How do we estimate the required irrigation water for an agricultural area?
For each month of the year, we estimate the monthly evapotranspiration of the area and substruct the monthly rainfall.
1. Evaportranpiration
What is the meaning of the term Evapotranspiration?
Evapotranspiration (ET) of a crop is the total water lost due to evaporation from the soil and transpiration from the plants. Since evaporation and transpiration take place simultaneously and somehow correlate, it more practical to consider them as one process and estimate the Evapotranspiration (ET)
What is Evaporation? What controls the rate of evaporation?
Evaporation is the change of water from the liquid state to the vapor state. Evaporation rate depends on:
1. The abundance of water (e.g. evaporation of a lake is larger than the evaporation of wet soil)
2. The available thermal energy (e.g. evaporation is larger during the daytime than during the night)
3. The ease of vapor transfer to the atmosphere (e.g. evaporation is larger for stronger winds and at lower atmospheric pressure)
What is Transpiration?
Transpiration is the process by which the water absorbed by plants is released to the atmosphere in the form of water vapor. The water vapor is released from the stomas (tiny openings on the leaves). Around 97–99.5% of the water that plants absorb from the soil is released to the atmosphere through transpiration.
What controls the rate of transpiration?
Plants control the rate of transpiration by increasing or decreasing the openings of their stomas. When there is abundant water in the soil, transpiration reaches its maximum possible, which is called potential transpiration. In dry conditions, the stomas close, and transpiration is reduced is order to save water and allow the plant to survice with less water
Question: How do plants adapt to extend their survival in dry conditions?
2. Calculate the Potential Evapotranspiration (PET)
Potential Evapotranspiration (PET) is the maximum evapotranspiration that can take place at a field when there is an unlimited water supply (saturated soil). In order to calculate the PET for a particular corp at certain location under certain environmental conditions, we first calculate the Reference Potential Evapotranspiration (PETo) and multiply it by the crop coefficient (kc)
Reference Potential Evapotranspiration (PETo) is the maximum evapotranspiration that can take place at a field covered by grass (i.e. the reference crop) when there is an unlimited water supply (saturated soil). In other words, it is the maximum evapotranspiration for a grass-covered field at at certain location under certain environmental conditions.
Blaney-Criddle method is an empirical equation for estimating the monthly PETo. The input parameters are the average temperature (Ta) and the percentage of sunshine hours (p).
PETo = 0.254 ∗ p (32+1.8 Ta) [mm/month]
PET = kc∙ PETo [mm/month]
p = το ποσοστό ωρών ηλιοφάνειας του συγκεκριμένου μήνα σε σχέση με το σύνολο των ωρών ηλιοφάνειας του έτους. Εξαρτάται από το γεωγραφικό πλάτος, και τον μήνα του έτους. Δίνεται σε μορφή πίνακα.
Τα = μέση μηνιαία θερμοκρασία [°C]
kc = ο φυτικός συντελεστής (αδιάστατος, εξαρτάται από την καλλιέργεια)
The Penman-Monteith (1965) method is the most widely used method for the calculation of PETo. Above we shocased the use of the Blaney-Criddle method because it is computational very easy to apply compare to the Penman-Monteith method