GIS/SATELLITE REMOTE SENSING: The Law of the River
Mapping of Lower Colorado River Region Ensures Equitable Water Distribution
By Anthony Montesano

It's a simple matter of water. In the Colorado River valley between the east end of Lake Mead and the international border with Mexico, the river is the primary source of water for agricultural, domestic, municipal, industrial, hydroelectric-power generation and recreational use. Most of the lower Colorado River's water, about 96 percent of the annual supply, flows into the lower basin at Lees Ferry, the official dividing line of upper and lower regions of the Colorado River. The dividing line was determined by the U.S. Department of Interior which assigned two separate entities of the U.S. Bureau of Reclamation (USBR) to administer to each region. In the U.S., accounting for the use and distribution of water from the Colorado River is required by law. Water in the river has been divided among seven states and Mexico in accordance with documents and laws collectively known as "The Law of the River."
      The Law of the River was developed over a 75-year period and encompasses the Colorado River Compact of 1922, the Boulder Canyon Project Act of 1928 and Arizona v. California, the U.S. Supreme Court Decree of 1964, placing the responsibility upon the Secretary of the Interior to provide complete, detailed and accurate records of waters used from the Colorado River. The Law of the River requires the U.S. to operate the lower Colorado River with an eye to three priorities: 1) regulation, improvement of navigation and flood control, 2) irrigation and domestic usage and 3) water power. Water cannot be released from the river unless there is a valid beneficial use for the water and even then, the water is released at a time and in such a way as to meet the delivery need and maximize other benefits including power production.

Real-Life "Milagro"
In John Nichols' novel The Milagro Beanfield War, building development in the small town of Milagro threatens to displace local New Mexican farmers and further cut off their water usage. When, in an act of defiance, one of the locals diverts water to irrigate his bean field, the town supports him because of their resentment of water usage laws that favor the rich and a small war between farmers and building developers begins to erupt.
      This fictional account, which was later made into an Academy Award-winning film, could very well be playing itself out in the lower Colorado River region today if not for the Law of the River and government's strict adherence to it.
      A major issue in the lower Colorado River region is the sharing of the river between the U.S. and Mexico. In 1944, the International Boundary and Water Commission (IBWC) was formed to maintain the international boundary, partition waters and deal with border sanitation issues between the U.S. and Mexico. The Mexican Water Treaty of 1944 between the U.S. and Mexico (which also established the IBWC) divided the surface waters of the river that run across the international boundary between Mexico and the U.S. Specific minimum quantities of water are detailed in the treaty, but the treaty has no definition of the quality of water delivered to Mexico or any requirement as to when the water has to be delivered.
      Over the years, Mexicans have often declared the division of water to be unfair and complained about the poor quality of the water as well. Mexicans have pointed out that the Colorado River is managed not to deliver optimum water supply for agricultural and domestic uses downstream in Mexico, but rather to optimize hydroelectric power production. The results, say the Mexicans, are irregular and inefficient water deliveries to Mexico.

A Satellite Solution
To help monitor water usage, the U.S. government turned to Pacific Meridian Resources - a leading user of GIS and image processing for mapping forests, agricultural lands, wetlands, vegetation and land use change - to help improve the current monitoring methods. Pacific Meridian Resources' tasks were two-fold: first was to develop methods for using GIS and satellite image processing to map and monitor crops and other vegetation growing near the Colorado River from Hoover Dam to the Mexican Border; second was to train USBR personnel in the use of methods. I
      rrigation of crops accounts for a huge portion of the water used in the region. Therefore, the project methods concentrated on mapping seasonal crop types so the USBR can relate crop type to water utilized by each crop, and thereby assess seasonal water usage. By integrating information from field visits, existing maps, satellites and other remote sensing platforms, Pacific Meridian is able to create detailed classifications of the entire project area including crop type, riparian and desert vegetation, and open water.
      "We began by collecting 30-meter Landsat TM multispectral and SPOT 10-meter black-and-white images," said Kass Green, president of Pacific Meridian. The USBR used the SPOT imagery to map field boundaries, while the TM imagery was used to identify crops. "Satellite images are effective for mapping agricultural areas for two reasons. First the footprint or extent of a satellite image is large, covering 10,000 square miles. Secondly, multi-spectral satellite images reveal types of energy that humans can't see with the naked eye such as infrared and thermal."
      Mid-infrared portions of the spectrum are particularly sensitive to vegetation type. "This sensitivity facilitates distinguishing one crop from another quickly and accurately," said Green, "because different kinds of crops display different kinds of spectral responses across the electromagnetic spectrum." Fifteen different crop types (including alfalfa, cotton, wheat, corn, melons, tomatoes, crucifers and dates) are mapped four times a year in May, August, November and March.

Processing the Images
The Landsat TM data was provided by Space Imaging EOSAT and recorded using the Sony DIR-1000M Tape Drive. The DIR-1000M which combines a maximum capacity of 100GB of uncompressed data per cassette with a sustained transfer rate of 128Mb/sec using Sony's DFC-1800N variable rate buffer interface regularly provides back-up and archiving for the satellite data at Space Imaging EOSAT's ground station in Norman, Okla.
      When accessing the differences in crops grown so close together, the integrity of the satellite images were paramount to accurate analysis. The interleaved, double Reed-Solomon encoding/decoding of the Sony DIR-1000M provides powerful error correction, yielding an extremely low error rate of 1 x 10-13 assuring the integrity of the image downloaded to the earth stations by Space Imaging EOSAT and analyzed by Pacific Meridian Resources. The DIR-1000M employs rotary helical scanning, an extremely precise recording technology which Sony continues to advance for video applications and more recently for digital data recording. A read-after-write facility, by which data are simultaneously verified, makes it possible to monitor any errors in real time and provide pristine, accurate images.
      Using proprietary image processing algorithms developed in ERDAS Imagine, Pacific Meridian turned the image into a map of crop type, riparian and desert vegetation, and open water. The crop type map is created four times a year, allowing the USBR to continuously monitor water usage.
      Ground verification and accuracy assessment are also conducted each time crops are mapped. Map accuracies consistently exceed 93 percent. "When we started this project, we were unsure if we could map crops adequately," said Green. "Most of the scientific literature said that it couldn't be done. However, the spectral integrity of the imagery, new algorithms, and the strong link between crop type and spectral response resulted in very accurate maps."
      Pacific Meridian is continuing to map the crops and other land cover of the lower Colorado River area for the USBR. The methods developed for this project are almost entirely automated now. New investigations include first applying the process to new areas and second testing the impact on accuracy of including radar or hyperspectral imagery.
      By using satellite image processing techniques, the USBR is able to monitor crops less expensively and more consistently.

About the Author:
Anthony P. Montesano is a freelance writer specializing in articles about the high-tech, communications and entertainment industries. He may be reached through his company Montike Publications, Inc. P.O. Box 86-4101 Ridgewood, NY 11385.

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