FLOOD07038

• County: Statewide
• Community: All
• Stream Name: All
• Basin: South Platte
• Title: ASFPM Abstracts, etc. 1994
• Date: 1/1/1994
• Prepared For: ASFPM
• Prepared By: ASFPM
• Floodplain - Doc Type: Educational/Technical/Reference Information

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CHOOSING A HYDROLOGIC MODEL FOR FLOOD
FORECASTING

by
David C. Curtis
DC Consulting

ABSTRACT

INTRODUCTION

Sooner or later, almost every ALERT system user must determine whether or not to use a
hydrologic forecast model. These programs use observed and, in some cases, forecast rainfall
amounts to estimate the amount of water flowing through a stream system.

The choice of which hydrologic model to use is not an easy one to make. There are several
different models of different (:omplexity and level of difficulty to use. This paper offers a brief
overview of some of the available hydrologic models and some of the key issues to consider
during the selection process. The models discussed include: API, ADVIS, Table Look-up,
Sacramento Soil Moisture Ac(:ounting, HEC-IF, and SSARR.

RAINFALL ESTIMATION

Before model selection, one VI~ry important element, rainfall estimation, must be considered. The
phrase commonly heard in the computer industry, "Garbage in. Garbage out." is equally
applicable to flash flood forecasting. Good model performance, no matter what model is used,
cannot be expected without a. good measurement system. The implication for forecast system
design is to invest in the ml:asurement and detection systems first, then consider hydrologic
models.

FORECAST MODELS

The purpose of a forecast model is to estimate future river flows and elevations based on
observed or forecast amounts of rainfall. In flash flood situations, certain portions of the forecast
hydrograph are more important than others. Accurate forecasts of the rising limb, the time to
hydro graph peak, and the magnitude of the peak are critical. These are the elements of model
output that have the most impact on the flood warning. The model implemented in a flood
warning system must consistently perform well in these three areas.

There are many different hydrologic forecast models in use. The most commonly used models in
local flood warning systems fall into two categories: simple index-type models and conceptual
rainfall-runoff models. Index models keep a running index that reflects current moisture
conditions. The moisture ind,~x, a "time of year" index, current rainfall, and rainfall duration is