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 production potential under the given environment. Section 3. The major climatic factors see Fig.
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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 production potential under the given environment. Section 3. The major climatic factors see Fig. The lowest values are found when it is cool, humid and cloudy with little or no wind. 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. For the Sunken Colorado pan, the K pan varies between 0. The K pan is high if: The K pan is low if: the pan is placed in a fallow area the pan is placed in a cropped area the humidity is high i.
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 Class A evaporation pan and the Sunken Colorado pan. 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 the 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 is inaccurate: in windy, dry, sunny areas, the ETo is underestimated up to some 60 percent , while in calm, humid, clouded areas, the ETo is overestimated up to some 40 percent.
If in a local meteorological station the daily minimum and maximum temperatures are measured, the mean daily temperature is calculated as follows: Step 2 : Determination of the mean daily percentage of annual daytime hours: p To determine the value of p. Table 4 is used. To be able to determine the p value it is essential to know the approximate latitude of the area: the number of degrees north or south of the equator see Fig. Table 5 can be used. In section 3. 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. 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.
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 - 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.
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 mid - season stage: this period starts at the end of the crop development stage and lasts until maturity; it includes flowering and grain-setting. The late season stage: this period starts at the end of the mid season stage and lasts until the last day of the harvest; it includes ripening.
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: Initial stage : 20 days Crop development stage : 30 days Mid-season stage : 30 days Late season stage : 20 days Total days For the "maximum" growing period of days the following values apply: respectively 25, 35, 70 and 20 days.
Should, under certain local circumstances, the duration of the growing period be days, the duration of the growth stages could be estimated as follows: Initial stage : 25 days Crop development stage : 35 days Mid-season stage : 40 days Late season stage : 20 days Total days With respect to Table 7 the following should be noted: 1.
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 respectively. 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. In other words: it is important to obtain preferably locally an accurate estimate of the total growing period.
The duration of the four growth stages can be estimated with the help of Table 7. Note: 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 four 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. This section 3. Crop Total growing period days Initial stage Crop dev. Note: 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. As a consequence the ETo and the Kc values do not correspond. Note: all months are assumed to have 30 days. In summary: 3. However there are some crops that do not directly fit this model: their crop factor Kc is determined in a different way.
The determination of their crop factor is explained in this section. Once the crop factor Kc has been determined, the same procedure as described in the sections 3. The special cases include: - alfalfa, pasture and clover - bananas - citrus - rice - sugarcane - cacao, coffee, tea - olives - grapes Alfalfa, Pasture and Clover Alfalfa, pasture and clover are regularly cut during the year.
Just after cutting they are in the "initial stage", while just before the next cutting, they are in the "late season stage". To determine the crop water need it is best to use an average value of the crop factor Kc. The average Kc values are given in Table 9. One year after planting, the first harvest takes place, after which the shoots that have produced are removed. Meanwhile young shoots have fully developed and take over the production. The Kc values for the first 6 months after planting are indicated in Table If there is no weed control, a Kc value of 0.
Rice For paddy rice the values indicated in Table 11 should be used. It is best to use locally available data. If such data are not available.
Tables 12a and 12b can be used. Table 12a refers to a virgin sugarcane crop from establishment to first harvest, which is assumed to take 18 months. Table 12b refers to a ratoon crop, which is the regrowth after the harvest of the virgin crop. The regrowth is assumed to take 12 months. Sometimes, depending on local practices, a virgin crop is followed by 2 or 3 ratoon crops.
The values indicated in the table provide a rough estimate and should only be used if the crop water needs cannot be calculated more accurately due to lack of data.
The Blaney—Criddle equation named after H. Blaney and W. Criddle is a method for estimating reference crop evapotranspiration. The Blaney—Criddle equation is a relatively simplistic method for calculating evapotranspiration. When sufficient meteorological data is available the Penman—Monteith equation is usually preferred. However, the Blaney—Criddle equation is ideal when only air-temperature datasets are available for a site.
Water Management in Horticultural Crops (1+1)