Thinning Increases Runoff

Thinning Increases Runoff

27 June 2014

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USA — Thinning thick, overgrown forests in northern Arizona could increase runoff for downstream cities by up to 12 percent in most years, according to a groundbreaking, years-long study by researchers from Northern Arizona University’s Ecological Restoration Institute.

The thinning initially had the same effect as increasing rainfall by about two inches annually. However, six years after the cut, the benefit declined to the equivalent of an extra half an inch.

The rough estimates emerged from a decades-long study of the impact of forest thinning projects on the Beaver Creek Experimental Watershed.

Beaver Creek tumbles off the Mogollon Rim to merge with the Verde River.

The Tonto National Forest watershed produces about 350,000 acre-feet of runoff annually. A 12 percent increase would produce an extra 42,000 acre-feet of water annually, which would end up in Verde and Salt River reservoirs, for eventual use in the Valley.

The study represents one of the few careful estimates of the extra water projects like the Four Forest Restoration Initiative (4FRI) could produce.

The project would contract with a timber company to thin about a million acres over the course of 10 to 20 years, reducing tree densities from 800 to 1,000 per acre to something like 50 to 100 per acre.

The 4FRI project represents the best chance to protect forested communities from wildfires, but it has fallen years behind schedule and never attracted the support of lawmakers from urban areas. However, the Beaver Creek study suggests that a massive thinning project may offer one of the most cost-effective ways to provide water for a region facing severe water shortage as temperatures rise and reservoirs dwindle.

The Forest Service awarded the 4FRI contract to one timber company three years ago but switched to a different company last year. The original plan called for thinning 30,000 to 50,000 annually, but after three years the contractor has thinned only a few thousand acres.

The researchers said the actual yield of extra water on a larger scale would depend on several factors. For instance, in an especially dry year even in a thinned forest the existing trees would use most of the extra water to survive. But even though thinning might not increase runoff in dry years, it would probably reduce tree death by limiting competition for the available water.

In addition, the water savings from the thinning disappears within 5-10 years as brush and saplings grow back. That means any thinning effort would have to either cycle through the same terrain every 5-10 years or forest managers would have to restore a natural fire regime to keep brush cleared. Currently, the 4-FRI plan calls for a roughly 20-year cycle on the thinning projects.

The ponderosa pine forest remains vital to the state’s water balance, noted Frances O’Donnel in a summary of the outcome of the study. The ponderosa forests cover just 20 percent of the Salt River watershed, but produce half of the water that ends up in the Salt River.

The estimates came from measuring stream flow in the experimental plots. The researchers had detailed measurements compiled in the period from 1950 to 1980. This gave them a way to calculate the baseline relationship between rainfall and stream flow. Then they thinned various portions of the experimental forest and measured the effect on the water flowing in the streams.

The researchers concluded, “Though more study is needed, our best hypothesis is that reducing understory regrowth following thinning will maximize runoff. Therefore, land managers wishing to maximize the hydrologic benefits of forest restoration should consider including maintenance treatments in restoration plans.”

The study suggests that thinning 50,000 or 100,000 acres annually could produce significant benefits, both in terms of tree survival and extra runoff. The only way to produce anything close to the maximum water yield on the Tonto Forest of 42,000 acre-feet would be to thin perhaps 200,000 acres annually or to return the frequent, low-intensity ground fires that kept brush and trees thinned before a century of grazing, logging and fire suppression unhinged the system.

However, the onset of the worst drought in centuries has made increasing runoff far more critical for the Valley, which has almost two-thirds of the state’s population and all of its political clout.

The Valley faces a water crisis, thanks to rising average temperatures and declining winter snow packs.

The U.S. Bureau of Reclamation recently projected an unprecedented reduction in water deliveries from Lake Mead as the giant Colorado River reservoir falls to levels not seen since it started filling up in 1938. Managers of the Central Arizona Project warned Phoenix and Tucson to slash water use or they will likely face reductions as early as 2019. If current trends continue, they would lose 29 percent of the 1.5 million acre-feet they receive by 2029.

Arizona cities get about a quarter of their water from the Colorado River.

The Salt and Verde River reservoirs hold about 2 million acre-feet, although they’ve also fallen to about half full. In a wet year, the reservoirs can store enough to get the Valley through three or four years with little rain.

The seven western states with rights to the Colorado River already use more water than flows in a normal year. Only wet years that fill the string of massive reservoirs keep the system operating.

However, the pronounced warming trend linked to the build up of heat-trapping pollutants in the atmosphere has both reduced average rainfall and decreased runoff by increasing evaporation.

The surface of Lake Mead has dropped to 1,085 feet above sea level — about half its capacity. If water levels fall another 85 feet, the intakes that provide water to Las Vegas, Phoenix and Tucson won’t work any more — drastically reducing deliveries.

Arizona is now in the worst drought since the start of record-keeping 110 years ago. Estimates based on tree ring data suggest this drought may be the worst seen in 500 to 1,000 years. The tree ring data suggests the drought could continue for another 20 or 30 years, with only occasional normal rainfall years.

So far this year, Payson has received about two inches of rain — about 20 percent of normal. All of Gila County is in either severe or extreme drought, according to the National Weather Service.

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