Solar radiation is the principal energy source for physical, biological and chemical processes, such as, snow melt, plant photosynthesis, evapotranspiration, crop growth and is also a variable needed for biophysical models to evaluate risk of forest fires, hydrological simulation models and mathematical models of natural processes [1]. An accurate knowledge of the solar radiation data at a particular geographical location is of vital importance [2]. The separation of solar irradiance into diffuse and beam components is necessary for a wide range of solar engineering tasks [3] and is now a key feature of several canopy scale models of photosynthesis [4]. Now a large project named “Soil, Hydrology, Atmosphere, Ecology and Human Activity” has been enabled in the Heihe river basin, which is one of the largest inland river basin of Northwest China, with an area of 13×104 km2. Many research items of the project, such as glacier hydrology, permafrost hydrology, snow hydrology and ecohydrology, need incident solar radiation as an input variable. In another word, hourly solar radiation fluxes, including global irradiance, direct irradiance and diffuse irradiance, with a spatial resolution of 1 km×1 km, is required in Heihe river basin in 2002.

Since solar radiation data is scarce in or near the Heihe river basin, simple radiation models based on climatic variables [5], [6], [7], [8], [9] M.I. Hijazin, The diffuse fraction of hourly solar radiation for Amman/Jordan, Renew Energy 13 (1998) (2), pp. 249–253. Abstract | View Record in Scopus | Cited By in Scopus (7)[9], [10], [11] and [12] cannot be used. Sounding meteorological data is also absent, therefore, it is not possible to use classical radiation transfer models. To circumvent the problem, a simple parameterized radiation transfer model is proposed by using NCEP/NCAR data under actual weather and terrain conditions.

This paper describes an hourly incident solar radiation model, which could be used to estimate long-term global radiation, direct radiation and diffuse radiation with high spatial and temporal resolution under its de facto weather and terrain in large regions. The model is based on parameterized radiation transfer theory, and has referred to some data from the NCEP/NCAR (National Centers Environmental Prediction/National Center for Atmospheric Research) and some information about topography. The model was successfully used to calculate hourly instantaneous solar irradiance by a spatial resolution of 1 km×1 km in Alberts projection, in the Heihe river basin with a drainage area of 130,000 km2 in 2002. Now that only observed global radiation data at three automatic stations is available in Heihe river basin, global radiation at the three stations is used to validate the model. The three automatic stations are deployed in the mountain (Xishui), in the oasis (Linze), and in the desert area (Erjinaqi), respectively. The measured hourly instantaneous global radiation data do not comply with the calculated series at Xishui, with a determination coefficient R2=0.71. While at Linze and Erjinaqi stations, the determination coefficients are 0.90 and 0.91, respectively. The main reason why large errors are observed at Xishui station is that total cloud percent data from the NCEP/NCAR do not have a high spatial and temporal resolution. Also the spatial resolution of the observed data is not consistent with the calculated values. According to the model numerical test, topography is an important factor affecting model results on uneven land surfaces. Besides, in arid desert regions with even land surfaces, the 6 hourly model results agree with NCEP/NCAR global radiation data and measured data well.