USGS

Water Resources of Colorado

Classification of Irrigated Land Using Satellite Imagery, the High Plains Aquifer, Nominal Date 1992

By Sharon L. Qi, Alexandria Konduris, David W. Litke, and Jean Dupree

Available from the U.S. Geological Survey, Information Services, Box 25286, Federal Center, Denver, CO 80225, USGS Water-Resources Investigations Report 02-4236, 35 p., 19 figs.

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Abstract

Satellite imagery from the Landsat Thematic Mapper (nominal date 1992) was used to classify and map the location of irrigated land across the High Plains aquifer. The High Plains aquifer underlies 174,000 square miles in parts of Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming. The U.S. Geological Survey is conducting a waterquality study of the High Plains aquifer as part of the National Water-Quality Assessment Program. To help interpret data and select sites for the study, it is helpful to know the location of irrigated land within the study area. To date, the only information available for the entire area is 20 years old. To update the data on irrigated land, 40 summer and 40 spring images (nominal date 1992) were acquired from the National Land Cover Data set and processed using a band-ratio method (Landsat Thematic Mapper band 4 divided by band 3) to enhance the vegetation signatures. The study area was divided into nine subregions with similar environmental characteristics, and a band-ratio threshold was selected from imagery in each subregion that differentiated the cutoff between irrigated and nonirrigated land. The classified images for each subregion were mosaicked to produce an irrigated land map for the study area. The total amount of irrigated land classified from the 1992 imagery was 13.1 million acres, or about 12 percent of the total land in the High Plains. This estimate is approximately 1.5 percent greater than the amount of irrigated land reported in the 1992 Census of Agriculture (12.8 millions acres). This information was also compared to a similar data set based on 1980 imagery. The 1980 data classified 13.7 million acres as irrigated. Although the change in the amount of irrigated land between the two times was not substantial, the location of the irrigated land did shift from areas where there were large ground-water-level declines to other areas where ground-water levels were static or rising.


Table of Contents

Foreword

Abstract

Introduction

Purpose and Scope

Acknowledgments

Description of Study Area

Previous Efforts to Classify Irrigated Land

Local and State Efforts

Regional Efforts

National Efforts

Classification of Irrigated Land for 1992

Description of National Land-Cover Data

Data Preprocessing

Applying the Agricultural Mask

Calculating the Band Ratio

Collection of Signatures

Selection of Threshold Value

Ground-Reference Information

Refinement of Irrigated Land Estimates

Description of Irrigated Land

Location of Irrigated Land

Comparison of Irrigated Land Estimates from Satellite Imagery and Agricultural Statistics

Comparison of 1992 Irrigated Land to 1980 Irrigated Land

Summary

References Cited

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