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In addition, those states are getting a hand from scientists at the USGS Earth Resources Observation and Science (EROS) Center.

The bill signed April 16, 2019, enables the Interior Secretary to implement a basin-wide agreement that had been negotiated for more than five years among Arizona, California, Colorado, Nevada, New Mexico, Utah, and Wyoming. The plan is to cut back water usage between 2020 and 2026 to protect levels of the Colorado River’s two largest reservoirs—Lake Mead and Lake Powell. The agreement, called the Colorado River Drought Contingency Plan, was implemented on May 20, 2019.

Climate variability and increasing use by a growing population has created a supply and demand imbalance and pushed system reservoirs to critically low levels. Lake Powell is at 45 percent full today; Lake Mead at 40 percent. Prolonged drought is an ongoing factor. But annual Consumptive Uses and Losses Reports (CULR) done by BOR since 1971 also highlight the increasing influences that crops and livestock, municipal needs, industrial uses, and mining have had on water use, too.

In December 2012, a report called the Colorado River Basin Water Supply and Demand Study that projected future water supply and demand scenarios on the Colorado River through 2060 identified potential inconsistencies within the lower basin datasets used in the CULR findings and committed BOR to resolve and correct any methodological inconsistencies with those datasets.

That review process fell to the Water Accounting and Verification Group in Boulder City, NV, which produces the lower basin portion of the CULR for BOR. The group was asked to look at the entire period for the reports, to identify previous errors, and to improve the dataset that’s been used to understand historical water use, identify trends in that use, and forecast potential future use.

It turns out EROS has been able to assist in that effort.

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Because Jeremy Dodds and his colleagues in the Water Accounting and Verification Group work with remotely sensed data, “we had been monitoring many of these new technologies using thermal-based and satellite data to directly calculate water use,” Dodds said. “As we were looking into these, we became aware” of the work of Gabriel Senay and his team at EROS.

Senay and his colleagues use remotely sensed data scaled to Landsat 30-meter resolution and energy-balance principles to model the rates of evaporation and transpiration—called evapotranspiration, or ET—on fields. As water and nutrients are transported from the soil to the plant’s leaves, ET regulates leaf temperature through evaporative cooling, which balances incoming energy from sunlight and outgoing energy from evaporation. A fully transpiring vegetated area expending energy can appear up to 40 degrees Fahrenheit cooler than bare areas with little evaporation.

When used in combination with local weather datasets for irrigated lands, Senay’s model can provide increasingly accurate and repeatable estimates of water consumption—the amount of combined water that either evaporates from the soil and vegetation surfaces or is transpired by plants. Considering that 60 to 70 percent of water use in the Colorado River’s lower basin tributaries is for irrigation, Dodds and his colleagues see important information to be gained through Senay’s work.

Along the mainstem of the Colorado River, the BOR relies upon a robust network of stream gages as the primary source for monitoring water uses.  But the Lower Basin tributaries to the Colorado are a different story. With far fewer gages, they require an alternate method of calculating consumptive water uses.

“I think the datasets we’ve been receiving (from Senay) are going to be a great improvement to the types of estimates that we’ve been able to do in the past,” Dodds said. “I have great confidence in Gabriel’s ability to quickly and accurately produce quality data.”

Prior agricultural estimates offered only a basin-wide look at consumptive water uses. Senay’s team can take a field-by-field view of consumption. It can sift through the length of the Landsat archive and bring consistent assessments to the use of water for more than the past 40 years. In fact, his team has already provided datasets in five-year increments from 1985 to 2015, and is finishing up work on additional datasets from 1975 and 1980.

“I think this kind of analysis ... is a very powerful tool for understanding the amount, really, of how much water is being used by different irrigation districts,” Senay said.

Dodds said he has shown representatives from each state comparisons of the newer remote-sensing technology to older methods with which they likely were more familiar. It wasn’t unusual for Senay’s methods to show up to 10 percent less water use in some areas compared to the BOR’s internal estimates, “which made sense because Senay’s method is estimating actual ET, while our estimates calculate potential ET and has a tendency to overestimate the amount of water actually consumed by a crop,” Dodds said.

Those representatives liked what they saw, Dodds added. And he likes the potential of Senay’s model for monitoring agricultural water use as the Lower Basin states incorporate several water-saving measures that were enacted by the Drought Contingency Plan with the intent to boost Lake Mead elevations.

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Among those measures:

• The Metropolitan Water District of Southern California and the Imperial Irrigation District, for example, have a history of paying farmers to fallow fields or implement on-farm water conservation activities;
• California is also reducing its reliance on Lake Mead through the use of desalinated water;
• New water conservation activities have also been enacted through the Southern Nevada Water Authority and several Arizona water users, where many cities are paying for turf removal and its replacement with rock, mulch, and natural vegetation to reduce water use.

As negotiations start up in the next year among all Upper and Lower Basin states to develop operational guidelines for 2026 and beyond, Dodds said he has no doubt that Senay’s data will be used in modeling projections that will assist water managers as they look at the Colorado River’s use long term.

“Gabriel has done a great job with us so far, and he’s been a pleasure to work with,” Dodds said. “I don’t have any doubt but that his work is going to help us a lot.”