Solar Radiation Calculator

The Solar Radiation Calculator applies the Hargreaves-Samani empirical model to estimate daily solar radiation from temperature range and extraterrestrial radiation. This approach is widely adopted in FAO Irrigation and Drainage Paper 56 for regions lacking pyranometer data. Solar Radiation drives photosynthesis, evapotranspiration, and crop growth modeling.

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Scientific Foundation

The Hargreaves-Samani equation (Rs = kRs × Ra × √(Tmax - Tmin)) correlates diurnal temperature range with atmospheric transmissivity. Ra is computed using Julian day, latitude, solar declination, sunset hour angle, and inverse Earth-Sun distance — all standard astronomical formulas from FAO-56. Typical kRs values: 0.16–0.19 inland, slightly higher coastal due to humidity effects.

Importance of Solar Radiation Estimation

Solar radiation is the primary energy input for crop growth, driving photosynthesis and determining potential evapotranspiration (ET0) in the Penman-Monteith or Hargreaves ET models. Accurate Rs estimates support irrigation scheduling, crop yield forecasting, solar PV feasibility in rural areas, and climate change impact studies on agriculture. In data-scarce regions like parts of Bangladesh, this tool fills critical gaps without expensive equipment.

User Guidelines

• Use accurate latitude (positive north, negative south).
• Day of Year: Jan 1 = 1, use calendar converter if needed.
• Enter reliable daily Tmax/Tmin (from weather stations or averages).
• kRs: start with 0.16; calibrate locally if possible (0.17–0.19 common).
• Results are estimates — validate against measured data when available.

When and Why to Use This Tool

Use when measured solar radiation is missing but temperature data exists — common in developing regions or historical datasets. Ideal for estimating ET0 in FAO Penman-Monteith limited-data mode, designing irrigation systems, assessing solar energy potential for farms, or modeling crop water requirements in variable climates.

Purpose of the Solar Radiation Calculator

This tool provides fast, science-based estimates of solar radiation to support sustainable agriculture, water management, and energy planning. It democratizes access to FAO-validated methods, helping farmers, researchers, and agronomists make informed decisions without specialized instruments.