Turning dreams into reality: challenges in flow-ecology relationships
Turning dreams into reality Challenges to developing flow-ecologicalrelationships to support streamflow management Julian D. Olden Cathy A. Reidy Liermann School of Aquatic & Fishery Sciences University of Washington, Seattle, WA
Water and the vitality of rivers• Flow variability shapes the physical, chemical and biological attributes and functioning of riverine systems – Channel form and habitat complexity Freshwaters Illustrated – Life-history patterns – Lateral and longitudinal connectivity – Resistance to species invasions• At the same time, human societies modify natural flow regimes to provide dependable ecological services and to Freshwaters Illustrated seek protection from floods and droughts
Water links society and nature• Global concerns about the sustainability of human water use practices have grown markedly in recent years• People clearly benefit from the direct use of water, but there are limits to the amount of water that can be withdrawn from river systems• There is a need to understand the relationships between river flow regimes and the in-stream aquatic and riparian ecosystems that they support Wallace et al. (2003)
Challenge Synopsis• Many of the hierarchical linkages between the ecological response to hydrology are poorly understood, inadequately validated, and remain expressed largely as descriptive hypotheses rather than predictive or quantitative models. What must we know, what can we reasonably learn, and what do we simply need to treat as uncertainty in the development of flow:ecology relationships for river management?
Major hurdles to linking ecological responses to riverine hydrology Devising testable hypotheses from general principles Generating modelsInforming decision that are realistic, support tools mechanistic and defendable Accounting for uncertainty (data, parameter, model, knowledge)
Characterizing flow regimesHydrologic metrics characterize statistical properties of the long- term hydrologic regime of rivers based on multi-year time series of discharge data.Important considerations for metric selection: – Sensitivity to natural and human drivers of hydrologic change – Uncertainty in their estimation (bias and precision) – Amenability to direct or indirect management actions – Independence from other hydrologic indices (X)The difficulty in developing predictive flow:ecology relationships may be due in part to the inappropriate selection or uncertain estimates of hydrologic metrics used as model predictors.
Uncertainty in metric estimationUncertainly in metric estimation is a function of:• Length of flow record• Period of flow record• Number of years of overlap Sauk R. Kettle R. Quinault R. Stehekin R. 1.0 Skokomish R. Standardised MSE averaged across all hydrologic metrics 0.8 American R. Naselle R. East Fork Lewis R. 0.6 Cispus R. 0.4 0.2 0.0 0 5 10 15 20 25 30 35 40 Number of years
Uncertainty in metric estimation 15 year period of record 0.6Standardized MSE 0.5 0.4 0.3 0.2 0.1 0.0 Magnitude Frequency Duration Timing RC General recommendations (Kennard et al., in press, River Res. Appl.) • Metric estimation must be based on at least 15 years of discharge data • Metric estimation should be based on overlapping discharge records contained within a discrete temporal window (ideally >50%) • Metric uncertainty varies greatly and should be accounted for when developing flow:ecology relationships.
Characterizing ecological responseIdeally, ecological response variables should be: – Sensitive to existing or proposed flow alterations – Differentially sensitive to other sources of human impact – Amenable to monitoring – Valued by societySome composite ecological indices may be useful if they correlate with human-induced changes in streamflow (e.g., Lotic- invertebrate Index for Flow Evaluation: Extence et al. 1999)Recent studies have demonstrated that ecological responses to flow variation and alteration can be inferred based on the biological attributes of species (e.g., resource and habitat utilization, life history)
Characterizing ecological response• Taxon-free metrics allows for the comparison of species compositions that naturally differ due to biogeographic constraints on regional species pools• The sensitivity and time scale of ecological change will depend on the organism, or group of organisms, in questionHabitat specialists Habitat generalists Freeman and Marcinek (2006)
Scales of ecological responses• There has been few attempts to reconcile the mismatch in scales between the hydrological change and the ecological response. Seasonal timing Inter-annual variability & predictability Flood magnitude 50 50 Flood 50 10 frequency Rate of 10 rise & fall 10 1 1 Discharge 0.1 0.1 1 0.01 0.01 Low flow Flood duration duration 0.001 0.001 0.1 1990 April m n 1975 1980 1985 1990 1995 2000 r t Au Sp Wi Su
Scales of ecological responses • Species occurrence at a particular location and time may be shaped by long-term flow dynamics, whereas the relative abundance or biomass of species is more likely driven by short-term flow events. Mary River, Australia Presence-Absence (87%) Abundance (62%) Biomass (58%) 14 14 14 12 12 12Relative contribution (%) 10 10 10 8 8 8 6 6 6 4 4 4 2 2 2 0 0 Long-term flow regime Short-term flow events Long-term flow regime Short-term flow events 0 Long-term flow regime Short-term flow events Data from Kennard et al. (2007)
Spatial and temporal mismatch 30• Limited spatial and temporal 25 # gauge-sample pairs coverage of both stream gauges and 20 biological samples 15 10 5• e.g., WA REMAP: summer surveys 0 (July-Sept) in 1993-1994 (Coastal) 0 5 10 15 20 and 1999-2000 (Cascades) 20 # of gauge-sample pairs 15• Observations made on a single scale 10 can, at best, capture only those riverine patterns and processes 5 pertinent to that scale of observation 0 0 5 10 15 20 25 30 Watercourse distance (km)
Multiple limiting factors• The development of robust flow–ecology relationships will need to take into account the role that other environmental factors play in shaping ecological patterns in streams and rivers• Our ability to disentangle the effects of multiple limiting factors will benefit from recent statistical advancements and targeted monitoring activities• We must carefully reconcile the advantages of more complex modeling approaches with the need to develop assessable science that informs decision support tools Swift (1979)
Summary• Until we better understand the relationships between ecological health and the magnitude of hydrological alteration, we will not be able to mount a convincing argument in support of environmental flow management.General recommendations• Agreement on a core suite of headline ecological and hydrologic indices• Modeling that recognize the scales of ecological responses to hydrologic alteration• Greater data sharing among the academic community• Initiation of new monitoring programs that target
Acknowledgments• Tim Beechie, Chris Konrad, NOAA Fisheries