Evapotranspiration and microclimate at a low-level radioactive-waste disposal site in northwestern Illinois

Abstract

From July 1982 through June 1984, a study was made of the evapotranspiration and microclimate at a low-level radioactive-waste disposal site near Sheffield, Bureau County, Illinois. Vegetation at the site consists of mixed pasture grasses, primarily awnless brome (Bromus inermis) and red clover (Trifoleum pratense). Three methods were used to estimate evapotranspiration: (1) an energy budget with the Bowen ratio, (2) an aerodynamic profile, and (3) a soil-based water budget. For the aerodynamic-profile method, sensible-heat flux was estimated by a profile equation and evapotranspiration was then calculated as the residual in the energy-balance equation. Estimates by the energy-budget and aerodynamic-profile methods were computed from hourly data and then summed by days and months. Yearly estimates (for March through November) by these methods were in close agreement: 648 and 626 millimeters, respectively. Daily estimates reach a maximum of about 6 millimeters. The water-budget method produced only monthly estimates based on weekly or biweekly soil-moisture content measurements. The yearly evapotranspiration estimated by this method (which actually included only the months of April through October) was 655 millimeters. The March-through-November average for the three methods of 657 millimeters was equivalent to 70 percent of total precipitation. Continuous measurements were made of incoming and reflected shortwave radiation, incoming and emitted longwave radiation, net radiation, soil-heat flux, soil temperature, horizontal windspeed, and wet- and dry-bulb air temperature. Windspeed and air temperature were measured at heights of 0.5 and 2.0 meters (and also at 1.0 meter after September 1983). Soilmoisture content of the soil zone was measured with a gamma-attenuation gage. Annual precipitation (938 millimeters) and average temperature (10.8 degrees Celsius) at the Sheffield site were virtually identical to long-term averages from nearby National Weather Service stations. Solar radiation averaged 65 percent of that normally expected under clear skies. Net radiation averaged 70.1 watts per square meter and was highest in July and negative during some winter months. Wind direction varied but was predominately south-southeasterly. Wind speed at the 2-meter height averaged 3.5 meters per second and was slightly higher in winter months than the rest of the year. The amount of water stored within the soil zone was greatest in early spring and least in late summer. Seasonal and diurnal trends of evapotranspiration rates mirrored those of net radiation; July was usually the month with the highest evapotranspiration rate. The ratio of sensible- to latentheat fluxes (commonly called the Bowen ratio) for the 2-year study period was 0.38, as averaged from the three methods. Monthly Bowen ratios fluctuated somewhat but averaged about 0.35 for late spring through summer. In fall, the ratio declined to zero or to slightly negative values. When the ratio was negative, the latent-heat flux was slightly greater than the net radiation because of additional energy supplied by' the cooling soil and air. Evapotranspiration calculated by the three methods averaged 75 percent of potential evapotranspiration, as estimated by the Penman equation. There was no apparent seasonal trend in the relation between actual and potential evapotranspiration rates.