DAILY ENERGY-BUDGET AND PENMAN EVAPORATION RATES AT UPPER KLAMATH-AGENCY LAKE, KLAMATH COUNTY, OREGON

Formulation of numerical hydrologic models, in an attempt to better understand the ground-water flow system of Upper Klamath Basin, requires an appropriate set of boundary conditions. One condition that is poorly known is the rate of evaporation from Upper Klamath-Agency Lake. Daily energy-budget and Penman evaporation rates were determined from 5-June-2003 to 30-September-2003 to quantify evaporation rates and to assess the assumptions associated with the Penman method for use in an area characterized as having semi-arid, high-steppe climate patterns and a ground water-dominated hydrologic system. Upper Klamath and Agency Lakes are large shallow lakes that lie east of the Cascade Mountains. At 1262.8 m stage (NGVD 29), the total area of the lakes is 283 km², with a total volume of 0.8 km³ and a mean depth of 2.7 m. Modes of energy required for energy-budget evaporation rates include net radiation, water-advected energy, conduction from lakebed sediments, and changes in energy stored in the water body. Daily flux of net radiation ranged from 8 W m^-2 to 241 W m^-2; net flux of water-advected energy ranged from -16 W m^-2 to 3 W m^-2; energy contributed from lakebed sediments equate to values ranging from -5 W m^-2 to 8 W m^-2; and changes in energy stored in the water body varied from -89 W m^-2 to 111 W m^-2. Bowen ratios used in energy-budget evaporation rates ranged from -0.06 to 0.41 resulting in daily energy-budget evaporation rates ranging between 0 mm and 10 mm. Respective monthly totals for June through September, not accounting the first four days of June, are 4.107 x 10⁷ m³, 4.916 x 10⁷ m³, 3.051 x 10⁷ m³, and 1.963 x 10⁷ m³. Penman evaporation rates rely on data representative of net radiation, wind speed, air temperature, and relative humidity. Daily values for net radiation ranged from -13 W m^-2 to 266 W m^-2; wind speeds between 0.8 m s^-1 and 3.7 m s^-1; air temperatures between 7.11 °C to 26.24 °C; and relative humidity between 0.37 to 0.89. Daily Penman evaporation rates ranged from 0 mm to 9 mm resulting in respective monthly totals for June through September of 5.767 x 10⁷ m³, 6.326 x 10⁷ m³, 4.398 x 10⁷ m³, and 2.829 x 10⁷ m³. Root-mean-square error and mean absolute errors of the two methods are 0.2 mm day^-1 and 2.0 mm day^-1, respectively. Results indicate that the Penman method overestimates monthly evaporation totals between 29% and 44%.