Solar panels are not the only things that convert solar radiation into energy. Plants have been doing this for billions of years.
How can farmers make use of solar radiation data? A huge decision farmers face is which crop to plant. This depends on a number of factors, two of which are soil moisture and the plant's rate of evapotranspiration (ET). Both of these are greatly affected by solar radiation.
Soil moisture is basically the amount of water the soil contains. As a farmer, you wouldn't want to plant a crop that needs a lot of water in an arid location, where the soil does not hold the reservoir of water needed to sustain a plant through the hottest and driest season. One of the main factors determining the amount of water in the soil is the rate of evaporation from the soil, which of course is affected by the amount of solar radiation. Irrigation solves this problem up to a point - farmers still need to understand the effect of solar radiation in order to fine-tune the amount of irrigation needed, so as not to under or over water their crops.
Evapotranspiration is the same as evaporation, only it is limited to the water that evaporates through the plant. Imagine placing a plastic bag over a plant and tying it around the base so that only the plant is in the bag and none of the soil. The water that condenses on the inside of the plastic bag is a product of evapotranspiration. All plants behave differently, so understanding which plants lose more water to evapotranspiration is critical to a farmer's decision making.
Although both soil moisture evaporation and plant evapotranspiration are significantly affected by the amount of solar radiation, another important factor is humidity (the amount of water held in the air). The lower the humidity, the greater the influence of solar radiation. If the air is very dry, then the soil and plant will lose more water to the air than they would on a more humid day, all other factors remaining the same.
Most farms are located outside urban areas, therefore finding solar radiation data is not an easy task. One remarkable network of on-site instruments measuring solar radiation is CIMIS in California. However, if your site is more than 25km from an on-site instrument, then satellite data becomes more accurate. Moreover, the cost of instrument installation and the complexity of acquiring data sets are additional obstacles (instruments need to be calibrated every 2-3 years and the cost of installation is around $5000 and up). In contrast, satellite data can be acquired less expensively and faster, making it an ideal solution for the agricultural community.
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