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Scientific Investigations Report 2007-5211

Infiltration and Runoff Measurements on Steep Burned Hillslopes Using a Rainfall Simulator with Variable Rain Intensities

By David A. Kinner and John A. Moody

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Scientific Investigations Report
2007-5211 PDF (20.7 MB)
Abstract

Multiple rainfall intensities were used in rainfall-simulation experiments designed to investigate the infiltration and runoff from 1-square-meter plots on burned hillslopes covered by an ash layer of varying thickness. The 1-square-meter plots were on north- and south-facing hillslopes in an area burned by the Overland fire northwest of Boulder near Jamestown on the Front Range of Colorado. A single-nozzle, wide-angle, multi-intensity rain simulator was developed to investigate the infiltration and runoff on steep (30- to 40-percent gradient) burned hillslopes covered with ash. The simulated rainfall was evaluated for spatial variability, drop size, and kinetic energy. Fourteen rainfall simulations, at three intensities (about 20 millimeters per hour [mm/h], 35 mm/h, and 50 mm/h), were conducted on four plots. Measurements during and after the simulations included runoff, rainfall, suspended-sediment concentrations, surface ash layer thickness, soil moisture, soil grain size, soil lost on ignition, and plot topography.

Runoff discharge reached a steady state within 7 to 26 minutes. Steady infiltration rates with the 50-mm/h application rainfall intensity approached 20–35 mm/h. If these rates are projected to rainfall application intensities used in many studies of burned area runoff production (about 80 mm/h), the steady discharge rates are on the lower end of measurements from other studies. Experiments using multiple rainfall intensities (three) suggest that runoff begins at rainfall intensities around 20 mm/h at the 1-square-meter scale, an observation consistent with a 10-mm/h rainfall intensity threshold needed for runoff initiation that has been reported in the literature.

Version 1.0

Posted March 2008


Suggested citation:

Kinner, D.A., and Moody, J.A., 2008, Infiltration and runoff measurements on steep burned hillslopes using a rainfall simulator with variable rain intensities: U.S. Geological Survey Scientific Investigations Report 2007–5211, 64 p.



Contents

Abstract

Introduction

Runoff from Burned Watersheds

Purpose and Scope of Study

Research Site

Methods

Types of Rainfall Simulators

Rainfall Simulator Design

Simulated Rainfall Calibration and Evaluation

Study Area Characteristics

Plot Experiments

Plot Topography and Stratigraphy

Rainfall Simulations

Soil Moisture

Overland Flow Velocities and Suspended Sediment

Simulated Rainfall Characteristics

Spatial Variability

Drop Characteristics

Incident Rainfall

Temporal Variability

Study Area

Topography

Surficial Features

Rainfall-Simulation Experiments

Plot Topography

Plot Stratigraphy

Ash and Soil Characteristics

Soil Moisture

Plot Runoff

Overland Flow Velocities and Flow Paths

Suspended Sediment

Summary

Acknowledgments

References



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