online_blaney_criddle: Potential evapotranspiration by the Blaney-Criddle method. Formulas. p = f(m,L,H) f = p(t + ) · a,b = f(i,RH,w) [ET]o = a + bf. The estimation of potential evapotranspiration is achieved by adopting empirical approaches, such as the Thornthwaite equation, the Blaney-Criddle formula. Notes: Evapotranspiration was calculated using a modified Blaney-Criddle method. Average effective rainfall is the amount that is useful to crops in an average.

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The crop water need ET crop is defined as the depth or amount of water needed to meet the water loss through evapotranspiration.

In other words, it is the amount of water needed by the various crops to grow optimally. The crop water need always refers to a crop grown under optimal conditions, i. The crop thus reaches its full meethod potential under the given environment.

Blanry crop water need mainly depends on: The major climatic factors see Fig. The lowest values are found when it is cool, humid and cloudy with little or no glaney. The influence of the climate on crop water needs is given by the reference crop evapotranspiration ETo. The ETo is usually expressed in millimetres per unit of time, e. Grass has been taken as the reference crop.

Definition of the reference crop evapotranspiration ETo: ETo is the rate of evapotranspiration from a large area, covered by green grass, 8 to 15 cm tall, which grows actively, completely shades the ground and which is not short of water see Fig. The best known pans are the Class A evaporation pan circular pan Fig.

For example, each morning at 7 o’clock a measurement is taken. If the water level rises too much due to rain water is taken out of the pan see Fig. Of course the water in the pan and the grass do not react in exactly the same way to the climate. Therefore a special coefficient is used K pan to relate one to the other.

The pan coefficient, K pan, depends on: For the Sunken Colorado pan, the K pan varies between 0. The K pan is high if: The K pan is low if: If the pan factor is not known the average value could be used see box. If more accuracy is required, the pan factors given in Annex 1 should be applied.

These values, however, only refer to the Methor A evaporation pan and the Sunken Colorado pan. Some examples 1 Type of pan: Pan Evaporation Method 3.

There are a large number of theoretical methods to determine the ETo.

Many of them have been determined and tested locally. If such local formulae are available they should be used. If such local formulae are not available one of the general theoretical methods has to be used.

The most commonly used theoretical method is the modified Penman method which is described in detail in FAO Irrigation and Drainage Paper This method, however, is rather complicated and beyond ,ethod scope of this manual.

Here only the Blaney-Criddle method is given.


The Blaney-Criddle method is simple, using measured data on temperature only see also Fig. It should be noted, however, that this method is not very accurate; it provides a rough estimate or “order of magnitude” only. Especially under “extreme” climatic conditions the Blaney-Criddle method methid inaccurate: Determination of the mean daily temperature: T mean The Blaney-Criddle method always refers to mean monthly values, both for the temperature and the ETo. If in blxney local meteorological station the daily minimum and maximum temperatures are measured, the mean daily temperature is calculated mfthod follows: Determination of the mean daily percentage of annual daytime hours: Table 4 is used.

To be able to determine the p value it is essential to know the approximate latitude of the area: Calculate ETo, using the formula: April From Table 4: Table 5 can be used.

Blaney–Criddle equation | Revolvy

The influence of the climate is given by the reference crop evapotranspiration ETo; the reference crop used for this purpose is grass see Fig. In other words, this section discusses the relationship between the reference grass crop and the crop actually grown in the field. The relationship between the reference grass crop and the crop actually grown is given by the crop factor, Kc, as shown in the following formula: Kc, maize is higher than 1. Cucumber, also fully developed, will use less water than the reference grass crop: Kc, cucumber is less than 1.

Kc and the growth stage of the crop A certain crop will use more water once it is fully developed, compared to a crop which has just recently been planted. Kc and the climate The climate influences the duration of the total growing period and the various growth stages. In a cool climate a certain crop will grow slower than in a warm climate. Thus, to determine the crop factor Kc, it is necessary, for each crop, to know the total length of the growing season and the lengths of the various growth stages.

Blaney–Criddle equation

The determination of the Kc values for the various growth stages of the crops involves several steps: Step 1 – Determination of the total growing period of each crop Step 2 blanet Determination of the various growth stages of each crop Step 3 – Determination of the Kc values for each crop for each of the growth stages The 3 steps mentioned above are dealt with in the sections 3.

It is mainly dependent on: Only if no data are available locally should Table 6 be used. As can be seen from Table 6 there is a large variation of values not only between crops but also within one crop type. In general it can be assumed that the growing period for a certain crop is longer when the climate is cool and shorter when the climate is warm. The total growing period is divided into 4 growth stages see Fig. The crop development stage: The blnaey – season stage: The late season stage: Table 7 shows the duration of the various growth stages for some of the major field crops.

For each crop the “minimum” and “maximum” duration of total growing period see also Table 6 have been taken and sub-divided in the various growth stages. This growing period corresponds with the following duration of growth stages: Should, under certain local circumstances, the duration of the growing period be blamey, the duration of the growth stages could be estimated as follows: The table always refers to “sown” crops.


When the crop is transplanted, the length of the initial stage should be reduced. Stage crop development stage mid-season stage late season stage 15 days 45 days 70 days 30 days 2. When a crop is harvested “green” or “fresh” the late season stage is short.

Compare, for example, green beans with dry beans Table 7. The duration of the late season stage is 10 and 20 days merhod. If a crop is planted in the winter or is growing in the cool season the total growing period is long.

The same is the case with the individual lengths of growing stages. The difference will be most pronounced for the stage during which the temperature is the lowest. It should be kept in mind that the influence of variations in the total growing period on the crop water need is very important.

Less important is the choice of the various lengths of growth stages. The duration of the four growth stages can be blaneyy with the help of Table 7.

The sum of the four growth stages should always equal the total growing period. Table 8 indicates per crop the Kc values for each of the balney growth stages. In fact, the Kc is also dependent on the climate and, in particular, on the relative humidity and the windspeed. The values indicated above should be reduced by 0. The values should be increased by 0. Estimate the duration of the various growth stages, using Table 7.

Blaney–Criddle equation – Wikipedia

Crop Total growing period days Initial stage Crop dev. Indicate on table, as per example below, the ETo values and the duration of the growth stages. When calculating the crop water needs, all months are assumed to have 30 days. For the calculation of the reference crop evapotranspiration ETo, section 3. It can be seen from the table above that the mehod and growth stages do not correspond. As a consequence the ETo and the Kc values do not correspond. It is thus necessary to determine the Kc on a monthly basis, which is done as follows: The Kc values are rounded to the nearest 0.

Calculate, on a monthly basis, the crop water need, using the formula: Calculate the monthly and seasonal crop water needs. However there are some crops that do not directly fit this model: Metho determination of their crop factor is explained in this section.

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