PROCESS-BASED, DISTRIBUTED WATERSHED MODELS. New generation Source waters and flowpaths Physically based. Objectives. Use distributed hydrologic modeling to improve understanding of the hydrology, water balance and streamflow variability.
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Distributed models incorporate the effects of topography through direct used of
the digital elevation data during computation, along with process-level knowledge.
The general tendency of water to flow downhill is however subject to macroscale conceptualization
National Weather Service - Hydro17
DAILY MODE COMPONENTS
Q = P - ET ± S
Runoff Precip Met Vars Ground Water
Soil Moisture Reservoirs
Basin Chars Snow & Ice
Water use Soil Moisture
Hydrologic Response Units - HRUs
HRU Delineation Based on:
- Slope - Aspect
- Elevation - Vegetation
- Soil - Precip Distribution
Total head = h + x + p
di/dt = K [(h + x + p) / x]
I = x (mt -m0) h<<p
[Green & Ampt]
Surface ( 6 hrus )
Subsurface ( 2 reservoirs )
Ground water (1 reservoir)
Beven, K., R. Lamb, P. Quinn, R. Romanowicz and J. Freer, (1995), "TOPMODEL," Chapter 18 in Computer Models of Watershed Hydrology, Edited by V. P. Singh, Water Resources Publications, Highlands Ranch, Colorado, p.627-668.
“TOPMODEL is not a hydrological modeling package. It is rather a set of conceptual tools that can be used to reproduce the hydrological behaviour of catchments in a distributed or semi-distributed way, in particular the dynamics of surface or subsurface contributing areas.”
which in turn influences runoff generation:
ln(A /tan b )
where ln is the natural logarithm, A is the area drained
per unit contour or the specific area, and tan b is the slope
Regions of the landscape that drain large upstream areas or that
are very flat give rise to high values of the index; thus areas with
the highest values are most likely to become saturated during a
rain or snowmelt event and thus are most likely to be
areas that contribute surface runoff to the stream.
Upslope contributing area a
Numerical Evaluation with the D Algorithm
Specific catchment areaa is the upslope area per unit contour length [m2/m m]
Tarboton, D. G., (1997), "A New Method for the Determination of Flow Directions and Contributing Areas in Grid Digital Elevation Models," Water Resources Research, 33(2): 309-319.) (http://www.engineering.usu.edu/cee/faculty/dtarb/dinf.pdf)
Specific catchment areaa [m2/m m]
(per unit coutour length)
where z is local water table depth (m)
f is a scaling parameter (m-1):
shape of the decrease in K with depth
(1) Delineate the basin from GRID DEM
(2) Characterize stream flow direction, stream channels, and modeling response unit (MRU)
(3) Parameterize input parameters for spatially distributed models such as TOPMODEL and TOP_PRMS model
MODELS WITH A GRAIN OF SALT!
DON’T HAVE TOO MUCH
CONFIDENCE IN MODELS!