H12A-01 INVITED
Long-term Experimental Networks for Stream Ecosystem Studies: the Lotic Intersite Nitrogen Experiment (LINX) and the Stream Experimental and Observatory Network (STREON) Component of the National Ecological Observatory Network
LINX and STREON represent large experimental networks to understand the dynamics of stream ecosystems. These inter-site coordinated experiments represent a shift towards research whose goal is to understand how the drivers of ecosystem structure and function change across major continental-scale environmental gradients. LINX consisted of two consecutive, inter-site studies involving 15N-tracer additions to streams to determine rates and mechanisms of nitrogen cycling at the scale of entire stream reaches. The LINX studies, which lasted for a decade, were the first to use the 15N addition approach to determine gross rates of ammonium and nitrate uptake, nitrification, and denitrification under ambient conditions in stream ecosystems across several biomes and many types of land use. The LINX studies documented the important roles of stream hydrology (flow and transient storage zones), chemistry (N concentrations), and biology (in- stream gross primary production and respiration rates) in controlling N uptake and retention. Stream network models based on field results showed that streams are important sites for N retention in the landscape. Although LINX demonstrated the importance of experiments conducted across stream networks, these studies were limited in duration and could not address questions dealing with long-term impacts. STREON, the stream component of the planned National Ecological Observatory Network (NEON), is intended to use long-term observations and experimental nutrient and consumer organism manipulations to understand controls and forecast changes in stream ecosystems. The observations, to be conducted in a network of more than 20 streams across the U.S. and Puerto Rico over 30 years, will involving state-of-the- art in situ sensors and data communication devices as well as periodic sampling and analyses to determine hydrologic, geomorphologic, biogeochemical, and biological properties and processes. The experiments, to be conducted at about 10 sites over at least 10 years, will involve N and P additions and secondary consumer exclosures to provide a better understanding of how eutrophication and consumer extinction and extirpation, two of the most pervasive forms of environmental change, interact to affect stream ecosystems. Together, the STEON observational and experimental studies will provide critical information on effects of such stresses as climate change, land use change, invasive species, and N deposition on stream ecosystems.
H12A-02
Increased Dissolved Organic Carbon (DOC) in Central European Streams is Generated by Ionic Strength Reductions Rather than Decreasing Acidity or Climate Change
DOC trends were investigated at two forested catchments (part of the GEOMON network) in western Czech Republic. They represent geochemical end-members of ecosystem sensitivity to acidification (acid-sensitive granitic Lysina catchment vs. acid-resistant serpentinitic Pluhuv Bor catchment). Despite very different bedrocks, soils and stream chemistry, mean discharge-weighted DOC concentrations were similar at both catchments between 1993-2007: 18.8 mg/L at Lysina and 20.2 mg/L at Pluhuv Bor. Between 1993 and 2007 DOC increased significantly (linear regression) at both catchments: the mean annual increase was 0.42 mg/L/yr (p<0.001) at Lysina and 0.43 mg/L/yr (p<0.001) at Pluhuv Bor. Thus we observed a 64% increase in DOC at Lysina and 65% at Pluhuv Bor. The long-term increase in DOC was correlated with a decrease in ionic strength (IS) at both catchments (p<0.001), which resulted from declining atmospheric deposition. Deposition of 30-39 kg S/ha/yr was observed between 1991-1993 and it declined to 7-11 kg S/ha/yr in 2004-2006, but only granitic Lysina was significantly acidified by this acidic deposition. Well-buffered Pluhuv Bor showed no decrease in soil or stream pH, as all incoming SO4 from the atmosphere was buffered by exchangeable cations in the magnesium-rich soils. Acid input was only manifested by rising IS, leading us to conclude that declining acidity did not play a role in the observed DOC increase at Pluhuv Bor. Data from low flow events throughout the study period further supports this conclusion. At acidic Lysina, DOC increased and IS decreased during low flow periods (25th precentile of runoff) despite the fact that pH did not change. At well-buffered Pluhuv Bor, acidity decreased during low flow, but IS and DOC remained unchanged. Climate change is not driving the increase in DOC at our study sites, because neither temperature, annual precipitation nor discharge (annual or weekly) show statistically significant trends during the study period.
H12A-03
Variations of fluvial export of large wood and sediment along the precipitation and latitude at the watershed scale
The fluvial export of large wood (LW) is almost impossible to measure at gauging stations in small watersheds, as its occurrence is generally associated with episodic and large flood events. However, in Japan, the annual stream transport volume of LW, as well as sediment volume to reservoirs, has been monitored by local reservoir management offices. The objectives of this study were to (1) elucidate the effects of precipitation variability regulating the LW and sediment export at the watershed scale, and (2) examine changes in LW and sediment export pattern along the latitudinal gradient of the Japanese archipelago. Annual precipitation was selected as one of the most important factors in explaining the variation in LW and sediment export at each reservoir site. In order to examine the precipitation intensity influencing LW and sediment export, we investigated not only an annual precipitation but also sums of daily precipitation equal to and greater than 10, 20, 30, c , 80, 90 and 100 mm. The fluvial export of LW and sediment produced and redistributed by various disturbance processes (such as landslides, debris and snow avalanches, debris flows, floods, wildfires, windstorms and tree mortalities) should be highly variable according to a period with and without huge and infrequent precipitation events. This study showed that sum of daily precipitation equal to and greater than 40 mm as the most important variable explaining the variation in LW export. Amounts of LW fluvial export in southern and central Japan are probably influenced by typhoon events and localized torrential downpours. Thus, LW pieces stored on the channel floor were consistantly removed, and thereby fluvial export is supply-limited and its accumulation may be less than in northern Japan where fewer large precipitation events occur. This situation may be reflected in lower unit LW export in the low latitudinal zone than in the high latitudinal zone when compared at the same intensity. Conversely, northern Japan (higher than 38 degree in latitude) receives snowfalls every year and rarely experiences typhoons and torrential downpours. LW pieces recruited by bank erosion, tree mortality and windthrow are accumulated on valley floors, and may be easily transported once infrequent flood occurs (transport-limited). This might be the reason why unit LW export in northern Japan was greater than that in southern and central Japan at the same intensity.
H12A-04
A Combined Statistical and Mechanistic Modeling Approach to Study the Effect of Clear- cutting in a Snow Dominated Watershed
Scientific investigations to predict the impact of land cover changes have traditionally employed paired watershed studies in which streamflows are monitored before and after land cover changes. The paired watershed approach requires monitoring periods that are often too long to address urgent forest management questions such as those related to pine beetle infestations. Because internal hydrological processes are not revealed using this black-box approach, the why and why not is often left to speculation. In recent years, distributed models have been used more often to address the influence of land cover changes on hydrology. However, most model applications rely on stream discharge at the outlet and meteorological or internal process data at one or a few locations, so that the why and why not is still left to speculation. Detecting and simulating the effects of land cover changes requires an improved procedure to measure inputs and internal processes in watersheds. In this study we introduce the experimental design to predict the influence of clear-cutting for a 17.4 km2 forested watershed in south-eastern British Columbia (Canada) with a distributed hydrological model. The study design includes discharge measurements at 10 nested sub- basins, 8 climate stations to capture variability in climate forcing, and spatial snow surveys at 25 locations in bi-weekly intervals during snow cover depletion. The input and internal process data from 4 years are used to stepwise calibrate the distributed hydrological soil and vegetation model (DHSVM). The ability of DHSVM to reproduce the main topographical gradients for snow accumulation and snow melt, the two most important hydrologic processes in a snow dominated watershed, is tested by spatial regression analysis with measured snow water equivalents. For different clear- cut scenarios, a stream flow time series is simulated, from which the peak flows are extracted. The experimental design enables us not only to predict the effect of harvesting different equivalent cut areas (ECA), but also how to optimally place cut blocks in a watershed in order to maximise ECA and minimize the effects on streamflow.
H12A-05
Lessons Learned from Recent Paired Watershed Assessments of Forestry Operations in Georgia
Published criticisms of the paired watershed approach are as old as the method itself and have made many valid points. Paired watersheds are difficult to replicate and incorporate neither the natural variability of hydrologic and water quality behavior of forested watersheds, the variability in treatment application, nor the variability in legacy effects of prior land uses. Recently completed paired watershed studies at B.F.Grant Experimental Forest (a repeat of the 1970s Hewlett study with modern BMPs) and at the International Paper Southlands Experimental Forest, both in Georgia, illustrate all of these problems. Data from these studies show that streams draining adjacent first order watersheds with similar topography, soils, and forest cover can have very different peak flow and low flow behavior as well as different valley and stream morphology. Nevertheless, paired watershed studies have contributed detailed mechanistic understanding of how forest operations impact stream conditions and have contributed to specific improvements in BMPs whose effectiveness has been demonstrated in several ways. Hewlett's 1970s B.F. Grant study clearly showed the need to establish wider and denser streamside management zones for temperature protection and also further demonstrated the need to minimize stream crossings and improve techniques for temporary crossings. The recent B.F. Grant and Southlands studies have provided validation of the effectiveness of modern BMPs and have raised new research avenues. Taken together, paired watershed experiments throughout the southeast have revealed some responses and issues that are consistent across physiographic provinces and some that are particular to physiographic provinces.
H12A-06
Paired Watershed Projects in Australia: Long-Term Studies and Short-Term Politics
Long-term paired watershed have been helpful in shedding light in controversial areas of forest hydrology in Australia. Recent unpublished results from two particular studies Croppers Creek (plantations, fire impacts) and Coranderrk (water use of regrowth eucalypts on Melbourne's watersheds) are considered in some detail to illustrate where forest hydrology and politics have converged. In each case the paired watershed studies are providing new information unobtainable in other ways. Paired watershed studies have been used in Australian forestry in 1954. Since then twenty eight distinct projects involving stream-flow measurement on 112 catchments have been used by Australian forestry and land management agencies. The set of catchments gauged has been growing (somewhat unsteadily) at, on average, two watersheds per year. In some cases projects considered "dead" have been resurrected because of new demands for information relating to drought and fire impacts. An analysis of citations from paired watershed studies indicate that these projects have contributed approximately one third of the Australian publications on forest hydrology. Further, this information has stimulated many other studies on related topics. The technique is undeniably useful from the hydrology point of view but the volatility of Australian land management agencies in the last two decades has sometimes compromised the utility of this work. Implications of this for the generation of current and future knowledge are considered.
H12A-07
A multi-scale modelling procedure to quantify hydrological impacts of upland land management
Recent UK floods have focused attention on the effects of agricultural intensification on flood risk. However, quantification of these effects raises important methodological issues. Catchment-scale data have proved inadequate to support analysis of impacts of land management change, due to climate variability, uncertainty in input and output data, spatial heterogeneity in land use and lack of data to quantify historical changes in management practices. Manipulation experiments to quantify the impacts of land management change have necessarily been limited and small scale, and in the UK mainly focused on the lowlands and arable agriculture. There is a need to develop methods to extrapolate from small scale observations to predict catchment-scale response, and to quantify impacts for upland areas. With assistance from a cooperative of Welsh farmers, a multi-scale experimental programme has been established at Pontbren, in mid-Wales, an area of intensive sheep production. The data have been used to support development of a multi-scale modelling methodology to assess impacts of agricultural intensification and the potential for mitigation of flood risk through land use management. Data are available from replicated experimental plots under different land management treatments, from instrumented field and hillslope sites, including tree shelter belts, and from first and second order catchments. Measurements include climate variables, soil water states and hydraulic properties at multiple depths and locations, tree interception, overland flow and drainflow, groundwater levels, and streamflow from multiple locations. Fine resolution physics-based models have been developed to represent soil and runoff processes, conditioned using experimental data. The detailed models are used to calibrate simpler 'meta- models' to represent individual hydrological elements, which are then combined in a semi-distributed catchment-scale model. The methodology is illustrated using field and catchment-scale simulations to demonstrate the the response of improved and unimproved grassland, and the potential effects of land management interventions, including farm ponds, tree shelter belts and buffer strips. It is concluded that the methodology developed has the potential to represent and quantify catchment-scale effects of upland management; continuing research is extending the work to a wider range of upland environments and land use types, with the aim of providing generic simulation tools that can be used to provide strategic policy guidance.