variance generally increased to meso-scale flows (0.58350 < R² > 0.82470; n = 32), and then, decreased during high flows (0.34912 < R² > 0.61805; n = 32). More specifically, the greatest observed influence of urban land use on flow frequency increased from a 0.2 to 1 year return period in low density urban areas, to a 1 to 2 year return period in high density urban areas in small Missouri streams. Thus, results indicate that management efforts should focus on reducing the frequency of 1 year events in low density urban land use areas, and 2 year events in high density urban areas to secure environmental services of small urban streams in Missouri, USA. These results hold important implications for other regions globally, where urban land use has increased the frequency of streamflow response.

A method for advancing understanding of streamflow and geomorphological characteristics in mixed-land-use watersheds

Methods are needed to quantify stream geomorphological response to land use and hydroclimatic variability. The method applied herein incorporated channel measurements from a physical habitat assessment (channel width, bankfull width, thalweg depth, and estimated cross-sectional area), and streamflow data collected via an experimental watershed study, to identify factors contributing to longitudinal variation in stream morphology in a mixed-land-use watershed of the central U.S. Channel and bankfull width ranged from 0.8 m and 1.8 m, respectively, at the headwaters, to 70 m and 74 m, respectively, mid-watershed. Minimum thalweg depth (0.2 m) was observed at the headwaters, while the maximum (8.6 m) was observed at the mouth. Mann Kendall results indicated a significant positive trend (p < 0.001) for each of the three metrics over the entire length of the stream. However, smaller sections of the creek exhibited contrasting trends consistent with channel widening and incision. Cross-sectional area significantly (p < 0.001) increased from the headwaters to the mouth. However, two reaches exhibited drastic reductions in cross-sectional area, which could indicate reduced channel capacity and localized flood hazard. The longitudinal pattern of channel width, bankfull width, and cross-sectional area showed the strongest (R² > 0.7), significant (p < 0.05) correlations with the estimated longitudinal pattern of 99th percentile flows, while thalweg depth correlated most strongly with 75th percentile flows (R² = 0.77, p < 0.001). Collectively, results emphasize the importance of high flows to channel morphology, but identify other factors (e.g. land use, geology, physiography) that variously contribute to observed stream geomorphology. Furthermore, results demonstrate the capacity of the method to provide detailed, quantitative characterizations of physical and hydrologic features, and to identify potential drivers of channel morphology in contemporary mixed-land-use watersheds.

Flow class analyses of suspended sediment concentration and particle size in a mixed-land-use watershed

Knowledge gaps remain concerning fundamental suspended sediment physical processes/relationships, such as particle size class dynamics and hydroclimatic variability. Streamwater grab samples were collected four times per week (Oct. 2009-Feb. 2014) at nested-scale gauging sites (n = 5), representing contrasting dominant land use practices. Streamflow was monitored in situ. Grab samples were analyzed for total suspended sediment concentration and mean particle size using laser particle diffraction. Comparisons were performed of suspended sediment parameters corresponding to different streamflow classes (i.e. 20th, 40th, 60th, 80th, and 99th percentile flows). Average suspended sediment concentrations displayed a decreasing trend from the predominately agricultural headwaters to the urbanized mid-watershed, and a subsequent increase to the suburban lower watershed. Results indicated significant (p < 0.05) differences in concentrations corresponding to different flow classes, with concentrations at more urban sites displaying greater "sensitivity" to streamflow variability. Significant (p < 0.05) differences between concentrations at different sites were found, but concentrations became progressively more similar (p > 0.05) at higher flows. Mean particle size results displayed significant differences (p < 0.05) between flow classes at every site. Notably, results showed a decrease in particle size during progressively higher flows, despite expectations based on stream velocity/competence relationships. Significant (p < 0.05) spatial differences in particle size were found between sites, specifically for flows within the 20th and 40th percentile flow class. However, the spatial pattern was weakened at higher flows (60th, 80th, and 99th percentile flow classes) as sites displayed greater statistical similarity. Collectively, results highlight the compounding influences of streamflow variability and land use practices on suspended sediment regimes; and considering unexpected results regarding relationships between particle size and flow, emphasize the need for continued research concerning particle size dynamics.

Spatiotemporal variability of suspended sediment particle size in a mixed-land-use watershed

Given existing knowledge gaps, there is a need for research that quantitatively characterizes spatiotemporal variation of suspended sediment particle size distribution (PSD) in contemporary watersheds. A five-year study was conducted in a representative watershed of the central United States utilizing a nested-scale experimental watershed study design, comprising five gauging sites partitioning the catchment into five sub-watersheds. Streamwater grab samples were collected four times per week, at each gauging site, for the duration of the study period (Oct. 2009-Feb. 2014). Samples were analyzed using laser particle diffraction. Significantly different (p<0.05) suspended sediment PSDs were observed at monitoring sites throughout the course of the study. For example, results indicated greater proportions of silt at site #5 (65%), relative to other sites (41, 32, 29, and 43%, for sites #1-#4, respectively). Likewise, results showed greater proportions of sand at sites #2 and #3 (66 and 68%, respectively), relative to other sites (57, 55, and 34%, for sites #1, #4, and #5, respectively). PSD spatial variability was not fully explained by hydroclimate or sub-watershed land use/land cover characteristics. Rather, results were strengthened by consideration of surficial geology (e.g. supply-controlled spatial variation of particle size). PSD displayed consistent seasonality during the study, characterized by peaks in the proportion of sand (and aggregates) during the winter (i.e. 70-90%), and minimums during the summer (i.e. 12-38%); and peaks in the proportion of silt particles in the summer (i.e. 61-88%) and minimums in the winter (i.e. 10-23%). Likely explanations of results include seasonal streamflow differences. Results comprise distinct observations of spatiotemporal variation of PSD, thereby improving understanding of lotic suspended sediment regimes and advancing future management practices in mixed-land-use watersheds.

Improving understanding of mixed-land-use watershed suspended sediment regimes: Mechanistic progress through high-frequency sampling

Given the importance of suspended sediment to biogeochemical functioning of aquatic ecosystems, and the increasing concern of mixed-land-use effects on pollutant loading, there is an urgent need for research that quantitatively characterizes spatiotemporal variation of suspended sediment dynamics in contemporary watersheds. A study was conducted in a representative watershed of the central United States utilizing a nested-scale experimental watershed design, including five gauging sites (n=5) partitioning the catchment into five sub-watersheds. Hydroclimate stations at gauging sites were used to monitor air temperature, precipitation, and stream stage at 30-min intervals during the study (Oct. 2009-Feb. 2014). Streamwater grab samples were collected four times per week, at each site, for the duration of the study (Oct. 2009-Feb. 2014). Water samples were analyzed for suspended sediment using laser particle diffraction. Results showed significant differences (p<0.05) between monitoring sites for total suspended sediment concentration, mean particle size, and silt volume. Total concentration and silt volume showed a decreasing trend from the primarily agricultural upper watershed to the urban mid-watershed, and a subsequent increasing trend to the more suburban lower watershed. Conversely, mean particle size showed an opposite spatial trend. Results are explained by a combination of land use (e.g. urban stormwater dilution) and surficial geology (e.g. supply-controlled spatial variation of particle size). Correlation analyses indicated weak relationships with both hydroclimate and land use, indicating non-linear sediment dynamics. Suspended sediment parameters displayed consistent seasonality during the study, with total concentration decreasing through the growing season and mean particle size inversely tracking air temperature. Likely explanations include vegetation influences and climate-driven weathering cycles. Results reflect unique observations of spatiotemporal variation of suspended sediment particle size class. Such information is crucial for land and water resource managers working to mitigate aquatic ecosystem degradation and improve water resource sustainability in mixed-land-use watersheds globally.

Land use change impacts on floods at the catchment scale: Challenges and opportunities for future research

Research gaps in understanding flood changes at the catchment scale caused by changes in forest management, agricultural practices, artificial drainage, and terracing are identified. Potential strategies in addressing these gaps are proposed, such as complex systems approaches to link processes across time scales, long-term experiments on physical-chemical-biological process interactions, and a focus on connectivity and patterns across spatial scales. It is suggested that these strategies will stimulate new research that coherently addresses the issues across hydrology, soil and agricultural sciences, forest engineering, forest ecology, and geomorphology.

Quantifying loading, toxic concentrations, and systemic persistence of chloride in a contemporary mixed-land-use watershed using an experimental watershed approach

A nested-scale experimental watershed study was implemented to quantify loading and persistence of chloride in an urbanizing, mixed-land-use watershed. A Midwest USA (Missouri) watershed was partitioned into five sub-basins with contrasting dominant land use. Streamwater was tested for chloride concentration four days per week from October 2009 through May 2014 at each site. Monitoring sites included co-located gauging and climate stations recording variables at 30-minute intervals. Results indicate significant (p<0.01) differences in chloride concentrations and loading between sites. Loading consistently increased from the forested headwaters (average=507kgday^-1) to primarily urban watershed terminus (average=7501kgday^-1). Chloride concentrations were highest (average=83.9mgL^-1) with the greatest frequency of acutely toxic conditions (i.e. 860mgL^-1) mid-watershed. This finding is in-part attributable to the ratio of chloride application to streamflow volume (i.e. increasing flow volume with stream distance resulted in chloride dilution, offsetting increased percent urban land use with stream distance). Results highlight the important, yet often confounding, interactions between pollutant loading and flow dynamics. Chloride peaks occurred during late winter/early spring melting periods, implicating road salt application as the primary contributor to the chloride regime. Floodplain groundwater analysis indicated seasonal sink/source relationships between the stream and floodplain, which could contribute to chronic toxicity and persistent low Cl^- concentrations in streamwater year-round. Results hold important implications for resource managers wishing to mitigate water quality and aquatic habitat degradation, and suggest important water quality limitations to stream restoration success in complex urban aquatic ecosystems.

Continuous and event-based time series analysis of observed floodplain groundwater flow under contrasting land-use types

There is an ongoing need to improve quantitative understanding of land-use impacts on floodplain groundwater flow regimes. A study was implemented in Hinkson Creek Watershed, Missouri, USA, including equidistant grids of nine piezometers, equipped with pressure transducers, which were installed at two floodplain study sites: a remnant bottomland hardwood forest (BHF) and a historical agricultural field (Ag). Data were logged at thirty minute intervals for the duration of the 2011, 2012, 2013, and 2014 water years (October 1, 2010-September 30, 2014). Results show significant (p<0.001) differences between Darcy-estimated groundwater flow at the two study sites. Although median flow values at the two sites were similar (0.009 and 0.010mday(-1) for the Ag and BHF sites, respectively), the BHF displayed a more dynamic flow regime compared to the Ag site. Maximum flow values were 0.020 and 0.049mday(-1) for the Ag and BHF sites, respectively. Minimum flow values were -0.018 and -0.029mday(-1) for the Ag and BHF sites, respectively. The BHF showed greater magnitude, longer duration, and more frequent negative flows, relative to the Ag site. Event-based analyses indicated a more seasonally responsive flow regime at the BHF, with higher flows than the Ag site during the wet season and lower flows than the Ag site during the dry season. Notably, the seasonal pattern of relative site flow differences was consistent across a wide range of precipitation event magnitudes (i.e. 8-45mm). Results are by majority attributable to greater rates of plant water use by woody vegetation and preferential subsurface flow at the BHF site. Collectively, results suggest greater flood attenuation capacity and streamwater buffering potential by the BHF floodplain, relative to the Ag, and highlight the value of floodplain forests as a land and water resource management tool.

The role of environmental land use conflicts in soil fertility: A study on the Uberaba River basin, Brazil

In the Uberaba River basin (state of Minas Gerais, Brazil), pastures for livestock production have invaded areas of native vegetation (Cerrado biome), while already existing pastures were invaded by crop agriculture, with an expansion of sugar cane plantations in the most recent years. In some areas of the basin, these land use changes were classified as environmental land use conflicts because the new uses were not conforming to land capability, i.e. the soil's natural use. Where the areas in conflict became dense, some soil properties have changed significantly, namely the organic matter content and the exchangeable potassium concentration, which have decreased drastically (5kg/m(3) per 10% increase in the conflict area) threatening the fertility of soil. Besides, these changes may have triggered a cascade of other environmental damages, specifically the increase of soil erosion and the degradation of water quality with negative impacts on aquatic biodiversity, related to a disruption of soil organic matter structural functions. Because half the Uberaba catchment has been considered is a state of accentuated environmental degradation, not only caused by environmental land use conflicts, conservation measures have been proposed and requested for immediate implementation across the watershed.