estimate_Rs_from_airTemDiff {simET}

Estimating solar radiation data derived from air temperature differences

Description

The difference between the maximum and minimum air temperature is related to the degree of cloud cover in a location. Clear-sky conditions result in high temperatures during the day (Tmax) because the atmosphere is transparent to the incoming solar radiation and in low temperatures during the night (Tmin) because less outgoing longwave radiation is absorbed by the atmosphere. On the other hand, in overcast conditions, Tmax is relatively smaller because a significant part of the incoming solar radiation never reaches the earth's surface and is absorbed and reflected by the clouds. Similarly, Tmin will be relatively higher as the cloud cover acts as a blanket and decreases the net outgoing longwave radiation. Therefore, the difference between the maximum and minimum air temperature (Tmax - Tmin) can be used as an indicator of the fraction of extraterrestrial radiation that reaches the earth's surface. This principle has been utilized by Hargreaves and Samani to develop estimates of ETo using only air temperature data.

Usage

estimate_Rs_from_airTemDiff(Ra, Tmax, Tmin, locations)

Arguments

Ra

extraterrestrial radiation [MJ m-2 d-1].

Tmax

maximum air temperature.

Tmin

minimum air temperature.

locations

The adjustment coefficient kRs is empirical and differs for interior' or 'coastal' regions.

Value

A vector for solar radiation

Note

The temperature difference method is recommended for locations where it is not appropriate to import radiation data from a regional station, either because homogeneous climate conditions do not occur, or because data for the region are lacking. For island conditions, the methodology of Equation 50 is not appropriate due to moderating effects of the surrounding water body. Caution is required when daily computations of ETo are needed. The advice given for Equation 49 fully applies. It is recommended that daily estimates of ETo that are based on estimated Rs be summed or averaged over a several-day period, such as a week, decade or month to reduce prediction error.

References

Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. FAO Irrigation and drainage paper No. 56. Rome: Food and Agriculture Organization of the United Nations, 1998.