Andrew J. Guswa

Ecosystem services: Challenges and opportunities for hydrologic modeling to support decision making

Ecosystem characteristics and processes provide significant value to human health and well-being, and there is growing interest in quantifying those values. Of particular interest are water-related ecosystem services and the incorporation of their value into local and regional decision making. This presents multiple challenges and opportunities to the hydrologic-modeling community. To motivate advances in water-resources research, we first present three common decision contexts that draw upon an ecosystem-service framework: scenario analysis, payments for watershed services, and spatial planning. Within these contexts, we highlight the particular challenges to hydrologic modeling, and then present a set of opportunities that arise from ecosystem-service decisions. The paper concludes with a set of recommendations regarding how we can prioritize our work to support decisions based on ecosystem-service valuation.

Seasonal variation in the stable isotopic composition of precipitation in the tropical montane forests of Monteverde, Costa Rica

reveal a seasonal signal that may be used to trace water through the hydrologic cycle. Deuterium excess indicates that water evaporated from land is an important flux to the region during the transitional and dry seasons when winds from the Caribbean slope dominate. Following the shift to convective rainfall at the start of the wet season, when the western equatorial winds influence the Pacific slope of Costa Rica, d excess values become depressed. Yet as the wet season progresses, d excess begins to climb. These data suggest that several months of rain are needed following an acute dry season on the northern Pacific slope before a terrestrial evaporative signal is detected in wet season precipitation. The evaporative flux may result from a wet season expansion of surface water bodies and flooding of seasonal wetlands.

Storage and release of road-salt contamination from a calcareous lake-basin fen, western Massachusetts, USA.

Road salt (NaCl) applications to highways have increased stream sodium and chloride concentrations due to retention within watersheds. The mechanisms for retention and export of Na(+) and Cl(-) from different environments are not fully understood. This field study examines the hydrologic and cation exchange processes that store and release Na(+) and Cl(-) from a calcareous fen adjacent to a highway. Despite high concentrations of Ca(2+) and Mg(2+), elevated salt concentrations enable Na(+) to occupy up to 15% of the cation exchange capacity of shallow peat. Calculations of selectivity coefficients show that Na(+) preferentially exchanges with Mg(2+), and Na(+) can be desorbed under more dilute conditions caused by precipitation and snowmelt. Detailed sampling of surface and ground waters during three snowmelt events illustrate early releases of Na(+) and Cl(-) at the onset of melting, with maximum fluxes coinciding with peak discharge. From 7 March through 4 April 2005, the flux of dissolved salt exiting the wetland amounts to 13% (Na) and 17% (Cl) of annual rock salt applied to the highway. For all of 2005, the total salt mass leaving the wetland via Kampoosa Brook is similar to the amount of road salt applied; 50% of the annual salt efflux occurred during the snowmelt season of March through May. In general, exported Na(+) and Cl(-) correlate with the number of lane miles of highway crossing the watershed. Large rain events outside of winter months are more effective than snowmelt with reducing dissolved salts because snowmelt also introduces contamination. For this and other wetlands having alkaline geochemistry and high flushing rates, management strategies that reduce rock salt amounts to roadways will assist with reducing salt contamination to levels less toxic to vegetation and aquatic organisms.

Potential effects of landscape change on water supplies in the presence of reservoir storage

This work presents a set of methods to evaluate the potential effects of landscape changes on water supplies. Potential impacts are a function of the seasonality of precipitation, losses of water to evapotranspiration and deep recharge, the flow-regulating ability of watersheds, and the availability of reservoir storage. For a given reservoir capacity, simple reservoir simulations with daily precipitation and streamflow enable the determination of the maximum steady supply of water for both the existing watershed and a hypothetical counter-factual that has neither flow-regulating benefits nor any losses. These two supply values, representing land use end-members, create an envelope that defines the water-supply service and bounds the effect of landscape change on water supply. These bounds can be used to discriminate between water supplies that may be vulnerable to landscape change and those that are unlikely to be affected. Two indices of the water-supply service exhibit substantial variability across 593 watersheds in the continental United States. Rcross, the reservoir capacity at which landscape change is unlikely to have any detrimental effect on water supply has an interquartile range of 0.14–4% of mean-annual-streamflow. Steep, forested watersheds with seasonal climates tend to have greater service values, and the indices of water-supply service are positively correlated with runoff ratios during the months with lowest flows.

Curve Number Approach to Estimate Monthly and Annual Direct Runoff

AbstractThis paper establishes a novel approach to estimate monthly and annual direct runoff by combining the curve number method of the Natural Resources Conservation Service with an exponential d...

Authentic, exemplary, and diverse

These words, spoken by President Ruth Simmons in 1999, became the rallying cry for the establishment of an innovative engineering program at Smith College. Tragically, the percentage of engineering degrees awarded to women – just 19% – has not changed since.1 This lack of diversity is an issue of both equity2 and competitiveness, as a growing body of evidence supports the superior performance of diverse teams.3 Attracting and retaining those currently not engaged in the pursuit of engineering presents an opportunity to unleash untapped creativity and innovation. In their paper, ‘Liberal Studies in Engineering – A Design Plan,’ Bucciarelli and Drew propose a bachelor of arts in Liberal Studies in Engineering that would subject exemplary engineering content to study from the ‘perspectives of the humanities, arts, and social sciences as well as engineering.’4 The authors hint that such a program may increase the diversity of the engineering population by approaching technical subjects from a perspective ‘in tune with authentic engineering practice.’5 Does this assertion have merit? In their 2011 paper, Cech and co-authors demonstrate that career-fit confidence – ‘the confidence that a profession’s career path is consonant with one’s individual interests and values’ – is an important predictor of persistence in engineering for both men and women.6 While first-year women have lower career-fit confidence than men, this confidence may be developed through engagement with authentic examples of engineering projects that broaden a mistakenly narrow perception of the field to include competencies such as leadership, communication, and teamwork.7 Thus, the proposal of Bucciarelli and Drew has the potential to diversify engineering if technical material is presented in an authentic way. In light of this, I suggest an interpretation of exemplary engineering content that expands beyond ‘the engineering sciences,

Spatial Scaling of Soil Moisture, Evapotranspiration, and Leakage

An outstanding issue in ecohydrological modeling is scaling nonlinear plant-level interactions among soil, vegetation, and water to larger spatial scales. Spatial heterogeneity in precipitation and vegetation exert significant control on scaling properties, especially in water-limited ecosystems. Computational results indicate that relationships between spatially averaged variables controlling soil-moisture dynamics are non-unique at larger averaging scales, even when unique, non-hysteretic relationships are defined at the plant level. The complexity of these relationships evolves with increasing averaging area based on the characteristics of the spatial heterogeneity. Through detail simulation studies, we can identify a threshold scale for soil moisture, evapotranspiration, and leakage beyond which the non-unique relationships will vary only slowly as averaging area becomes larger. The threshold scale, which is analogous to the concept of a representative elementary area, enables identification of a large-scale relationship that is meaningful with respect to the characteristics of the system. We use numerical simulations to assess the effects of storm spatial structure, rainfall intensity and frequency, and soil and vegetation characteristics on threshold-scale values of the relevant variables. Results are generalized by relating the threshold scales to a dimensionless group of parameters that includes length scales characteristic of the heterogeneity and the models resolution. We then evaluate how the computationally derived non-unique relationships relate to analytical approaches to obtain spatially averaged functional relationships. Specific comparison of empirically obtained upscaled functions (based on the non- unique relationships) with the so-called statistical dynamic approach provides guidelines to account for spatial heterogeneity in soil, plant, and climate systems.