An evaluation of the net radiation sub-model in the ASCE standardized reference evapotranspiration equation: Implications for evapotranspiration prediction

Net radiation (Rn) is a key component of the surface energy balance, but it is expensive and difficult to measure accurately. For these reasons, Rn is often predicted in evapotranspiration (ET) calculations with a model requiring measurements of incoming shortwave radiation, air temperature, and vapor pressure. We compared Rn predictions from the Rn sub-model used in the American Society of Civil Engineers (ASCE) standardized reference ET equation to mean Rn measurements from five 4-component reference net radiometers. The radiometers were part of a recent comparison study of multiple net radiometer models conducted over irrigated and clipped turfgrass in northern Utah (Blonquist et al., 2009). In the Rn model, net shortwave radiation is determined by direct measurement of solar radiation and an assumed value of albedo for the surface (0.23 for fully vegetated surfaces), and net longwave radiation is calculated with a Brunt (1932, 1952) approach for predicting net surface emissivity, calculated from near surface vapor pressure. Additionally, the ratio of measured incoming shortwave radiation to predicted clear-sky shortwave radiation is used as a surrogate variable for cloud cover in the net longwave radiation calculation. Relative to the reference Rn measurements (average of five 4-component net radiometers), modeled Rn was high during the day by an average of 8.6% and high in magnitude (more negative) at night by an average of 13.4% over hourly time intervals. Daily total Rn calculated by summing the hourly model predictions was always higher than the reference measurements, by an average of 8.1%, whereas daily total Rn calculated from the model over daily time intervals was closer to the reference measurements, 2% high on average. The model Rn error during the day was partly caused by the assumption in the model that surface albedo is a constant value of 0.23. Measurements showed albedo ranged from approximately 0.21 at solar noon to 0.30 near the beginning and end of the day, with a mean value of 0.23. However, most of the model Rn error was due to the prediction of net longwave radiation, where the empirical equation in the model typically yielded values that were too low in magnitude (less negative), by approximately 20% on average, but the error was dependent on time of day. The Rn error at night was largely caused by the inability to measure the surrogate for cloud cover at night, which relies on measurement of solar radiation from a previous time period of sufficient solar zenith angle. All five of the net radiometer models tested in the comparison study matched the mean of the reference net radiometers better than the ASCE model. When modeled hourly Rn was used to calculate ET over hourly time intervals, or when hourly ET values were summed to yield daily ET, ET was typically high, by 6% on average, relative to ET calculated from measured reference Rn. When modeled Rn was calculated over daily time intervals and used to calculate ET over daily time intervals, ET was more accurate, 1% high on average, relative to ET calculated over daily time intervals from measured reference Rn. While new models for Rn are being developed, the sub-model in the ASCE standardized reference ET equation has been in use for the past two decades in thousands of ET stations. As newer models are developed we hope to use this data set to evaluate them.