Add Antecedent Precipitation Index (API) to rainfall data to track rainfall loading

Tempest Owners Feature Requests
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The Antecedent Precipitation Index (API) calculates cumulative rainfall over time applying a decay factor to account for evaporation and drainage factors influenced by solar energy and temperature. In other words, the API goes with precipitation and down based on how much has likely evaporated or infiltrated, giving you a better picture of the current soil moisture load. Although this measure may not be directly comparable across different soil types, it can be very useful for comparing daily conditions at the same location over time.
This data should be a discrete value available via API as well as a line on the rainfall chart.

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API is an overall and generalized average condition measurement, the soil type wouldn’t impact the calculation but it would impact how you interpret the measurement.

Typical API Values and Their Meaning
• Low API (0–0.5 in or 0–12 mm) → Very dry soil, increased risk of drought stress, high water absorption potential.
• Moderate API (0.5–1.5 in or 12–38 mm) → Moist soil, normal conditions, moderate runoff potential.
• High API (1.5–3 in or 38–76 mm) → Saturated soil, risk of standing water, increased runoff.
• Extreme API (>3 in or >76 mm) → Flood conditions likely, soil is unable to absorb more water, high flood/landslide risk.

Specific locations and soil conditions will impact how you interpretat the API

Example API Irrigation Thresholds by Soil Type (inches of water equivalent):
Sandy Soil:
• Irrigation Needed: API < 0.3 in (8 mm)
• Monitor Conditions: API 0.3–0.7 in (8–18 mm)
• No Irrigation Needed: API > 0.7 in (18 mm)
Loamy Soil:
• Irrigation Needed: API < 0.5 in (13 mm)
• Monitor Conditions: API 0.5–1.0 in (13–25 mm)
• No Irrigation Needed: API > 1.0 in (25 mm)
Clay Soil:
• Irrigation Needed: API < 0.8 in (20 mm)
• Monitor Conditions: API 0.8–1.5 in (20–38 mm)
• No Irrigation Needed: API > 1.5 in (38 mm)

Sandy soils drain quickly and need frequent irrigation at low API values.
Loamy soils hold water better but still require irrigation when API drops below 0.5 inches.
Clay soils retain moisture for longer, so they need watering less often but in larger amounts when API falls below 0.8 inches.

For a home lawn or garden, tracking API alongside actual soil moisture measurements (using a moisture meter or simple touch test) can help fine-tune irrigation needs.

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