Infiltration and Runoff from Terrestrial Hydrology Models
Objectives
Hydrologic modeling within SRBEX is concerned with the interaction and
behavior of the hydrologic cycle as it pertains to the land surface. To
this end, we seek to develop a terrestrial hydrology model (THM) capable
of realistically simulating the response of watersheds of diverse sizes
to storm events.
Current Activities
The initial thrust of this effort has been twofold:
- Examining available soils and landcover datasets to determine their
suitability for determining runoff curve numbers, and
- Developing, calibrating, and integrating the overland flow and
channel routing components of the THM.
Several soils and landuse/landcover classification databases have been
investigated for their potential value in estimating hydrologic
abstractions such as runoff curve number. The STATSGO soils database includes information
such as soil-texture class, hydrologic soil group, available water
capacity, and permeability. The STATSGO saturated permeability values,
which may represent laboratory rather than field measurements, are very
high compared to those normally used for hydrologic modeling; as an
interim measure, each STATSGO soil type is being assigned a permeability
obtained from other sources. The higher resolution SSURGO (Soil Survey
Geographic) database is being examined as an alternative source of soils
information; unfortunately, these data have been digitized for only a
few counties within the Susquehanna River Basin. In particular, SSURGO
data have been combined with the 25-meter EPA EMAP landuse/landcover data to
estimate curve numbers for the Mahantango Creek basin and the WE-38
intensive study area within it.
The current version of the THM estimates rainfall infiltration and,
ultimately, runoff by one of 3 methods: a phi-index or constant
infiltration routine, the SCS curve number method, or the Green-Ampt
equation. It approximates overland flow using the forward finite
difference solution to the kinematic wave equation, and incorporates a
modified Muskingum channel routing routine. Groundwater, reservoir, and
snowmelt components will be added later.
In addition to testing and calibration, current model development and
research focus on stabilizing the channel routing scheme, determining
the sensitivity of the rainfall network requirements, and investigating
the applicability of the three alternative infiltration methods.
Results
The THM has been run for several rainfall events in the WE-38 intensive
study area, using observed precipitation data, with very good results.
Hydrograph peaks, which are obviously the result of the immediate
response of overland flow, are matched quite well. Hydrograph volumes
are matched reasonably well, particularly when there is an attempt to
account for baseflow contributions. There is need to enhance this
aspect of the model, as the current THM does not contain a groundwater
or through-flow component.
A comparison of runoff hydrographs predicted using the Green-Ampt
equation and using Philip's equation with actual stream gage data (Lukhele, 1994) concluded that
Green-Ampt performed better than Philip's equation. Specifically, the
Green-Ampt method is better able to account for precipitation and
infiltration under variable rainfall intensities.
Last change: 8 May 1995,
R. A. White / raw@essc.psu.edu