Documents
Resources
Learning Center
Upload
Plans & pricing Sign in
Sign Out

Spatially Distributed Hydrologic Modeling and Scale Issues

VIEWS: 0 PAGES: 30

									 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.
   – Test and validate model components and complete
     model against internal and spatially distributed
     measurements.
   – Evaluate the level of complexity needed to provide
     adequate characterization of streamflow at various
     scales.
   – Quantify spatial heterogeneity of inputs (rainfall,
     topography, soils - where data exist) and relate this
     to heterogeneity in streamflow.
   – Role of groundwater? Fracture flow?
Distributed models incorporate the effects of topography through direct used of
the digital elevation data during computation, along with process-level knowledge.
Hydrological processes within a
catchment are complex, involving:
   •   Macropores
   •   Heterogeneity
   •   Fingering flow
   •   Local pockets of saturation
The general tendency of water to flow
downhill is however subject to
macroscale conceptualization
 TOP_PRMS

  PRMS


National Weather
Service - Hydro17



  TOPMODEL
       Terrain Based Runoff
   Generation Using TOPMODEL
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.”
      TOPMODEL and GIS
• Surface saturation and soil moisture deficits
  based on topography
  – Slope
  – Specific Catchment Area
  – Topographic Convergence
• Partial contributing area concept
• Saturation from below (Dunne) runoff
  generation mechanism
Saturation in zones of convergent topography
Topographic index is used to compute the depth to the water table,
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.
Topographic Definition                         Numerical Evaluation
Specific catchment area a is the upslope       with the D¥ Algorithm
area per unit contour length [m2/m Þ m]

             Stream line



                                Contour line


                                   aa
                         tin g are
                con tribu
       Ups lope




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)
Hydraulic conductivity (K) decreases with depth




where z is local water table depth (m)
f is a scaling parameter (m-1):
        shape of the decrease in K with depth
      TOPMODEL assumptions
• The dynamics of the saturated zone can be approximated
  by successive steady state representations.
• The hydraulic gradient of the saturated zone can be
  approximated by the local surface topographic slope, tanb.
• The distribution of downslope transmissivity with depth is
  an exponential function of storage deficit or depth to the
  water table


        -   To lateral transmissivity [m2/h]
        -   S local storage deficit [m]
        -   z local water table depth [m]
        -   m a parameter [m]
        -   f a scaling parameter [m-1]
    Topmodel - Assumptions
              • The soil profile at each point
                has a finite capacity to transport
                water laterally downslope.



                e.g.

                 or
     D   Dw
S
                         Topmodel - Assumptions
Specific catchment area a [m2/m Þ m]
(per unit contour length)
                                        • The actual lateral discharge is
                                          proportional to specific
                                          catchment area.


                                        • R is
                                           – Proportionality constant
                                           – may be interpreted as “steady state”
                                             recharge rate, or “steady state” per
                                             unit area contribution to baseflow.
                         D         Dw
  S
                      Topmodel - Assumptions
Specific catchment area a [m2/m Þ m]
(per unit coutour length)
                                        • Relative wetness at a point and
                                          depth to water table is
                                          determined by comparing qact
                                          and qcap



                                        • Saturation when w > 1.
                                          i.e.
                         D         Dw
  S
         Topmodel
Specific catchment area a [m2/m Þ m]
(per unit coutour length)




                             z
                      D          Dw
S
       GL4 CASE STUDY: OBJECTIVES

• to test the applicability of the TOP_PRMS model for
  runoff simulation in seasonally snow-covered alpine
  catchments

• to understand flowpaths determined by the TOP_PRMS
  model

• to validate the flowpaths by comparing them with the
  flowpaths determined by tracer-mixing model
RESAERCH SITE
             GIS WEASEL
• Simplify the treatment of spatial
  information in modeling by providing
  tools (a set of ArcInfo 8 commands) to:


(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
  PROCEDURES FOR DELINEATION AND
        PARAMETERIZATION

• DEM (10 m) was converted from TIN to GRID format using
  ArcInfo 8 commands
• a pour-point coverage was generated using location information
  of gauging stations
• DEM and the pour-point coverage were overlaid to delineate the
  basin
• DEM slope and direction were re-classified to extract the
  drainage network
• a base input parameter file and re-classified DEM were used to
  derive parameters needed for TOP_PRMS model
DELINEATION FOR GREEN LAKE 4

                       •   Delineated
                           basin area:
                           220ha

                       •   Matches the
                           real basin

                       •   Three HRU
                           (MRU)
                           delineated
                           (one stream
                           tributary one
                           MRU)
                    INPUT DATA
• Measured
  discharge

• Measured
  precipitation

• Measured
  temperature

• Measured
  solar radiation
SIMULATED SNOWMELT VS. RUNOFF
          Green Lake 4
MONTHLY WATER BUDGET
SENSITIVITY ANALYSIS AND
PARAMETER CALIBRATION
   COMPARISON OF TOPOGRAPHIC
PARAMETERS IN GLV WITH LOCH VALE
   PROBLEM ON RUNOFF SIMULATION

• Runoff peaks in May and June failed to be captured by the model
• The modeled runoff tells us that a large amount of snowmelt was
  infiltrated into soil to increase soil water storage
• However, the reality is that there were runoff peaks in May and
  June as observed
• It is hypothesized that a large amount of the snowmelt produced
  in May and June may contribute to the stream flow via overland
  and topsoil flowpaths due to impermeable barrier of frozen soils
  and basal ice
               Summary and Conclusions
• Modeling system centered on TOPMODEL for representation
  of spatially distributed water balance based upon topography
  and GIS data (vegetation and soils).
• Capability to automatically set up and run at different model
  element scales.
• Encouraged by small scale calibration, though physical
  interpretation of calibrated parameters is problematic.
• Large scale water balance problem due to difficulty relating
  precipitation to topography had to be resolved using rather
  empirical adjustment method.
• Results provide hourly simulations of streamflow over the
  entire watershed.
    WARNING: TAKE ALL
MODELS WITH A GRAIN OF SALT!




    DON’T HAVE TOO MUCH
   CONFIDENCE IN MODELS!

								
To top