3.1 Plant Observation Method
3.2 Estimation Method
3.3 Simple Calculation Method
3.4 Conversion of mm/day into litres/sec.ha
3.5 Adjusting the irrigation schedule to actual rainfall
The accurate determination of an irrigation schedule is a time-consuming and complicated process. The introduction of computer programs, however, has made it easier and it is possible to schedule the irrigation water supply exactly according to the water needs of the crops. Ideally, at the beginning of the growing season, the amount of water given per irrigation application, also called the irrigation depth, is small and given frequently. This is due to the low evapotranspiration of the young plants and their shallow root depth. During the mid season, the irrigation depth should be larger and given less frequently due to high evapotranspiration and maximum root depth. Thus, ideally, the irrigation depth and/or the irrigation interval (or frequency) vary with the crop development (Figure 8).
When sprinkler and drip irrigation methods are used, it may be possible and practical to vary both the irrigation depth and interval during the growing season. With these methods it is just a matter of turning on the tap longer/shorter or less/more frequently.
When surface irrigation methods are used, however, it is not very practical to vary the irrigation depth and frequency too much. With, in particular, surface irrigation, variations in irrigation depth are only possible within limits. It is also very confusing for the farmers to change the schedule all the time. Therefore, it is often sufficient to estimate or roughly calculate the irrigation schedule and to fix the most suitable depth and interval; in other words, to keep the irrigation depth and the interval constant over the growing season. In this Chapter, three simple methods to determine the irrigation schedule are briefly described: plant observation method, estimation method and simple calculation method. In the last section of this chapter, some remarks are made about taking into account actual rainfall in irrigation scheduling.
The plant observation method is the method which is normally used by farmers in the field to estimate "when" to irrigate. The method is based on observing changes in plant characteristics, such as changes in colour of the plants, curling of the leaves and ultimately plant wilting.
In the estimation method section, a table is provided with irrigation schedules for the major field crops grown under various climatic conditions.
The simple calculation method is based on the estimated depth (in mm) of the irrigation application, and the calculated irrigation water need of the crop during the growing season.
The plant observation method determines "when" the plants have to be irrigated and is based on observing changes in the plant characteristics, such as changes in colour of the plants, curling of the leaves and ultimately plant wilting. The changes can often only be detected by looking at the crop as a whole rather than at the individual plants. When the crop comes under water stress the appearance changes from vigorous growth (many young leaves which are light green) to slow or even no growth (fewer young leaves, darker in colour, and sometimes greyish and dull).
Some crops (such as cassava) react to water stress by changing their leaf orientation: with adequate water available, the leaves are perpendicular to the sun (thus allowing optimal transpiration and production). However, when little water is available, the leaves turn away from the sun (thus reducing the transpiration and production).
To use the plant observation method successfully, experience is required as well as a good knowledge of the local circumstances. A farmer will, for example, know where the sandy spots in the field are, which is where the plants will first show stress characteristics: the colour changes and wilting are more pronounced on the sandy spots.
An example of the plant observation method is given in Figures 9 and 10. The sugarcane in Figure 9 suffers heavily from water shortage: the leaves are stiff (bent towards the centre) and curled. Figure 10 shows the same sugarcane when enough moisture is available: the lower leaves are hanging, thus exposing them fully to the sunlight and allowing maximum evapotranspiration (water use of the plants) and crop production.
Figure 9. Sugarcane suffering from water shortage
Figure 10. The same sugarcane when enough moisture is available
The disadvantage of the plant observation method is that by the time the symptoms are evident, the irrigation water has already been withheld too long for most crops and yield losses are already inevitable. It is important to note that it is not advisable to wait for the symptoms. Especially in the early stages of crop growth (the initial and crop development stages), irrigation water has to be applied before the symptoms are evident (see Chapter 1).
Another indicator of water availability is the leaf temperature. If the leaves are cool during the hot part of the day (Figure 11), the plants do not suffer from water stress. However, if the leaves are warm, irrigation is needed. Special devices (infra-red thermometers) have been developed to measure the leaf temperature in relation to the air temperature. However, they must be calibrated for specific conditions before being used to determine the irrigation schedule.
Figure 11. Leaf temperature
Another method used to determine the irrigation schedule involves soil moisture measurements in the field. When the soil moisture content has dropped to a certain critical level, irrigation water is applied. Instruments to measure the soil moisture include gypsum blocks, tensiometers and neutron probes. Their use, however, is beyond the scope of this manual.
3.2.1 Estimating the Irrigation Schedule
3.2.2 Adjusting the Irrigation Schedule
Section 3.2.1 includes a table to estimate irrigation schedules of field crops for various soil types and climates. Section 3.2.2 explains how the values thus found can be adjusted when used under different circumstances.
In this section, a table is provided to estimate the irrigation schedule for the major field crops during the period of peak water demand; the schedules are given for three different soil types and three different climates. The table is based on calculated crop water needs and an estimated root depth for each of the crops under consideration. The table assumes that with the irrigation method used the maximum possible net application depth is 70 mm.
With respect to soil types, a distinction has been made between sand, loam, and clay, which have, respectively, a low, a medium and a high available water content. With respect to climate, a distinction is made between three different climates.
|
Shallow and/or sandy soil |
In a sandy soil or a shallow soil (with a hard pan or impermeable layer close to the soil surface), little water can be stored; irrigation will thus have to take place frequently but little water is given per application. |
|
Loamy soil |
In a loamy soil more water can be stored than in a sandy or shallow soil. Irrigation water is applied less frequently and more water is given per application. |
|
Clayey soil |
In a clayey soil even more water can be stored than in a medium soil. Irrigation water is applied even less frequently and again more water is given per application. |
|
Climate 1 |
Represents a situation where the reference crop evapotranspiration ETo = 4 - 5 mm/day. |
|
Climate 2 |
Represents an ETo = 6 - 7 mm/day. |
|
Climate 3 |
Represents an ETo = 8 - 9 mm/day. |
An overview indicating in which climatic zones these ETo values can be found is given below:
REFERENCE CROP EVAPOTRANSPIRATION (mm/day)
|
Climatic zone |
Mean daily temperature |
||
|
low |
medium |
high |
|
|
Desert/arid |
4 - 6 |
7 - 8 |
9 - 10 |
|
Semi-arid |
4 - 5 |
6 - 7 |
8 - 9 |
|
Sub-humid |
3 - 4 |
5 - 6 |
7 - 8 |
|
Humid |
1 - 2 |
3 - 4 |
5 - 6 |
It is important to note that the irrigation schedules given in Table 3 are based on the crop water needs in the peak period. It is further assumed Chat little or no rainfall occurs during the growing season. Some examples on the use of Table 3 are given below.
EXAMPLES
1. Estimate the irrigation schedule for groundnuts grown on a deep, clayey soil, in a hoc and dry climate.Firstly, the climatic class has to be identified: climate 3 (ETo = 8-9 mm/day) represents a hot climate. Table 3 shows that for climate 3 the interval for groundnuts grown on a clayey soil is 6 days and the net irrigation depth is 50 mm. This means that every 6 days the groundnuts should receive a net irrigation application of 50 mm.
2. Estimate the irrigation schedule for spinach grown on a loamy soil, in an area with an average temperature of 12º C during the growing season.
The average temperature is low: climate 1 (ETo = 4-5 mm/day). Table 3 shows, with climate 1, for spinach, grown on a loamy soil an interval of 4 days and a net irrigation depth of 20 mm.
3. Estimate the irrigation schedule of sorghum grown on a sandy soil, in an area with a temperature range of 15-25º C during the growing season (Figure 12).
The average temperature is medium: climate 2 (ETo = 6-7 mm/day). Table 3 shows, with climate 2 for sorghum grown on a sandy soil, an irrigation interval of 6 days and a net irrigation depth of 40 mm.
Table 3. ESTIMATED IRRIGATION SCHEDULES FOR THE MAJOR FIELD CROPS DURING PEAK WATER USE PERIODS
a. Adjustments for the non-peak periods
The irrigation schedule, which is obtained using Table 3, is valid for the peak period; in other words, for the mid-season stage of the crop.
During the early growth stages, when the plants are small, the crop water need is less than during the mid-season stage. Therefore, it may be possible to irrigate during the early stages of crop growth, with the same frequency as during the mid-season, but with smaller irrigation applications. It is risky to give the same irrigation application as during the mid-season, but less frequently; the young plants may suffer from water shortage as their roots are not able to take up water from the lower layers of the root zone.
Dry harvested crops or crops which are allowed to die before harvest (for example grain maize) need less water during the late season stage than during the mid-season stage (the peak period). During the late season stage, the roots of the crops are fully developed and therefore the same amount of water can be stored in the root zone as during the mid-season stage. It is thus possible to irrigate during the late season stage less frequently but with the same irrigation depth as during the peak period.
In summary, in order to save water, it may be feasible to irrigate, during the early stages of the crop development, with smaller irrigation applications than during the peak period. During the late season stage it may be feasible to irrigate less frequently, in particular if the crop is harvested dry.
When adjusting the irrigation schedule for the non-peak periods, it should always be kept in mind that the irrigation schedules must be simple, in particular in surface irrigation schemes where many farmers are involved. It will often be necessary to discuss with the farmers, before implementing the irrigation schedule, the various alternatives and come to an agreement which best satisfies all parties involved (Figure 13).
Figure 13.Discussing the irrigation schedule with the farmers
b. Adjustment for climates with considerable rainfall daring the growing season
The schedules obtained from Table 3 are based on the assumption that little or no rainfall occurs during the growing season. If the contribution from the rainfall is considerable during the growing season, the schedules need to be adjusted: usually by making the interval longer. It may also be possible to reduce the net irrigation depth. It is difficult to estimate to which values the interval and the irrigation depth should be adjusted. It is therefore suggested to use the simple calculation method (section 3.3), instead of the estimation method, in the case of significant rainfall during the growing season. Alternatively it is possible to adjust the irrigation schedule to the actual rainfall; this is further discussed in section 3.5.
c. Adjustment for local irrigation practices or irrigation method used
It may happen that the net irrigation depth obtained from Table 3 is not suitable for the local conditions. It may not be possible, for example, to infiltrate 70 mm with the irrigation method used locally. Tests may have shown that it is only possible to infiltrate some 50 mm per application.
In such cases, both the net irrigation depth and the interval must be adjusted simultaneously. For example, suppose that maize is grown on a clayey soil in a moderately warm climate. According to Table 3, the Interval is 10 days and the net irrigation depth is 70 mm. This corresponds to an irrigation water need of 70/10 = 7 mm/day.
Instead of giving 70 mm every 10 days, it is also possible to give:
63 mm every 9 days
56 mm every 8 days
49 mm every 7 days
42 mm every 6 days etc.
This means that in the above example an interval of seven days is chosen with a net application depth of 49 mm.
d. Adjustment for shallow soils
A soil which is shallow can only store a little water, even if the soil is clayey. For shallow soils - sandy, loamy or clayey - the column "shallow and/or sandy soil" of Table 3 should be used.
e. Adjustment for salt-affected soils
In the case of irrigating salt-affected soils, special attention needs to be given to the determination of the irrigation schedule. This topic will be dealt with in a separate training manual (Drainage and Salinity) in this series.
Figure 14. Appropriate irrigation schedules help to produce good crops
3.3.1 Application of the Simple Calculation Method
3.3.2 Adjusting the Simple Calculation Method for the Peak Period
3.3.3 Calculation Example Irrigation Scheduling
Section 3.3.1 gives a simple calculation method for the irrigation schedule; this schedule is based on the entire growing season. Section 3.3.2 explains how to adjust the schedule to the period of peak water demand. In section 3.3.3 a calculation example is given.
The simple calculation method to determine the irrigation schedule is based on the estimated depth (in mm) of the irrigation applications, and the calculated irrigation water need of the crop over the growing season.
Unlike the estimation method (see section 3.2), the simple calculation method is based on calculated irrigation water needs. Thus, the influence of the climate, i.e. temperature and rainfall, is more accurately taken into account. The result of the simple calculation method will therefore be more accurate than the result of the estimation method.
The simple calculation method to determine the irrigation schedule involves the following steps that are explained in detail below:
|
Step 1: |
Estimate the net and gross irrigation depth (d) in mm. |
|
Step 2: |
Calculate the irrigation water need (IN) in mm, over the total growing season. |
|
Step 3: |
Calculate the number of irrigation applications over the total growing season. |
|
Step 4: |
Calculate the irrigation interval in days. |
Step 1: Estimate the net and gross irrigation depth (d) in mm
The net irrigation depth is best determined locally by checking how much water is given per irrigation application with the local irrigation method and practice. If no local data are easily available, Table 4 can be used to estimate the net irrigation depth (d net), in mm. As can be seen from the table, the net irrigation depth is assumed to depend only on the root depth of the crop and on the soil type. It must be noted that the d net values in the table are approximate values only. Also the root depth is best determined locally. If no data are available, Table 5 can be used which gives an indication of the root depth of the major field crops.
Table 4. APPROXIMATE NET IRRIGATION DEPTHS, IN mm
|
|
Shallow rooting crops |
Medium rooting crops |
Deep rooting crops |
|
Shallow and/or sandy soil |
15 |
30 |
40 |
|
Loamy soil |
20 |
40 |
60 |
|
Clayey soil |
30 |
50 |
70 |
Table 5. APPROXIMATE ROOT DEPTH OF THE MAJOR FIELD CROPS
|
Shallow rooting crops (30-60 cm): |
Crucifers (cabbage, cauliflower, etc.), celery, lettuce, onions, pineapple, potatoes, spinach, other vegetables except beets, carrots, cucumber. |
|
Medium rooting crops (50-100 cm): |
Bananas, beans, beets, carrots, clover, cacao, cucumber, groundnuts, palm trees, peas, pepper, sisal, soybeans, sugarbeet, sunflower, tobacco, tomatoes. |
|
Deep rooting crops (90-150 cm): |
Alfalfa, barley, citrus, cotton, dates, deciduous orchards, flax, grapes, maize, melons, oats, olives, safflower, sorghum, sugarcane, sweet potatoes, wheat. |
Not all water which is applied to the field can indeed be used by the plants. Part of the water is lost through deep percolation and runoff. To reflect this water loss, the field application efficiency (ea) is used. For more detail on irrigation efficiencies, see Annex 1. The gross irrigation depth (d gross), in mm, takes into account the water loss during the irrigation application and is determined using the following formula:
d gross = gross irrigation depth in mm
d net = net irrigation depth in mm
ea = field application efficiency in percent
If reliable local data are available on the field application efficiency, these should be used. If such data are not available, the following values for the field application efficiency can be used:
|
- for surface irrigation |
: ea = 60% |
|
- for sprinkler irrigation |
: ea = 75% |
|
- for drip irrigation |
: ea = 90% |
If, for example, tomatoes are grown on a loamy soil, Tables 4 and 5 show that the estimated net irrigation depth is 40 mm. If furrow irrigation is used, the field application efficiency is 60% and the gross irrigation depth is determined as follows:
Step 2: Calculate the irrigation water need (IN) in - over the total growing season
This has been discussed in detail in Volume 3. Assume that the irrigation water need (in mm/month) for tomatoes, planted 1 February and harvested 30 June, is as follows:
|
|
Feb. |
Mar. |
Apr. |
May |
June |
|
IN (mm/month) |
67 |
110 |
166 |
195 |
180 |
The irrigation water need of tomatoes for the total growing season (Feb-June) is thus (67 + 110 + 166 + 195 + 180 =) 718 mm. This means that over the total growing season a net water layer of 718 mm has to be brought onto the field.
If no data on irrigation water needs are available, the estimation method (section 3.2) should be used.
Step 3: Calculate the number of irrigation applications over the total growing season
The number of irrigation applications over the total growing season can be obtained by dividing the irrigation water need over the growing season (Step 2) by the net irrigation depth per application (Step 1).
If the net depth of each irrigation application is 40 mm (d net = 40 mm; Step 1), and the irrigation water need over the growing season is 718 mm (Step 2), then a total of (718/40 =) 18 applications are required.
Step 4: Calculate the irrigation interval (INT) in days
Thus a total of 18 applications is required. The total growing season for tomatoes is 5 months (Feb-June) or 5 x 30 = 150 days. Eighteen applications in 150 days corresponds to one application every 150/18 = 8.3 days.
In other words, the interval between two irrigation applications is 8 days. To be on the safe side, the interval is always rounded off to the lower whole figure: for example 7.6 days becomes 7 days; 3.2 days becomes 3 days.
CONCLUSION
In this example, the irrigation schedule for tomatoes is as follows:
d net = 40 mm
d gross = 65 mm
interval = 8 days
When using the simple calculation method to determine the irrigation schedule, it is advisable to ensure that the crop does not suffer from undue water shortage in the months of peak irrigation water need.
For instance, in the above example the interval is 8 days, while the net irrigation depth is 40 mm. Thus every 30 days (or each month): 30/8 x 40 mm = 150 mm water is applied. The amount of water given during each month (d net) should be compared with the amount of irrigation water needed during that month (IN).
The result is shown below. The "IN" values represents the irrigation water needs, while the "d net" values represent the amount of water applied. The "d net - IN" values show whether too much or too little water has been applied:
|
|
Feb |
Mar |
Apr |
May |
June |
Total |
|
IN (mm/month) |
87 |
110 |
166 |
195 |
180 |
718 |
|
d net (mm/month) |
150 |
150 |
150 |
150 |
150 |
750 |
|
d net - IN (mm/month) |
+83 |
+40 |
-16 |
-45 |
-30 |
+32 |
The total net amount of irrigation water applied (750 mm) is more than sufficient to cover the total irrigation water need (718 mm). However, in February and March too much water has been applied, while in April, May and June, too little water has been applied.
Care should be taken with under-irrigation (too little irrigation) in the peak period as this period normally coincides with the growth stages of the crops that are most sensitive to water shortages (see Chapter 2).
Figure 15. Check if enough water is given during the peak months
To overcome the risk of water shortages in the peak months, it is possible to refine the simple calculation method by looking only at the months of peak irrigation water need and basing the determination of the interval on the peak period only.
In the example given above for tomatoes, this means looking at the months April, May and June:
|
Months of peak irrigation water need |
Apr |
May |
June |
Sub-total |
|
IN (mm/month) |
166 |
195 |
180 |
541 |
The total irrigation water need from April to June (90 days) is 541 mm, while the net irrigation depth is 40 mm. Thus 541/40 = 13.5 (rounded 14) applications are needed. Fourteen applications in 90 days means one application every 6.4 (rounded 6) days. Calculated this way the irrigation schedule for the tomatoes would be:
d net = 40 mm
d gross = 65 mm
interval = 6 days
Over the total growing period of 150 days, this means 150/6 = 25 applications, each 40 mm net and thus in total 25 x 40 = 1000 mm.
The overall result of adjusting the irrigation schedule to the months of peak irrigation water demand is shown below:
|
|
Feb |
Mar |
Apr |
May |
June |
Total |
|
IN (mm/month) |
67 |
110 |
166 |
195 |
180 |
718 |
|
d net (mm/month) |
200 |
200 |
200 |
200 |
200 |
1000 |
|
d net - IN (mm/month) |
+133 |
+90 |
+34 |
+5 |
+20 |
+282 |
This way of determining the irrigation schedule avoids water shortages in the month of peak water needs but on the other hand also results in a higher seasonal irrigation water application.
It is possible to combine the two schedules. In this way some water is saved, and there are no water shortages in the peak period, but it is a bit more complicated for the farmers.
The result of the combined irrigation schedule for the whole growing season is as follows:
|
|
Feb |
Mar |
Apr |
May |
June |
Total |
|
IN (mm/month) |
67 |
110 |
166 |
195 |
180 |
718 |
|
d net (mm/month) |
150 |
150 |
200 |
200 |
200 |
900 |
|
d net - IN (mm/month) |
+83 |
+40 |
+34 |
+5 |
+20 |
+182 |
In summary:
Feb-Marchd net = 40 mm
d gross = 65 mm
Interval = 8 daysApril-May-June
d net = 40 mm
d gross = 65 mm
Interval = 6 days
QUESTION: Determine the irrigation schedule for groundnuts:
1. Based on the total growing period.
2. Based on the months of peak irrigation water need.
3. Based on a combination of the two schedules above (1 and 2).
GIVEN:
|
Crop |
: groundnuts |
|
Soil type |
: loam |
|
Irration method |
: furrow irrigation |
|
Field application efficiency |
: 60% |
|
Total growing period |
: 130 days |
|
Planting date |
: 15 July |
|
Irrigation water need (IN) |
: |
|
|
July* |
Aug |
Sept |
Oct |
Nov** |
Total |
|
IN (mm/month) |
38 |
115 |
159 |
170 |
45 |
527 |
* as of 15 July
** up to 25 November
ANSWER 1: irrigation schedule for groundnuts, based on the total growing period
Step 1: Determine the net and gross depth (d) in mm of the irrigation applications
Table 5 shows that groundnuts have a medium root depth. Grown on a loamy soil, the net irrigation depth (d net) will thus be approximately 40 mm (Table 4).The gross irrigation depth (d gross) can be calculated using the following formula:
The field application efficiency (ea) is 60% and the net irrigation depth (d net) is 40 mm.
Thus:
Step 2: Calculate the irrigation water need (IN) in mm over the total growing season
The irrigation water need over the total growing season of 130 days (15 July - 25 November) is 38 + 115 + 159 + 170 + 45 = 527 mm (see data).
Step 3: Calculate the number of irrigation applications over the total growing season
The number of applications equals the seasonal irrigation water need (Step 2) divided by the net irrigation depth (Step 1). Thus the number of applications is 527/40 = 13.2 = rounded 13 applications.
Step 4: Calculate the irrigation interval in days
A total of 13 applications is given during the total growing period of 130 days. The interval is thus 130/13 = 10 days.
IN SUMMARY:
The irrigation schedule for groundnuts, based on the total growing period is:
d net = 40 mm
d gross = 65
Interval = 10 days
The comparison of the irrigation water required (IN) and the irrigation water applied (d net) is given below:
|
|
July* |
Aug |
Sept |
Oct |
Nov** |
Total |
|
IN (mm/month) |
38 |
115 |
159 |
170 |
45 |
527 |
|
d net (mm/month) |
60 |
120 |
120 |
120 |
100 |
520 |
|
d net - IN (mm/month) |
+22 |
+5 |
-39 |
-50 |
+55 |
-7 |
* July: 15 days only, as the planting date is 15 July
** Nov.: 25 days only, as the last day of the harvest is 25 November
ANSWER 2: Irrigation schedule for groundnuts based on months of peak irrigation water need
As can be seen from the table above, the months of peak Irrigation water need are September and October. In this example the irrigation schedule will be based on these two months.
Step 1: Estimate the net and gross depth (d) in mm of the Irrigation applications. The net and gross depth (d) are calculated in the same way as in Answer 1.
Thus:
d net = 40 mm
d gross = 65 mm (rounded)
Step 2: Calculate the irrigation water need over the months of peak irrigation water need
The months of peak irrigation water need are September and October, and during these two months the IN (159 + 170) = 329 mm.
|
|
Sept |
Oct |
Total |
|
IN (mm/month) |
159 |
170 |
329 |
Step 3: Calculate the number of irrigation applications during the peak months
The number of applications is 329/40 = 8.2, rounded 8 applications.
Step 4: Calculate the irrigation interval in days
8 applications are given during the peak months September and October i.e. during 60 days: the interval = 60/8 = 7.5 = rounded 7 days.
IN SUMMARY:
The irrigation schedule for groundnuts, based on the months of peak irrigation water need is:
d net = 40 mm
d gross = 65 mm
Interval = 7 days
The comparison of the irrigation water required (IN) and the irrigation water applied (d net) is given below:
|
|
July* |
Aug |
Sept |
Oct |
Nov** |
Total |
|
IN (mm/month) |
38 |
115 |
159 |
170 |
45 |
527 |
|
d net (mm/month) approx. |
85 |
170 |
170 |
170 |
140 |
735 |
|
d net - IN (mm/month) |
+47 |
+55 |
+11 |
0 |
+95 |
+208 |
* July: 15 days only, as the planting date is 15 July
** Nov.: 25 days only, as the last day of the harvest is 25 November
There are no water shortages in the peak months, but the total amount of water applied is high.
ANSWER 3: Irrigation schedule for groundnuts combining the two previous schedules
It is possible to combine the two schedules obtained in answer 1 and answer 2. For the non-peak period, the Answer 1 schedule is used. For the peak period, Answer 2 is used. The result is shown below.
|
|
July* |
Aug |
Sept |
Oct |
Nov** |
Total |
|
IN (mm/month) |
38 |
115 |
159 |
170 |
45 |
527 |
|
d net (mm/month) approx. |
60 |
120 |
170 |
170 |
100 |
620 |
|
d net - IN (mm/month) |
+22 |
+5 |
+11 |
0 |
+55 |
+93 |
July, August, November
d net = 40 mm
d gross = 65 mm
interval = 10 days
September, October
d net = 40 mm
d gross = 65 mm
interval = 7 days
Similarly, other schedules can be determined by trial and error. The objective should be to best match the required amount of water with the amount actually given. The schedules thus obtained, however, should not be too difficult for the farmer to implement.
In the previous sections it has been explained how to determine the irrigation depth of each irrigation application (in mm) and the interval between two irrigation applications (in days). From these figures it is, however, not easy to visualize what the flow of Irrigation water to a block of, for example, one hectare would be. Below a "rule of thumb" is given on how to convert an irrigation depth and interval into a continuous water flow.
8.64 mm/day = 1.0 litre/sec.hectare
In other words, an irrigation application of 8.64 mm per day corresponds to a continuous water flow of 1.0 litre per second per hectare. Further details of the conversion are given in the Scheme Irrigation Supply Training Manual. Table 6 may assist with the conversion of mm/day into l/sec.ha.
EXAMPLE: Determine the continuous water flow when the gross irrigation depth is 64 mm and the interval is 8 days, for an area of 50 ha.
ANSWER: 64 mm every 8 days is 64/8 = 8 mm/day; 8 mm/day corresponds to 0.93 l/sec.ha. For an area of 50 ha the net continuous flow would be: 50 x 0.93 = 46.5 l/sec.
Table 6. CONVERSION OF MM/DAY INTO L/SEC.HA
|
mm/day |
l/sec.ha |
l/sec.ha |
mm/day |
|
2 |
0.23 |
0.2 |
1.7 |
|
3 |
0.35 |
0.3 |
2.6 |
|
4 |
0.46 |
0.4 |
3.5 |
|
5 |
0.58 |
0.5 |
4.3 |
|
6 |
0.69 |
0.6 |
5.2 |
|
7 |
0.81 |
0.7 |
6.0 |
|
8 |
0.93 |
0.8 |
6.9 |
|
9 |
1.04 |
0.9 |
7.8 |
|
10 |
1.16 |
1.0 |
8.6 |
|
12 |
1.39 |
1.2 |
10.4 |
|
14 |
1.62 |
1.4 |
12.1 |
|
16 |
1.85 |
1.6 |
13.8 |
|
18 |
2.08 |
1.8 |
15.6 |
|
20 |
2.31 |
2.0 |
17.3 |
The estimation method (section 3.2) to determine the irrigation schedule can only be used when no significant rainfall occurs during the growing season. The simple calculation method (section 3.3) is based on the average irrigation water need of the crop which is the average crop water need minus the average effective rainfall. This method is used when designing and implementing an irrigation system with a "rotational" water supply: each field receives a certain amount of water on dates that are already fixed in advance. The rotational supply takes into account the average rainfall only and thus does not take into account the actual rainfall; this results in over-irrigation in wetter than average years and under-irrigation in drier than average years. In surface irrigation systems the rotational water supply method is most commonly used.
There are also water supply methods which allow the irrigation water to be distributed "on demand". The farmer can take water whenever necessary. In this case it is possible to take the actual rainfall into account and thus give the correct amount of irrigation water even in drier or wetter years. With this method of irrigation scheduling, however, the rainfall has to be measured on a daily basis (for details see Annex II). The net irrigation depth (d net) has to be determined in accordance with the irrigation method used (see section 3.3.1). In addition, the crop water need has to be known on a daily basis for each month of the growing season (see Volume 3). As soon as the accumulated water deficit exceeds the value of the net irrigation depth, irrigation water is supplied.
An example is given below for a situation with a crop water need (CWN) of 8 am/day and a net irrigation depth (d net) of 45 mm. As soon as the accumulated deficit exceeds the d net (= 45 mm), irrigation water is supplied. Note that the "deficit" can never be positive; maximum zero.
|
day |
CWN |
Rain |
d net |
|
Accumulated deficit |
|
1 |
8 |
- |
- |
|
-8 |
|
2 |
8 |
- |
- |
(-8-8) |
-16 |
|
3 |
8 |
- |
- |
(-16-8) |
-24 |
|
4 |
8 |
- |
- |
(-24-8) |
-32 |
|
5 |
8 |
- |
- |
(-32-8) |
-40 |
|
6 |
8 |
- |
45 |
(-40-8+45) |
-3 |
|
7 |
8 |
- |
- |
(-3-8) |
-11 |
|
8 |
8 |
12 |
- |
(-11-8+12) |
-7 |
|
9 |
8 |
24 |
- |
(-7-8+24) |
0 |
|
10 |
8 |
- |
- |
(0-8) |
-8 |
|
11 |
8 |
- |
- |
(-8-8) |
-16 |
|
12 |
8 |
- |
- |
(-16-8) |
-24 |
|
13 |
8 |
4 |
- |
(-24-8+4) |
-28 |
|
14 |
8 |
- |
- |
(-28-8) |
-36 |
|
15 |
8 |
- |
- |
(-36-8) |
-44 |
|
16 |
8 |
- |
45 |
(-44-8+45) |
-7 |
|
17 |
8 |
- |
- |
(-7-8) |
-15 |
|
etc. |
|
|
|
|
|
In the above example of adjusting the irrigation schedule to the actual rainfall, irrigation takes place on day 6, on day 16, etc. with on each occasion a net irrigation depth of 45 mm.
