![]() Their generality leads to complexity and this translates to longer execution times when you have a global spatial domain and millions of data cells (pixels). Both libraries, pyephem and astral, are targeted at more general problems than calculating solar transits on Earth, however. The Python developer community ostensibly has at least two choices of open-source libraries for calculating sunrise and sunset times (I did not look at Skyfield or AstroPy). I recently ran into this issue when developing a new photosynthesis model in Python. Further, if our model has a large spatial domain-say, continental to global scales-we inevitably need to calculate sunrise and sunset at different latitudes, across different longitudes, and possibly taking into account very different elevations. But the approach is attractive because the calculation of sunrise and sunset times is fairly elaborate. However, this empirical threshold might vary between sites and seasons it's ultimately arbitrary. This is the approach used in photosynthesis models like MODIS MOD17. Consequently, we would want to integrate hourly VPD data (such as from a numerical weather model) only for those hours when the sun is up (the photoperiod) and we would therefore need to know the timing of sunrise and sunset.Īn alternative approach to such day-length calculations, particularly if we're already integrating data from a weather model, would be to use solar irradiance-an empirical threshold on down-welling short-wave radiation, for instance, would be an acceptable proxy for the sun is in the sky. ![]() ![]() ![]() As VPD is a key driver of transpiration and photosynthesis, if we want to correctly estimate the impact of VPD on these processes, we may need to estimate VPD as it is experienced by plants in the heat of the day. Vapor pressure deficit (VPD), as one important example, has strong diurnal variation, tending to decrease in the evening as the ambient temperature declines. In Earth system models that run on daily or weekly time steps, there are many quantities that are we may wish to calculate only when the sun is in the sky. ![]()
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