Available Water
Click chart to add irrigation events
How much water your soil is gaining or losing, from local rainfall and evapotranspiration.
This calculator models the daily water balance in your soil by combining two data streams from Open-Meteo: historical and forecast precipitation (how much rain fell or is expected) and reference evapotranspiration (ET0), which estimates how much water leaves the soil through plant transpiration and surface evaporation each day.
The difference between precipitation and ET tells you whether your soil gained or lost water on any given day. Over time, these daily gains and losses accumulate in the soil moisture bucket — a simplified model of the available water in your root zone.
Precipitation shows daily rainfall in inches or millimeters. Evapotranspiration shows the daily ET demand — how much water the atmosphere wants to pull from the soil. Net is the difference: positive means the soil gained water, negative means it lost more than it received. The available water chart tracks the cumulative effect, showing how full your soil's water reservoir is over time. Click the available water chart to add irrigation events and see how they affect the forecast.
The 7-day forecast uses the ECMWF ensemble (50 model runs with slightly different initial conditions). Instead of a single prediction, you see the median outcome with p10–p90 range lines showing the spread of plausible outcomes. This is especially important for precipitation — ET forecasts are tight, but rain can vary enormously between ensemble members. The available water chart runs all 50 members through the full soil model independently, so the confidence band reflects how weather uncertainty propagates through the bucket.
The model uses a 120-day spin-up period to establish realistic starting conditions. It tracks water between field capacity (FC, the most water soil can hold after drainage) and wilting point (WP, where plants can no longer extract water). The region between these is your Total Available Water (TAW).
As soil dries, the model applies the FAO-56 stress coefficient (Ks): ET runs at full rate until readily available water is depleted (50% of TAW), then ramps linearly down to zero at the wilting point. When available water drops below 50% of TAW, most crops and landscapes benefit from irrigation.
ET × (multiplier) adjusts the ET rate for your situation. The default of 1.0 assumes a well-watered grass lawn. Drought-tolerant plants may use 0.5–0.7, vegetables 0.8–1.2, and potted plants in full sun may need 1.3 or higher due to increased exposure from wind and radiant heat.
Soil type determines how much water the soil can hold per inch of depth. Sand holds the least (~0.06 in/in), clay loam holds the most (~0.19 in/in). Root depth controls how much of that soil column plants can access. Shallow-rooted turfgrass (6–8 in) dries out quickly, while deep-rooted shrubs (18–24 in) have a much larger reservoir.
Historical precipitation and ET come from ERA5 reanalysis (~25 km grid). Because these are gridded products rather than local rain gauges, localized thunderstorms may be underrepresented. Forecasts use the ECMWF IFS ensemble (50 members). ET0 is calculated using the FAO Penman-Monteith equation. Geocoding is provided by Nominatim (OpenStreetMap data, ODbL license).