Modeling Summer Month Hydrological Drought Probabilities in the United States Using Antecedent Flow Conditions

Climate change raises concern that risks of hydrological drought may be increasing. We estimate hydrological drought probabilities for rivers and streams in the United States (U.S.) using maximum likelihood logistic regression (MLLR). Streamflow data from winter months are used to estimate the chance of hydrological drought during summer months. Daily streamflow data collected from 9,144 stream gages from January 1, 1884 through January 9, 2014 provide hydrological drought streamflow probabilities for July, August, and September as functions of streamflows during October, November, December, January, and February, estimating outcomes 5‐11 months ahead of their occurrence. Few drought prediction methods exploit temporal links among streamflows. We find MLLR modeling of drought streamflow probabilities exploits the explanatory power of temporally linked water flows. MLLR models with strong correct classification rates were produced for streams throughout the U.S. One ad hoc test of correct prediction rates of September 2013 hydrological droughts exceeded 90% correct classification. Some of the best‐performing models coincide with areas of high concern including the West, the Midwest, Texas, the Southeast, and the Mid‐Atlantic. Using hydrological drought MLLR probability estimates in a water management context can inform understanding of drought streamflow conditions, provide warning of future drought conditions, and aid water management decision making.     

Streamflow Changes in the South Atlantic,United States During the Mid‐ and Late 20th Century1

Abstract: Repeated severe droughts over the last decade in the South Atlantic have raised concern that streamflow may be systematically decreasing, possibly due to climate variability. We examined the monthly and annual trends of streamflow, precipitation, and temperature in the South Atlantic for the time periods: 1934‐2005, 1934‐1969, and 1970‐2005. Streamflow and climate (temperature and precipitation) trends transitioned ca. 1970. From 1934 to 1969, streamflow and precipitation increased in southern regions and decreased in northern regions; temperature decreased throughout the South Atlantic. From 1970 to 2005, streamflow decreased, precipitation decreased, and temperature increased throughout the South Atlantic. It is unclear whether these will be continuing trends or simply part of a long‐term climatic oscillation. Whether these streamflow trends have been driven by climatic or anthropogenic changes, water resources management faces challenging prospects to adapt to decadal‐scale persistently wet and dry hydrologic conditions.     

Characterization of Storm Flow Dynamics of Headwater Streams in the South Carolina Lower Coastal Plain1

Epps, Thomas H., Daniel R. Hitchcock, Anand D. Jayakaran, Drake R. Loflin, Thomas M. Williams, and Devendra M. Amatya, 2012. Characterization of Storm Flow Dynamics of Headwater Streams in the South Carolina Lower Coastal Plain. Journal of the American Water Resources Association (JAWRA) 1‐14. DOI: 10.1111/jawr.12000 Abstract: Hydrologic monitoring was conducted in two first‐order lower coastal plain watersheds in South Carolina, United States, a region with increasing growth and land use change. Storm events over a three‐year period were analyzed for direct runoff coefficients (ROC) and the total storm response (TSR) as percent rainfall. ROC calculations utilized an empirical hydrograph separation method that partitioned total streamflow into sustained base flow and direct runoff components. ROC ratios ranged from 0 to 0.32 on the Upper Debidue Creek (UDC) watershed and 0 to 0.57 on Watershed 80 (WS80); TSR results ranged from 0 to 0.93 at UDC and 0.01 to 0.74 at WS80. Variability in event runoff generation was attributed to seasonal trends in water table elevation fluctuation as regulated by evapotranspiration. Groundwater elevation breakpoints for each watershed were identified based on antecedent water table elevation, streamflow, ROCs, and TSRs. These thresholds represent the groundwater elevation above which event runoff generation increased sharply in response to rainfall. For effective coastal land use decision making, baseline watershed hydrology must be understood to serve as a benchmark for management goals, based on both seasonal and event‐based surface and groundwater interactions.     

The 2007‐2009 Drought in Athens,Georgia, United States: A Climatological Analysis and an Assessment of Future Water Availability1

Campana, Pete, John Knox, Andrew Grundstein, and John Dowd, 2012. The 2007‐2009 Drought in Athens, Georgia, United States: A Climatological Analysis and an Assessment of Future Water Availability. Journal of the American Water Resources Association (JAWRA) 48(2): 379‐390. DOI: 10.1111/j.1752‐1688.2011.00619.x Abstract: Population growth and development in many regions of the world increase the demand for water and vulnerability to water shortages. Our research provides a case study of how population growth can augment the severity of a drought. During 2007‐2009, a drought event that caused extreme societal impacts occurred in the Athens, Georgia region (defined as Clarke, Barrow, Oconee, and Jackson counties). An examination of drought indices and precipitation records indicates that conditions were severe, but not worse than during the 1925‐1927, 1954‐1956, and 1985‐1987 drought events. A drought of similar length to the 2007‐2009 drought would be expected to occur approximately every 25 years. Streamflow analysis shows that discharge levels in area streams were at a record low during 2007 before water restrictions were implemented, because of greater water usage caused by recent population increases. These population increases, combined with a lack of water conservation, led to severe water shortages in the Athens region during late 2007. Only after per capita usage decreased did water resources last despite continuing drought conditions through 2009. Retaining mitigation strategies and withdrawal levels such as seen during the height of the drought will be an essential strategy to prevent water shortages during future extreme drought events. The key mitigation strategy, independent local action to restrict water use in advance of state‐level restrictions, is now prohibited by Georgia State Law.     

Reconstructed Streamflows for the Headwaters of the Wind River,Wyoming, United States1

Abstract: Tree rings offer a means to extend observational records of streamflow by hundreds of years, but dendrohydrological techniques are not regularly applied to small tributary and headwaters gages. Here we explore the potential for extending three such gage records on small streams in the Wind River drainage of central Wyoming, United States. Using core samples taken from Douglas fir (Pseudotsuga menziesii), piñon pine (Pinus edulis), and limber pine (Pinus flexilis) at 38 sites, we were able to reconstruct streamflows for the headwaters of the Wind River back to 1672 AD or earlier. The streamflow reconstructions for Bull Lake Creek above Bull Lake; the Little Popo Agie River near Lander, Wyoming; and Wind River near Dubois, Wyoming explained between 40% and 64% of the observed variance, and these extended records performed well in a variety of statistical verification tests. The full reconstructions show pronounced inter‐annual variability in streamflow, and these proxy records also point to the prevalence of severe, sustained droughts in this region. These reconstructions indicate that the 20th Century was relatively wet compared to previous centuries, and actual gage records may capture only a limited subset of potential natural variability in this area. Further analyses reveal how tree‐ring based reconstructions for small tributary and headwaters gages can be strongly influenced by the length and quality of calibration records, but this work also demonstrates how the use of a spatially extensive network of tree‐ring sites can improve the quality of these types of reconstructions.     

Periphyton Nutrient Limitation and Maximum Potential Productivity in the Beaver Lake Basin,United States1

Ludwig, Andrea, Marty Matlock, Brian Haggard, and Indrajeet Chaubey, 2012. Periphyton Nutrient Limitation and Maximum Potential Productivity in the Beaver Lake Basin, United States. Journal of the American Water Resources Association (JAWRA) 48(5): 896‐908. DOI: 10.1111/j.1752‐1688.2012.00657.x Abstract: The objectives of this study were to measure periphytic growth responses to enrichment with nitrogen (N), phosphorus (P), and simultaneous N and P using in situ bioassays in streams draining Beaver Reservoir Basin, Northwest Arkansas; compare periphytic growth responses measured with in situ bioassays with a range of land use and point sources; and test the lotic ecosystem trophic status index (LETSI) as a simplifying metric to compare effects of nonpoint‐source pollutant‐limiting variables of N, P, and sediment across the basin. P limitation was observed at sites across a transect of stream orders throughout the basin; however, at the two sites with highest ambient nitrogen concentrations, limitation was often coupled with nitrogen limitation. Nutrients were at nonlimiting levels at both of two sites below wastewater treatment plants in all seasonal deployments. A Michaelis‐Menten growth equation described LETSI as a function of ambient PO₄‐P concentrations (p < 0.05); the midpoint (LETSI of 0.50) corresponded with a PO₄‐P concentration of approximately 3 μg/l. Change‐point analysis indicated a threshold point at LETSI of 0.80 and 15 μg/l PO₄‐P. These low values show that the periphytic community has a high affinity for available P, and that the watershed as a whole is sensitive to available nutrient inputs.     

County‐Scale Rainwater Harvesting Feasibility in the United States: Climate,Collection Area,Density, and Reuse Considerations

Roof rainwater harvesting (RWH) has the potential to augment water supplies for urban and suburban uses throughout the United States (U.S.). Studies of the performance of RWH at the building and city scales in the U.S. are available, but a countrywide overview of the potential performance of RWH at the county scale has not been done before. Three approaches were taken: (1) assess the viability of RWH in terms of the rainfall that could be captured in relation to the water demand in each county (excluding agriculture), (2) evaluate the performance of a “typical” domestic RWH system across all counties with metrics related to its ability to supply the potable and nonpotable demand, and (3) evaluate the effect of adding a 50% rainwater reuse component to the analysis. We find RWH could be a viable supplemental water source in the U.S., particularly in counties of the Pacific Northwest, Central, and Eastern regions (percent demand covered >50%). Low population density counties have the potential to meet their annual water needs with RWH, while high‐density counties could only source a small portion (~20%) of their annual demand with RWH. Typical RWH systems in counties in the Central and Eastern U.S. performed better than in Western counties. Adding a reuse component can be a key factor in making RWH attractive in many areas of the country. This work can inform future water infrastructure investment and planning in the U.S.     

Adaptation of Climate Model Projections of Streamflow to Account for Upstream Anthropogenic Impairments

A statistical procedure is developed to adjust natural streamflows simulated by dynamical models in downstream reaches, to account for anthropogenic impairments to flow that are not considered in the model. The resulting normalized downstream flows are appropriate for use in assessments of future anthropogenically impaired flows in downstream reaches. The normalization is applied to assess the potential effects of climate change on future water availability on the Rio Grande at a gage just above the major storage reservoir on the river. Model‐simulated streamflow values were normalized using a statistical parameterization based on two constants that relate observed and simulated flows over a 50‐year historical baseline period (1964–2013). The first normalization constant is a ratio of the means, and the second constant is the ratio of interannual standard deviations between annual gaged and simulated flows. This procedure forces the gaged and simulated flows to have the same mean and variance over the baseline period. The normalization constants can be kept fixed for future flows, which effectively assumes that upstream water management does not change in the future, or projected management changes can be parameterized by adjusting the constants. At the gage considered in this study, the effect of the normalization is to reduce simulated historical flow values by an average of 72% over an ensemble of simulations, indicative of the large fraction of natural flow diverted from the river upstream from the gage. A weak tendency for declining flow emerges upon averaging over a large ensemble, with tremendous variability among the simulations. By the end of the 21st Century the higher‐emission scenarios show more pronounced declines in streamflow.     

Sociopolitical Conditions for Successful Water Quality Trading in the South Nation River Watershed,Ontario, Canada1

O’Grady, Dennis, 2011. Sociopolitical Conditions for Successful Water Quality Trading in the South Nation River Watershed, Ontario, Canada. Journal of the American Water Resources Association (JAWRA) 47(1):39‐51. DOI: 10.1111/j.1752‐1688.2010.00511.x Abstract: The South Nation River watershed has a regulated water quality trading program. Legally, wastewater dischargers must not discharge any increased loading of phosphorus (P) into receiving waters. New wastewater systems are now choosing trading instead of traditional P removal technology, and point source dischargers are buying P credits from rural landowners, primarily farmers. These credits are generated by constructing nonpoint source pollution control measures. Mathematical formulae are used to calculate the credits of P removed by each measure. A successful trading program requires several conditions, including community agreement, legislative backing, credit and cost certainty, simplified delivery and verification, written instruments, and legal liability protection. South Nation Conservation, a community‐based watershed organization, is the broker handling the transactions for these P credits. The program is run by a multi‐stakeholder committee, and all project field visits are done by farmers and not paid professionals. An independent evaluation showed higher opinions for the broker and regulatory agency, and most farmers were willing to, or had already, recommended the program to other farmers.     

Characterizing Drought in Irrigated Agricultural Systems: The Surface Water Delivery Index (SWDI)

Quantifying surface water shortages in arid and semiarid agricultural regions is challenging because limited water supplies are distributed over long distances based on complex water management systems constrained by legal, economic, and social frameworks that evolve with time. In such regions, the water supply is often derived in a climate dramatically different from where the water is diverted to meet agricultural demand. The existing drought indices which rely on local climate do not portray the complexities of the economic and legal constraints on water delivery. Nor do these indices quantify the shortages that occur in drought. Therefore, this research proposes a methodological approach to define surface water shortages in irrigated agricultural systems using a newly developed index termed the Surface Water Delivery Index (SWDI). The SWDI can be used to uniformly quantify surface water deficits/shortages at the end of the irrigation season. Results from the SWDI clearly illustrate how water shortages in droughts identified by the existing indices (e.g., SPI and PDSI) vary strongly both within and between basins. Some surface water entities are much more prone to water shortages than other entities based both on their source of water supply and water right portfolios.     

ANALYSIS OF POND SEEPAGE FOR APPLICATION IN FISHERIES AND AQUACULTURE1

ABSTRACT: The purpose of this research was to examine through modeling and experimentation if seepage out of a pond through stratified soil can be predicted, and effectively collected and managed to augment streamflow during a low precipitation period extending three months or more. The 55 m² experimental pond with sandy/loamy banks was excavated to hardpan, and its bottom was approximately 0.7 above the water table. Output from a mathematical model containing both bottom and bank seepage elements agreed with experimental data, and showed that as compared to bottom seepage, the bank seepage contributed approximately 25 percent of the total seepage. Seepage collection (as measured from a circumscribing ditch) linearly varied with stage (r² < 0.99). There was an 8 to 22 percent over‐collection at the lower pond stages, and a 9 to 45 percent under‐collection at the highest stage. As an example of its utility, the model was applied to estimate the pond size and shape needed to supply a hypothetical stream and maintain fish stocks during a three‐month low‐precipitation period. Future work will focus on nutrient transport and removal.     

Hydrologic and Morphologic Variability of Streams With Different Cranberry Agriculture Histories,Southern New Jersey,United States1

Procopio, Nicholas A., 2010. Hydrologic and Morphologic Variability of Streams With Different Cranberry Agriculture Histories, Southern New Jersey, United States. Journal of the American Water Resources Association (JAWRA) 46(3):527-540. DOI: 10.1111/j.1752-1688.2010.00432.x Abstract: The creation of reservoirs and the modification of stream channels are common practices used to facilitate the efficient production of cranberries. The potential impacts to hydrologic and geomorphic aspects of streamflow and channel structure have not been adequately assessed. In this study, the streamflow regime of 12 streams and the channel morphologies of 11 streams were compared for study sites in the Pinelands region of New Jersey with upstream active-cranberry bogs, upstream abandoned-cranberry bogs, and basins with no apparent agricultural history. Flow regime metrics included measures of low-flow, median-flow, and bankfull discharge, two measures of streamflow variability (spread and a modified Richards-Baker Flashiness index), and the frequency of overbank flooding. Stream-channel morphology metrics included average bank slope, average bankfull width, average bankfull depth, average bankfull width-to-depth ratio, and average bankfull area. No significant differences between stream types were apparent for any of the metrics. Basin-area normalized streamflow values of all 12 study sites were highly correlated to each other. Significant relationships existed between some of the flow-regime and channel-morphology metrics. Due to the lack of significant differences between stream types, it appears that neither historic nor current cranberry agricultural practices considerably influence flow regimes or the channel morphology of streams in the New Jersey Pinelands.     

Inventory of Interbasin Transfers in the United States

Interbasin transfers (IBTs) are man‐made transfers of water that cross basin boundaries. These transfers are used to distribute water resources according to supply and demand. The objectives of this work were to quantify the number of IBTs that exist in the United States (U.S.) and to examine the distribution of IBTs and potential causes associated with any observed clustering of IBTs. Defining “basin” was important to enable determination of which transfers qualify as “interbasin.” A variety of definitions are employed by states, with no federal definition. The most recent national studies of IBTs were conducted by the U.S. Geological Survey (USGS) in 1985 and 1986 using USGS Hydrologic Unit Code (HUC) definitions of basins. To build a 2016 inventory of IBTs in the U.S., and to identify where they most commonly occur, the USGS National Hydrography Database (NHD) was utilized in conjunction with the Watershed Boundary Dataset (WBD). Transfers across HUC6 basin boundaries were considered interbasin. Geographical information analysis with the NHD and WBD databases revealed that there are a total of 2,161 man‐made waterways crossing HUC6 basin boundaries in the U.S. IBTs are somewhat concentrated: Florida, Texas, and North Carolina account for over 50% of the total identified IBTs. For some states, identified IBTs are locally clustered. Analysis of these clusters suggests a variety of reasons that IBTs have been built, including population, drainage, and agricultural factors.     

Spatial Distribution of Water Supply in the Coterminous United States1

Abstract: Available water supply across the contiguous 48 states was estimated as precipitation minus evapotranspiration using data for the period 1953‐1994. Precipitation estimates were taken from the Parameter‐Elevation Regressions on Independent Slopes Model (PRISM). Evapotranspiration was estimated using two models, the Advection‐Aridity model and the Zhang model. The evapotranspiration models were calibrated using precipitation and runoff data for 655 hydrologically undisturbed basins, and then tested using estimates of natural runoff for the 18 water resource regions (WRR) of the 48 contiguous states. The final water supply coverage reflects a mixture of outputs from the two evapotranspiration models. Political, administrative, and land cover boundaries were mapped over the coverage of mean annual water supply. Across the entire study area, we find that 53% of the water supply originates on forested land, which covers only 29% of the surface area, and that 24% originates on federal lands, including 18% on national forests and grasslands alone. Forests and federal lands are even more important in the West (the 11 western contiguous states), where 65% of the water supply originates on forested land and 66% on federal lands, with national forests and grasslands contributing 51%.     

Coupling of the Water Cycle with Patterns of Urban Growth in the Baltimore Metropolitan Region,United States

Regional municipal water plans typically do not recognize complex coupling patterns or that increased withdrawals in one location can result in changes in water availability in others. We investigated the interaction between urban growth and water availability in the Baltimore metropolitan region where urban growth has occurred beyond the reaches of municipal water systems into areas that rely on wells in low‐productivity Piedmont aquifers. We used the urban growth model SLEUTH and the hydrologic model ParFlow.CLM to evaluate this interaction with urban growth scenarios in 2007 and 2030. We found decreasing groundwater availability outside of the municipal water service area. Within the municipal service area we found zones of increasing storage resulting from increased urban growth, where reduced vegetation cover dominated the effect of urbanization on the hydrologic cycle. We also found areas of decreasing storage, where expanding impervious surfaces played a larger role. Although the magnitude of urban growth and change in water availability for the simulation period were generally small, there was considerable spatial heterogeneity of changes in subsurface storage. This suggests that there are locally concentrated areas of groundwater sensitivity to urban growth where water shortages could occur or where drying up of headwater streams would be more likely. The simulation approach presented here could be used to identify early warning indicators of future risk.     

Reconstructions of Columbia River Streamflow from Tree‐Ring Chronologies in the Pacific Northwest,USA

We developed Columbia River streamflow reconstructions using a network of existing, new, and updated tree‐ring records sensitive to the main climatic factors governing discharge. Reconstruction quality is enhanced by incorporating tree‐ring chronologies where high snowpack limits growth, which better represent the contribution of cool‐season precipitation to flow than chronologies from trees positively sensitive to hydroclimate alone. The best performing reconstruction (back to 1609 CE) explains 59% of the historical variability and the longest reconstruction (back to 1502 CE) explains 52% of the variability. Droughts similar to the high‐intensity, long‐duration low flows observed during the 1920s and 1940s are rare, but occurred in the early 1500s and 1630s‐1640s. The lowest Columbia flow events appear to be reflected in chronologies both positively and negatively related to streamflow, implying low snowpack and possibly low warm‐season precipitation. High flows of magnitudes observed in the instrumental record appear to have been relatively common, and high flows from the 1680s to 1740s exceeded the magnitude and duration of observed wet periods in the late‐19th and 20th Century. Comparisons between the Columbia River reconstructions and future projections of streamflow derived from global climate and hydrologic models show the potential for increased hydrologic variability, which could present challenges for managing water in the face of competing demands.     

Trends in Pesticide Concentrations in Streamsof the Western United States, 1993‐20051

Johnson, Henry M., Joseph L. Domagalski, and Dina K. Saleh, 2010. Trends in Pesticide Concentrations in Streams of the Western United States, 1993‐2005. Journal of the American Water Resources Association (JAWRA) 00(0):1‐22. DOI: 10.1111/j.1752‐1688.2010.00507.x Abstract: Trends in pesticide concentrations for 15 streams in California, Oregon, Washington, and Idaho were determined for the organophosphate insecticides chlorpyrifos and diazinon and the herbicides atrazine, s‐ethyl diproplythiocarbamate (EPTC), metolachlor, simazine, and trifluralin. A parametric regression model was used to account for flow, seasonality, and antecedent hydrologic conditions and thereby estimate trends in pesticide concentrations in streams arising from changes in use amount and application method in their associated catchments. Decreasing trends most often were observed for diazinon, and reflect the shift to alternative pesticides by farmers, commercial applicators, and homeowners because of use restrictions and product cancelation. Consistent trends were observed for several herbicides, including upward trends in simazine at urban‐influenced sites from 2000 to 2005, and downward trends in atrazine and EPTC at agricultural sites from the mid‐1990s to 2005. The model provided additional information about pesticide occurrence and transport in the modeled streams. Two examples are presented and briefly discussed: (1) timing of peak concentrations for individual compounds varied greatly across this geographic gradient because of different application periods and the effects of local rain patterns, irrigation, and soil drainage and (2) reconstructions of continuous diazinon concentrations at sites in California are used to evaluate compliance with total maximum daily load targets.     

Mitigating Groundwater Impacts of Residential Wells through Small‐Scale Distributed Storage in the Skagit River Basin

The exemption for groundwater wells for residential uses from the prior appropriations system, common in the western United States, has eroded in Washington State since about 2000 due to a series of legal cases. Water markets can allow the transfer of an existing water right, typically from an agricultural use, to compensate for the effect of a new residential well. But water must be legally and physically available in a way suitable to satisfy mitigation requirements. A recent court case in the Skagit basin in Northwestern Washington State has effectively halted residential development in rural areas of the basin because no suitable water rights are available to purchase for mitigation. This paper presents and examines the cost‐effectiveness of various water supply mitigation strategies. We find a small‐scale, distributed stream‐side storage system for augmenting instream flow purchased from downstream sources is relatively cost‐effective to mitigate against the effects of domestic groundwater use compared to more common alternatives. We consider transporting water to storage sites by both small‐gauge pipe and by truck. Overall, trucking water to stream‐side storage and release points tends to be more cost‐effective to mitigate against indoor‐use only given current subbasin housing densities, whereas piping for direct streamflow augmentation is more cost‐effective for higher mitigation needs associated with indoor and outdoor use and higher housing densities.     

Spatial Considerations in Wet and Dry Periods for Phosphorus in Streams of the Fort Cobb Watershed,United States

The Fort Cobb Watershed in Oklahoma has diverse biogeophysical settings and provides an opportunity to explore the association of water quality with a diverse set of landscapes during both wet (April 2007‐December 2009) and dry (January 2005‐March 2007) periods. The objective of this work was to identify spatial patterns in phosphorus (P) (soluble reactive P [SRP] and bioavailable P [BAP]) associated with landscape metrics for two distinct streamflow regimes. Spatial autocorrelation of P was evaluated using contiguous (side‐by‐side) and upstream (upstream:downstream) connectivity matrices. Biogeophysical metrics were compiled for each contributing area, and were partitioned based on association to P concentrations. Results for both SRP and BAP indicated that spatial autocorrelation was present (p < 0.05). There was more spatial autocorrelation and stream P concentrations were three to five times higher in the Wet phase than in the Dry phase (p < 0.05). Analysis with recursive partitioning resulted in higher R² with spatial autocorrelation than without spatial autocorrelation and indicated that lateral metrics (topography, soil, geology, management) were better predictors for SRP than instream metrics. During Wet phase, lateral metrics indicative of rapid surface and subsurface water movement were associated with higher P stream concentrations. This research demonstrated that we can detect landscapes more vulnerable to P losses and/or contaminations in either drought or very wet periods.     

Identification of Putative Geographically Isolated Wetlands of the Conterminous United States

Geographically isolated wetlands (GIWs) are wetlands completely surrounded by uplands. While common throughout the United States (U.S.), there have heretofore been no nationally available, spatially explicit estimates of GIW extent, complicating efforts to understand the myriad biogeochemical, hydrological, and habitat functions of GIWs and hampering conservation and management efforts at local, state, and national scales. We used a 10‐m geospatial buffer as a proxy for hydrological or ecological connectivity of National Wetlands Inventory palustrine and lacustrine wetland systems to nationally mapped and available stream, river, and lake data. We identified over 8.3 million putative GIWs across the conterminous U.S., encompassing nearly 6.5 million hectares of wetland resources (average size 0.79 ± 4.81 ha, median size 0.19 ha). Putative GIWs thus represent approximately 16% of the freshwater wetlands of the conterminous U.S. The water regime for the majority of the putative GIWs was temporarily or seasonally flooded, suggesting a vulnerability to ditching or hydrologic abstraction, sedimentation, or alterations in precipitation patterns. Additional analytical applications of this readily available geospatially explicit mapping product (e.g., hydrological modeling, amphibian metapopulation, or landscape ecological analyses) will improve our understanding of the abundance and extent, effect, connectivity, and relative importance of GIWs to other aquatic systems of the conterminous U.S.