**3. Results**

determine relevant articles for this study. Overall, 305 articles were selected. Gray literature was omitted from this review. Our search was focused on articles that had framed their work explicitly in the ecosystem service concept (i.e., measuring the supply and demand of ecosys-

**Figure 2.** Flow diagram of the methodology and selection process of the systematic review following the Preferred

A total of 305 articles were screened to determine relevant articles for this study (**Figure 2**) [20]. In addition, articles were excluded if they used the concept of ecosystem service to justify or explain the study, but did not actually assess ecosystem services. Overall, 150 were selected after excluding duplicates. Then, only articles that carried out assessments of ecosystem services from supply and demand perspective were considered (n = 99 studies). In this second selection process, the exclusion criteria included factors related to the type of valuation methods based on the multidimensional assessment of ecosystem services [8]. After this final selec-

We classified all studies using the supply–demand framework of ecosystem services [16, 18] and grouped them by major watersheds (hydrologic unit code, level 2; HUC-2). Data collection was organized based on the general characteristics of this chapter, and the variables and methods

tion, 68 articles were kept for the quantitative review (**Figure 2**) [20].

tem services) across US watersheds [21].

Reporting Items for Systematic Reviews (PRISMA).

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**2.2. Data collection and structure**

#### **3.1. Analysis of published articles**

The number of articles assessing ecosystem services from supply and demand perspectives in the USA increased exponentially after 2010 (**Figure 3A**), with only six articles published before 2004. From 2001 to 2010, the average rate of publication was around two articles per year. Thereafter, the publication rate rose to 11 articles per year. Most of the selected articles (60 articles) had a biophysical or an environmental perspective followed by economic (28 articles), interdisciplinary assessments (24 articles), and sociocultural assessments (14 articles) (**Figure 3B**). Only a few studies actually produced maps of ecosystem services. Almost half of the studies (45 articles) used empirical data for quantifying ecosystem services (**Figure 3C**). Over a third of studies performed modeling data analysis, and only 16 articles conducted theoretical approaches. From all the selected articles, 38 articles were carried out at a local scale, followed by 25 articles at a regional scale, and seven at a national scale (**Figure 3D**). Local scale was defined when the study covered just one US state, regional scale when for two US states, and national when it covered more than two US states.

#### **3.2. Ecosystem services values and frameworks employed**

Results show that over 78% of all studies did not use or mention any ecosystem services framework to structure goals, 21% used the [10] framework, and only 1% used the supply and demand frameworks (**Figure 4A**). Overall, considering the [10] classification of ecosystem services, we found that regulating services was the class most commonly quantified or valued (82%), followed by provisioning (41%) and cultural ecosystem services (21%) (**Figure 4B**). However, over half of the studies (52%) included more than one ecosystem service type in the analysis.

Using the Common International Classification of Ecosystem Services (CICES, www.cices.eu), we found that the regulating services were the most frequently studied category; however, the number of articles including cultural services in their assessments was higher than those studying provisioning services (**Figure 5**). Overall, the review identified a total of 308 ecosystem services studied. Among the regulating services, filtration, sequestration, storage and accumulation by ecosystems,

**Figure 3.** (A) Number of publications 2001–2014 that quantified ecosystem services across U.S. watersheds; (B) number of publications by authors' discipline(s); (C) number of articles by type of analysis, and (D) number of articles by spatial scale.

**3.3. Ecosystem services across US watersheds**

Ecosystem Services (CICES).

The 68 studies evaluated in our dataset covered 18 of the 21 HUC-2 US watersheds (**Figure 6**). The assessments predominantly focused on ecosystem services delivered by watersheds located in the eastern half of the USA, with the three most studied watersheds being the South Atlantic-Gulf (HUC 03, N = 15, the Mid-Atlantic (HUC 02, N = 8), and the Upper Mississippi (HUC 07, N = 17)). By contrast, the US watersheds with no studies were located in northern and western regions, respectively, the Souris-Red-Rainy (HUC 09, N = 0) and the Upper Colorado (HUC 14, N = 5) (**Figure 6**). Watershed regions including the Pacific Northwest (HUC 17), the Missouri (HUC 10), the Arkansas-White-Red (HUC 11), the Texas-Gulf HUC 12), and the Lower Mississippi (HUC 08) were well represented with 10–12 articles per watershed (**Figure 6**).

**Figure 5.** Number of articles assessing ecosystem services based on the Common International Classification of

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We found differences across US watersheds in relation to the number of studies implementing the assessment of the supply and demand side of ecosystem services (**Figure 7**). Results show that 47 articles performed studies of the supply of ecosystem services and 19 articles implemented assessment of the social demand of ecosystem services. From the supply perspective,

**Figure 4.** (A) Number of articles using different ecosystem services frameworks; (B) percentage of articles based on ecosystem service categories. Each article can be represented in multiple categories.

habitat maintenance, and chemical conditions of freshwaters were the services most studied, while disease control, pest control, and storm protection were the least studied (**Figure 5**). There were no studies that addressed pollination or seed dispersal. Regarding provisioning services, filtration and sequestration by biota, water for non-drinking purposes, and raw material were the most studied while groundwater for drinking purposes and physical and experimental use of plants and animals were the least studied. Genetic pools and raw medicines were not studied. Finally, in terms of cultural services, we found that recreation, existence value, and esthetic values were the most studied while educational and cultural heritage were the least studied (**Figure 5**).

**Figure 5.** Number of articles assessing ecosystem services based on the Common International Classification of Ecosystem Services (CICES).

#### **3.3. Ecosystem services across US watersheds**

habitat maintenance, and chemical conditions of freshwaters were the services most studied, while disease control, pest control, and storm protection were the least studied (**Figure 5**). There were no studies that addressed pollination or seed dispersal. Regarding provisioning services, filtration and sequestration by biota, water for non-drinking purposes, and raw material were the most studied while groundwater for drinking purposes and physical and experimental use of plants and animals were the least studied. Genetic pools and raw medicines were not studied. Finally, in terms of cultural services, we found that recreation, existence value, and esthetic values were the

**Figure 4.** (A) Number of articles using different ecosystem services frameworks; (B) percentage of articles based on

**Figure 3.** (A) Number of publications 2001–2014 that quantified ecosystem services across U.S. watersheds; (B) number of publications by authors' discipline(s); (C) number of articles by type of analysis, and (D) number of articles by spatial

scale.

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most studied while educational and cultural heritage were the least studied (**Figure 5**).

ecosystem service categories. Each article can be represented in multiple categories.

The 68 studies evaluated in our dataset covered 18 of the 21 HUC-2 US watersheds (**Figure 6**). The assessments predominantly focused on ecosystem services delivered by watersheds located in the eastern half of the USA, with the three most studied watersheds being the South Atlantic-Gulf (HUC 03, N = 15, the Mid-Atlantic (HUC 02, N = 8), and the Upper Mississippi (HUC 07, N = 17)). By contrast, the US watersheds with no studies were located in northern and western regions, respectively, the Souris-Red-Rainy (HUC 09, N = 0) and the Upper Colorado (HUC 14, N = 5) (**Figure 6**). Watershed regions including the Pacific Northwest (HUC 17), the Missouri (HUC 10), the Arkansas-White-Red (HUC 11), the Texas-Gulf HUC 12), and the Lower Mississippi (HUC 08) were well represented with 10–12 articles per watershed (**Figure 6**).

We found differences across US watersheds in relation to the number of studies implementing the assessment of the supply and demand side of ecosystem services (**Figure 7**). Results show that 47 articles performed studies of the supply of ecosystem services and 19 articles implemented assessment of the social demand of ecosystem services. From the supply perspective,

**Figure 6.** Number of articles evaluating ecosystem services across major U.S. watersheds. Only 18 of the 21 HUC-2 U.S. watersheds showed results. Legend: New England (HUC 01), Mid-Atlantic (HUC 2), South Atlantic-Gulf (HUC 3), Great Lakes (HUC 4), Ohio (HUC 5), Tennessee (HUC 6), Upper Mississippi (HUC 7), Lower Mississippi (HUC 8), Souris-Red-Rainy (HUC 9), Missouri (HUC 10), Arkansas-White-Red (HUC 11), Texas-Gulf (HUC 12), Rio Grande (HUC 13), Upper Colorado (HUC 14), Lower Colorado (HUC 15), Great Basin (HUC 16), Pacific Northwest (HUC 17), California (HUC 18).

using either modeling techniques or proxies, a total of 137 ecosystem services were assessed: 60 regulating, 42 provisioning, and 35 cultural services. From the social demand perspective, using either sociocultural or economic valuation techniques, a total of 60 ecosystem services were assessed: 26 regulating, 16 provisioning, and 22 cultural ecosystem services.

The major US watersheds with the greatest number of studies implementing biophysical assessment of the ecosystem services supply were located in southeastern and midwestern regions (**Figure 7A**). Overall, all watershed regions included supply assessment of the three classes of services, that is, regulating, provision, and cultural, with the exception of the Ohio and Tennessee regions that only included provisioning and regulating services. The watershed regions that were most studied from the supply perspective included the Upper Mississippi (HUC 07), the Missouri (HUC 10), and the South Atlantic-Gulf (HUC 03). The Souris-Red-Rainy (HUC 09) and the Upper Colorado (HUC 14) were the regions that were least studied using the supply dimension.

Studies that assessed the social demand of ecosystem services (i.e., implementing sociocultural or economic valuation) were concentrated in the eastern half of the country (**Figure 7B**). Overall, all watershed regions included assessment of the three classes of services, that is, regulating, provision, and cultural, with the exception of the Texas-Gulf region that only included cultural services. The most-studied major watersheds from the social demand perspective included the Upper Mississippi (HUC 07), the South-Atlantic (HUC 03), and the Mid-Atlantic (HUC 02). The remaining watersheds, with the exception of the Pacific Northwest (HUC 17), the Great Lakes (HUC 04), and the Lower Mississippi (HUC 08), had less than six studies on the social demand of ecosystem services.

**4. Discussion**

Basin (HUC 16), Pacific Northwest (HUC 17), California (HUC 18).

Water resources management and planning in the USA face the challenge of not only ensuring the needs for humans but also preserving ecosystem health, which has a direct connection to human well-being through ecosystem services [4, 6]. This meta-analysis provides a

**Figure 7.** Number of studies evaluating the biophysical supply (A) and social demand (B) of ecosystem services across major U.S. watersheds. Only 18 of the 21 HUC-2 U.S. watersheds showed results. Legend: New England (HUC 01), Mid-Atlantic (HUC 2), South Atlantic-Gulf (HUC 3), Great Lakes (HUC 4), Ohio (HUC 5), Tennessee (HUC 6), Upper Mississippi (HUC 7), Lower Mississippi (HUC 8), Souris-Red-Rainy (HUC 9), Missouri (HUC 10), Arkansas-White-Red (HUC 11), Texas-Gulf (HUC 12), Rio Grande (HUC 13), Upper Colorado (HUC 14), Lower Colorado (HUC 15), Great

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**Figure 7.** Number of studies evaluating the biophysical supply (A) and social demand (B) of ecosystem services across major U.S. watersheds. Only 18 of the 21 HUC-2 U.S. watersheds showed results. Legend: New England (HUC 01), Mid-Atlantic (HUC 2), South Atlantic-Gulf (HUC 3), Great Lakes (HUC 4), Ohio (HUC 5), Tennessee (HUC 6), Upper Mississippi (HUC 7), Lower Mississippi (HUC 8), Souris-Red-Rainy (HUC 9), Missouri (HUC 10), Arkansas-White-Red (HUC 11), Texas-Gulf (HUC 12), Rio Grande (HUC 13), Upper Colorado (HUC 14), Lower Colorado (HUC 15), Great Basin (HUC 16), Pacific Northwest (HUC 17), California (HUC 18).

#### **4. Discussion**

using either modeling techniques or proxies, a total of 137 ecosystem services were assessed: 60 regulating, 42 provisioning, and 35 cultural services. From the social demand perspective, using either sociocultural or economic valuation techniques, a total of 60 ecosystem services

**Figure 6.** Number of articles evaluating ecosystem services across major U.S. watersheds. Only 18 of the 21 HUC-2 U.S. watersheds showed results. Legend: New England (HUC 01), Mid-Atlantic (HUC 2), South Atlantic-Gulf (HUC 3), Great Lakes (HUC 4), Ohio (HUC 5), Tennessee (HUC 6), Upper Mississippi (HUC 7), Lower Mississippi (HUC 8), Souris-Red-Rainy (HUC 9), Missouri (HUC 10), Arkansas-White-Red (HUC 11), Texas-Gulf (HUC 12), Rio Grande (HUC 13), Upper Colorado (HUC 14), Lower Colorado (HUC 15), Great Basin (HUC 16), Pacific Northwest (HUC 17), California (HUC 18).

The major US watersheds with the greatest number of studies implementing biophysical assessment of the ecosystem services supply were located in southeastern and midwestern regions (**Figure 7A**). Overall, all watershed regions included supply assessment of the three classes of services, that is, regulating, provision, and cultural, with the exception of the Ohio and Tennessee regions that only included provisioning and regulating services. The watershed regions that were most studied from the supply perspective included the Upper Mississippi (HUC 07), the Missouri (HUC 10), and the South Atlantic-Gulf (HUC 03). The Souris-Red-Rainy (HUC 09) and the Upper Colorado (HUC 14) were the regions that were

Studies that assessed the social demand of ecosystem services (i.e., implementing sociocultural or economic valuation) were concentrated in the eastern half of the country (**Figure 7B**). Overall, all watershed regions included assessment of the three classes of services, that is, regulating, provision, and cultural, with the exception of the Texas-Gulf region that only included cultural services. The most-studied major watersheds from the social demand perspective included the Upper Mississippi (HUC 07), the South-Atlantic (HUC 03), and the Mid-Atlantic (HUC 02). The remaining watersheds, with the exception of the Pacific Northwest (HUC 17), the Great Lakes (HUC 04), and the Lower Mississippi (HUC 08), had less than six

were assessed: 26 regulating, 16 provisioning, and 22 cultural ecosystem services.

least studied using the supply dimension.

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studies on the social demand of ecosystem services.

Water resources management and planning in the USA face the challenge of not only ensuring the needs for humans but also preserving ecosystem health, which has a direct connection to human well-being through ecosystem services [4, 6]. This meta-analysis provides a comprehensive inventory of watershed-scale ecosystem services knowledge across major US watersheds. More specifically, our analysis summarizes the scientific literature since 2000 within the context of the number of studies investigating the biophysical supply and social demand for ecosystem services. We found a temporal trend in the number of publications similar to that found from international studies following the global development trend in this research area [3, 22]. Our results emphasize the urgent need to implement interdisciplinary frameworks that take into account the interdependent social, economic, and biophysical dynamics of shared water resources and the need for using integrative approaches to capture different value domains [18, 23].

knowledge types, capabilities, demographics, rights, and value systems [32, 33]. We also identified methodological limitations in current ecosystem services research conducted across major US watersheds. Most of the studies were focused on a single ecosystem service without investigating the potential implications that trade-offs between multiple ecosystem services may have in watershed management [3, 4]. Many recent investigations have showed that investigations on single ecosystem services may result in producing a knowledge gap that can only be solved by integrative and holistic approaches for the assessment of multiple ecosystem services [22, 34, 35]. Understanding the different tradeoffs among ecosystem services should include assessments of both the supply and societal demand of ecosystem services [15–17]. Thus, we need to integrate multiple indicators, data sources, and methods in order to assess the suite of ecosystem services from supply to social demand

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Overall, we found that the use of the supply and demand framework of ecosystem services for watershed-scale studies in the USA has been extremely limited. The majority of the watershed case studies were found in the eastern half of US, with very few in the Southwest. Studies implementing biophysical assessment of the ecosystem services supply were located in the Southeast and Midwest, while studies investigating the social demand of ecosystem services were concentrated along the east coast of the USA. In addition to identifying the gaps in our knowledge of watershed-scale ecosystem services across the USA, we call attention to the scale issue in ecosystem services research, which describes the mismatch between the scale at which ecosystem services are provided and the scale at which those services are used, valued, or managed [16]. Future studies should not only address multiple spatial and temporal scales; they should also assess different stakeholder scales, from the individual to the community to

Understanding and quantifying tradeoffs between ecosystem services, considering their ecological, cultural, and economic value, is a key challenge for water resources management and planning in the USA [36] and beyond [37]. Our study demonstrates the knowledge gap across US watersheds in terms of integrating biophysical, sociocultural, and economic dimensions to assess the biophysical supply and social demand for services, which is key for increasing public awareness of the importance of river systems in maintaining human well-being [3, 38]. Moving forward, we would like to see more comprehensive ecosystem service studies at watershed scales using integrative (yet standard) approaches to assess tradeoffs at multiple

This research was primarily funded by the Oklahoma Biological Survey and the South-Central Climate Science Center (SC-SCC) at the University of Oklahoma (US). AJC and CQS

across different spatial and temporal and stakeholder scales [32, 33].

**5. Conclusions**

the municipality to the state, and beyond.

spatiotemporal and stakeholder scales.

**Acknowledgements**

Overall, our results showed that the number of studies investigating regulating and provisioning services was higher relative to those investigating cultural services. This finding is consistent with similar studies across the globe, where research on the supply and demand of ecosystem services has focused mainly on provisioning and regulating services [24, 25]. In the Mediterranean region, for example, [21] showed that provisioning services attracted much more scientific attention, which is also consistent with most of the findings related to the assessment of ecosystem services in European landscapes [13, 23]. Furthermore, using the CICES classification, we found that from a total of 308 ecosystem services studied across all US watersheds, regulating services (e.g., filtration, sequestration, storage and accumulation by ecosystems, habitat maintenance, and chemical conditions of freshwaters) were most commonly studied, while cultural services (e.g., educational and cultural heritage) were the least studied. As recently highlighted by the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES), there is an urgent need for global efforts by governments, academia, and society to promote knowledge of earth's biodiversity and ecosystems, with the aim of informing sustainable policy and management of natural resources [26, 27]. One of the key components of the IPBES approach is the notion of nature's contributions to people, which recognizes the critical role that culture plays in defining all links between humans and ecosystems. We therefore argue that there is also a need to recognize the important role that cultural services and values can play in water resources management in the USA and the need to operationalize the role of indigenous and local knowledge in understanding watershed's contribution to people [26, 28].

Different disciplines have traditionally assessed ecosystem services separately [18, 24], which has led to the conclusion that ecosystem services values are multidimensional, and thus their evaluation must be conducted from the ecological, social, and economic perspective [23, 28, 29]. Although we found a small percentage of studies that used this multidisciplinary approach in their assessments, our results showed that most of the studies conducted across US watersheds implemented a biophysical approach, which points out the gap of integrating different approaches into ecosystem service research [30, 31]. We believe that this gap is due to the absence of a shared theoretical framework, as we found that over 78% of all studies in the USA did not use a standard ecosystem services framework. In a recent article, [32] concluded that integrated valuation of ecosystem service supply and demand still faces challenges in understanding the tradeoffs among ecosystem services. With regard to ecosystem service demand, it is necessary to use systematic methods for different stakeholders (beneficiaries, impairers, and managers) because of their different knowledge types, capabilities, demographics, rights, and value systems [32, 33]. We also identified methodological limitations in current ecosystem services research conducted across major US watersheds. Most of the studies were focused on a single ecosystem service without investigating the potential implications that trade-offs between multiple ecosystem services may have in watershed management [3, 4]. Many recent investigations have showed that investigations on single ecosystem services may result in producing a knowledge gap that can only be solved by integrative and holistic approaches for the assessment of multiple ecosystem services [22, 34, 35]. Understanding the different tradeoffs among ecosystem services should include assessments of both the supply and societal demand of ecosystem services [15–17]. Thus, we need to integrate multiple indicators, data sources, and methods in order to assess the suite of ecosystem services from supply to social demand across different spatial and temporal and stakeholder scales [32, 33].
