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. . DRAFT ~) . ) APPLICATION OF REMOTE SENSING AND GIS TO ESTIMATE IRRIGATION WATER USE IN THE UPPER GUNNISON RIVER BASIN IN COLORADO ~ (~ en ~ INTRODUCTION Pursuant to Public Law 90-537, the U.S. Bureau of Reclamation (BOR) prepares reports of annua~ consumptive uses and losses in the Colorado River System. The report for the 1976-1980 period indicates that irrigated agriculture typically accounts for twd-thirds of beneficial consumptive uses and losses (Bureau of Reclamation, 1982). Reliable estimates of irrigated acreage are needed to estimate this water use. The present project is an effort of the BOR in cooperation with the USGS to develop ~dvanced methods for estimating water use. Knowledge of the annual amount of consumptive water use and uniform techniques of estimation are needed to equitably partition Colorado River Basin water among Colorado River Compact states. Remote sensing an~ GIS techniques were used previously by BOR to support water use estimation in the Upper Green River Basin (Verdin. 1985; Verdin. et. al.. 1985). These techniques have been expanded upon to estimate irrigated acreages in the Upper Gunnison River basin. The Upper Gunnison River basin, located in southwest Colorado (see Figure 1) is 1,059,390 hectares; (2,617,770 acres) in area. The basin is divided into three U.S. Geological Survei hydrologic units: 14020001, 14020002, and 14020003. The basin contains parts of! five counties: Delta. Montrose. Hinsdale, Saguache, and Gunnison. Elevations in thei basins range from 5,000 to 13.000 feet above sea level, with irrigated agricult~re found typically at 5.000 to 9,000 feet above sea level. The growing season is short, typically only 100 to 140 days, depending on elevation, with the possibility of; frost at any time. Crop variety is limited; pasture grass. native hay and alfalfa ~ccount for nearly all acreage. although some small grains are present. In many ways, the Upper Gunnison basin is typical of many of the headwater subbasins of the Upper Colorado. METHODOLOGY Photo interpretation Field reconnaissance was conducted by photointerpreters in the summer of 1987, to familiarize themseives with conditions in the basin. Irrigation practice in the Upper Gunnison basin was found to be primarily by flooding in well established irrigation fields. Systems to collect runoff from hillsides and retain the water in detention ponds for later irrigation were noted in several areas. These collection systems included check dams across drainages and scoring of hillsides with small ditches to direct water flow. Some ditches directed water through sagebrush areas that were incidentally irrigated as the water passed to pasture fields. The predominant crop types observed we~e grass/hay (e.g. smooth brome. timothy, fescues, and blue grass) and alfalfa. Small grains noted were barley and oats. Through field reconnaisance the extent of irrigated lands was determined, and photointerpreters were to identify and develop rules for differentiating them from spectrally similiar riparian vegetation. Color infrared (CIR) 1:24,OOO-scale NHAP (National High Altitude Photography)