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been used by the USBR to subdivide the county data into hydrologic unit data in the Gunnison River
<br />Basin.
<br />Table 1 in Section 6.0, Supporting Materials, shows the comparison between the total county irrigated
<br />acreage listed by the CAS and the on e used by the USBR in computing the Colorado River System
<br />Consumptive Uses and Losses Report, 1985-1990 data file for the Gunnison River basin for the years
<br />1985 through 1990. Table 2 shows the irrigated acreage from 1985 to 1990 for the counties and their
<br />subdivision into hydrologic units in the Gunnison River basin as given by the USBR.
<br />Table 1 shows that the irrigated acreage used by the USBR is much higher than the irrigated acreage
<br />reported in the CAS. The CAS seem to be consistently lower than the values used by the USBR. For
<br />illustration, the total irrigated acreage for the Delta County, which falls entirely inside the Gunnison
<br />River basin for the years 1985 to 1990, as listed by CAS are 44,500, 55,670, 54,570, 57,170, 60,070, and
<br />56,570 acres; however, the acres used by the USBR for the same area and time period are 86,160,
<br />77,310, 70,700, 74,610, 81,250, and 74,950.
<br />In fact, the USBR identified the small values in the CAS data and improved the estimated irrigated
<br />acreage by adding pasture crops acreage given by the U.S. Department of Agriculture (USDA). The data
<br />the USBR added to the CAS are shown in the shaded cells in Table 1. Table 1 also shows the differences
<br />between the total irrigated acreage recorded by the CAS and the USBR for the same period.
<br />GIS and Irrigated Acreage
<br />To increase the accuracy of the estimate of irrigated acreage in Colorado?s portion of the Colorado River
<br />basin, aerial photos of the irrigated areas where digitized. The resulting maps were sent to water division
<br />offices for field verification. This process was repeated twice, and the final irrigated maps are being
<br />used to calculate the irrigated acreage in the Gunnison River basin. The tables and figures in the Section
<br />6.0, Supporting Materials, show that there are significant differences between the irrigated acreage and
<br />crop type given by the GIS, the USBR, and the CAS data.
<br />Table 3 gives the total irrigated acreage in the Gunnison River basin using GIS 1993 data. Figures 1, 2,
<br />and 3 show the irrigated acreage and crop type used by each source in Delta county only. Figure 4 shows
<br />a comparison between GIS and CAS data . The GIS 1993 gives higher total crops acreage than CAS
<br />1993 for both Delta and Gunnison Counties, where the total county irrigated acreage fall entirely inside
<br />the Gunnison River basin. A comparison between crop type used by the three different data sources,
<br />USBR, CAS and GIS 1993, is shown in Table 4.
<br />Since there are no GIS data available for historical years 1985 through 1990 and there is also no clear
<br />approach from USBR data to break down the CAS county statistics, a meeting between the CRDSS CU
<br />Team and USBR personnel was held to discuss different approaches to recalculate the irrigated acreage
<br />and crop type in each hydrologic unit for the years 1985 through 1990. The results of this meeting were
<br />to use two approaches to estimate the hydrologic unit irrigated acreage from CAS 1985 through 1990
<br />data using the GIS 1993 data.
<br />Table 5 shows some results of using the first approach in Delta County only. In this approach, the crop
<br />name and irrigated acreage distribution from GIS 1993 were preserved and applied to CAS 1985 data.
<br />The result shown in Table 5 indicates that GIS 1993 gives significantly higher irrigated acreage in Delta
<br />County for 1993 (81,071 acres) compared with CAS 1993 (46,200 acres). Also, the estimated value from
<br />the GIS for 1985 was improved to 78,088 acres compared to 44,500 acres listed by CAS 1985 acreage.
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