ID | 389 |
---|---|
Citation | Creed, I, Sass, G, Beall, F, Buttle, J, Moore, D and Donnelly, M. 2011. Scientific theory, data and techniques for conservation of water resources within a changing forested landscape. SFMN, Edmonton, AB. |
Organization | SFMN |
URL | http://www.sfmn.ales.ualberta.ca/Publications/~/media/sfmn/Publications/StateofKnowledgeReports/Documents/SOK2011Water2CreedetalEn.pdf |
Abstract/Description or Keywords | This State of Knowledge (SOK) report builds on a companion SOK report entitled モHydrological principles for conservation of water resources within a changing forested landscapeヤ, which presents a suite of hydrological principles that provide a foundation for the development of sustainable forest management strategies to conserve water resources. With this report, our objective is to review the state of science resources (including data and tools) behind the sustainable management of forests from the perspective of conserving water resources and minimizing adverse effects resulting from forest management activities. We also provide a current synthesis of field studies and available datasets, as well as the scientific achievements and challenges facing the application of digital tools including digital terrain analysis, remote sensing and hydrological modelling. Finally, we provide recommendations for scientists, policy makers and resources managers with respect to future research and monitoring endeavours, analysis of integrated datasets and training of the next generation of forest hydrologists and forest managers to promote the practice of sustainable forest management. Where does sustainable forest management come from? Sustainable forest management strategies for the conservation of water resources are based on the results of hundreds of scientific studies from Canada and abroad. These studies have sought to understand the consequences of management activities on hydrological processes in forested landscapes. In order to do so, research has been conducted at a series of experimental study sites, most being headwater catchments, spanning a diverse range of forested regions. In Canada, there are approximately 50 sites which have produced most of the forest hydrology research. Do forest management activities change hydrological processes? They potentially do, but the effects are variable across spatial and temporal scales. Different forest types will respond differently to forest harvesting, and the responses in a given forest will differ over the course of a year. Additionally, global change is altering how we look at the effects of forest harvesting, because it is modifying the fundamental hydrological processes occurring within our forests. Future research should focus on maintaining and expanding the monitoring network of sites that are examining the long-term hydrological and biogeochemical characteristics of a wide variety of natural and harvested forests. These sites could be used to investigate how we can translate results from experimental catchments to larger regional drainage basins. Specifically, it is important to learn more about how the effects of forest harvesting change with scale and how they compare with other land use/land cover changes. It is also important to look beyond issues of annual water yield and consider other aspects of streamflow regimes including streamflow magnitudes (high and low flows), timing of streamflows, frequencies, and the durations of streamflow events. The effects of forest harvesting on forest hydrology and biogeochemistry Water quantity and quality issues, especially as they relate to the provision of safe and sufficient drinking water supply will be increasingly important under a changing climate, particularly since many predict an increased frequency and severity of extreme weather events. For these reasons, it is crucial to have a clear understanding of the impacts of forest harvesting on water. The key questions asked by hydrologists working in managed forests are: 1. How do forest management activities alter hydrological processes?; and 2. How do we mitigate the impacts? We now have a fairly good grasp on many of these changes, although knowledge gaps remain, specifically for subsurface (ground water) hydrological processes. Changes in hydrological processes In general, basins that have been harvested produce greater water yields. This response, however, is highly variable and is affected by such factors as climate, geology, soils, topography and vegetation. Additionally the effects of forest harvest are not static, and diminish over time as the forest re-grows. Some potential reasons why greater water yields can be expected in harvested areas are: ユ Higher snow accumulation, accompanied by more rapid snow melt in the spring due to higher radiation exposure. ユ Higher melt rates in regions subject to mid-winter rain-on-snow events, due to an increase in wind-driven inputs of sensible and latent heat to the snow surface. ユ Higher soil water content during the snow-free season as a result of fewer trees intercepting rain and snow, and an overall reduction in evapotranspiration due to reductions in leaf area. Increased amounts of water in the soil in harvested watersheds translate into higher water tables, increased groundwater recharge, and greater groundwater discharge to streams and rivers; although the magnitude of these effects are highly variable. Higher soil water could also lead to increased subsurface flow and change the rate at which water is able to infiltrate into the soil. If infiltration rates have been altered through forestry operations, there can be resulting increases in overland flow. The magnitude of all these effects, however, depends on the techniques used to remove the trees (e.g., soil compaction from equipment, roads). The issue of increased peak flows due to forest harvesting, and the associated risk of flooding, is a contentious issue in hydrology. However, there is general consensus that forest harvesting has a proportionally larger effect on smaller rather than larger basins, and that effects are generally observed when more than 20% of a basin area is harvested. There is also general consensus that low flows during dry periods increase following harvesting due to increased soil and groundwater recharge, at least for the first five to ten years following harvest. Again, these effects are highly variable and longer-term effects (10 years +) are virtually unknown. Water quality Water quality issues are complex and interconnected. In fact, the effects of forest harvesting can be difficult to tease apart from issues such as climate, vegetation and land use history. Also, there are many indicators that represent quality based on different variables and that affect the ecosystem in diverse ways. Stream temperature is an important water quality indicator that influences such properties as dissolved oxygen levels, and can be modified by several factors related to forest harvesting, including increased solar radiation at the water surface. Sediment concentrations can impact the quality of drinking water and habitat quality for fish spawning. Sediments can also act as a vector for some nutrients and contaminants. Sedimentation is mostly a result of the construction of roads and stream crossings for logging operations. In general, sediment yields peak shortly after harvesting and decline with forest regrowth. The maximum sediment supply to streams, however, may not occur immediately after harvesting, particularly in the case of landslides. Nutrient and contaminant levels in forested streams do not all react in the same way to forest harvesting, although there are general trends for most water chemistry indicators; and, in many cases, a lag is observed between forest harvesting and biogeochemical response. For example, loss of vegetation causes nutrient and contaminant concentrations to increase followed by a decline as result of uptake by forest regrowth. Canada's national hydrological datascape Knowledge generated by research regarding hydrological processes of forests and the effects of forest management activities on these processes are fundamentally based on hydrological datasets. These datasets provide forest hydrologists and managers with spatially distributed information about water quantity and quality, which can be used as a baseline to measure the response of hydrological systems to forestry operations within a watershed. The majority of Canada's forested watersheds are not gauged; thus managers must make informed decisions with the data that are available. Currently, there are two main sources of hydrological information in Canada: 1. Experimental study sites targeting locally-focused hydrological issues and/or research; and 2. Water Survey of Canada (WSC) river and lake hydrometric gauging sites (for discharge and lake level monitoring). Experimental study sites Many of the Canadian experimental watersheds were established by Federal and Provincial agencies along with university scientists in the 1970s. Several hundred scientific papers have resulted from these research watersheds at the local scale, but there have been few attempts to place these results in a national context and extrapolate results from these studies to other sites that are not gauged. One of the most valuable research efforts is the HydroEcological Landscapes and Processes (HELP) project. It identified 46 research sites across Canada and compiled, organized, and synthesized the knowledge into a framework to quantify hydrological, geomorphic, and ecologic processes in forests. Most of these 46 sites measure streamflow at the watershed outlet water temperature, lake out-flows, as well as air temperature and precipitation on a daily basis. Approximately half of the HELP's research sites are still active, many with invaluable long-term data records; however, maintaining funding for these sites has been difficult. Water Survey of Canada (WSC) The national Water Survey of Canada hydrometric archive operated by Environment Canada contains more than 2000 stations in forested regions across the country. Approximately 700 stations are still active, although there are gaps and areas of uncertainty in data records. Small basins in the boreal forest have only minimal coverage with few sites having records greater than 35 years. To move forward, it is important to consolidate existing data and to increase data availability. The number of active gauging stations and experimental watersheds are decreasing, but developing relationships among government, university, and local stakeholder groups can help maintain or even improve upon the resources currently available. Methods used for hydrological research The earliest forest hydrology studies used field based methods to study hydrological processes and how they are affected by management activities; however, the recent proliferation of powerful desktop computing coupled with new remote sensing instruments has opened up a completely different vantage point for understanding and managing forests. The fields of digital terrain analysis, remote sensing and hydrological modelling have great relevance for understanding the hydrological patterns and processes of forested ecosystems. These tools can be readily integrated into forest planning strategies since they are applicable to the study and management of forests at broad spatial and long temporal scales. This does not mean, however, that the classic monitoring framework is obsolete, since digital tools will always rely on field corroboration. The future of both forest hydrology and forest management, from a hydrological perspective, will require an integrated planning and monitoring framework using all these tools, at a range of spatial scales from hillslopes to continental drainage basins. Bird's-eye-view of forest hydrology: Novel approaches using remote sensing techniques Remote sensing uses devices that detect electromagnetic radiation to observe phenomena from a distance. A variety of remote sensing techniques can be used to estimate components of the water budget at different spatial and temporal scales, giving forest managers additional information regarding the distribution of water in forest catchments to facilitate planning decisions. Most early remote sensing sensors were optical; however, radiation in those wavelengths cannot penetrate clouds and vegetation. Consequently, these sensors are primarily used to monitor open areas of water such as lakes and wetlands. Alternatively, microwave radiation can penetrate vegetation and clouds, allowing a more un-restricted observation of forested landscapes. Both of these sensors measure the reflectance or backscatter of electromagnetic radiation from the objects that they observe. Selecting the appropriate sensor is an important first step in any remote sensing study. Some of the factors that managers should consider are the record-length, spatial resolution of imagery, and how radiation will interact with the object of interest and the atmosphere. Unfortunately, most airborne and satellite sensors were not specifically designed for hydrological applications, so data must be analyzed with care. Often, multiple sources of imagery can be combined to answer specific questions. Visible and infrared imagery, and more recently microwave sensors can provide some information about how water enters a forested catchment through rainfall; the spatial resolution, however, is usually quite coarse. Visible and infrared imagery is also useful to determine the leaf area index which provides a good estimate of the amount of rainfall interception by the canopy. This is one of the most useful applications of remote sensing in hydrology and is used extensively in hydro-ecological models. Light Detection And Ranging LiDAR data can also provide information about the canopy structure, including gaps, height, soil bulk density, and surface area. Remote sensing is able to provide new information about water stored in a landscape. While optical sensors can be used to estimate the area of surface water in a landscape, active microwave sensors and imaging radars can penetrate canopies and clouds and operate during day or night, giving more and higher resolution information about wet areas beneath the canopy. It is difficult, however, to monitor groundwater using remote sensing because the electromagnetic radiation used by current sensors cannot penetrate the ground. Alternative methods can be used to estimate groundwater recharge and discharge. There are also a variety of methods to measure the different ways that water leaves forested catchments. Satellitederived vegetation indices, surface temperature and surface albedo have been used to map daily evapotranspiration at approximately 1-km spatial resolution, and much work is being done to improve these estimates. A disadvantage, however, is that discharge can only be estimated in rivers that are wide enough to be detected by sensors. One particularly useful application of remote sensing for forest management is in the designation of hydrologically relevant buffer zones, which mitigate the effects of land use activities on nearby surface waters. Remote sensingderived maps can be used to assess the organization of surface flowpaths prior to the design and placement of such buffer strips. To move forward, it is essential that new sensors, especially microwave, microgravity, and airborne geophysical sensors, are designed with the specific purpose of sensing hydrological phenomena. There is also a wealth of existing remote sensing data that in many cases is either inaccessible, discarded due to storage issues, or goes unused due to complexity of interpretation. A coordinated public and private effort is needed to archive these data and increase public access. There also needs to be increased interdisciplinary training for non-experts so that remote sensing analysis can be adopted by a broader audience. Digital terrain analysis approaches for tracking hydrological and biogeochemical pathways and processes in forested landscapes Digital terrain analysis (DTA) is an increasingly valuable tool used to map the movement of water and nutrients across forested landscapes. DTA holds great promise for forest managers since it can be used to identify critical features that should not be disturbed by management activities. Currently, DTA is being used to map hydrologically sensitive areas (e.g., wetlands and small streams) and minimize the impact of placing roads, culverts and cut blocks. DTA is effective because in many landscapes, topography controls water flow by directing water from high elevations to low elevations due to gravity. Topography also forces water to converge and diverge due to the shape of the land surface. However, the dominance of topography as a primary control on water flow needs to be carefully considered for each landscape as there could be other biophysical factors that influence water flow, such as geology, soils, and vegetation. In order to understand how DTA can help in the management of forests, it is important to have a basic understanding of the principles of this technique. In most cases, a square grid with a defined resolution is laid over top of an area, and for each cell the elevation is measured using photogrammetry, laser altimetry (LiDAR) or interferometric techniques. The resultant surface is called a Digital Elevation Model (DEM). DEMs are used to model how a drop of water would move across these digital surfaces. The preferred source of DEMs is LiDAR, as the laser beams can penetrate the forest canopy and record very accurate surface elevations. It is important to consider the spatial resolution of DEMs since most DTA techniques are very sensitive to the spatial resolution at which they were derived. The hydrological features being modeled should be properly matched against the spatial resolution of the DEM, since overly-coarse resolutions will misrepresent patterns, while overly-fine resolutions will contain too much detail to be useful. DTA techniques have been developed to map hydrological pathways and areas of hydrological storage. For example, LiDAR-derived DEMs have significantly improved our ability to map canopy-shaded rivulets and wetlands within headwater forests. Not only do we have a better idea where streams begin, but we also have better assessments of where water is stored on the surface and how these different features are connected via surface saturated areas. This improved mapping of hydrological features has allowed for improved predictions of streamflow, including the different components of the hydrograph and water transit times through catchments. DTA can also be used to identify hydrological controls on the formation of biogeochemical pools. Many of the DTA techniques that track the movement of water from land to surface waters are applicable for monitoring nutrients due to the close interrelationship between water and nutrient movement. For example, there is a rich literature on applying DTA techniques to predict the export of nutrients from catchments. DTA has also found use in the prediction of land-atmosphere transfer of nutrients with considerable focus on the production of greenhouse gases including carbon dioxide, methane, and nitrous oxide. Looking into the future, the advances in the vertical and horizontal accuracy of DEMs must continue and be made broadly available across different forest types. Additionally, an improved integration and validation with field-based measurements, remote sensing, and distributed hydrological modelling, complemented by the establishment of global benchmark datasets, will further improve the applicability of DTA data. Improved imaging of subsurface features, such as bedrock topography, would substantially improve our ability to model hydrological processes in terrain where surface topography is not the dominant control on water flow (e.g., boreal plains). Virtual ecosystems: Using hydrological models to understand the past and predict the future of water in forests Quantitative ecological models are mathematical descriptions of ecosystems processes. These models serve as virtual laboratories, providing opportunities to manipulate components and functions more easily than in natural systems. In forest hydrology, quantitative ecological models can be used to understand the water and associated nutrient and sediment cycles. In order to select the appropriate model when using quantitative methods, it is important to have clear what the overall goal of modelling is. It is also important to consider the context, function, performance, and evaluation of available models. Defining the model context places the proposed modelling exercise into a larger scientific, management, policy, and geographic context; the latter characterized by scale and an expectation of uncertainty. The function of the model determines which part of the hydrological cycle needs to be considered, what level of detail is needed to describe the hydrological processes, and what modelling resources are required. Once a potential model has been selected, its performance is optimized by adjusting key parameters to improve model fit. Finally, it is important to evaluate the model by inspecting and analyzing the output and determining the error of the computed data. Hydrological models can be used in different ways to aid in the decision making process of managers: 1. Models can be used to define the reference condition of forests that have not been monitored for long enough for determining the range of natural variation in hydrological properties (usually streamflow). Hydrological models can also be used for virtual experiments such as simulating the possible impacts of harvesting activities. Experiments can be designed to separate the effects of multiple disturbances, such as climate, fire, harvesting, or acid rain. 2. In terms of forecasting, models can be used to simulate possible futures under changing environmental, management, and climatic scenarios. For example, global circulation models are used to predict future climate scenarios that will potentially impact the hydrological cycling of water within forests. It can be difficult, however, to apply these models for planning and management at the regional and local scales. There are several modelling methods that allow continental climatic information to be downscaled to local levels, but it can be difficult to incorporate all of the effects of climate change accurately, including all the ecosystem processes that will be altered, into a locally applicable model. Recommendations Based on the current state of science and analytical tools reviewed in this document, we make the following recommendations in order to improve our future understanding of forest hydrology in Canada: ユ Create a national data archive for forest hydrology: A centralized database would encourage cross-site comparisons and synthesis of meta-experiments. This will greatly expand knowledge generation and increase the incorporation of these resources into forest management planning and operations. Due to most hydrological data being funded by federal and provincial governments, we advocate for the public distribution and access of these dataset at no cost. ユ Continue existing and establish new monitoring networks: Many crucial questions in forest hydrology, especially those related to climate change, can only be addressed by analyzing long-term datasets. Canada needs to take a lesson from the USA where there is a network of long-term observation sites. ユ Encourage and adopt emerging technologies for quantitative analysis: The integration of remote sensing imagery in hydrological models will continue to be a critical research area. Further integration of digital terrain analysis, remote sensing, and distributed simulation modelling data sets using GIS-based spatial decision support systems will be especially important for managers and other decision makers. We need improved sensors for in-situ sampling as well as for airborne and satellite remote sensing of various hydrological phenomena. ユ Promote integration of datasets at relevant scales: We need to integrate spatially explicit forest modelling processes with hydrological tools to enhance our ability to upscale results from headwater catchments to regional drainage basins, consider in-stream as well as lake and wetland processes, and forecast the cumulative effects of forest management activities on water resources through time. Develop a national watershed classification system: An ecohydrology-based watershed classification system would enable planners to recognize the spatial variability in and between different catchments to refine forest management prescriptions based on site conditions and associated hydrological processes, as well as to minimize the potential for adverse effects from harvest or renewal activities on water resources and lotic ecosystems. ユ Promote hydrological training and knowledge transfer: The goal of this report was in part to promote the application and adoption of the hydrological knowledge and techniques presented herein within forest management. We need enhanced knowledge transfer from product developers and users in digital terrain analysis, remote sensing, and distributed simulation modelling to forest hydrologists and managers. |
Information Type | report |
Regional Watershed | All |
Sub-watershed if known | |
Aquifer # | |
Comments | |
Project status | complete |
Contact Name | Irena Creed |
Contact Email | [email protected] |