Evaluation of two new models of net radiometers

Radiant energy from the sun and the earth’s atmosphere is the source of available energy at the earth’s surface. The sum of this energy, or net radiation, is largely used to evaporate water, heat the air and soil, and drive photosynthesis. Net radiation is a key component to the surface energy balance, but has traditionally been difficult and expensive to measure accurately. Two new net radiometer models have been released in the past year. We compared and evaluated these models using two replicate Kipp and Zonen model CNR 1 net radiometers as references.  We also compared several less expensive older model net radiometers.  Additionally, we predicted net radiation from solar radiation, air temperature, and absolute humidity measurements using a model. In the model, net longwave radiation is calculated with a Brunt (1932; 1952) approach for predicting net emissivity (e). The ratio of measured solar radiation to predicted clear-sky solar radiation is used as a surrogate for cloud cover. Net shortwave radiation is determined by direct measurement of solar radiation and the albedo of the surface. Our findings generally indicate that net radiometer accuracy increases with increasing cost of the radiometer. Prediction of net radiation from the model yielded adequate results for some applications, such as evapotranspiration predictions and irrigation scheduling, but the model does not work very well at night due to some simplifying assumptions. For global climate change studies where accurate net radiation  measurements are required, a higher end net radiometer is recommended. Accurate net radiation measurements depend on proper placement of the sensor, proper leveling, and routine maintenance to keep the sensing surfaces clean.