Variable streamflow response to forest disturbance in the western United States — USFS

Disturbances that do not remove the entire canopy, such as this beetle-caused tree mortality, tend to result in decreased rather than increased water yield.

Click the link to read the article on the USFS website (Sara A. Goeking and David Tarboton):

Forest disturbance is expected to lead to increased streamflow – but in very dry watersheds, the opposite is often true.

Forest disturbance is typically expected to lead to increased runoff – and therefore more water available for aquatic ecosystems and people – because loss of forest vegetation results in less water being taken up and transpired by plants. However, recent studies in the western U.S. have found no change or even decreased streamflow following forest disturbance due to drought and insect epidemics.

We investigated streamflow response to forest cover change using hydrologic, climatic, and forest data for 159 watersheds in the western U.S. during 2000–2019. Forest change and disturbance were quantified in terms of net tree growth (total growth volume minus mortality volume) and mean annual mortality rates, respectively, from the U.S. Forest Service’s Forest Inventory and Analysis database. Annual streamflow was analyzed using multiple methods to understand the contributions of precipitation, temperature, aridity, and tree mortality.

Many watersheds exhibited decreased annual streamflow even as forest cover decreased. This decreased streamflow was not attributable to precipitation and temperature changes in many disturbed watersheds, yet streamflow change was not consistently related to disturbance, suggesting drivers other than disturbance, precipitation, and temperature.

Multiple regression analysis indicated that although change in streamflow is significantly related to tree mortality, the direction of this effect depends on aridity. Specifically, forest disturbances in wet, energy-limited watersheds (i.e., where annual precipitation exceeds potential evapotranspiration [PET]) tended to increase streamflow, while post-disturbance streamflow more frequently decreased in dry, water-limited watersheds (where the PET to precipitation ratio exceeds 2.35).

Effect of tree mortality and aridity on change in annual streamflow (ΔQ) for 2000–2009 vs. 2010–2019, based on 159 watersheds. PET=potential evapotranspiration; P=precipitation.

Key Findings

  • While streamflow often increased following forest disturbance, it decreased in some watersheds.
  • The direction of streamflow response to forest disturbance (increase vs. decrease) is dependent on aridity.
  • Tree mortality during 2000-2019 was highest in arid watersheds – the same watersheds where disturbance tends to result in decreased streamflow.
  • Forest disturbances in wet, energy-limited watersheds tended to increase streamflow, while post-disturbance streamflow more frequently decreased in dry, water-limited watersheds.

Other Resources

Disturbance effects on water yield in western coniferous forests

Using Forest Inventory & Analysis data for broad-scale assessments of vegetation effects on water resources

Featured Publications

Variable streamflow response to forest disturbance in the western US: A large-sample hydrology approach

Goeking, Sara A. ; Tarboton, David G. , 2022

Leery of open forum, #water group struggles to inform public: #Wyoming’s #ColoradoRiver Working Group serves as a conduit between water users and state engineer’s office — WyoFile #GreenRiver #COriver #aridification

Members of the public packed the Sublette County Public Library in Pinedale Sept. 27, 2022 for a water meeting organized by the Wyoming State Engineer’s Office. (Dustin Bleizeffer/WyoFile)

Members of a working group created by Gov. Mark Gordon to “disseminate information” and “act as a sounding board for the public and stakeholders” regarding Colorado River Compact issues reported Monday mounting public frustration about access to information. 

The Colorado River Working Group, formed in 2021, essentially acts as a consulting body and communications conduit between water users in the Green River and Little Snake River basins and the State Engineer’s Office. 

At a meeting of the group on Monday members said constituents are confused. Members also reported fielding complaints from stakeholders who can’t get the information they need to stay abreast of the fast-moving and complex topic that stands to impact water users in the state.

At the same meeting, State Engineer Brandon Gebhart insisted the body isn’t subject to the state’s open meetings laws and said he’s hesitant to take questions from the public during working group meetings. Though Monday’s meeting was open to the public — as were six previous meetings — none have been live-streamed or otherwise made available to anyone not in attendance, according to the engineer’s office.

That’s by design, according to Gebhart. 

“I’m a little concerned that if we start one of these [live-streamed presentations] that we wouldn’t get through any of the topics before the questions start coming in,” Gebhart told working group members. In a follow-up with WyoFile Tuesday, Gebhart added, “My general concern about doing public webinars is being unable to get through the numerous and complex topics we need to cover if we get slowed down by multiple public questions.”

Chris Brown of the Wyoming Attorney General’s Office discusses the implications of the Colorado River Compact with water users in Pinedale Sept. 27, 2022. (Dustin Bleizeffer/WyoFile)

The working group’s meetings are intended to hash out information and discuss how to disseminate it with water users, Gebhart said. The group’s outreach is primarily done directly between the group’s members and their constituents.

Though there was no formal call for public comments or questions at the Monday meeting, members of the working group, SEO and the attorney general’s office did field some questions from members of the public in attendance.

Under pressure

The main topic of discussion Monday was how the SEO is scrambling to entice eligible water users to take part in a conservation program that pays them to voluntarily leave water in streams that flow to the Colorado River. 

Explaining the program and eligibility requirements to myriad water users is complicated, particularly as many in the ag community are leery of government-sponsored programs aimed at reducing water use, according to the SEO. A tight timeframe makes the effort more challenging. The Upper Colorado River Commissionannounced a call for System Conservation Pilot Program proposals Dec. 14 with a filing deadline of Feb. 1.

The SEO, which is overseeing the program in Wyoming, is eager to enroll as many participants as possible, according to the agency. The state and its upper basin partners need to demonstrate progress in cultivating various voluntary water conservation efforts to build a case against the potential for mandated cuts under the Colorado River Compact or federal intervention. The agency is relying on members of the working group to help field questions and explain the potential benefits of the program. But so far, confusion reigns, members indicated.

Rep. Albert Sommers irrigates his ranch near Pinedale from where he trails cattle to Union Pass, seen on the horizon (Angus M. Thuermer Jr./WyoFile)

“Conservation districts — they really don’t know enough about what’s going on and they can’t ask enough questions,” Rep. Albert Sommers (R-Pinedale), a member of the working group, told fellow members. “There just needs to be more formal outreach in the country.”

Industrial water users in southwest Wyoming — trona mines, natural gas processors and electrical power utilities — “are yearning for information,” working group member Aaron Reichel of Genesis-Alkali said.

Sen. Larry Hicks (R-Baggs), also a member of the working group, said “there’s a lot of concerns with this System Conservation Pilot Project.” Concerns include “the timeframe to get [information], who to contact, who’s going to answer these questions to put together an application, what’s eligible — all those questions. I’m just getting inundated with this stuff because of the timeframe of this.”

Working group structure

Gordon, anticipating the need to protect the interests of Wyoming water users from the impacts of the Colorado River crisis, formed the Colorado River Working Group in 2021 and appointed eight members. The group includes two representatives for municipal water users, one for agriculture, one for environmental interests, two for industrial water users and two legislators — Sen. Hicks and Rep. Sommers.

Gordon “tasked members with helping to more broadly disseminate information about key Colorado/Green/Little Snake River Basin issues to interested stakeholders, and for members to provide insights as Wyoming navigates important river issues,” Gebhart told WyoFile via email, adding that the SEO relies on the working group to enhance its own public outreach efforts.

In forming the group, Gordon agreed to the SEO’s suggestion that it not be subject to the state’s open meeting laws, according to Gebhart, though the group has decided to mostly adhere to open meetings standards so far. 

Gordon’s office didn’t directly answer what justifies the working group’s exemption from the state’s open meetings laws. As a gubernatorial appointed group convened by a state agency to address issues with a critical public resource the body would appear at a glance to be obligated to operate transparently — but such quasi-governmental groups can and do exist, according to Bruce Moats, a Wyoming attorney who specializes in First Amendment and Wyoming media law.

“The group appears to exist in a kind of a gray area,” Moats said. “The question is, why is it necessary to have the option to close meetings [to the public] when you have exemptions under the public meetings law that allow for that. Just why?” 

At the urging of group members Monday, Gebhart agreed to consider hosting a webinar that provides members of the public the chance to ask questions about Colorado River issues and the SEO’s efforts to enroll water users in the SCPP.

“We are not trying to limit information getting to the public,” Gebhart told WyoFile. “Ultimately, our goal is to get more, and accurate, information to those potentially affected by the current situation.”

Wyoming rivers map via

Aridification/Saturation Watch — @Land_Desk #snowpack (January 17, 2023)

West snowpack basin-filled map January 17, 2023 via the NRCS.

We just did the snowpack data a couple weeks ago, so I’m not going to do it again. But holy relentless train of atmospheric rivers is there ever a lot of moisture falling from the sky. Look at these rainfall totals! Still, it’s interesting that of those nine precipitation totals, only three set new 16-day records. Yes, this is some wacky weather, but it’s not unprecedented, yet.

Judging from snowpack levels across the West, it would appear that each successive range of mountains wrings a little less moisture out of the clouds than the previous one. So California is getting buried, Utah is having one powder day after another, and Colorado is looking pretty darned good.

Pretty good in northern Colorado, too, but surprisingly not way ahead of last year at this time. This is for Rabbit Ears Pass:

And all of this new snow on top of that stuff that fell back in October and November and then rotted = avalanche danger. So be careful out there.