monitoring the effectiveness of erosion control efforts on kahoolawe, hawaii
DESCRIPTION
by Dr. Scot IzukaResearch Hydrologist, USGSat the Hawaii Water Quality Conference 2008TRANSCRIPT
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MONITORING EROSION ON MONITORING EROSION ON KAHO‘OLAWEKAHO‘OLAWE
U.S. Department of the InteriorU.S. Geological Survey
By Scot Izuka, USGSBy Scot Izuka, USGSPresented at the Presented at the Hawaii Water Quality Hawaii Water Quality Conference Conference March 25, 2008March 25, 2008
A USGS study in cooperation with the Kaho‘olawe Island Reserve CommissionA USGS study in cooperation with the Kaho‘olawe Island Reserve Commission
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Area = 45 mi2
Uninhabited Rainfall ~ 25 in/yrNo perennial streams
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400 AD – Earliest human habitation 400 AD – Earliest human habitation (archaeological evidence)(archaeological evidence)
Early times – grass and trees grew in Early times – grass and trees grew in thick soil on most of islandthick soil on most of island
Wild goats (~1800 to 1990s)Wild goats (~1800 to 1990s)
Sheep and cattle (1858 to 1952)Sheep and cattle (1858 to 1952)
Target bombing by U.S. military (1941 Target bombing by U.S. military (1941 to 1993)to 1993)
UXO clearing (1998-2003) 74% of UXO clearing (1998-2003) 74% of island cleared; 9% to depth of 4 feetisland cleared; 9% to depth of 4 feet
Restoration projects, including Restoration projects, including revegetation (2001 – present)revegetation (2001 – present)
(Sources: Kaho‘olawe Island Reserve Commission; (Sources: Kaho‘olawe Island Reserve Commission; Macdonald and others, 1983)Macdonald and others, 1983)
History History
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HakioawaHakioawaDenudation by grazing
Upper elevations bare
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Restoration EffortsRestoration Efforts
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Motivation for StudyMotivation for Study
Need to assess effectiveness of restoration Need to assess effectiveness of restoration efforts in reducing erosion on Kaho‘olaweefforts in reducing erosion on Kaho‘olawe
ObjectiveObjective
Monitor erosion and sediment transport in Monitor erosion and sediment transport in Hakioawa and Kaulana watersheds, which are Hakioawa and Kaulana watersheds, which are currently undergoing restorationcurrently undergoing restoration
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ApproachApproach
Periodic measurements of soil erosion at Periodic measurements of soil erosion at selected locations (both in restoration and selected locations (both in restoration and non-restoration areas)non-restoration areas)
Monitor streamflow and suspended-sediment Monitor streamflow and suspended-sediment discharge at mouths of streams discharge at mouths of streams
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Helicopter View of Bare AreaHelicopter View of Bare Area
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BadlandsBadlands
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Badlands Erosion in Soft MaterialBadlands Erosion in Soft Material
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Pedestal RocksPedestal Rocks
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Upper Horizon is Removed in Many Upper Horizon is Removed in Many AreasAreas
Remnant Remnant hummock of hummock of upper soil upper soil horizonhorizon
Soil stripped, leaving Soil stripped, leaving underlying hardpanunderlying hardpan
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Erosion Monitoring MethodErosion Monitoring Method
Install transects (“pins”)Install transects (“pins”)
Measure ~ every 6 Measure ~ every 6 months for 3 yearsmonths for 3 years
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HakioawaHakioawaKaulanaKaulana
HakioawaHakioawa
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Restoration Restoration startedstarted
No restoration No restoration
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RillRill InterfluveInterfluve
HummockHummock
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PRELIMINARY RESULTSPRELIMINARY RESULTS
FIELD-MEASUREMENT FIELD-MEASUREMENT DATESDATES
1.1. January 2007January 2007
2.2. September 2007September 2007
3.3. March 2008March 2008
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Rainfall Between MeasurementsRainfall Between Measurements
Dec. 2007
NWS Gage 499.6 Kaho‘olawe
Data from National Climatic Data CenterData from National Climatic Data Centerand National Weather Serviceand National Weather Service
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Change Between January 2007 and Change Between January 2007 and March 2008March 2008
(negative = erosion)(negative = erosion)
Rill 7 -2 0Interfluve 0 -1 -1Hummock NA -6 -6All 2 -2 -1
Average Change (mm/yr)Restoration
startedNo
RestorationAllFeature
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Statistical Significance?Statistical Significance?
Erosion rate of restoration sites was Erosion rate of restoration sites was statistically less than that of non-statistically less than that of non-restoration sites (one-tailed test, 95% restoration sites (one-tailed test, 95% confidence interval)confidence interval)
Erosion rate of rills was not Erosion rate of rills was not significantly different from that of significantly different from that of interfluves (two-tailed test, 95% interfluves (two-tailed test, 95% confidence interval)confidence interval)
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Even So, Rills Show Extremes of Even So, Rills Show Extremes of Erosion/Deposition Erosion/Deposition
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H15H15
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H35H35
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H37H37
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Where Are the Biggest Changes? Where Are the Biggest Changes?
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H25H25
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H14H14 H19H19
H17H17
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H41H41
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Some Variation in Rills and Some Variation in Rills and Interfluves not Accounted for in Interfluves not Accounted for in
Sampling DesignSampling Design
Are the transects representative of Are the transects representative of the entire watershed?the entire watershed?
Assume that we have enough Assume that we have enough transects to eliminate bias in transects to eliminate bias in computed statisticscomputed statistics
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Restoration Effect Likely to Mature over Restoration Effect Likely to Mature over TimeTime
Plants are small at Plants are small at presentpresent
Erosion rate likely to Erosion rate likely to change in the future change in the future as plants growas plants grow
Erosion data Erosion data collected today can collected today can be used basis for be used basis for comparison in futurecomparison in future
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No Conclusions, Just ObservationsNo Conclusions, Just Observations
Rills show extreme variability in deposition Rills show extreme variability in deposition and erosionand erosion
Erosion rate less extreme in interfluves but Erosion rate less extreme in interfluves but mean is not statistically different from rillsmean is not statistically different from rills
Erosion rate high in hummocksErosion rate high in hummocks
Erosion rate statistically lower in restoration Erosion rate statistically lower in restoration than in non-restoration areas (but there is than in non-restoration areas (but there is question of whether transect data are question of whether transect data are representative of the whole watershed)representative of the whole watershed)
Erosion rate is likely to change as plants Erosion rate is likely to change as plants grow; data collected today can be basis for grow; data collected today can be basis for comparison with a future studycomparison with a future study
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Stream Gage and Automatic Stream Gage and Automatic Sediment SamplerSediment Sampler
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SummarySummary
All preliminary – after only one year of All preliminary – after only one year of erosion monitoring erosion monitoring
Rills show extremes for both deposition Rills show extremes for both deposition and erosion, but the mean change was and erosion, but the mean change was not statistically different from interfluvesnot statistically different from interfluves
Erosion also high in hummocksErosion also high in hummocks
At this stage, erosion rate in restoration At this stage, erosion rate in restoration areas is statistically lower than in non-areas is statistically lower than in non-restored areasrestored areas
More monitoring to come!More monitoring to come!