Prospectus for a CUAHSI Hydrologic Observatory:

The Potomac River Basin and Western Shore Chesapeake Bay Drainage

Text Box: Figure 1. Locator map
1. Spatial Extent of Hydrologic Observatory

We propose as a CUAHSI long-term hydrologic observatory the Potomac River Basin plus fringe western Chesapeake drainage as shown on Figures 1 and 2. The total area comprising this chosen grouping of basins is approximately 52,000 km²and spans five physiographic provinces -- the Appalachian Plateau, Valley and Ridge (including the Great Valley), Blue Ridge, Piedmont, and Atlantic Coastal Plain. The elevation drops from about 1000 m in the Appalachian Mountain headwaters to sea level at the Chesapeake Bay. The total population of the proposed HO area as of 2000 was 8.26 million, with the highest density in the Baltimore-Washington metropolitan region. As summarized in Table 1, the chosen basin area is composed of 45% forested, 32% agriculture, 5.7% developed, and 4.8% open water land use. Parts of four states ≠ Maryland, Virginia, Pennsylvania, West Virginia ≠ and the entire District of Columbia are included in the study area.

Figure 2. Watersheds comprising the proposed CUAHSI Potomac Hydrologic Observatory. See http://cuereims.umbc.edu/website/PotomacHO/viewer.htm for an interactive version of this map.

Table 1. Land use, area, and population of the Potomac HO.

Basin	Potomac	Md Western Shore	Patuxent	Rappahannock	Total	Percent
Area (km²)	38,019	4,325	2,479	7,369	52,191	100.0
Developed	1,816	754	264	135	2,968	5.7
Agriculture	12,077	1,489	850	2,279	16,695	32.0
Forested	21,888	1,582	1,049	4,193	23,471	45.0
Open Water	1,500	350	158	497	2,505	4.8
Wetland	427	106	135	181	850	1.6
Barren	311	47	23	80	461	0.9

Population (2000)	5,243,322	2,188,148	590,769	240,754	8,262,993

1.1 Scientific Rationale

There is a long history of hydrologic investigations as well as a remarkably dense network of monitoring stations for both basic research and assessment of societal needs within the nominated study area. For this reason, we believe that the Potomac River Basin and other nearby watersheds draining to the western shore of Chesapeake Bay offer a valuable set of targets for establishing an NSF-funded Hydrologic Observatory. All five of the CUAHSI science topics are addressed by current and prospective research agendas. The inherent heterogeneities imposed by natural landscape patterns spanning five physiographic provinces are overlain with the historical legacy of four centuries of human disturbance, including a wave of deforestation, agricultural land use, and land abandonment leading to reforestation contemporaneous with some of the most rapid expansion of urban and suburban land use in the United States. The availability of new tools for spatially and temporally intensive characterization of atmospheric phenomena and land-surface patterns is anticipated to help us in developing a new understanding of the driving phenomena over a variety of spatial and temporal scales, which are expected to lead to much-improved modeling and predictive capability.

1.2 Site Characteristics

Mean annual precipitation in the Potomac River basin ranges from a minimum of approximately 750 mm in portions of the Valley and Ridge physiographic province to maxima of more than 1250 mm in high elevation regions along both the eastern and western margins of the Valley and Ridge province (the Blue Ridge on the east and the Appalachian Plateau on the west). The annual cycle of precipitation throughout the region is characterized by modest seasonal variations with a summer maximum, whereas the average annual cycle of runoff is characterized by a spring peak and a late summer/early autumn minimum. High-intensity precipitation events associated with convective systems or tropical cyclones may interrupt this pattern any time between May and November, with rain-on-snow events occasionally causing severe floods in winter. The heterogeneities in rainfall distribution associated with complex terrain are poorly understood and of fundamental importance for all aspects of the hydrologic cycle.

Equally striking are the heterogeneities in hydrologic response across physiographic province boundaries. The Great Valley (including, most prominently, the Shenandoah Valley) is underlain by carbonates with extensive karst development and maintains high base flow throughout the late summer and early autumn, providing water supply for the Washington metropolitan area during the annual minimum flow period. The western Valley and Ridge and Appalachian Plateau provinces, by contrast, have low base flow and flashy response to precipitation events. Some of the largest unit-discharge flood peaks in the eastern U.S. have occurred in Blue Ridge catchments underlain by crystalline rock. The Piedmont is characterized by varying hydrologic response driven in part by contrasts between the deeply weathered saprolites and rolling topography of the crystalline Piedmont and the dense soils in shale-dominated Triassic basins. This contrast is typical of the Piedmont region from North Carolina through NJ. Superimposed on these natural variations are the hydrologic consequences of land-use history, including intensive agriculture and rapid urbanization during the period from 1950 to the present. The Coastal Plain is groundwater-dominated, tends to have smaller watersheds draining separately to tidewater rather than to a single main stream, and is characterized by strong base flow and relatively low unit-discharge flood peaks.

Land cover within the study area shows strong contrasts by physiographic province as well. The high-relief Blue Ridge, Valley and Ridge and Plateau provinces are dominated by second-growth forest (including substantial areas of National Forest in Virginia and West Virginia) with patches of upland pasture, and cropland and urban development concentrated along the narrow valley bottoms. The low-relief carbonate-dominated Great Valley is dominantly agricultural, and in fact the Shenandoah Valley is the largest producer of turkeys in the U.S. with attendant water-quality consequences from manure management practices. The eastern foothills of the Blue Ridge and the western Piedmont are also agricultural landscapes, but with growing urban development radiating outward from the Washington and Baltimore metropolitan areas, and the eastern Piedmont is in a more advanced stage of transformation from agriculture to urban land cover. The Coastal Plain, still primarily agricultural, is also characterized locally by some of the most rapid transformation of land to residential and urban uses in the country, particularly along the Chesapeake Bay shores from the mouth of the Potomac River to Baltimore.

Sediment yields are strongly associated with current and historical patterns of land development, with the lowest values in the forested landscapes of the Appalachian provinces and the highest values in the agricultural and urban portions of the Piedmont. Because the precipitation regime of the humid eastern U.S. has much higher rainfall rates than the 17^th- and 18^th-century colonists had experienced in Europe, farming practices transplanted from Europe led to very high rates of soil erosion, truncating upland soil profiles and depositing large volumes of sediment in the valley bottoms where they still reside. The more recent wave of urban development triggered a new round of soil erosion, followed by stream-channel widening and export of stored sediment as hydrologic regimes became more flashy. The role of sediment storage and remobilization as an influence on the timing of sediment delivery to tidewater is a major concern for understanding impacts on estuarine systems.

A wealth of existing instrumented field sites forms a rich network of resources that will be woven together as part of this effort. The Potomac River Basin is a USGS NAWQA site; the NSF-funded Baltimore LTER (also funded as a NSF CLEANER site) is located on the Western Chesapeake fringe; both USGS and USDA research headquarters (Reston and Beltsville) are located in the region; the Interstate Commission on the Potomac River Basin (ICPRB) oversees interstate management of the Potomac River; the Smithsonian Environmental Research Center maintains a number of field sites, and major research universities located in the region all also maintain active field sites. This effort also represents a significant partnership with the US Geological Survey MD-DE-DC District office. Most recently, researchers utilizing the Delmarva Peninsula (not yet included in the study area) have expressed an interest in joining this effort, with potential resources to be committed by the State of Delaware (personal communication with Paul Imhoff, U. Delaware).

2. Existing Data Infrastructure

The existing monitoring network necessary to support a Hydrologic Observatory includes federally-operated networks of precipitation gages, stream gages, monitoring wells, and meteorological stations collecting data on components of the radiation budget for use in estimating evapotranspiration. These are supplemented by additional gages maintained by other agencies, institutions and individual investigators, some of which are listed below with links to supporting information. Additional supporting geospatial data, such as high-resolution topographic and land-use information and remote-sensing imagery, are available from multiple sources.

For the states of Maryland, Virginia and West Virginia there is a robust network of precipitation gages including 126 stations in the NOAA/NWS cooperative observer 15-minute precipitation network, numerous NWS hourly precipitation stations, and 377 stations in the Integrated Flood Observaton and Warning System (IFLOWS) network. In addition there are several weather radars collecting data covering portions of the Hydrologic Observatory study area, and current research activities involve development of strategies for calibration of radar data to provide high-resolution precipitation fields within selected portions of the study area.

One of the most important aspects of the existing data infrastructure in the Potomac watershed is the availability of a particularly dense backbone network of high-altitude IFLOWS precipitation gages, with the largest number situated in the Appalachian Highlands of West Virginia. Within the South Branch Potomac River basin of West Virginia there are 36 gages within an area of 3850 km². Another 28 IFLOWS gages are located within the Shenandoah watershed in Virginia, which is the major tributary to the Potomac with a drainage area of 7820 km². This network, although not originally designed to serve research needs and despite some flaws that need to be corrected, offers an excellent opportunity, when coupled with high-resolution weather-radar scan data, to provide new insight and understanding of processes associated with orographic precipitation.

Radiation and flux data are important for estimation of evapotranspiration within the study area. Two flux towers located within the Maryland portion of the study area (one operated by the Baltimore Ecosystem Study and one operated by the Smithsonian Environmental Research Center) are collecting components of the radiation balance, and additional meteorological stations operated by the Baltimore Ecosystem Study also collect radiation budget data. Another monitoring station at Canaan Valley, WV, near the western edge of the Potomac River watershed, is currently listed as a proposed node in the SURFRAD radiation monitoring network (http://www.srrb.noaa.gov/surfrad/index.html).

The U.S. Geological Survey has been conducting hydrologic investigations in the proposed study area since the early 1890s. Ongoing long-term data collection in 2004 included 163 stream-gaging stations and 785 observation wells, and is summarized in tables 2 and 3. Several major USGS investigations are active in the proposed study area. The National Water Quality Assessment (NAWQA) Program has included the Potomac River Basin and Delmarva Peninsula as a study unit since 1991 (http://md.water.usgs.gov/watershed/MD200/) and encompasses parts of all of the physiographic provinces listed above. USGS is working with the Chesapeake Bay Program to improve the Bay watershed model, using the Potomac River Basin as a prototype. The Chesapeake Bay River Input Monitoring Program (http://va.water.usgs.gov/chesbay/RIMP/) includes 3 of its 9 stations on the Potomac, Rappahannock, and Patuxent Rivers, and has been active since 1984. Other major USGS projects include investigations of surface-water, ground-water, and water-quality problems and processes in the Anacostia River Basin, Shenandoah Valley, and Maryland and Virginia Coastal Plains. Efforts are also underway to determine the sources, transport, and residence time of sediment delivered to portions of the Bay impacted by sediment (http://md.water.usgs.gov/watershed/MD172/index.html). USGS is working with Montgomery County, Maryland, and USEPA to assess ecological impacts of development and is a partner in the Baltimore Ecosystem Study, which is part of a dense urban stream-gaging network of 41 stations. The Baltimore work also includes an assessment with USEPA of hydrologic impacts of stream restoration.

Table 2. Daily USGS streamflow gaging stations operated in 2004 in the Potomac, Rappahannock, Patuxent, and West Chesapeake Hydrologic Units. [Main-stem stations are those with relatively large drainage areas and/or located at the mouth of primary tributaries. HCDN is the USGS long-term Hydro-Climatic Data Network. * indicates that some main-stem stations were not included in computing mean drainage area.]

River Basin (Hydrologic Unit Codes)	Number of stations (total)	Number of stations (main-stem)	Number of HCDN stations	Mean drainage area (km²)	Mean years of record
02070001, 02070002, 02070003, 02070004 Potomac River Basin upstream of Shenandoah River	36	9	10	947*	47
02070005, 02070006, 02070007 Shenandoah River Basin	22	4	4	453*	47
02070008, 02070009 Potomac River Basin from Shenandoah River to Fall Line	20	3	7	212*	39
02070010, 02070011 Potomac River Basin below Fall Line	21	0	3	82	36
02080103, 02080104 Rapidan and Rappahannock River Basins	8	2	4	639*	62
02060003 Upper Chesapeake (Gunpowder, Patapsco, and other River Basins)	46	2	3	65	19
02060004, 02060006 Lower Chesapeake (Patuxent and other River Basins)	10	1	1	225	35
All stations	163	21	32	337	37

Table 3. USGS groundwater observation wells operated in 2003 in the Potomac, Rappahannock, Patuxent, and West Chesapeake Hydrologic Units.

River Basin (Hydrologic Unit Codes)	Number of wells (total)	Maximum depth (m)	Mean depth (m)	Mean number of observations
02070001, 02070002, 02070003, 02070004 Potomac River Basin upstream of Shenandoah River	211	366	73	80
02070005, 02070006, 02070007 Shenandoah River Basin	67	215	94	96
02070008, 02070009 Potomac River Basin from Shenandoah River to Fall Line	24	270	76	188
02070010, 02070011 Potomac River Basin below Fall Line	157	519	162	135
02080103, 02080104 Rapidan and Rappahannock River Basins	8	223	169	288
02060003 Upper Chesapeake (Gunpowder, Patapsco, and other River Basins)	110	197	34	238
02060004, 02060006 Lower Chesapeake (Patuxent and other River Basins)	208	537	141	91
All wells	785	537	107	123

In addition to the stream gages mentioned above, there are multiple other monitoring activities associated with the Baltimore Ecosystem Study (BES), part of the NSF-funded Long-Term Ecological Research (LTER) network supported by major contributions from the U.S. Forest Service. Major hydrologic research themes include the influence of land use and development patterns on biogeochemical cycling and export of nutrient and carbon from nested catchments along the rural to urban gradient, the role of impervious area runoff, altered drainage flowpaths and infrastructure as an influence on urban water budgets and water quality, effects of urbanization on riparian zone groundwater with influence on riparian vegetation and  nutrient retention, and flood response to convective precipitation in small urban watersheds.  Current monitoring activities are summarized in a linked table.

Other institutions and organizations that are currently involved in hydrologic monitoring within the Hydrologic Observatory watershed boundaries include the Smithsonian Environmental Research Center (SERC) in Edgewater, MD; the USDA Agricultural Research Service facility in Beltsville, MD; the Appalachian Laboratory of the University of Maryland Center for Environmental Science (UMCES) in Frostburg, MD; University of Virginiaπs Shenandoah Watershed Study (SWAS) and the Virginia Trout Stream Sensitivity Study (VTSSS) in Shenandoah National Park and in the George Washington and Jefferson National Forests; the Interstate Commission on the Potomac River Basin (ICPRB) ; the Fernow Experimental Forest in Parsons, WV, a short distance west of the Potomac basin; the Canaan Valley Institute in Canaan Valley (and several other locations), WV; the Maryland Biological Stream Survey (MBSS) of the Maryland Dept. of Natural Resources (DNR); the U.S. EPA Office of Research and Development; and numerous local government agencies, two of which (Baltimore County and Montgomery County, MD) are represented here. Academic researchers currently involved in monitoring activities for research purposes are affiliated with University of Maryland College Park, Princeton University, UMBC, University of North Carolina-Chapel Hill, Institute of Ecosystem Studies, University of Delaware, Indiana University, and Howard University. Repositories of geospatial data for landscape characterization and modeling are also essential elements of the data infrastructure, including the Regional Earth Science Applications Center (RESAC) at University of Maryland College Park, the Center for Urban Environmental Research and Education (CUERE) at UMBC, and the Mid-Atlantic Integrated Assessment (MAIA) team at EPA.

Multiple groups are also engaged in assessing and synthesizing the data collected from monitoring efforts in order to identify appropriate management objectives and to develop strategies for achieving those objectives. ICPRB is developing an assessment framework to merge monitoring data collected by its member jurisdictions (Maryland, Pennsylvania, Virginia, West Virginia, and the District of Columbia) so that the data can be used to assess the health of stream biota in a consistent, basin-wide manner; ICPRB also is responsible for coordinating water-management efforts to ensure the sustainability of drinking water supplies derived from the Potomac River during times of drought. The EPA Chesapeake Bay Program is currently engaged in developing a watershed-wide non-tidal monitoring network that will include stream monitoring at 100-150 sites, many of which will be located within the Hydrologic Observatory study area. That data collection effort is intended to provide data in sufficient quantity and quality to calculate stream loads of nutrients and sediment, and associated temporal trends. The CBP also has contacts with the broad community of agencies that are involved in Bay restoration and with the stakeholders involved.

3. Proposed Core Data

The existing data infrastructure as described above, even though incomplete, already collects and makes available much of the information that would be considered core data in any Hydrologic Observatory. Rather than repeat the listing of these items, we have interpreted this element of the prospectus outline as an invitation to identify new or innovative core data products that are anticipated to be included in the plan of the Hydrologic Observatory. As it is impossible to monitor everything simultaneously within feasible budget limits, the set of choices must be constrained using primary research thrusts or themes as a guide. The themes listed below represent four ≥clusters≤ of topics that are also related to important CUAHSI science topics. Each of them is also related to critical water-resource issues within the particular geographic region encompassed by this Hydrologic Observatory.

Orographic precipitation mechanisms, hillslope hydrology, runoff generation and groundwater recharge. This cluster of topics treats the relationships between hydrometeorology, topographic pattern (particularly in mountainous environments), processes that govern watershed hydrologic response during extreme events (both floods and droughts), and processes that control the availability of water for water-supply purposes during dry periods. Particular focus is on those portions of the HO falling within the Blue Ridge, Great Valley, Valley and Ridge, and Appalachian Plateau provinces.
Sediment sources, storage, and delivery, floodplain processes, and fate of sediment-associated contaminants. This cluster of topics treats the processes responsible for routing of sediment from source to ultimate destination; the role of sediment as a transporting medium for a variety of chemical constituents; the role of valley-bottom landforms as storage reservoirs for sediment, as landforms that modulate the timing and delivery of flood flows, and as critical riparian habitat. Sediment is also identified as one of the most important causes of water-quality degradation and adverse effects on habitat in the Chesapeake Bay.
Biogeochemical cycling and the sources, pathways, and delivery of nutrients from nontidal uplands to estuarine waters. Excess nutrient delivery to tidal waters is generally recognized as the most important cause of excessive phytoplankton growth, changes in food-web composition and organic-matter processing, and hypoxia and associated negative impacts (e.g. ≥dead zones≤) in Chesapeake Bay and its tributaries. Agriculture is the largest source of excess nutrients, with urban land use and sewage treatment also playing an important role. There are multiple pathways and time scales for delivery of nutrients, with P traveling primarily in association with sediment and N traveling primarily in dissolved form. N in particular has been a focus of many of the existing water-quality monitoring efforts. Recent analyses of monitoring data indicate that 20 years of restoration efforts have not caused measurable reductions in nutrient flux, raising questions about adequacy of existing models for prediction of nutrient transport and delivery.
Urbanization and transformation of hydrologic landscapes and processes; the role of infrastructure in urban water budgets; effect of impervious area and altered flow paths on biogeochemical processes. In addition to ongoing research activities in the Baltimore-Washington metropolitan area (described above), there are also nodes of rapid urban and residential development in coastal-plain areas near Chesapeake Bay where utilization of groundwater resources and potential contamination associated with urban development are key issues.

We propose the following core data collection efforts motivated by these four clusters of topics.

3.1 Cluster 1 - Core data to quantify orographic precipitation mechanisms, hillslope hydrology, runoff generation and groundwater recharge

Precipitation gage records at existing sites and at selected new locations will be archived and made publicly available.
Time series of research-level radar reflectivity products will be generated from the weather radars at Sterling, VA, State College, PA, Pittsburgh, PA, Charleston, WV, and a new weather radar installation to be placed at a high-altitude location in order to capture data from areas that are presently shaded by mountainous terrain.
The radar reflectivity records and precipitation gage records will be used to generate a continuous rainfall rate field for the entire study area with 5-minute, 1-km² resolution.
A real-time network of groundwater monitoring stations with automatic water-level recorders and telemetry will be created, augmenting the small number of existing sites with enough wells to provide a synoptic picture of water-table patterns and changes over time.
Remote sensing characterization of temporal and spatial patterns of soil moisture will be developed.
Small mountain watersheds will be selected for more intensive study and will be instrumented using dense networks of piezometers with water-level recorders to characterize processes associated with groundwater recharge and runoff generation. These will be located in proximity to stream gages. Locations to be selected will include watersheds on the western margin of the Blue Ridge, where infiltration through quartz talus slopes brings moderately acidic water in contact with the carbonates of the Shenandoah and Hagerstown valleys, generating karst systems that have high storage capacity and that sustain baseflow over summer dry periods, ensuring reliable water supply for the Washington metropolitan area.

3.2 Cluster 2 ≠ Core data to quantify sediment sources, storage, and delivery, floodplain processes, and fate of sediment-associated contaminants

A series of continuous sediment monitoring stations will be established with the goal of characterizing sediment yields and loads at the Fall Line, at physiographic province boundaries, and at major land-cover boundaries. Placement of these stations will include consideration of the merits of a nested design for use in mass-balance estimates.
LiDAR data will be collected at high resolution along major valleys and selected smaller streams to characterize channel and floodplain topography, surface roughness, and vegetation patterns; these can be used for hydraulic modeling and for habitat assessment, and can be repeated to assess channel changes and spatial patterns of change in the volume of sediment stored in floodplains. There is some potential for the utilization of large-footprint LiDAR systems (LVIS) for describing the three-dimensional structure of vegetation canopy in forested areas; there are also tradeoffs in the types of LiDAR systems used with respect to the spatial resolution of the data points collected.
Rapid-response field surveys using a total station will be coupled with aerial reconnaissance to investigate changes in floodplains and sediment deposits following large flood events.
Additional sources of data (e.g. investigation of isotopic tracers, dendrochronology, etc.) will be supported by individual investigator awards, as they cannot be collected as part of a routine monitoring effort.

3.3 Cluster 3 ≠ Core data to quantify biogeochemical cycling and the sources, pathways, and delivery of nutrients from non-tidal uplands to estuarine waters

Surface-water sampling points included in the Potomac/Delmarva Peninsula NAWQA study will continue to be monitored for nutrients in both particulate and dissolved form, and sampling will include both base flow and storm samples.
Regular sampling of nitrogen and phosphorus species will be conducted at a selected network of groundwater monitoring wells chosen in collaboration with the NAWQA study.

3.4 Cluster 4 ≠ Core data to quantify urbanization and transformation of hydrologic landscapes and processes; the role of infrastructure in urban water budgets; effect of impervious area and altered flow paths on biogeochemical processes, for selected urban areas

The existing network of precipitation gages, meteorological stations and stream gages supporting activities of the Baltimore Ecosystem Study will continue to be maintained, with an emphasis on high-resolution (1- and 5-minute) streamflow data and a dense network of precipitation gages for calibration of radar rainfall observations.
Optical current meters will be emplaced on bridges at strategic locations in watersheds of particular interest; a network of remotely-triggered low-cost video cameras for capturing information about stages, inundated areas and flow patterns during floods will also be set up.
LiDAR and high-resolution aerial photography will be used to generate detailed representations of planimetric and topographic patterns for use in modeling, habitat assessment, survey control, etc.
Monitoring will include sites chosen for the purpose of describing the role of infrastructure, e.g. storm sewers, sanitary sewers, detention ponds, infiltration basins, etc. Water-quantity monitoring will include the use of weirs and automatic flow meters in inlet and outlet pipes. Water-quality monitoring will include collection of data on constituents (chloride, coliforms, nutrients, caffeine, fluoride, and trace organic chemicals such as pharmaceuticals and hormones) that may be used to assess the effect of leaking infrastructure.
Investigation of urban groundwater will be supported by installation of piezometer nests with water-level recorders, ≥peepers≤ to characterized pore-water chemistry, deeper monitoring wells at selected locations.
Monitoring of habitat conditions, detrital organic matter, benthic macroinvertebrate and fish populations, and stream microbiota.

4. Example Science Questions

The topics used as drivers for core data collection are repeated here with accompanying science questions. The CUAHSI science themes -- (1) Linking hydrologic and biogeochemical cycles; (2) Sustainability of water resources; (3) Hydrologic and ecosystem interactions; (4) Hydrologic extremes; and (5) Fate and transport of chemical and biological contaminants -- are threaded through these topics.

4.1 Orographic precipitation mechanisms, hillslope hydrology, runoff generation, and groundwater recharge

Hydrologic extremes in the Potomac River basin have played a high profile role in the development of water resources in the United States. The record flood peaks in the Potomac River in 1889 and 1936 were major stimuli for fresh approaches to solving the nationπs flood hazard problems. Extreme floods with nominal recurrence intervals of 100 to 500 years have continued to occur somewhere in the area covered by the Potomac and nearby watersheds of the central Appalachian region with a frequency of about once every 10 years, and in addition to associated local hazards these events have significant impacts on regional sediment yields, water quality, and living resources in receiving waters. The severe drought of the early 1960s crystallized the debate over water supply in the humid regions of the United States, and particularly after the severe drought of 2000-2002 this debate remains of current importance with concern over potential impacts of climate change coupled with growing demand associated with population growth and urban sprawl.

The water supply for the Federal Government and the Washington D.C. metropolitan region is tied to the largely unregulated flow of the Potomac River. The municipal demand for water supply has risen to the point that extended droughts, such as those in the early 1960s and early 1930s, would overtax the supplies of the unregulated Potomac River. Innovative water management and forecasting tools have been developed for the Potomac River basin and these developments have reduced the pressures for construction of major dams and reservoirs. Innovative management tools rely on solid scientific understanding of the hydrology of the basin and there are gaps in this understanding that are directly linked to reliability of water supply. Of particular importance is groundwater recharge and its control of late summer and fall base flow.

Example science questions

How do spatial variability and spatial heterogeneity of rainfall over the Potomac River Basin control the distribution of extreme floods over basin scales ranging from 1 to 10,000 km²?
How do spatial variability and spatial heterogeneity of rainfall and infiltration control patterns of groundwater recharge, base flow and temporal distribution of runoff in the Potomac River basin?

4.2 Sediment sources, storage, and delivery, floodplain processes, and fate of sediment-associated contaminant

The ≥sediment yield problem≤, which concerns the declining trend in the volume of soil eroded on hillslopes compared with the sediment transported by the river out of the basin, has a long history in the Potomac. Major studies in the Potomac include Grace Brushπs investigations of sedimentation rates in the Potomac estuary and their links to centuries of changing agricultural practice, Robert Meadeπs studies of the sediment yield problem of major East Coast rivers using old (19^th century) and recent suspended sediment observations from the Potomac, M. Gordon Wolmanπs study of the cycle of erosion associated with urbanization, and Wark and Kellerπs study of the variation of soil erosion with land use. Because sediment has been identified as one of the two most important pollutants affecting the Chesapeake Bay, and because it serves as a carrier for a variety of associated contaminants, there are broad implications associated with the changing relationships between upland erosion, hydrologic response to climate patterns, and remobilization of sediment from storage at intermediate locations in the landscape. The problem is central to assessing the role of non-point source pollution on receiving waters, like the Chesapeake Bay.

The transport of sediment and nutrients is dominated by major flood events. It was estimated that flooding from Hurricane Fran in 1996 transported sediment and nutrients to the Potomac estuary exceeding the mean load for 10 years. Although sediment yields typically are highest in the Piedmont, sediment sources in the Blue Ridge or Valley and Ridge play a dominant role during extreme hydrometeorological events that generate widespread mass wasting.

Example science questions

What is the distribution of travel times of sediment eroded from upland portions of the watershed to the Potomac Estuary and the Chesapeake Bay?
How does this pattern vary with particle-size distribution of sediment and how does this affect delivery of associated nutrients and contaminants?

4.3 Biogeochemical cycling and the sources, pathways, and delivery of nutrients from non-tidal uplands to estuarine waters

A major algal bloom in the Potomac River in 1983, virtually within view of the Capital, illustrated the coupled hydrologic and biogeochemical controls of water quality. The algal bloom in the upper Potomac estuary was likely due to release of phosphorus from benthic sediments in the upper estuary. The coupled transport of sediment and nutrients and the complex cycling of nitrogen and phosphorus were implicated as major players in the algal bloom. Excess nutrient inputs and resulting excess algal production are chronic problems in the Potomac River.

The pattern of agricultural and forested land in the upper Potomac basin presents opportunities to compare the effects of a variety of land uses. Agriculture is the main source of nutrient inputs to the upper Potomac watershed (via fertilizer, import of feeds, N-fixing crops and processed through livestock wastes). Atmospheric deposition is an important source of N also. Despite extensive monitoring efforts and special studies (e.g. NAWQA), there are still important questions about processes affecting nutrient retention in watersheds and about pathways and time scales of nutrient delivery by surface water and groundwater.

Example science questions

How does spatial and temporal distribution of nutrient sources and delivery from upland watersheds to tidewater vary with land cover and geology?
What are the characteristic travel times and dominant pathways followed by nutrients derived from different source areas?

4.4 Urbanization and transformation of hydrologic landscapes and processes; the role of infrastructure in urban water budgets; effect of impervious area and altered flow paths on biogeochemical processes

Urbanization has resulted in striking heterogeneities in hydrologic response in the region. Andersonπs classic study of urbanization and its impacts on hydrologic response was carried out in the northern Virginia suburbs of Washington, D.C. The Baltimore Ecosystem Study provides an invaluable regional resource for urban hydrology studies, as do other ongoing projects in the counties surrounding Washington and along the I-270 corridor. Considerable attention has focused on water quality in the Anacostia River and its links to urban hydrology, and significant resources are being invested in restoration plans for the Anacostia.

Example science questions

∑       How does infrastructure affect the water balance and water cycle in urban drainage basins?

∑       What are the dominant controls of flash floods in urban drainage basins?

∑       How does leaking infrastructure affect urban water quality including toxics, pathogens, and emerging contaminants (e.g. pharmaceuticals, hormones)?