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Water transfers from alfalfa to cities

Justin Huntington Published on 30 May 2014

Nevada is the driest state in the U.S., is the fastest-growing state in the nation over the last 10 years and is the most urbanized state in the nation (a majority of people live in Las Vegas and Reno).

The economic growth and arid climate of Nevada has created a desperate need to secure additional water resources for water municipalities and water authorities, not only to diversify their water portfolio but also to allow for future growth.

Like most western U.S. states, the majority of available Nevada water is used for agriculture. Roughly 85 percent of permitted water rights, both surface and groundwater, is consumed by irrigated agriculture.

Nevada’s hydrographic basins are fully appropriated near urban areas, meaning that additional water rights will likely need to be obtained through an application to change the manner of use from irrigation to municipal, as well as applications for interbasin transfers from rural areas to populated areas.

These issues are putting Nevada on the cutting edge of water law, hydrological, environmental and economic impacts analyses.

Nevada’s arid climate is optimal for producing high-quality dairy and beef hay, where alfalfa and grass hay make up roughly 95 percent of Nevada’s crop acreage.

Alfalfa is the highest water-using crop grown in Nevada, and the estimated evaporation and transpiration (ET) from alfalfa can define the upper limit for a change in the manner of use from irrigation to some other use (municipal, environmental, industrial).

These issues have made estimating water use from alfalfa a central focus for many proposed water transfers.

During the past decade, the use of Landsat satellite imagery for estimating crop acreage and water use has increased significantly.

This is due, in part, to a reduction in cost over the last decade, where imagery cost $4,000 per satellite image during the 1980s and 1990s, was reduced to about $600 per image during the 2000s and now is free of charge starting in 2009.

The free access to imagery, coupled with significant advances in processing hardware, software and analysis techniques, has catapulted the determination of ET via satellites to the forefront of ET estimation techniques, especially where water consumption from large areas, such as agriculture, has to be quantified.

The Metric ET processing software (mapping ET at high resolution using internalized calibration) got its start in 2000 at the University of Idaho (UI) with collaboration from Dutch scientists and support by NASA.

Since then, the use and development of Metric has spread through partnerships with the Desert Research Institute (DRI), the University of Nebraska and New Mexico Tech.

Metric keys off the moderately high-resolution Landsat satellite to provide information on vegetation vigor and water use for each 30m x 30m square (100 foot x 100 foot) of the earth’s surface.

Metric utilizes “energy balance” computations that require images of the surface temperature for each 30m “pixel” of the surface. The energy balance determines the amount of ET consumed by each irrigated field by noting the amount of energy consumed.

“It is a lot like keeping account of how much heat energy is needed on one’s stove top to boil dry a pot of coffee,” says Rick Allen, professor of water resources engineering at UI.

“A field of alfalfa has the same requirements: to evaporate and transpire water from the crop requires a fixed amount of energy. That ET is required for water and nutrients to flow from the soil through the plants and into the air to support plant growth.”

In collaboration with UI, Desert Research Institute and Nevada Division of Water Resources (NDWR) are actively applying Metric to estimate crop water use in Nevada.

Metric employs a relatively complicated computation process requiring 140 equations that are fortunately largely hidden from the operator and, for the most part, are automatically applied to each pixel.

Carson Valle, NV  Tomp. serface map

The surface energy balance of Metric relies on Landsat surface temperature, shown in Figure 1 for Carson Valley, Nevada, for an image acquired on August 15, 2009, where blue colors represent cool temperatures starting at 80ºF and orange colors represent hot temperatures, up to 120ºF.

Similar to a swamp cooler on a house, evaporation pulls heat from the surface; hence, agricultural fields are much cooler than non-agricultural and desert areas shown in Figure 1.

Also shown in Figure 1 is the growing season estimate of ET for agricultural fields, most of which are alfalfa, pasture grass, grass hay and some spring grain.

It is remarkable how much information we can gather from the surface temperature images of Landsat that point to how much water has been consumed by each irrigated field.

The accuracy in estimation of ET has proven to be very good, based on tests against on-the-ground ET measurements by the U.S. Geological Survey.

As Figure 1 illustrates, not all fields have the same water use, and there is a large range. This range is important when considering potential transfers of irrigation water rights to a new use that require transfer of “wet water” as opposed to “paper water.”

For example, a potential water-rights purchase and transfer of water from a field that has historically only been using a relatively small amount of water is not as valuable in terms of providing additional water supply.

The ability to document the actual consumption is very valuable. Actual historical consumptive use information on a field-by-field level derived by Landsat is arguably the only way to objectively evaluate the “wet” water value of water-rights purchases in terms of the potential to provide additional water supply.

Also, high consumption of water is generally directly tied to high yields for alfalfa, so monitoring ET can provide feedback to growers on why yields might be varying across a field or from field to field.  FG

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