INTRODUCTION TO THE INTEGRATED HYDROLOGIC MODEL (IHM)
Jeff Geurink, Water Resources Engineer, Tampa Bay Water, Clearwater, Florida,
Geurink@tampabaywater.org ; Ken Trout, Research Hydrologist, University of
South Florida, Tampa, FL, trout@eng.usf.edu ; Mark Ross, Associate Professor,
University of South Florida, Tampa, FL, mross@eng.usf.edu
Abstract: The public domain Integrated Hydrologic Model (IHM) combines the EPA surface
water model HSPF and the USGS groundwater model MODFLOW with near simultaneous
integration and fundamentally new interpretations of vadose zone process dynamics. The two
model components are concurrently run, linked through an interface that provides a surfacewater to groundwater transition that maintains strict water balance and smoothness in physical
processes. The computational elements of the surface-water components are combinations of
hydrologically distinct landforms made up of specific land use and soil types. Traditionally,
HSPF has been a lumped-parameter model where parame ters were averaged over a basin or
smaller pervious area. The approach used in the IHM is to disaggregate basins into discrete
landform element s that have similar hydrologic properties. These landforms can include
impervious areas, irrigated areas, areas with different soils or significantly different depth-towater table, and areas with different types of land (vegetative) cover or different land uses. Using
discrete landforms within basins allows significant distributed parameter analysis. To facilitate
this level of discretization for regional domains the model has been uniquely written to maintain
efficient computational structure. IHM integration code interprets and manages fluxes and
storages across the component model interfaces, accounting for disparate discretization of land
segments, hydrography (HSPF) and ground water (MODFLOW).
INTRODUCTION
Hydrologic modeling of areas where there is a significant temporal flux and storage connection
between surface water and groundwater requires simultaneous modeling of both systems. This
simultaneous or integrated surface and groundwater modeling is necessary to understand and
capture the interaction between the surface and groundwater systems, to adequately evaluate the
hydroperiod and stresses on wetlands, to reproduce saturation-excess runoff, and to add water
balance constraints that are absent in single system models. Research is currently being
completed to develop, test and apply a new integrated surface-groundwater model, called the
Integrated Hydrologic Model (IHM) (Ross et al. 2004), which is intended to address weaknesses
of previous models and extend the functionality of current integrated modeling to simulate
coastal plain systems.
IHM is designed to provide an advanced predictive capability of the complex interactions of
surface water and groundwater features in shallow water-table environments. The model can be
characterized as deterministic, extremely-distributed-parameter, semi- implicit and real-time with
variable time steps and feature specific spatial discretization. The model components explicitly
account for all significant hydrologic processes including precipitation, interception,
evapotranspiration, runoff, recharge, irrigation flux applied to land, stream flow, wetland
hydroperiod, baseflow, interflow groundwater flow, and all the component storages of surface,
vadose and saturated zone. Input requirements include precipitation and potential
evapotranspiration time series, surface topologic features (i.e. land use, soils, topography and
derived slopes), irrigation fluxes, hydrography characteristics, rating conditions, hydrogeologic
parameters of the groundwater system and information about well pumping and surface-water
diversions. Output includes detailed water balance information on all major hydrologic
processes, including surface water and groundwater flows to wetlands, streams and lakes,
evapotranspiration losses from all storages, reach stage, soil moisture, recharge to the
groundwater system and storage, heads and fluxes in the groundwater system.
The integration codes of IHM exist entirely in the public domain, as do the component models,
HSPF (Bicknell, et al., 2001) and MODFLOW (Harbaugh and McDonald, 1996). However,
commercial database utilities Microsoft Access®, and user-preference model utilities (e.g.,
Groundwater Vistas® and WIN HSPF) for regional watershed applications are useful, necessary
and facilitated by IHM for highly discretized model domains. The model also uses extensive
geographical information system capabilities (e.g., ARCGIS). Where adequate data exist, a GIS
can provide considerable time savings and analysis benefits over conventional means of
developing model data sets (Ross and Tara, 1993). Because model parameters of integrated
models are more constrained, the potential exists for the results from an independent surface
water or groundwater model to be improved with an integrated model strategy (Geurink, et al.,
1995). However, IHM requires extensive data to be correctly applied and calibrated. Significant
costs can be incurred in collecting and manipulating this detailed data, and this must be
understood before embarking on any integrated modeling strategy (Geurink, et al., 1997).
IHM OVERVIEW
IHM version 1.0 consists of a comprehensive surface-water model, a groundwater model,
integration and vadose zone components, and a supporting Microsoft Access database.
Hydrological Simulation Program—FORTRAN, HSPF V. 12 (Bicknell, et al., 2001) satisfies the
surface-water- modeling component, However, unique deterministic interpretations of
parameters, especially in the vadose zone, and code changes have been made. This model is
distributed and supported by the Environmental Protection Agency and Aqua Terra Consultants
and has widespread application for regional, sub-regional and urbanized hydrologic basin
investigations in the U.S. The groundwater component model, MODFLOW-96 (Harbaugh and
McDonald, 1996) supported by the U.S. Geological Survey, has been widely used. Much of IHM
is integration software to provide for the interaction of the two component models, HSPF and
MODFLOW, as well as special interpretation and algorithms specific to the dynamic boundary at
the interface between the vadose zone and the water table. Of great value is the maintenance of
generic compone nt codes HSPF and MODFLOW such that nearly full capabilities of both
models are maintained and third party software is still applicable.
Within IHM, the user can elect to perform surface-water-only simulations with full
implementation of HSPF, or alternatively, groundwater-only simulations using MODFLOW with
pre-determined or post-HSPF user-defined recharge and potential groundwater ET. In fact, a
typical application is to pre-calibrate the surface water and groundwater applications in this
stepwise fashio n prior to performing comprehensive integrated simulations. IHM is intended for
the complete assessment of hydrologic water budgets for surface/groundwater interactions.
Surface Water: HSPF was developed to simulate the hydrologic and water quality processes on
pervious and impervious land surfaces, and transport using reservoir routing within streams and
reservoirs. HSPF uses irregularly shaped watershed sub-basins that can be discretized into
multiple land segments with pervious and impervious land parcels (PERLNDS and IMPLNDS).
IHM allows multiple land segment types, based on user-defined, similarly behaved, hydrologic
response units (HRUs), to discretize land-based hydrologic processes within each sub-basin.
Each land segment type in a sub-basin is comprised of many land parcels that form one pervious
simulation unit (PERLND) which may include some disconnected (drains onto pervious
surfaces) impervious areas. Connected or “effective” (draining contiguously over impervious
surfaces) impervious areas across all land segment types within the sub-basin are aggregated into
one or more IMPLND units. Hydrologic response from each sub-basin is then the aggregated
response of the contributing PERLND and IMPLND simulation units.
Hydrologic response of all wetland and open water features is simulated with coupled HSPF
reaches (RCHRES ) and MODFLOW river package (RIV) reaches. Three general categories of
reaches are supported including conditionally-connected, connected, and routing. Conditionallyconnected reaches represent wetland or open water bodies that are usually isolated from
downstream drainage features except on a seasonal basis or under extreme high- water levels.
Connected reaches represent water bodies that consistently discharge to downstream water
bodies. Routing reaches receive water from the conditionally-connected and connected reaches.
Discharges from each land segment are routed to one or more reaches (on a percentage basis)
based on the reach categories and conditions of the sub-basin.
While the complete HSPF and MODFLOW codes comprise hundreds of subroutines, IHM reconceptualizes only those subroutines relevant to the hydrologic water balance integration of
HSPF and MODFLOW. Other features of the component codes have been maintained.
Groundwater: MODFLOW (Harbaugh and McDonald, 1996) was developed to simulate
groundwater flow in quasi-three dimensions using a block-centered, finite-difference approach to
a variably discretized Cartesian space. It provides for simulation of unconfined and confined
groundwater- flow conditions. The river package of MODFLOW is used in conjunction with the
HSPF RCHRES module to simulate flux transfer between surface-water bodies and groundwater
that is dependent on temporally- varying stage. Within IHM, wells, springs, rivers, lakes,
wetlands, drains, boundary conditions, evapotranspiration and variable hydrogeologic conditions
can be defined for multi-aquifer systems, subject to the limitations of MODFLOW. The
RCHRES module of HSPF maintains water balance and determines heads considering the
dynamic groundwater fluxes determined by the MODFLOW RIV package. The simulated depth
from the RCHRES is used in part to determine stage for the MODFLOW RIV package which
calculates the dynamic flux interaction between water bodie s and groundwater.
Evapotranspiration: Evapotranspiration (ET) is an important element of the hydrologic cycle
and is the dominant component of the annual rainfall of a region (e.g., ET return is as high as 70
or 80% of precipitation in Florida; (Bidlake, et al., 1993). ET can be the most difficult budget
term to assess. While both HSPF and MODFLOW have ET subroutines, which are often used
separately, IHM actually employs both in a unique interpretation and hierarchical approach with
stricter adherence to the physics of ET. IHM accounts for evapotranspiration by first specifying a
potential atmospheric evaporation-rate time series based on open pan data or other meteorologic
data.
For the integrated simulations, HSPF is used to simulate the distribution of ET among principal
surface-water storages, including interception, depression and vadose zone storage. Both vadose
zone and saturated groundwater ET are dictated by vegetative cover characteristics, including
plant coefficient, root-zone (rhizosphere) depth, soil characteristics and depth to the water table.
The extinction depth and maximum ET surface for the evapotranspiration package of
MODFLOW are distinctively defined.
Model Databases: Management and analysis of the large volume of data which supports IHM
requires GIS and other data base tools. These tools are used before, during and after a simulation.
Although the IHM can be run without the use of a GIS, the extensive spatial data requirements
make that impracticable for all but the simplest of model conceptualizations. Areas of subbasins, land segments, reaches and grid cells must be determined. Slopes must be calculated,
topographic elevations must be assigned and data and observation points must be located. Plant
communities and soil types must be mapped to estimate ET parameters (e.g, root zone, plant
coefficients etc.) and recharge variation. Hydrography characteristics must be summarized for
the appropriate discretization and coupling. Summaries of the spatial data are stored in a
Microsoft Access® database. The Access® database has the capability to import data from many
GIS applications, either directly or through intermediate text files.
The surface-water component, HSPF, utilizes a unique time series data structure for both model
input and output. Time series data for HSPF are stored in a portable binary format called
Watershed Data Management files or WDM files. Many public domain programs exist to import,
export, analyze, graph, and transform data stored in WDM format (e.g., WDMUTIL, GENSCN,
ANNIE, IOWDM, HYSEP, SWSTAT, and HSPEXP). IHM utilizes the WDM file format and a
dynamic Access® database for efficient transfer of model input, output, and integration data.
The IHM integration database is implemented using Microsoft Access®. It has 13 tables, each
containing multiple columns (fields). The integration database contains spatial data for land
segments, water bodies and grid cells that are used for HSPF and MODFLOW input. The
database also includes temporal input data for boundary conditions and pumping wells that are
used for MODFLOW.
Integrating Software : Simulated results from HSPF and MODFLOW cannot be directly
integrated with each other because these models use different spatial discretization concepts. As
described earlier, HSPF uses irregularly shaped watershed sub-basins that are discretized into
multiple land segments with pervious and impervious land parcels originating from HRUs.
MODFLOW uses a block-centered, finite-difference approach to discretize space. For
integration, HSPF results (e.g., recharge) must be allocated to a nodal network for use by
MODFLOW, while MODFLOW results must be regrouped by land segment within each subbasin for use by HSPF. In IHM, recharge and ET fluxes use both regular grid cells and irregular
land segments within sub-basins. Therefore, the results from each discretization domain must be
manipulated prior to transfer to the other domain.
Temporal discretization also varies between HSPF and MODFLOW as a characteristic of the
different time scales of surface and groundwater processes. The integration time step of IHM, the
time interval over which time integrated model results are transferred from one component
model to the other, is specified to be the same as the stress period length of MODFLOW.
Physically-based surface-water runoff simulations are typically performed on hourly or less (15
minutes is preferred) increments, while groundwater response time scales are much longer,
requiring time steps of a day to multiple days. Also, surface water features, including lakes,
wetlands and streams, have a different characteristic timescale compared to rainfall/runoff
processes. Therefore, HSPF reaches (RCHRES ) can use a different time step length than what is
used for the land segments (PERLND, IMPLND). To provide time-step compatibility, IHM
integrating software aggregates HSPF results (e.g., in 15 minute, hourly or daily increments) into
MODFLOW stress periods, and MODFLOW results are partitioned into appropriate periods for
HSPF. Within a MODFLOW stress period, a time step length of less than the stress period length
can be specified for MODFLOW simulation. Integration and component model time-step lengths
are variable and user-specified.
The integrating software, including integral dynamic databases, provides the linkage between the
component hydrologic models used in IHM. Component model results are processed (integrated
or partitioned) and placed in the model dynamic database for use in the next time step. The codes
also dynamically pass some integration data. For example, HSPF lower zone storage parameters
are modified based on water-table heads from MODFLOW as the water table nears land surface.
There are numerous software checks made for internal errors, with warnings and halts provided
during the simulation when problems are found. Software also has been written to summarize
and ensure a water balance for the model components.
IHM Operation: For surface-water or groundwater-only simulation, the user can utilize
standard commercial pre-processors for the individual models, HSPF or MODFLOW mindful of
the unique component and parameter interpretations for IHM, including WinHSPF, GENSCN,
Groundwater Vistas, Visual MODFLOW, etc. After the user builds a data set and runs IHM,
standard commercial or IHM unique post-processing software can be used to aid output
assessment.
Updated values for recharge, baseflow, stream-stage relationships, soil moisture, depth-to-water
table and remaining potential evapotranspiration are passed to various integrating software
components throughout the numerical integration time step. Integration software updates
dynamic memory data and archived database results for the next component model, the next
integration time step, or for post-processing following a completed simulation. The integration
looping sequence is repeated until the simulation is completed. The user sets the total length of
integration, integration time step length, and time step/stress period length of all component
models.
LAND SEGMENTS/COMPUTATIONAL ELEMENTS
To better simulate the water balance and runoff/recharge processes of regional basins and
maintain model parameters based on physical and not “lumped” properties, the IHM
discretization of subbasins starts by subdividing areas into small hydrologic response units
(HRUs). These HRUs incorporate hydrologically unique land-use categories and soil conditions
combined with depth-to-water table consideration (ideally not combining shallow and deep
environments). IHM requires categorizing the different HRUs into fewer hydrologically similar
segments, usually by land use (e.g., typically 7-10 per basin). These subdivisions are refe rred to
as land segments. Each land segment within each model basin is comprised of an aggregation of
many discrete, unconnected HRUs. Unique to the IHM, full spatial resolution of HRUs (down to
the grid dimension) is maintained in the model for groundwater association. The calibration
process shifts from basin-by-basin to landform- (segment) based parameter adjustments thereby
maintaining direct association to and predictive capability for land use issues.
Land segments and surface water bodies are distinctly discretized for the HSPF and MODFLOW
components of IHM. An HSPF land segment (index j) is comprised of multiple HRUs which
exhibit similarity in hydrologic response over time for an applied stress. Stated earlier, an HRU
can be a unique combination of land use, soils, slope, predominant depth to water table and
possibly other characteristics. Model objectives, limitations on runtime, or other considerations
constrain the number of land segments which form the basis for PERLND simulation units in
HSPF. HRUs are aggregated in a consistent manner to maintain similarity in hydrologic response
and to stay within the defined limits for land segments for the model application. Each HRU can
contain imperviousness dependent on land use characteristics. As a consequence, each land
segment can contain imperviousness. Unless there exists detailed mapping of impervious areas,
only an area weighted percent imperviousness by land segment can be determined.
Imperviousness from all land segments within a sub-basin are aggregated into one or more
IMPLND elements in HSPF. However, the model is set up to accommodate a separate
impervious element for each appropriate land use category. In such a manner, the model could be
discretized down to the size of roof tops, driveways and grassed front lawns if warranted.
For MODFLOW, the entire model domain is discretized with rectangular or square, finitedifference cells (index i). Intersection of HSPF land segments with MODFLOW cells form
individual land fragments as shown in Figure 1 (HRUs inside & HRUs cut by the grid). In this
example, aggregation of HRUs is based on five general categories of pervious land use including
urban, irrigated, grass/pasture (non-irrigated), forested, and mined/disturbed. The wetland and
open water categories are represented as reaches. In IHM, a land segment of a particular type
(e.g. pasture grass land) is unique from one subbasin to the next. If, for no other reason, than it
could be in different antecedent moisture condition or metrological stress region. It is not
necessary for integration to maintain the individual land fragments of the same land segment
within a cell as there is no intra- grid detail for depth-to-water table. Therefore, like fragments of
the same land segment within a cell are grouped into aggregated land fragments referred to
simply as grid land fragments (index ij) within each cell as shown in Figure 2. Within a cell, the
number of land segments is thus limited excepting that there can be multiple subbasins.
Legend
Basin Boundary
Grid Cell
Land Use
Urban
Irrigated
Grass/Pasture
Forested
Open Water
Wetlands
Mined/Disturbed
Figure 1 Example of land fragments within grid cells
Basin k
Cell i
Legend
Basins
Basin
Grid
GridDiscretization
Cell
Agricultural
Grass/Pasture
Fragment i,j
Irrigated
Urban
Wetland
Forested
Other
Urban
.
Segment j
Figure 2 Example of sub-basins, land segments, land fragments and grid cells
CONCLUSION
An integrated hydrologic model using HSPF and MODFLOW has been formulated using small
hydrologically similar computational elements appropriate to the two models which provides for
computational efficiency sufficient to handle large regional (~103 km2 ) applications and adequate
distribution to maintain all parameter assignments within physical homogeneous landforms.
Considerable discretization (e.g., 106 HRU elements) is facilitated by GIS and integration
database utilities. Consistency and conservation of fluxes and storages between the model
components has been maintained. The model provides for better runoff/recharge prediction for
both Hortonian and saturation excess mechanisms. The model insures consistent plant ET
process distribution between surface storages and below ground storages and allows for smooth
transition between soil moisture fluxes supporting surface evaporation and subsurface ET.
Unique interpretation and code changes for a more physically-based vadose zone, plant and soil
parameters and water fluxes and storage ha ve been implemented in IHM. Variable time steps
(including integration time steps) are provided in the model to streamline model simulation time.
The model is being tested through calibration, verification and validation exercises on regional
and detailed field scale applications as well as simple analytical comparisons and/or vadose zone
solutions with Richard’s Equation.
REFERENCES
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U.S. Environmental Protection Agency, Athens, Georgis.
Bidlake, W. M., Woodham, W. M., and Lopez, M. A. (1993). Evapotranspiration from Areas of
Native Vegetation In West-Central Florida, U. S. Geological Survey Open-File Report 93415, U. S. Geological Survey, Tallahassee, Florida.
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Integrated Model for Comprehensive Water Management Evaluations - Task Three MultiScale Groundwater Model Development And Simulation, University of South Florida,
Tampa, Florida
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Flow Model, U.S. Geological Survey Open-File Report 96-486, Reston, Virginia.
Ross, M.A., and Tara, P.D. (1993). “Integrated Hydrologic Modeling with GIS,” American
Society of Civil Engineering, Journal of Water Resources Planning and Management,
119(2).
Ross, M., Geurink, J., Aly A., Tara, P., Trout, K., and Jobes, T. (2004). Integrated Hydrologic
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Environmental Engineering, University of South Florida, (online at
http://hspf.com/pub/him/IHM_Theory_Manual.pdf).
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Conceptualization in the HSPF-MODFLOW Integrated Models, Journal of the American
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