A Methodology for Evaluating and Ranking Water Quantity Indicators in Support of Ecosystem-Based Management

Abstract

Ecosystem-based Management (EBM) is an approach that includes different management priorities and requires a balance between anthropogenic and ecological resource demands. Indicators can be used to monitor ecosystem status and trends, and assess whether projects and/or programs are leading to the achievement of management goals. As such, the careful selection of a suite of indicators is a crucial exercise. In this paper we describe an indicator evaluation and selection process designed to support the EBM approach in Puget Sound. The first step in this process was the development of a general framework for selecting indicators. The framework, designed to transparently include both scientific and policy considerations into the selection and evaluation process, was developed and then utilized in the organization and determination of a preliminary set of indicators. Next, the indicators were assessed against a set of nineteen distinct criteria that describe the model characteristics of an indicator. A literature review was performed for each indicator to determine the extent to which it satisfied each of the evaluation criteria. The result of each literature review was summarized in a numerical matrix, allowing comparison, and demonstrating the extent of scientific reliability. Finally, an approach for ranking indicators was developed to explore the effects of intended purpose on indicator selection. We identified several sets of scientifically valid and policy-relevant indicators that included metrics such as annual-7 day low flow and water system reliability, which are supportive of the EBM approach in the Puget Sound.

Appendix A

Table 4 Potential indicators of surface water hydrologic flow regime for areas of management concern in the Puget Sound

Focusing the List of Candidate Indicators of Hydrologic Flow Regime

The PSP identified the following issues of potential concern related to the hydrologic flow regime in Puget Sound:

• Changes in hydrology related to land use;

• Climate change; and

• In-stream flows supportive of individual species and food webs, including stream and floodplain habitats (Puget Sound Partnership 2008b, a)

A literature review was performed based on these stated management concerns to determine: 1) which of the indicators of hydraulic alteration would be most appropriate based on the predicted or observed alternations related to land use change and climate change, and 2) which aspects of the flow regime are most relevant to the aquatic species in Puget Sound streams and rivers. The results are summarized below.

The Puget Sound basin includes at least 13 major river systems, which can be classified as rainfall-dominated, snowmelt-dominated, or transitional (Mote and others 1999; Beechie and others 2006; Ebbert and others 2000). Rainfall-dominated rivers exhibit peak flows during winter; snowmelt-dominated rivers have peak flows in late-spring and late-fall with low winter flows. Transitional rivers exhibit less pronounced high or low flows in the late-fall and late-spring, and winter. Hydrologic flow regimes in Puget Sound rivers have been altered through the construction of dams for flood control or power generation, or by changes in land cover and climate. Flows in the Skagit, Nisqually, Green, Skokomish, and Cedar rivers are regulated by dams (Puget Sound Partnership 2009a).

A.1 Indicators of Hydrologic Alteration – Climate Change

Puget Sound river hydrology may be affected by climate change. Precipitation in the region occurs predominately in the winter months. The accumulation of snow in the mountains is a primary storage mechanism, particularly for snowmelt-dominated and transitional river systems. It has been estimated that upwards of 70% of total stream discharge in the Western United States is from melting snowpack (Cayan 1996). Glacier-derived melt-water can also be important sources of summer flow (Bach 2002). Climate change assessments have predicted increased winter and spring temperatures resulting in decreased snowpack storage in the mountains, increased winter runoff as more precipitation falls as rain, and lower summer flows (Hamlet and others 2005; Hamlet and Lettenmaier 1999; Lettenmaier and others 1999; Mote and others 1999; Leung and others 2004; Barnett and others 2008). Climate change is not predicted to impact total annual precipitation (Elsner and others 2010). Hydrologic flow patterns may change from snowmelt-dominated and transitional to rainfall-dominated (Mote and others 1999). Decline in snowpack or changes in stream flow timing may be problematic for regional water supplies as most systems have been developed based on historic flow patterns (Milly and others 2008).

Results of modeling, multivariate statistical analysis, and snowpack temperature sensitivity analysis indicate that (1) there are real, long-term trends of decreasing snowpack, particularly in lower elevations, (2) the long-term trends are associated with changes in temperature and not rainfall, and (3) the snowpack will continue to decrease in the future (Hamlet and others 2005; Mote and others 2008; Barnett and others 2008; Mote and others 2003; Elsner and others 2010). It is not clear, however, if future trends will be readily discernable due to a high level of interannual variability (Casola and others 2009).

Related work found significant trends towards earlier runoff in many rivers and streams in the Pacific Northwest (Stewart and others 2004). While stream flow timing was partially controlled by long-term climate patterns such as the Pacific Decadal Oscillation, there remained a significant part of the variation that was explained by a longer-term warming trend in spring temperatures, possibly associated with anthropogenic forcings (Stewart and others 2005; Barnett and others 2008).

Based on the observed and predicted impacts of climate change, we concluded that it would be appropriate to evaluate measures of spring snowpack, stream flow timing, and summer low flows as indicators of hydrologic alteration (see Table 4).

A.2 Indicators of Hydrologic Alteration – Land Use/Urbanization

Alterations in land use can affect stream and river hydrology in various ways (Poff and others 1997). Urbanization is associated with the increase of impervious surface area, which can result in increases the severity and frequency of peak stream flows by reducing infiltration and increasing runoff; overall annual stream flow volumes are generally not affected (Konrad and others 2005; Cuo and others 2009; Konrad and Booth 2002; Booth 2005; Cuo and others 2008; Moscrip and Montgomery 1997; Paul and Meyer 2001). Urbanization my lead to lower base flows from reduced infiltration and the construction of storm drains (Simmons and Reynolds 1982; Cuo and others 2008). Logging of forested lands may increase annual flow by reducing evapotranspiration in the watershed though other hydrologic changes, such increasing flooding, are disputed (Bosch and Hewlett 1982; Bowling and others 2000; Alila and others 2009). Land use changes in river basins may lead to alterations in channel morphology which can exacerbate flooding potential without affecting in stream flow (Stover and Montgomery 2001).

Comparisons of a forested and a developed basin in the Puget Sound lowlands found a higher degree of runoff from the developed basin, suggesting lower rates of infiltration (Burges and others 1998). Similarly, others have found increased monthly stream flow, higher peak flows, and increased flood frequency in streams within developed or urbanized watersheds compared to watersheds which remained undeveloped (Leith and Whitfield 2000; Moscrip and Montgomery 1997). Stream flashiness, or the rate at which a stream responds to precipitation events in the watershed, has been shown to significantly increase in urbanized basins in the Puget Sound compared to undeveloped basins (Fleming 2007; Konrad and Booth 2002).

Cuo and others (2009) utilized a Distributed Hydrology-Soil-Vegetation Model in order to determine the relative effects of land cover and climate change on the flow patterns in Puget Sound streams. They found that the relative importance of temperature and land cover differed between the upland and lowland basins. In the lowland basins land cover changes were more important and generally resulted in higher peak flows and lower summer flows, primarily from increased runoff. Both land use change and climate effects were more important in the upland basins. Climate effects were more important in the transitional zones and resulted in higher winter flows, earlier spring peak flows, and lower summer flows.

Based on the observed and predicted impacts of land use alterations and urbanization, we concluded that it would be appropriate to evaluate measures of peak flow, summer low flow, and flashiness as indicators of hydrologic alteration (see Table 4).

A.3 Indicators of Hydrologic Alteration – Ecological Effects

Alterations of river and stream hydrology can affect aquatic ecosystems by changing physical habitats, disrupting the natural connectivity of habitats, or by facilitating the successful invasion of exotic species (Bunn and Arthington 2002). Native species may have evolved according to the pressures and timing of natural flow regimes; altering flow patterns may negatively affect those species (Waples and others 2009). Many studies have focused on evaluating the biological effects of altered hydrology in the Puget Sound, particularly with the regionally-iconic salmon species; these studies suggest that peak flows and flashiness may be important.

Increased peak flows may affect salmon survival by disrupting redds, increasing deposition of fine sediments and reducing dissolved oxygen transfer, reducing growth rates, or increasing downstream displacement and mortality (Waples and others 2009). Increases in peak flow particularly during the egg incubation period may significantly contribute to embryo mortality (Thorne and Ames 1987; Topping and others 2009; Schuett-Hames and others 2000; Greene and others 2005; Montgomery and others 1996; Kinsel and others 2008; Kiyohara and Zimmerman 2009). Conversely, increased summer flows have been shown to be positively correlated with coho run strength in Puget Sound likely due to increased habitat availability (Mathews and Olson 1980; Battin and others 2007).

Increased urbanization (i.e., land use changes) is also associated with decreased salmon populations (Moscrip and Montgomery 1997; Pess and others 2002; Bilby and Mollot 2008) and altered species compositions (Scott and others 1986). However, it is not yet clear whether hydraulic alterations are the primary driver, or whether the biological effects result from other potentially deleterious impacts such as increased pollution, nutrient runoff and/or channel straightening (Paul and Meyer 2001).

Several studies have evaluated the effects of urbanization and altered hydrology on stream benthic communities. Laboratory studies have shown that flow alternations are sufficient to alter benthic community structure (Bond and Downes 2003), though it has been difficult to demonstrate clear, statistically robust relationships between flow metrics and community structure, commonly evaluated through a benthic index of biological integrity (B-IBI). Field studies in Puget Sound lowland streams reported correlations between B-IBI and flashiness but not peak flow (Morley and Karr 2002; Booth and others 2004; Cassin and others 2005; DeGasperi and others 2009). None of the hydrologic indicators were good predictors of B-IBI. Similar work suggested that urbanization and increased total impervious area (TIA) may constrain the potential benthic diversity, and that patch size and road crossings may also be important (Booth and others 2004; Morley and Karr 2002; Alberti and others 2007). From a management perspective additional research may be required to utilize B-IBI as the justification for choosing specific metrics of hydrologic alteration for reporting purposes. There is no demonstrated relationship between B-IBI and the condition of vertebrate species (Morley and Karr 2002), which are clearly of higher management concern, nor is the natural variability well understood (Mazor and others 2009).

Although this literature review suggests that peak flows (specifically during periods of salmonid egg incubation), summer low flows, and flashiness may all be suitable indicators of hydrologic alteration due to their relationship to specific ecological responses, there remains considerable uncertainty. Poff and Zimmerman (2010) reviewed 165 papers to investigate the possibility of developing quantitative relationships between hydrologic alterations and ecological response and found that, while there were general relationships (e.g., a decline in fish abundance and diversity with alterations in flow magnitude) they were unable to support any quantitative relationships. Similarly, Bauer and Ralph (2001) evaluated the potential of incorporating aquatic habitat indicators, including those related to flow regime, into legal standards for water quality. They concluded that the effects of low flow on habitat were not sufficiently well understood for the development of numeric criteria, but could support the development of some narrative standards; the relationships between peak flows and habitat were even less certain.

Instream flow rules have been developed as a management response to the recognized need to balance competing human and ecological demands for freshwater resources (Petts 2009). The instream flow rules developed for many rivers and streams in the Puget Sound define specific flow and timing regimes, mainly by establishing minimum flow levels during specific times of the year. Due to the complexity of natural flow regimes, however, it is not clear if simplified instream flow rules are protective of native flora and fauna (Arthington and others 2006; Naiman and others 2008). The adoption of flow rules more considerate of all aspects of the natural flow regime has been suggested (Arthington and others 2006; Bunn and Arthington 2002; Naiman and others 2008; Petts 2009; Poff and Zimmerman 2010). Nonetheless, a measure of the effectiveness of water resources management may be the comparison of actual flows with those specified by the instream flow rules.

Due to a large degree of uncertainty, it is not possible to specifically determine a set of indicators based solely on ecological concerns. However, peak flows and summer low flow may be ecologically informative and are important to other aspects of water quantity (see Table 4). Tracking violations of instream flow rules may be of use. As such, we concluded that it would be appropriate to further evaluate these indicators.