Sources of solar radiation estimates and their effect on daily potential evaporation for use in streamflow modeling

Abstract

The accurate estimation of potential evaporation (PE), to in turn calculate evapotranspiration, is an important step in many hydrologic models. The National Weather Service (NWS) has used PE to obtain daily estimates of mean evapotranspiration in continuous rainfall-runoff models for river forecasting. The daily PE estimates are derived mainly from meteorological data gathered on a regular basis throughout the country. Solar radiation is one of the required input variables. Because of its widespread availability, sky cover is now used almost exclusively by NWS to estimate solar radiation. Over a period of time, a bias has developed between the long-term mean PE (computed using a combination of historical observed pan evaporation data and meteorological data) and PE estimated operationally using real-time meteorological data. This difference is a result of the use of sky cover based solar radiation estimates. These biased solar radiation estimates translate into long-term means of PE which are significantly lower than values using corresponding direct measurements of solar radiation or estimates of solar radiation using percent sunshine. A standard for PE has been established and verified to which long-term means can be compared. PE estimates derived from sky cover can be corrected to the standard using a ratio of long-term means. Many meteorological variables which have been measured or observed manually in the past are being converted to automatic observations. With the advent of automated sensors, which do not duplicate the manual sky cover observations, another source of solar radiation is necessary to model PE for use in river forecasting. Satellite estimates of solar radiation are compared with other means of measuring and estimating solar radiation. Available on a nationwide basis, satellite estimates produce values of solar radiation comparable to those obtained by direct measurement. Based on availability and accuracy, satellite estimates of solar radiation are recommended for use in the estimation of PE instead of estimates derived from sky cover or hours of sunshine (percent sunshine). This assumes that no direct observations of solar radiation are available.