The Lower Klamath and Tule Lake National Wildlife Refuges, located in the upper Klamath Basin of Oregon and California, encompass approximately 46,700 and 39,100 acres, respectively. Demand for water in the semiarid upper Klamath Basin has increased in recent years, resulting in the need to better quantify water availability and use in the refuges. This report presents an evaluation of water-use estimates for both refuges derived on the basis of two approaches. One approach used evaporation and evapotranspiration estimates and the other used measured inflow and outflow data. The quality of the inflow and outflow data also was assessed. Annual water use in the refuges, using evapotranspiration estimates, was computed with the use of different rates for each of four land-use categories. Annual water-use rates for grain fields, seasonal wetlands, permanently flooded wetlands with emergent vegetation, and open-water bodies were 2.5, 2.9, 2.63, and 4.07 feet per year, respectively. Total water use was estimated as the sum of the products of each rate and the number of acres in its associated land-use category. Mean annual (2003-2005) water use for the Lower Klamath and Tule Lake refuges was approximately 124,000 and 95,900 acre-feet, respectively. To estimate water deliveries needed for each refuge, first, annual precipitation for 2003-2005 was subtracted from the annual water use for those years. Then, an adjusted total was obtained by adding 20 percent to the difference to account for salinity flushing. Resulting estimated mean annual adjusted needed water deliveries in 2003-2005 for the Lower Klamath and Tule Lake refuges were 107,000 and 82,800 acre-feet, respectively. Mean annual net inflow to the refuges for 2003-2005 was computed by subtracting estimated and measured surface-water outflows from inflows. Mean annual net inflow during the 3-year period for the Lower Klamath refuge, calculated for a subsection of the refuge, was approximately 73,700 acre-feet. The adjusted needed water delivery for this section of the refuge, calculated from evapotranspiration estimates, was approximately 77,600 acre-feet. For the Tule Lake refuge, mean annual net inflow during the 3-year period was approximately 76,100 acre-feet, which is comparable to the estimated annual needed water delivery for the refuge of 82,800 acre-feet. For 1962-2005, mean annual net inflow to the Lower Klamath refuge was approximately 49,800 acre-feet, about 23,900 acre-feet less than for 2003-2005. Although mean April-September net inflows for 1962-2005 and 2003-2005 have remained fairly constant, annual net inflow has increased for October-March, which accounts for the difference. Consistently higher autumn and winter flow deliveries since the mid-1980s reflect a significant change in refuge management. More sections of the refuge are currently managed as seasonal wetlands than were in the 1960s and 1970s. Flow records for the Ady Canal at State Line Road, Klamath Straits Drain at State Line Road, and D Pumping Plant were evaluated for their data quality. On the basis of USGS flow-record criteria, all three flow records were rated as 'poor.' By definition, 95 percent of the daily flows in a record having this rating could be in error by more than 15 percent.