Coquille Subbasin Working Atlas - p8
Water Use and Availability
Page 6: Vegetation and Land Cover
Page 8: Water Use and Availability
Page 9: Fish Distribution & Habitat
Irrigation water rights
The number of acres allocated a primary irrigation right within a quarter-quarter section are shown in Allocation of Irrigation Rights map. This information is based on the Oregon Department of Water Resources (ODWR) water rights database and is current as of 1994. It is not a measure of actual irrigation activity, but shows the number of acres that can be irrigated under the existing allocation of irrigation rights. In some cases the listed number of acres that can be irrigated exceeds the total acres available. Generally, however, this data provides a reasonable indication of the number of acres that can legally be irrigated.
[click on map for larger version]
| Table 13: Allocation of Irrigation Rights | |||
| Subdivision | Total Acres | Irrigated | % of Total |
| Coquille | 109,044 | 5,794 | 5.3% |
| East Fork Coquille | 85,281 | 1,432 | 1.7% |
| Middle Fork Coquille | 193,112 | 1,160 | 0.6% |
| North Fork Coquille | 99,329 | 2,666 | 2.7% |
| South Fork Coquille | 174,080 | 2,624 | 1.5% |
| Total | 660,846 | 13,677 | 2.1% |
| Source: ODWR 1994 water rights database | |||
Most of the irrigation rights are clustered around the Coquille River between Broadbent and Riverton. There are a significant number of permitted irrigation sites just northwest and southwest of the city of Coquille, where a series of irrigation ditches are located. Table 13 shows the percentage of each subdivision allocated irrigation rights.
Water quantity
Estimated water availability for four months in an average year is shown in the Seasonal Water Availability map. This information is based on the Oregon Water Resources Department Water Availability Database (Cooper, 1993) which indicates water availability on a monthly basis by subtracting out-of-stream consumptive uses and in-stream water rights from natural streamflow. When the result of this calculation yields a positive number, water is available.
For each basin in the state, water availability is determined for a select number of watersheds, or water availability units. There are 76 water availability units in the Coquille subbasin. Water availability units are nested, and each water availability unit is included in downstream units. For water to be available in any given unit, then, it must also be available in all units downstream. If water is over-appropriated in a downstream water availability unit, no further appropriation can occur.
In the water availability calculation natural streamflow is represented by the amount of water flowing in the stream 80 percent of the time in any given month. This number is either obtained from streamflow records from gage stations located near the point of calculation or are estimated from statistical regressions relating streamflow to subbasin characteristics. Out-of-stream consumptive uses are calculated from water rights allocations. The difference between the natural streamflow and the consumptive uses is the net streamflow; the water remaining for in-stream flow or later appropriation.
The database for the Coquille summarizes the amount of the natural streamflow, consumptive uses, net streamflow, in-stream water rights, and the recommended flow for fish for each of the subdivisions. Comparison of net streamflow to recommended flow for fish indicates whether or not there is a sufficient water remaining in the stream to support fish. Many water availability units are over-appropriated or do not have sufficient flow quantities to meet in-stream needs. As indicated by the map display, monthly water availability decreases from February to August, and the entire basin is appropriated or over-appropriated from August through October. Water availability then increases from October to February. This view of year round water availability may be deceiving. If one unit is completely appropriated, all units upstream are also designated as having no available water, which may not always be the case.
