Evapotranspiration and the soil water balance
Daily water use by a planting and the store of available water in the root zone.
Quantities
| Symbol | Quantity | Unit |
|---|---|---|
ET₀ | reference evapotranspiration | mm/day |
K_c | crop coefficient | dimensionless |
ET_c | crop evapotranspiration | mm/day |
A | bed area | m² |
θ_FC, θ_WP | water content at field capacity and at wilting point | m³/m³ |
Z_r | rooting depth | m |
TAW | total available water | mm |
p | fraction of TAW extractable without reducing transpiration | dimensionless |
D | depletion relative to field capacity | mm |
Formulas
Crop evapotranspiration
ET_c = ET₀ · K_c
Daily use
V = ET_c · A
A depth of 1 mm over 1 m² corresponds to a volume of 1 L.
Total available water
TAW = 1000 · (θ_FC − θ_WP) · Z_r
Readily available water
RAW = p · TAW
Water balance
D_i = D_(i−1) + ET_c,i − P_i − I_i, 0 ≤ D_i ≤ TAW
P is precipitation and I irrigation, both in mm. Irrigation in litres is converted to a depth by dividing by the bed area.
Crop coefficient
K_c is given by three values (initial, mid-season, late season) together with the lengths of four development stages; linear interpolation is applied during the development and late stages. Values are taken from FAO-56 tables 11 and 12.
Soil hydrological constants
θ_FC and θ_WP follow from soil texture through FAO-56 table 19, which covers nine of the twelve USDA texture classes. The intervals given in the table are replaced by their midpoints. The fraction p is tabulated by crop in table 22 within 0.2…0.7; a value of 0.5 is used when interpreting a moisture sensor reading, because the reading refers to the bed rather than to a single planting.
Applicability and limitations
- Under cover, precipitation is excluded from the balance while ET₀ remains an open-ground value; use is therefore underestimated.
- Mulching and inter-row cultivation are not represented.
- The calculation yields daily use and current depletion; an irrigation schedule is not derived from it.
- For three USDA texture classes table 19 gives no hydrological constants; the balance is not computed for them.