SOUTH KOHALA — There is a common claim that one handful of healthy soil contains more microorganisms than there are people living on earth. But productive land and fertile soil are washing away at an alarming rate. North Hawaii is no stranger to erosion, and Pelekane Watershed is one such example.
Worldwide, 24 billion tons of fertile soil is lost each year due to erosion, according to the United Nations Convention to Combat Desertification. To put the figure into perspective, to reverse this staggering conundrum each person on the planet would need to be responsible for their own three tons of soil.
The good news is, governmental and non-governmental organizations in South Kohala are partnering with landowners and other stakeholders in an effort to learn more about erosion and mitigate its effects. Pelekane Bay Watershed is directly benefiting.
The Watershed stretches eight miles, beginning at sea level and climbing up the slopes of Kohala Mountain to 5,300 feet. The base of the Watershed, Pelekane Bay, is located just north of Spencer Beach Park and may reflect the most noticeable effects.
Since the building of Kawaihae Harbor in the 1960s, the bay’s natural flushing ability was reduced. Waters have become murkier through the decades since, flooded by sediment carried down Kohala Mountain streams. As a result, water quality and the coral reef ecosystem have become threatened.
Jonathan Stock, a research geologist and director at the United States Geologic Survey Innovation Center in Menlo Park, California, has spent the last 12 years working on watershed projects in Hawaii. Currently, he is measuring the rates of erosion in the Watershed by collecting data on such things as total rainfall, rain intensity and soil moisture to assess how much rainfall is available for plants versus runoff.
“We are primarily engaged in three activities: mapping, monitoring and modeling,” he said.
His goal is to help land managers develop methods that will reduce the amount of material that is transported from land to nearshore waters and coral reefs.
From an administrative standpoint, two USGS science centers are currently working together on the project: Geology, Minerals, Energy, &Geophysics Science Center in Menlo Park, California, and Pacific Island Ecosystems Research Center based in Volcano. The ecologists’ and geologists’ joint efforts have proven effective, providing valuable insights on landscape processes. Both were invited by Kohala Watershed Partnership and the National Oceanic and Atmospheric Administration to start the project nearly a year ago.
Permissions are now being secured to install field instruments and to access areas for measurements. In similar studies, scientists have utilized vegetation and surficial mapping, along with field experiments and sensor networks, to create a “sediment budget” for the landscape. Gathering the data is a necessary first step.
The current system in place is a series of 100 check dams installed in 2010 in the most severely damaged areas. Consisting of wire mesh covered by ground cloth and stacked rocks, the goal was to hold back sediment. The dams were added after Kohala Watershed Partnership received a $2.69 million federal stimulus grant for NOAA coastal restoration.
Stock considers the check dams temporary.
“We need to solve the problem with more sustainable solutions and think carefully how we can gracefully get the Pelekane Watershed back into health,” he said.
The crux of the research being done aims to identify the key culprits of the erosion in the Pelekane Watershed.
Gordon Tribble, director of the Pacific Island Ecosystems Research Center, stated, “The makai part of the watershed is fairly dry. That tends to result in less vegetation that can hold soils and have low rates of erosion. Some obvious things that disturb the vegetation are roads, fire and grazing by both managed and feral animals.”
Goats are known to denude all woody vegetation, so by 2011 Kohala Watershed Partnership successfully eliminated goats from 6,600 acres by installing 18 miles of goat-proof fencing.
One priority on Stock’s list is to “keep blue water blue.” This begins high on the mountain and may require a return to the “ridge-to-reef” concept rooted in the traditional ahupua’a land divisions of Hawaii.
A model to follow comes from a study done on the Kawela Watershed on Molokai. In a five-year period from 2008-13, USGS research led by Stock showed that vegetation increased from less than 1 percent cover to more than 55 percent, and erosion rates decreased tenfold. Furthermore, models were able to predict that a change in land management would reduce the total sediment load from approximately 6 tons per year, to 2 tons or less annually.
“We may not be able to solve the whole problem and stop sediment, but we definitely can reduce it,” he said.
For now, measuring the sediment load in Pelekane Watershed seems to be a wise first step.