Transpiration & Soil Evaporation, Split

The single crop coefficient lumps everything into one Kc, so ETc = Kc × ETo. The dual approach (FAO-56 Chapter 7) splits Kc into a basal crop coefficient Kcb for plant transpiration plus a soil-evaporation coefficient Ke, giving ETc = (Kcb + Ke) × ETo. The split lets you see water lost to bare-soil evaporation separately from water the plant actually transpires — which is exactly what this tool exposes.

Pick the crop, enter your reference evapotranspiration (ETo) and irrigation interval, then drag the day scrubber to the stage you are in. The tool reads Kcb from the FAO-56 dual table, computes Ke for your soil-wetting pattern, and returns the daily ETc in mm. For maize at mid-season with ETo of 6 mm/day, Kcb is about 1.15, so transpiration alone is roughly 6.9 mm/day.

Kcb is the ratio of crop transpiration to reference ET when the soil surface is dry but the plant has no water stress. It rises from about 0.15 at emergence to a mid-season peak (around 1.0–1.2 for most field crops) and falls again at senescence. It captures the plant's own water use, with no soil evaporation included.

Ke describes water evaporating from the exposed, wet soil surface. It is largest right after rain or irrigation when the topsoil is wet and the canopy is open, and it falls toward zero as the surface dries or the canopy closes. FAO-56 caps it: Ke = Kr × (Kc_max − Kcb), limited by the fraction of soil that is both exposed and wetted (few).

Early in the season the canopy is small, so most of the wet soil is exposed to the sun, and frequent light irrigations keep that surface wet. Both push Ke up. The tool shows the seasonal evaporation share; for widely-spaced or slow-establishing crops on frequently-wetted soil it can exceed 25–30% of total ETc — water that never reaches the plant.

Three levers dominate: wet less of the surface (drip wets roughly 30% versus 100% for sprinkler or flood), cover the soil (plastic or organic mulch between rows), and stretch the interval during the open-canopy initial stage so the topsoil dries between wettings. Each reduces few or Kr in the FAO-56 equation, lowering Ke directly.

Use the FAO Penman-Monteith reference evapotranspiration for grass at your location and season — typically 2–4 mm/day in cool or humid weather and 6–9 mm/day in hot, dry, windy conditions. Many weather services and the FAO CLIMWAT/CROPWAT datasets publish ETo; the tool multiplies it by (Kcb + Ke) to get your crop's ETc.

Yes — it changes Ke, not Kcb. A shorter interval keeps the topsoil wet more of the time, so cumulative soil evaporation rises; a longer interval lets the surface dry, cutting evaporation. Transpiration stays the same. That is why the seasonal evaporation share climbs as you shorten the interval in the tool.

They are the dual (basal) crop coefficients tabulated in FAO Irrigation and Drainage Paper 56, Chapter 7, Table 17, with stage lengths from Table 11. They are sub-humid-climate baselines (RH_min around 45%, wind around 2 m/s); FAO-56 gives adjustments for drier, windier conditions, but the table values are the standard starting point.

For irrigation scheduling and water accounting, yes — it is more physically accurate because it tracks the wetting and drying of the soil surface day by day. The single Kc is simpler and fine for seasonal planning, but it hides the evaporation losses that the dual method makes visible, and those losses are where most on-farm water savings are found.

Yes. Drip wets only a small fraction of the surface, so the exposed-and-wetted fraction (few) is small and Ke stays low even with frequent irrigation; sprinkler or flood wet the whole surface, raising Ke. Comparing the seasonal evaporation share between wetting patterns is one of the clearest reasons to choose drip.

No — it computes the potential ETc with no soil-water stress, which is the standard FAO-56 reference condition for sizing irrigation. If the crop is under-watered, actual ET falls below ETc; pair this with a yield-response (Ky) tool to see what a deficit costs in yield.