Title page for ETD etd-121698-112858

Type of Document Master's Thesis
Author Al-Smadi, Mohammad Ahmed
Author's Email Address malsmadi@vt.edu
URN etd-121698-112858
Title Incorporating Spatial and Temporal Variation of Watershed Response in a GIS-based Hydrologic Model
Degree Master of Science
Department Biological Systems Engineering
Advisory Committee
Advisor Name Title
Heatwole, Conrad D. Committee Chair
Carstensen, Laurence William Jr. Committee Member
Perumpral, John V. Committee Member
Wolfe, Mary Leigh Committee Member
  • geographic information system
  • time-area curve
  • peak flow rate
  • time to peak flow rate
  • temporal variability
  • excess rainfall
  • hydrologic modeling
  • hydrograph shape
  • spatial variability
Date of Defense 1998-11-20
Availability unrestricted

The hydrograph at the watershed outlet was simulated using the time-area curve

concept implemented in a geographic information system (GIS). The goal of this study was

to determine if hydrograph prediction accuracy would be improved by accounting for spatial

and temporal variation of excess rainfall. Three models with different methods of

estimating excess rainfall were developed: the Distributed Curve Number (DCN) model

uses a CN for each cell, generating spatially distributed excess rainfall using the

Soil Conservation Services curve number method (SCS, 1972); the Uniform Curve Number

(UCN) model uses a single "average" CN for the whole watershed, thus generating

a uniform excess rainfall; the Phi index model which uses the Phi-index method to

generate uniform excess rainfall.

With the aid of a GIS, the cumulative flow time to the watershed outlet is

estimated for each cell in the watershed and the isochrones of equal travel time are

developed. The time-area curve is developed in the form of an S curve. The spatially

distributed 1-hr unit hydrograph is derived from the S curve as the difference between

the S curve and its value lagged by 1-hr. The models used in this study describe the

physical processes and flow mechanisms. They also reflect effects of watershed

characteristics (slope, landuse, soil drainage potential) and excess rainfall intensity

on the resulting hydrograph at the watershed outlet. Surface flow is divided into

channel flow and overland flow based on the upstream drainage area. Flow is routed

to the watershed outlet through a channel network derived from the watershed Digital

Elevation Model (DEM).

The models developed were tested against observed rainfall-runoff data from

the 1153-ha Virginia Piedmont watershed (Owl Run). A total of 30 storms were simulated,

with statistical comparison of peak flow rate, time to peak flow rate, and the hydrograph

shape. The hydrograph shape was compared both visually and statistically. Results

indicated that the two models which account for temporal variation in excess rainfall

(DCN and UCN) predicted the output hydrograph much more accurately than the Phi model

which lacks the ability to capture the temporal variation of excess rainfall. For this

watershed, results showed that the spatial variability in excess rainfall which was

accounted for by the DCN model did not improve the prediction accuracy over the UCN

model which lacks that ability. However, a sensitivity analysis for the effect of the

spatial distribution of the excess rainfall indicated that can be a significant effect

of spatial distribution on the predicted hydrograph.

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