Major Sub-basins Research Articles

Flash flood dynamics and composition in a semiarid mountain watershed

Flash flood hydrographs were examined using water stable isotopes (deuterium and oxygen) and a plug‐flow lumped catchment model to assess the origin and routing processes of flood water in a semiarid basin in southwestern United States. Precipitation and stream water were sampled during storm and flood conditions at high and low elevations. Isotope mixing relationships readily determined the predominance of three sources for the representative summer monsoon events: high elevation precipitation from two major subbasins and base flow. Each flood progressed through a series of source water contributions, as indicated by several segments of linear mixing between these end‐members. We developed a plug‐flow lumped catchment model to test possible governing processes for specific watershed and forcing conditions. Results suggest two main findings: First, these flood events were generated primarily from event water runoff in high elevations that mixed at the flood bore with pre‐event base flow resident in the stream. The power, speed, and turbulence of the flood bore cause it to mix with, ride atop and push the resident in‐stream water in the front of the rising limb. Second, the timing and volume of flood waves from subbasins are identified by their combined isotopic signature at the basin outlet; this approach may provide an effective mesoscale constraint for rainfall‐runoff models.

Assimilation of GRACE Terrestrial Water Storage Data into a Land Surface Model: Results for the Mississippi River Basin

Abstract Assimilation of data from the Gravity Recovery and Climate Experiment (GRACE) system of satellites yielded improved simulation of water storage and fluxes in the Mississippi River basin, as evaluated against independent measurements. The authors assimilated GRACE-derived monthly terrestrial water storage (TWS) anomalies for each of the four major subbasins of the Mississippi into the Catchment Land Surface Model (CLSM) using an ensemble Kalman smoother from January 2003 to May 2006. Compared with the open-loop CLSM simulation, assimilation estimates of groundwater variability exhibited enhanced skill with respect to measured groundwater in all four subbasins. Assimilation also significantly increased the correlation between simulated TWS and gauged river flow for all four subbasins and for the Mississippi River itself. In addition, model performance was evaluated for eight smaller watersheds within the Mississippi basin, all of which are smaller than the scale of GRACE observations. In seven of eight cases, GRACE assimilation led to increased correlation between TWS estimates and gauged river flow, indicating that data assimilation has considerable potential to downscale GRACE data for hydrological applications.

Report on Mississippi‐Atchafalaya streamflow

A new report from the U.S. Geological Survey presents streamflow and nutrient flux data from 1960–2005 for the Atchafalaya River, the main stem of the Mississippi River, 9 major subbasins of the Mississippi, and 21 additional selected subbasins. Nutrient flux estimates are provided for 6 water‐quality constituents: dissolved nitrite plus nitrate, total organic nitrogen plus ammonia nitrogen, dissolved ammonia, total phosphorous, dissolved orthophosphate, and dissolved silica. The report, “Streamflow and Nutrient Fluxes of the Mississippi‐Atchafalaya River Basin and Subbasins for the Period of Record Through 2005,” is available at http://toxics.usgs.gov/pubs/of‐2007‐1080/

Estimating groundwater storage changes in the Mississippi River basin (USA) using GRACE

Based on satellite observations of Earth’s time variable gravity field from the Gravity Recovery and Climate Experiment (GRACE), it is possible to derive variations in terrestrial water storage, which includes groundwater, soil moisture, and snow. Given auxiliary information on the latter two, one can estimate groundwater storage variations. GRACE may be the only hope for groundwater depletion assessments in data-poor regions of the world. In this study, soil moisture and snow were simulated by the Global Land Data Assimilation System (GLDAS) and used to isolate groundwater storage anomalies from GRACE water storage data for the Mississippi River basin and its four major sub-basins. Results were evaluated using water level records from 58 wells set in the unconfined aquifers of the basin. Uncertainty in the technique was also assessed. The GRACE-GLDAS estimates compared favorably with the well based time series for the Mississippi River basin and the two sub-basins that are larger than 900,000 km2. The technique performed poorly for the two sub-basins that have areas of approximately 500,000 km2. Continuing enhancement of the GRACE processing methods is likely to improve the skill of the technique in the future, while also increasing the temporal resolution.

Fine‐Scale Genetic Structure of Bull Trout at the Southern Limit of Their Distribution

AbstractWe used six polymorphic microsatellite loci to analyze the population genetic structure of bull trout Salvelinus confluentus in the Boise River, Idaho, and we compared our results with previous data from similarly sized river systems in western North America. Within the Boise River, we found low genetic variation within and significant differentiation among sample sites. Two cohesive groups of populations were associated with the two major subbasins in this system, which we attributed to long‐term reduction of gene flow or distinct sources of colonization at this scale. We observed a significant pattern of isolation by distance in one subbasin and not in the other; this result suggests that the relative influences of gene flow and drift have differed between the two subbasins. Ecologically defined patches of suitable habitat were not good predictors of genetic variation among samples. Dams and other anthropogenic barriers have recently changed the potential for gene flow and genetic drift but were not associated with the major boundaries of genetic differentiation. There was some evidence of lost genetic variation in smaller patches that were physically isolated by both natural and anthropogenic barriers. We found a large range of within‐population genetic variation and among‐population genetic differentiation for bull trout from river basins across the species' range, but our estimates in the Boise River were the lowest (or among the lowest) observed. The relative roles of drift and gene flow appear to vary strongly at both fine and broad spatial scales. We cannot presume that the physical and ecological processes influencing the genetic population structure of bull trout in one region will accurately reflect those in another region; this may have important implications for conservation and management actions.

Controls of Tufa Development in Pleistocene Lake Bonneville, Utah

Prominent tufa localities along the Provo level (∼14,000 14C yr B.P.) shoreline in Pleistocene Lake Bonneville have been characterized in detail. Three types of tufa are recognized: capping tufa, beachrock, and capping tufa over beachrock. Capping tufa and beachrock are end members of a continuum based on variable clastic content. All three types typically occur on headland environments that had stable substrate and little sediment input. Tufa development correlates with bedrock exposure and landform orientation, which in turn are correlated ($$R^{2}=0.89$$ ) with the longest fetch directions in the basin. Tufa also tends to be located at major subbasin divides and in the western portion of the basin.

Combined application of δ13C and molecular ratios in sediment cores for PAH source apportionment in the New York/New Jersey harbor complex

Sub-basins within the New York/New Jersey (NY/NJ) Harbor host variable amounts and sources of polycyclic aromatic hydrocarbons (PAHs). Analyses of radionuclides 137Cs and 7Be were used to assign approximate dates to individual sections of sediment cores collected from four major sub-basins of the harbor complex. Sampling sites range from heavily contaminated Newark Bay and Passaic River to less contaminated Raritan Bay and Jamaica Bay. PAHs were identified with GC–MS and quantified by GC respectively. Gas chromatography–isotope ratio mass spectrometry (GC/IRMS) was applied to determine δ 13C values of pyrene ( δ 13C Py), a principal 4 ring parent PAH. Two end-member PAH source components, petroleum-related PAHs (PRPAH) and combustion-related PAHs (CRPAH), were calculated from a carbon isotopic mass balance equation based on assumptions that petroleum-related δ 13C Py is −29‰, and that combustion-derived δ 13C Py is −24‰. Temporal trends of PRPAH in these cores are similar to those of the petroleum-derived unresolved complex mixture (UCM), whereas CRPAH resembled trends of “Total” PAH (TPAH). Three molecular ratios, fluoranthene to fluoranthene plus pyrene (Fl/(Fl + Py)), high molecular weight 4–6 ring PAH to total PAH (Ring456/TPAH) and parent to total (Par/(Par + Alkyl)), correlate strongly with each other, and show a moderate to strong positive correlation with δ 13C Py. In contrast, the ratios of anthracene to phenanthrene plus anthracene (A/(Pa + A)), benzo[ a]anthracene to benzo[ a]anthracene plus chrysene (BaA/(BaA+Chy)), and indeno[1,2,3- cd]pyrene to indeno[1,2,3- cd]pyrene plus benzo[ ghi]perylene (IP/(IP + Bghi)) show no, weak, or even negative correlation with the four well correlated ratios ( δ 13C Py, Par/(Par + Alkyl), Fl/(Fl + Py), and Ring456/TPAH). Combined application of the four well correlated molecular indicators suggests that PAH contribution from combustion became more important in most areas of the NY/NJ Harbor during the last three decades of the 20th century.

Elemental gaseous mercury flux at the water/air interface over the Negro River basin, Amazon, Brazil

The amount of dissolved gaseous mercury (DGM) released annually into the atmosphere by water bodies in each of the 18 major sub-basins of the Negro River tributaries has been estimated for the wet season and adopted for the entire year. Using Remote Sensors (RS) and the Geographic Information System (GIS), an estimate was made for the total flooded area during the period of the wet season (May–July), which corresponds to around 10% of the total basin area. The estimates of DGM evasive flux values were made during four scientific campaigns and ranged from 0.09 to 14 μg m − 2 y − 1 . Mercury wet deposition to the same area was estimated using average values of the metal concentration in the rainwater (9.8 ng L − 1 ), which varies from 17 to 27 μg m − 2 year − 1 . Considering the flooded area of the Negro River Basin as 69 000 km 2, the total amount of mercury emitted by all bodies of water reaches nearly 0.26 ton year − 1 , which represents not more than 2% of the total mercury found in wet deposition in this same area.

Trophic state and microorganisms community of major sub-basins of the middle Rio Doce basin, southeast Brazil

Total phosphorus concentration was used to define the trophic state of the main sub-basins of the middle Rio Doce, in Minas Gerais State (southeast Brazil) and physical, chemical, and microbiological variables of water were analyzed during 2000 to 2001. The study evaluated changes in water quality caused by seasonality and human activities. Water temperature, conductivity, pH, total alkalinity, dissolved oxygen, and concentrations of NH4-N, NO2-N, NO3-N, total-N, PO4-P, total-P, DOC, and chlorophyll-a were measured in seven rivers stretches (sampling stations). Total yeasts, faecal and total coliforms, and heterotrophic bacteria were also determined. The studied areas were considered to be under oligotrophic to eutrophic conditions. The variables that presented highest positive correlation with faecal coliforms were total-P and total-N, and heterotrophic bacteria density was identified as a good parameter to differentiate the ecosystems. These results suggested the inclusion of the trophic level and the distinct activities within a watershed as important elements when proposing conservation and restoration areas.

Long-term open-water season stream temperature variations and changes over Lena River Basin in Siberia

This study systematically analyzes long-term (1950–1992) stream temperature records for the major sub-basins within the Lena River watershed in order to describe water temperature regimes over the various parts of the Lena watershed and document significant stream temperature changes induced by reservoir regulation, and by natural variations/changes. The results show that the open water season can be divided into three consecutive stages—“increasing temperature stage” in the early open water season, “stable temperature stage” in the mid-warm season, and “decreasing temperature stage” in the late open water season. Temperature conditions are similar over the Aldan and Upper Lena regions. However, stream temperatures at the Lena basin outlet are up to 8 °C lower than those over the southern sub-basins. This suggests that the latitudinal difference in climatic variables, such as air temperature, might be the major control on stream temperature regime. Results also demonstrate that the reservoir regulation has a strong influence on the regional water temperature regime and change in the regulated sub-basin. Reservoir regulation has increased (decreased) the downstream water temperatures in the Vilui valley during the early (mid) open water season. Trend analyses show consistent warming trends across the entire Lena River basin in the early open water season. This may indicate a response to earlier snowmelt over the Lena River watershed. Trend results also demonstrate that the Aldan tributary, without much human impact, experiences warming (cooling) trends in the first (second) half of the open water season, leading to a stream temperature regime shift toward early open water season. The upper Lena River has warming (cooling) trends in the early (mid-late) open water season. Over the regulated Vilui tributary, however, stream temperatures have significantly increased in the early and late parts of the warm season due to combined effects of natural changes and reservoir regulation. Over the Lena basin as a whole, strong positive correlations have been found between the basin mean monthly air and water temperatures during the warm season. Increasing water temperatures were observed during the early and mid-June. Because of stream temperature increase in this peak flow period, the Lena River heat flux has gone up by 23% in June. This may have considerable impact on the thermal conditions of the Laptev Sea in the early summer season.

Discharge Characteristics and Changes over the Ob River Watershed in Siberia

This study analyzes long-term (1936–90) monthly streamflow records for the major subbasins within the Ob River watershed in order to examine discharge changes induced by human activities (particularly reservoirs and agricultural activities) and natural variations. Changes in streamflow pattern were found to be different between the upper and lower parts of the Ob watershed. Over the upper Ob basin, streamflow decreases in summer months and increases in the winter season. The decreases in summer are mainly due to water uses along the river valley for agricultural and industrial purposes and to reservoir regulation to reduce the summer peak floods. The increases in winter streamflow are caused by reservoir impacts to release water for power generation over winter months. In the lower Ob regions, however, streamflow increased during midsummer and winter months and weakly decreased in autumn. These increases in summer flow are associated with increases in summer precipitation and winter snow cover over the northern Ob basin. Because of reservoir regulations and water uses in the upper parts of the Ob basin, it is a great challenge to determine hydrologic response to climate change and variation at the basin scale. Discharge records observed at the Ob basin outlet do not always represent natural changes and variations mainly due to impacts of large dams; they tend to underestimate the natural runoff trends in summer and overestimate the trends in winter and autumn seasons. This study clearly demonstrates regional differences in hydrologic response to climate changes and variations within a large watershed such as the Ob River. It also illustrates that, relative to climatic effects, human activities are sometimes more important and direct in altering regional hydrologic regimes and affecting their long-term changes particularly at both seasonal and regional scales. It is, therefore, necessary to consider human activities in regional/global environment change analyses and further examine their impacts in other large northern watersheds.

Streamflow changes over Siberian Yenisei River Basin

This study analyzes long-term (1935–99) monthly discharge data for the major sub-basins within the Yenisei River watershed in order to document significant streamflow changes induced by reservoir regulations and by natural variations/changes. The results show that both the unregulated upper basin and major lower streams of the watershed experienced streamflow decreases in the early melt period and discharge increases in the late melt season. These changes in snowmelt runoff pattern suggest a delay in snowcover melt in the Yenisei basin perhaps associated with cooling trends during the snowmelt months over central Siberia. This study also demonstrates that the reservoir regulation has significantly altered the monthly discharge regimes in northeast and the upper portions of the Yenisei basin. Constructions of four large dams in the northeast Yensiei regions reduced the summer peak flows in the Angara valley by 15–30% and increased the winter low flows by 5–30%. Operations of two large reservoirs in the upper Yenisei regions enhanced the winter flows by 45–85% and reduced the summer flows by 10–50%. These alterations lead to a streamflow regime change toward less seasonal variation over the eastern and lower Yenisei basin. Because of reservoir regulations, discharge records collected at the Yenisei basin outlet do not always represent natural changes and variations, they tend to underestimate the natural streamflow trends in summer and overestimate the trends in winter and fall seasons. Cold season discharge increase over the Yenisei river is not natural-caused, but mainly the effect of reservoir regulations in the Yenisei basin.

Suspended Sediment, C, N, P, and Si Yields from the Mississippi River Basin

The annual loads of C,N,P, silicate, total suspended sediment (mass) and their yields (mass area−1) were estimated for six watersheds of the Mississippi River Basin (MRB) using water quality and water discharge records for 1973 to 1994. The highest load of suspended sediments is from the Missouri watershed (58 mt km2 yr−1), which is also the largest among the six major sub-basins. The Ohio watershed delivers the largest load of water (38%). The Upper Mississippi has the largest total nitrogen load (32%) and yield (1120 kg TN km2 yr−1). The loading of organic carbon, total phosphorus and silicate from the Upper Mississippi and Ohio watersheds are similar and relatively high (range 2.1–2.5, 0.068–0.076, and 0.8–1.1 mt km2 yr−1, respectively). The yields of suspended sediments, total phosphorus, total nitrogen, and silicate from the Lower Mississippi watershed are disproportionately the highest for its area, which is the smallest of all the watersheds and has the weakest monitoring network. The loading from the Red and Arkansas watersheds are of lesser importance than the others for most parameters investigated. The total nitrogen loading to coastal waters increased an additional 150% since the early 1900s, and is now dominated by loads from the Upper Mississippi watershed, rather than the previously dominant Ohio watershed. An analysis of trends for 1973–1994 suggests variability among years, rather than uni-directional change for most variables among 11 key stations. Explanatory relationships were established or confirmed to describe TN and TP loadings in terms of the now largely human-created landscape arising mostly over the last 150 years.

Changes in Lena River streamflow hydrology: Human impacts versus natural variations

This study systematically analyzes long‐term (1936–1999) monthly discharge records for the major subbasins within the Lena River watershed in order to document significant streamflow hydrology changes induced by human activities (particularly reservoirs) and by natural variations/changes. The results show that the upper streams of the watershed, without much human impact, experience a runoff increase in winter, spring, and (particularly) summer seasons and a discharge decrease in fall season. These changes in seasonal streamflow characteristics indicate a hydrologic regime shift toward early snowmelt and higher summer streamflow perhaps due to regional climate warming and permafrost degradation in the southern parts of Siberia. The results also demonstrate that reservoir regulations have significantly altered the monthly discharge regime in the lower parts of Lena River basin. Because of a large dam in west Lena River, summer (high) flows at the outlet of the Vilui valley have been reduced by up to 55% and winter (low) flows have been increased by up to 30 times. These alterations, plus streamflow changes in the upper Lena regions, lead to strong upward trends (up to 90%) in monthly discharge at the basin outlet during the low‐flow months and weak increases (5–10%) in the high‐flow season. Monthly flow records at the basin outlet have been reconstructed by a regression method to reduce reservoir impacts. Trend analyses and comparisons between the observed and reconstructed monthly flows show that because of reservoir regulations, discharge records observed at the Lena basin outlet do not always represent natural changes and variations. They tend to underestimate the natural runoff trends in summer and overestimate the trends in both winter and fall seasons. Therefore the cold season discharge increase identified at the mouth of the Lena basin is not all naturally caused, but the combined effect of reservoir regulation and natural runoff changes in the unregulated upper subbasins. This study clearly illustrates the importance of human activities in regional and global environment changes and points to a need to examine human impacts in other large high‐latitude watersheds.

Advances in Holocene mountain geomorphology inspired by sediment budget methodology

The sediment budget, which links sediment sources to sediment sinks with hydroclimatic and weathering processes mediating the response, is applied to the analysis of sediments in three alpine lakes in British Columbia. We provide two ways of using the sediment budget as an integrating device in the interpretation of mountain geomorphology. These approaches differ in their resolution and ability to budget the major components of the fine-sediment cascade in glaciated environments. Taken together, they provide an integrated index of landscape change over the Holocene. The first example compares the hydroclimatic controls of lake sedimentation for the last 600 years (A.D. 1370–1998) preserved in varved sediments from two of the lake basins. This hydroclimatological approach incorporates contemporary monitoring, air photo analysis, and detailed stratigraphy of sedimentation events within a single varve to infer the timing, sources, and preferred pathways of fine-grained sediments reaching the lake basins. The results indicate that glaciers, hillslope, and channel instability within the major subbasins are the principal sediment sources to the lake basins. Transitory sediment storage of glacially derived sediments within the channels is believed to modulate the episodic and more frequent delivery of sediments from adjacent hillslope and fluvial storage sites and direct routing of glacial rock flour during years of prolonged glacial melt. The second example, relying on the phosphorus geochemistry of sediments in an alpine lake basin, considers the evolution of phosphorus forms (from mineral to occluded and organic fractions) as a function of the soil development, inherent slope instability, and repeated cycles of glaciation and neoglaciation over the Holocene. This geochemical approach demonstrates that both neoglaciation and full glaciation have essentially zeroed the system in such a way that a high proportion of mineral phosphorus remains in the present lake sediments and the bioavailability of phosphorus (a key to ecosystem development) is low. Both examples illustrate the importance of variable sediment sources; the seasonality, frequency, and magnitude of sediment transfers; and the profound influence of ice cover over contemporary, neoglacial and Pleistocene time scales. They also signal the value of including both clastic and dissolved components in the sediment budget.

The geometry of fan‐deltas and related turbidites in narrow linear basins

AbstractThe sediment distribution in three narrow, linear basins, two modern and one ancient, in Greece and Italy, was studied and related to changes in basin configuration. The basins are the Plio‐Quaternary Patras–Corinth graben, the Pliocene–Quaternary Reggio–Scilla graben and the middle Tertiary Mesohellenic piggy‐back basin. These basins were formed at different times and under different geodynamic conditions, but in each case, the tectonic evolution produced a narrow area in the basin where the water depth decreased dramatically, forming a strait with a sill. This strait divided the basin into major and minor sub‐basins, and the strait has a similar impact on sedimentary environments in all three basins, even though different depositional environments were formed along the initial basin axis.Predictions for the development of depositional environments in the two modern basins, especially in their straits, are based on the studied ancient basin. In the straits, powerful tidal flows will transport finer sediments to sub‐basins and trapezoidal‐type fan‐deltas will gradually fill up and choke the strait through time. In sub‐basins, according to basin depth, either deltaic (in the shallow minor sub‐basin) or turbiditic (in the deep major sub‐basin) deposits may accumulate. Moreover, an extensive shelf is likely to develop between the strait and major sub‐basin. This shelf will be cross‐cut by canyons and characterized by thin fine‐ to coarse‐grained deposits. These sediment models could be applied to analogous basin geometries around the world. Copyright © 2003 John Wiley & Sons, Ltd.

Microphytobenthic potential productivity estimated in three tidal embayments of the San Francisco Bay: A comparative study

In this paper we describe a three-step procedure to infer the spatial heterogeneity in microphytobenthos primary productivity at the scale of tidal estuaries and embayments. The first step involves local measurement of the carbon assimilation rate of benthic microalgae to determine the parameters of the photosynthesis-irradiance (P-E) curves (using non-linear optimization methods). In the next step, a resampling technique is used to rebuild pseudo-sampling distributions of the local productivity estimates; these provide error estimates for determining the significance level of differences between sites. The third step combines the previous results with deterministic models of tidal elevation and solar irradiance to compute mean and variance of the daily areal primary productivity over an entire intertidal mudflat area within each embayment. This scheme was applied on three different intertidal mudflat regions of the San Francisco Bay estuary during autumn 1998. Microphytobenthos productivity exhibits strong (ca. 3-fold) significant differences among the major sub-basins of San Francisco Bay. This spatial heterogeneity is attributed to two main causes: significant differences in the photosynthetic competence (P-E parameters) of the microphytobenthos in the different sub-basins, and spatial differences in the phase shifts between the tidal and solar cycles controlling the exposure of intertidal areas to sunlight. The procedure is general and can be used in other estuaries to assess the magnitude and patterns of spatial variability of microphytobenthos productivity at the level of the ecosystems.

Nitrogen budget in the Upper Mississippi River watershed

Abstract Nitrogen budget calculations performed for highflow and low-flow years in the major sub-basins of the Upper Mississippi River watershed show differences in nitrogen applications and discharges. Nitrogen budgets show that fertilizer is the most important input of nitrogen to the basins, but also show that atmospheric input and animal manures can be significant inputs of nitrogen to the basins. The transport of nitrogen from the land to rivers varies with the prevailing hydrologic conditions. The annual nitrogen budgets are not balanced. In years of high precipitation and river discharge, more nitrogen can be removed than had been applied that year, presumably from N stored in the soil or ground water. Storage of nitrogen in soils is a major unknown in the model, but calculations suggest that it is a significant reservoir of N.

Historical effects of El Nino and La Nina events on the seasonal evolution of the montane snowpack in the Columbia and Colorado River Basins

Snow‐water equivalent (SWE) data measured at several hundred montane sites in the western United States are used to examine the historic effects of El Nino and La Nina events on seasonal snowpack evolution in the major subbasins in the Columbia and Colorado River systems. Results are used to predict annual runoff. In the Columbia River Basin, there is a general tendency for decreased SWE during El Nino years and increased SWE in La Nina years. However, the SWE anomalies for El Nino years are much less pronounced. This occurs in part because midlatitude circulation anomalies in El Nino years are located 35° east of those in La Nina years. This eastward shift is most evident in midwinter, at which time, SWE anomalies associated with El Nino are actually positive in coastal regions of the Columbia River Basin. In the Colorado River Basin, mean anomalies in SWE and annual runoff during El Nino years depict a transition between drier‐than‐average conditions in the north, and wetter‐than‐average conditions in the southwest. Associations during La Nina years are generally opposite those in El Nino years. SWE anomalies tend to be more pronounced in spring in the Lower Colorado River Basin. Our predictions of runoff reveal modest skill for scenarios using only historic El Nino and La Nina information. Predictions based on the water stored in the seasonal snowpack are, in almost all cases, much higher than those based on El Nino‐Southern Oscillation (ENSO) information alone. However, combining observed midwinter snow conditions with information on seasonal snowpack evolution associated with ENSO improves predictions for basins in which ENSO signals exhibit strong seasonality.

Assessing the response of subgrid hydrologic processes to atmospheric forcing with a hydrologic model system

An integrated hydrologic model system (HMS) was developed and used to study hydrologic processes and systems responding to various climatic forcings. The modeling system operates with a time step of minutes to days to facilitate coupling with a mesoscale meteorological model (MM). The major emphasis with HMS is on the interaction among climate, land surface, surface water, and ground water. HMS utilizes spatially detailed information on climate, soil type, land use, digital elevation, and hydrologic parameters. The focus of this study was to improve the presentation of rainfall-runoff partitioning by implementing subgrid-scale spatial variability in precipitation and hydraulic conductivity. The practical application of HMS is demonstrated in the hydrologic simulation of a major sub-basin of the Susquehanna River Basin in Pennsylvania. Questions concerning data preparation, model calibration, and subgrid-scale spatial variability are addressed in the hydrologic simulation. The simulation without considering subgrid-scale spatial variability using both observed and MM5-simulated precipitations significantly underestimates the streamflow. The simulated and observed streamflows compare well in the simulation with consideration of the subgrid-scale spatial variability in hydraulic conductivity using observed precipitation. With the implementation of subgrid-scale spatial variability in precipitation and hydraulic conductivity, the simulated results using MM5-simulated precipitation were improved in terms of fit between the simulated and observed streamflows.