Government expert sees no increase in San Pedro riparian area vegetation

This was published yesterday in the Sierra Vista Herald Review. According to a sidebar in the article, today (Thursday May 16) Leslie Arianna Brand, an avian ecologist familiar with the SPRNCA, will discuss the critical importance of the riparian area to a multitude of avian species, both migratory and year-round residents, and the need to preserve and enhance the habitats the cottonwood and willow stands provide. The sidebar also states that she will also rebut the claims of defense witness Steve Carothers, SWAC aquatic and terrestrial biologist, who testified the government does not need as much water as it is claiming. Tomorrow is expected to be the last day of the trial. Special Master Judge Mark Brain is expected to reach a decision by next year. Shar Porier, who worked for the Herald Review when I was at the BLM from 2014

to 2016, has provided exceptional coverage of the trial. Given the recent release of the San Pedro Riparian National Conservation Area Proposed Resource Management Plan and Environmental Impact Statement and the lawsuit against the Villages at Vignetto in Benson, it will be very interesting to see the Special Master's decisions and what the implications are for all of those projects. Water will continue to be the most important issue in the future of Arizona whether its citizens realize it, or not.

PHOENIX — Since January, the federal government has been at odds with local governments and industry and recently reinvigorated the case to gain water rights for the continued preservation of the San Pedro Riparian National Conservation Area (SPRNCA).

With the court case in its final days, the U.S. Department of Justice, on behalf of the Department of the Interior and Bureau of Land Management, tried to cast doubt to the defense witnesses conclusions that vegetation has increased along the SPRNCA.

Justin Huntington, research professor at the Desert Research Institute, a member of the NASA/USGS Landsat team and principal of Huntington Hydrologic, took the stand Tuesday as the next to the last witness for the federal government in the trial.

He was questioned by DOJ attorney Dave Gehlert about the vegetation in the SPRNCA based on normalized difference vegetation index (NDVI). It is a simple indicator used to analyze remote sensing measurements from a satellite and assess whether an area being observed contains live green vegetation or not.

Huntington said there has not been a significant increase in SPRNCA vegetation.

His research showed there was some evidence of mesquite encroachment into grasslands of the terraces. However, he speculated losing the grass as a water user could balance out the water use of the mesquite.

“I saw no increase in riparian vegetation in the riparian,” he said. “I evaluated change in NDVI within the entire riparian corridor of the SPRNCA, including the floodplain of the entrenched active channel, and on the pre-entrenchment terraces — the whole riparian corridor with the floodplain excluded.”

The evaluation addressed NDVI for 1988, the year the SPRNCA was established, through 2017, the most recent year for which NDVI data was available, he said. “NDVI increased slightly over the whole riparian corridor and decreased slightly on the terraces, but neither change is statistically significant,” he said.

NDVI measures the chlorophyll in vegetation which reflects more near-infrared and green light compared to other wavelengths, Huntington said. It can assess whether the target being observed contains live green vegetation or not. Healthy vegetation absorbs most of the visible light that hits it, and reflects a large portion of the near-infrared light. Unhealthy or sparse vegetation reflects more visible light and less near-infrared light, so it makes it relatively easy to “see” how vegetation is responding to the climate, to temperatures.

He chose one cloud-free satellite image a year in the dry months of May or June to assess the vegetation index.

“I used an approach that paired multiple data sources, including remotely sensed satellite and aerial imagery, Geographic Information System (GIS) data, and ground-based field observations,” he said.

“This approach was chosen due to the large area that the riparian corridor occupies in the SPRNCA, along with the long time period required to adequately assess vegetation change, ideally about 30 years of continuous data.”

While there was some increase in vegetation in the southern reaches, the northern reaches appeared to have lost some, he added. It indicates vegetation growing in areas away from the active floodplain and on the pre-entrenchment terrace is not increasing as much in the south and is declining more in the north. Groundwater pumping may have an effect on the northern reaches.

“It is plausible that NDVI declines are being expressed in the north more so than in the south, since the predominant effects of the regional cone of depression will be manifested in the northern reaches of the SPRNCA due to groundwater gradients in the regional aquifer,” he said.

Huntington rebutted the testimony of Chris Garrett, an eco-hydrologist with SWCA Environmental Consulting, who said there was an extensive increase in vegetation within the SPRNCA.

It was a matter of the size of the pixel used in the imaging process, explained Huntington. Garrett used large pixels which made it hard to define detail. He used smaller pixels, which allowed him to “see” gaps in the canopy and the plant life popping up in them.

He also viewed the entire SPRNCA, while Garrett kept to a narrow band near the river “which effectively ignores the terraces and most of the mesquite trees that are the largest water users in the SPRNCA. The evaluation of measured riparian width does not capture plant density, cannot show the plants are using more water. It illustrates how much of the riparian community was excluded from the quantified NDVI analysis.”

Further, it was not possible to determine water use of the vegetation through NDVI, which Garrett said it could, Huntington said. If there is more vegetation, Garrett surmised it would use more water. Huntington said he was wrong.

“NDVI is not a good measure for water use,” he said. “None of the Garrett report’s findings allow for a conclusion that either riparian vegetation has increased within the SPRNCA, or that riparian vegetation uses more water since the SPRNCA was established.”

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