# 3. SCHEME IRRIGATION WATER NEED AND SUPPLY

## 3.1 Introduction

Chapter 2 outlined some of the basic aspects of water availability for irrigation. This chapter deals with the irrigation water need of an entire irrigation area, SINgross, and the supply of irrigation water to the entire irrigation area, SINop. The SINgross is the continuous flow of water required for good crop production during the irrigation season. SINop is the flow of water actually applied to the irrigation area, continuously or at intervals after taking the operational criteria into account.

Because there is always some water stored in the soil (rootzone of the crop), or on the field (in case of paddy rice), SINgross can be supplied at intervals without affecting crop production. However, over the long run, the supplied volumes of water should equal the needed volumes of water.

The SINgross may be determined quickly, just to get a rough idea of the volume of water needed. This quick method, which is explained in section 3.2 of this manual, is called the approximate method and is suitable for preliminary planning only, or when there is a lack of reliable data. The second, the calculation method is more accurate and thus suitable for the detailed planning of sound water management. The calculation method, explained in the section 3.4, uses fairly detailed information on the irrigation area's cropping system. (The concept of a cropping system will be introduced in section 3.3.)

## 3.2 The Approximate Method

The approximate method of determining the SINgross assumes a constant average irrigation need, INnet, for the entire season. The most simplified standard value of this INnet is 1 litre per second per hectare. This is equivalent to a daily water requirement of 8.6 mm (see Annex II). When the daily water requirement is 4.3 mm, the irrigation need would be 0.5 l/s.

Like most rules of thumb, this rule should be applied with caution. The proxy values of INnet in hot and dry climates can be three times as great as in humid climates. Typical values are presented in Table 1.

Table 1 APPROXIMATE AVERAGE INnet VALUES FOR DIFFERENT CLIMATES AND RICE

 Humid tropical climate 0.5 l/s/ha Monsoon climate wet season 0.5 l/s/ha Monsoon climate dry season 1.0 l/s/ha Semi-arid climate wet season 1.0 l/s/ha Semi-arid climate dry season 1.5 l/s/ha Arid climate 1.5 l/s/ha Rice 1.5 l/s/ha

The approximate values of the irrigation need for an entire area, SINnet may be calculated by multiplying the INnet with the area, Area (in hectares). Therefore, the equation representing the approximate Net Scheme Irrigation Need is:

 SINnet (l/s) = Area (ha) x INnet (l/s/ha)

 EXAMPLE: The area of a specific irrigation scheme on which several crops are grown is 50 ha. The estimated net irrigation requirement is 1 l/s/ha. Thus the SINnet for the entire scheme would be: 50 (ha) x 1 (l/s/ha) = 50 (l/s)

The net scheme irrigation need, SINnet is the amount of water needed to meet crop water needs of an entire scheme minus the effective rainfall. Water lost during delivery must be added before the gross scheme irrigation need, SINgross, can be determined.

The SINgross is calculated by dividing SINnet by the overall scheme irrigation efficiency (e), which is the product of the conveyance efficiency (ec) and the field application efficiency (ea). The subject of irrigation efficiencies is discussed at length in Annex 1 in Training Manual 4, "Irrigation Scheduling".

The present series of training manuals is mainly concerned with small irrigation schemes and tertiary units of larger schemes which use gravity irrigation methods. Gravity irrigation of sandy soils is not widespread. Therefore, a conveyance efficiency of about 85% and an application efficiency of about 60% have been selected, which result in an overall area irrigation efficiency of about 50%. This value for the overall efficiency will be used in all further calculations in this manual. It should also be kept in mind that the values mentioned above are only indicative values.

Once the scheme irrigation efficiency (e, expressed in percentages) is known, SINgross can be calculated with the following formula:

 SINgross (l/s) = 100/e x SINnet (l/s)

The formula above shows that as the efficiency drops (becomes smaller), more water has to be supplied to satisfy the irrigation need of a scheme.

 EXAMPLE: An irrigation scheme of 70 ha located in a semi-arid area is irrigated during the dry season. The average INnet can be estimated at 1.5 l/s/ha (see Table 1). The total net irrigation need of the scheme is: Area times INnet which is 70 x 1.5 = 105 l/s. The irrigation efficiency is 50% and thus the SINgross equals 100/e x SINnet, which equals 100/50 x 105, or 210 l/s.

Operational criteria

The amount of water that is supplied to an irrigation area should be equal to the gross scheme irrigation water need.

This would mean that, if the same flow of water is supplied continuously, second after second, hour after hour, and day after day without interruption throughout the entire growing season, the SINgross which is expressed in a continuous flow of litres per second would have been completely supplied, and SINop, operational scheme irrigation need, would equal SINgross.

However, water is not always supplied continuously. If irrigation water is applied only 12 out of a possible 24 hours per day, then during these 12 hours twice as much water must be given to supply the same total. Further, if irrigation does not take place every day, more water must be supplied on irrigation days in order to maintain the same total quantity per week. This is illustrated in the following examples and in Figures 17 and 18.

 EXAMPLE: An area has an irrigation need of 25 l/s. However, the water supply is available only 4 hours per day, every day. Since the farmers can irrigate only during 4 out of the 24 hours a day, the available time is only 4/24 or 1/6 of the total time. The supply should therefore be 6 times as great, or: 6 x 25, or 150 l/s.

Figure 17 - Irrigation water applied for 4 hours instead of 24 hours EXAMPLE: Another scheme also has a continuous irrigation need of 25 l/s. Although farmers irrigate the whole day (24 hours), they only do it 2 days a week. Now the amount of time available for irrigation is 2 days out of 7, which is 2/7 or 1/3.5. The supply should therefore be 3.5 times as much during the 2 irrigation days, to supply the same total amount. The actual irrigation flow should therefore be 3.5 x 25, or 87.5 l/s.

Figure 18 - Irrigation water applied for 2 days instead of 7 days EXAMPLE: What happens if the water requirement of 25 l/s has to be supplied in 8 hours per day and only 4 days per week? In this case, the total amount of time in which the irrigation water has to be supplied is 8/24 or 1/3 of a day during only 4/7 of a week. This leaves farmers with only 1/3 x 4/7, or 4/21 of the time in which to irrigate. During this time the actual flow should be 21/4 times the continuous requirement, or 21/4 x 25, which equals 131 l/s.

The ratios of hours per day and days per week during which the irrigation system functions are called, in this manual, the operational criteria. By using these operational criteria, the required scheme irrigation supply can be determined with the following formula:

 SINop (l/s) = SINgross/Top Top = d/7 x h/24

where:

SINop = operational scheme irrigation need
Top = operational criteria
d = number of irrigation days per week
h = number of irrigation hours per day
SINgross = gross scheme irrigation need

The formula above shows that the fewer the number of irrigation applications per week and the fewer the number of hours of irrigation per day, the larger the scheme's supply should be, and the larger should also be the capacity of the irrigation system.

 EXAMPLE: In an irrigation scheme, irrigation takes place 4 days per week, 9 hours per day. Thus the operational criteria, Top = 4/7 x 9/24 = 3/14. The SINgross is 50 l/s. According to the formula the SINop should be: 50 (l/s) / (3/14) = 233 l/s. SAMPLE PROBLEM: Estimate, using the approximate method, the SINop for the following scheme. ASSUMPTIONS: The scheme is situated in a monsoon climate. Irrigation is practised during the dry season. The area of the scheme is 20 ha, with loamy soil. Irrigation takes place 5 days per week for 10 hours per day. The scheme uses surface irrigation methods, and the canals are made of earth. CALCULATION: Step 1: Estimate the Net Irrigation Need, INnet According to Table 1, the approximate INnet for this climate should be 1 l/s/ha. Step 2: Estimate the Net Scheme Irrigation Need for the entire area. SINnet (l/s) = INnet (l/s/ha) x Area (ha) For an INnet of 1 l/s/ha and an area of 20 ha, the result is a SINnet of 20 l/s. Step 3: Estimate the scheme irrigation efficiency, e. The scheme irrigation efficiency (e) can be found by multiplying the ec and ea values in the following formula (the following calculation is used throughout this manual for the irrigation efficiency): e= ec x ea/100 = 85 x 60/100 » 50% Step 4: Calculate the gross scheme irrigation need, SINgross in l/s. The SINgross can be calculated with the following formula: SINgross = SINnet x 100/e In this case, SINgross = 20 x 100/50 = 40 l/s. Step 5: Calculate the operational scheme irrigation need, considering the following operational criteria. The scheme is irrigated 5 days per week for 10 hours per day, Top and SINop may be calculated according to the following formula: Top = d/7 x h/24 = 5/7 x 10/24 » 0.3 SINop (l/s) = SINgross / Top = 40 / 0.3 = 133 l/s

## 3.3 Cropping Pattern

In Chapter 3.2 a quick but rough estimation was made of a scheme's irrigation requirement, by using the rule of thumb for the INnet. A much more accurate prediction of a scheme's irrigation flow requirements can be made, if it is based on the scheme's cropping pattern. The cropping pattern, or cropping schedule of an irrigation area provides information, for a period of at least one season, on three important elements:

- which crops are grown
- when are they cultivated
- how many hectares of each crop are grown.

This information is written down, in detail, let us say for the area of one farmer during one year, and from this record there emerges a pattern. For example, the cropping pattern for farm A:

- onions cultivated from April 15 to September 15 on 1 ha
- potatoes from October 15 to February 15 on 1/2 ha, on the same plot as the onions
- cotton from July 10 to January 20 on 1 and 1/2 ha.

Information on the type of crop and the period of cultivation can be visualized in a line diagram.

Figure 19 - Line diagram of an example farm's crop calendar This crop calendar diagram shows that farmer A starts with onion cultivation in mid-April which he then harvests mid-September. Within one month he will plant potatoes on the same plot, which he expects to uproot in mid-February. Farmer A has a second plot on which he grows cotton over a six-month period from mid-July to the end of January of the following year.

If the area of each crop has to be shown, the lines representing individual crops should be replaced by bars where the height of each bar is a measure of the area of each crop. In the example below, for instance, an area of one hectare is represented by a bar-height of about 0.5 centimetre. The length of the bar indicates the period in which the crop is grown. A diagram with crop bars rather than crop lines is called a cropping pattern.

Figure 20 shows that farmer A grew 1 and 1/2 ha of cotton from 10 July to 20 January, 1 ha of onions from mid-April to mid-September and 1/2 ha of potatoes from mid-October to mid-February.

The diagram can indicate more crop husbandry dates, such as crop manuring and periods of weeding and transplanting, etc. It is also possible to draw up a cropping pattern diagram for larger areas, including a group of farmers, or an entire irrigation scheme. The only difference is that with a single farm, it was supposed that the crop was sown or planted on one specific day. The same assumption cannot be made in the case of a larger irrigation scheme. In fact, there are several reasons why farmers do not all start planting on the same day on an irrigation scheme:

- labour and machinery needed to prepare the land is in short supply, and each has to wait his turn;

- a farmer is not ready due to other activities, so he postpones planting the crop for a while;

- also, it is impossible to supply all the fields on an irrigation scheme with the water required to start a crop on the same day. The larger the scheme's area, the longer the time it takes to serve all the fields.

The cultivation of any crop on an irrigation scheme is thus spread over a certain period of time. This is called staggered cultivation or a staggered cropping pattern.

When, for example, the first farmer starts with onion cultivation on 1 April, and other farmers follow more or less regularly on the days after until the last farmer has planted his onions at the end of April, the crop calendar for onions is staggered over one month. In a cropping pattern diagram this staggering is depicted as below.

Figure 21 - Staggered cropping of onions Each line in the diagram represents a farm, or a certain area, and the day it starts onion cultivation. Of course; when crop planting dates are staggered, so also are the end of season dates.

In a normal cropping pattern diagram the subsequent lines are simplified, forming a parallelogram bar. The lower side represents the first farmer, or the area which is planted earliest. The upper side represents the last area to be planted and cropped.

Figure 22 - Parallelogram cropping pattern As with the cropping pattern diagram for one farmer, the extent of the area cultivated for each type of crop, across the entire scheme, can be indicated with the height of the parallelogram serving as a measure. The height of the vertical axis of the diagram represents the total area of an irrigation scheme, for example a scheme of 150 ha (see Figure 23). Onions are cultivated on 60 ha and they have a growing period of 5 months. The onion planting season starts in the beginning of April and is staggered over one month. Potatoes, with a growing period of 4 months, are grown on 30 ha starting 1 October. The cultivation period is staggered over one month. Cotton, with a growth period of 190 days, is grown on 75 ha from the beginning of July. Cotton cultivation is staggered over a period of 45 days.

An area which is empty at any given moment in the cropping pattern diagram is not cultivated at that time. For example, during the month of March nothing is being cultivated on the scheme; in mid-September 30 ha of onions have already been harvested and 15 ha of land have been left fallow.

## 3.4 The Calculation Method

In Section 3.2, the irrigation water need and the actual scheme irrigation flow requirement were estimated using the approximate method, which is based on a rule of thumb which says that one hectare of irrigated land needs a fixed flow of water during the irrigation season.

When accurate crop and climatic data are available, a much more precise prediction may be made of the gross scheme irrigation need, SINgross, while the operational scheme irrigation need, SINop, can be finely tuned to the real irrigation need. However, when a scheme has many crops, this more precise method also requires more complex calculations. A calculation method example is presented in this section to help field technicians and extension agents understand and become familiar with the underlying principles and the basic elements involved in calculating scheme water needs.

The calculation of an individual crop's irrigation water need, INnet, has already been explained in Training Manual 3, "Irrigation Water Needs". Using this calculation as a point of departure, which ends with the net irrigation need of an individual crop expressed in millimetres of water depth per month (mm/mon), this section will focus on determining the net scheme irrigation need SINnet (in litres per second) of an entire irrigation scheme with a variety of crops throughout the year.

### 3.4.1 Scheme irrigation need for a simple cropping system

Suppose, as in the former section, that 60 ha of onions are planted on an irrigation scheme of 150 ha and that the planting of onions is spread out evenly over one month. Planting is begun on 1 April and finished on 30 April.

The irrigation water need of the earliest planted onions during the month of April is 98 mm, which is 3.3 mm/day for 30 days (see Manual 3, Part II 4.9). Onions which are not planted until April 15 do not need water during the first 15 days of April. For the remaining 15 days of April, the irrigation water need is 15 x 3.3 mm, or 49 mm. Onions which are planted on the last day of April do not need any irrigation water in April at all.

This example indicates that the irrigation water need of the onions which are planted in between the earliest and the latest planted onions, lies in between the irrigation water need of the earliest and latest planted onions. In general, the average irrigation water need of an area cropped with onions, or any other crop, is the average of the irrigation water need of the earliest (INe) and the irrigation water need of the latest (INl) planting. This is also expressed in the formula below: This monthly irrigation need, INav , is presented as a water layer which is applied during that month on a certain area. However, water flows are not actually expressed in millimetres of water covering a certain area over a certain period of time, but in litres per second, or cubic metres per second. Therefore the INav per month should be converted into a current irrigation water flow. SINnet in litres per second (net scheme irrigation need) can be calculated as follows: or When the monthly irrigation need of an area with staggered crop cultivation must be determined, the irrigation needs of the earliest and of the latest plantings must be calculated. The following example is based on Training Manual 3, Chapter 5, data sheets 3 and 4. Field technicians and extension agents should consult this material before proceeding with this manual.

***
SAMPLE PROBLEM: Determine the irrigation need of onions. (Following is a list of assumptions to be used in synthesizing the information in the tables below.)

ASSUMPTIONS:

 Crop: Dry Onions Earliest planting: 1 April Total area: 60 ha Latest planting: 30 April

 Duration of crop growth phases Crop factors: (Kc) Initial stage: 15 days Kc 0.5 Crop devt. stage: 25 days Kc 0.75 Mid season: 70 days kc 1.05 Late season: 40 days Kc 0.85

Potential Evapotranspiration (ET0) in the example location:

 Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec ET0 (mm/day) 4.7 5.1 5.2 5.6 5.7 6.1 5.8 5.5 5.6 5.2 4.3 4.6

Effective rainfall (Pe) in the example place:

 Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Pe (mm/mon) 1 3 7 10 12 13 72 82 16 7 1 0

CALCULATION:

Step 1: Draw the cropping pattern. Step 2: Calculate the INe according to the method explained in Training Manual 3. Step 3: Calculate the INl in the same way. Step 4: Calculate the average monthly irrigation need INav.

 Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec INe mm/mon 98 150 179 102 59 INl mm/mon 99 161 111 83 127 INav mm/mon 49 125 170 107 71 64

Step 5: Determine SINnet in litres per second for each month with the following formula: For example, if April's INav equals 49 mm, then the SINnet equals 49 mm times the area (60 ha) divided by 260, or The same calculation should be made and recorded on the schedule below for the months of May, June, July, August and September.

 Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec SINnet l/s 11 29 39 25 16 15

***

Please note the variations for SINnet for each month in the above example. It is interesting to compare the results with the ones obtained by the approximate method. With the approximate method, only a single value for SINnet can be obtained. If INnet is assumed to be 1 l/s/ha, SINnet for 60 ha is 60 l/s, which is about 50% greater than the peak water need (39 l/s for June) calculated above.

The data contained in the foregoing tables and results of the calculations can be summarized on Data Sheet 1 which is given in Annex III.1.

### 3.4.2 Scheme irrigation needs for a multiple cropping system

On many irrigation schemes more than one kind of crop is grown at the same time, and over the course of a year. It is therefore often necessary to calculate irrigation needs for a multiple cropping system. Once the irrigation need for a single cropping pattern has been estimated, the irrigation need for a scheme with more than one crop can be determined by adding up the irrigation requirements for each month. The way this is done is much the same as with the calculation for a simple cropping pattern, following the three steps outlined below.

Step 1: Draw the multiple cropping system diagram, as explained in Section 3.3.

Step 2: Determine the scheme irrigation need for each crop separately, following the steps explained in Section 3.4.1. Use Data Sheet 1.

Step 3: Add up each month's calculated irrigation needs. The total scheme irrigation need should be expressed in litres per second.

Data Sheet 2, included in Annex III.2, is designed to calculate the scheme irrigation need for a multiple cropping system.

 SAMPLE PROBLEM: Determine the scheme irrigation need based on the following assumed data. ASSUMPTIONS: Crop 1 Crop 2 Crop 3 Name: onions cotton potatoes Area: 60 ha 75 ha 30 ha Planting period: 1 Apr-30 Apr 1 Jul-15 Aug 1 Oct-30 Oct Growing period: 150 days 190 days 120 days Climatic data are the same as the data used in the example Section 3.4.1. Scheme area op: 150 ha (see Figure 23) CALCULATION: Step 1: The multiple cropping pattern is drawn in the diagram included in Data Sheet 2 as was done in Figure 23. Note the scale of the vertical axis which indicates that the scheme area is 150 ha. Step 2: The determination of the irrigation needs for the three different crops can be done on Data Sheet 1 (see also Manual 3 Part II Chapter 5). This method was extensively explained in Section 3.4.1, and is thus not repeated here. The results are copied onto Data Sheet 2. Step 3: The net scheme irrigation need is found by simply adding up monthly irrigation needs for each crop. The results of the calculation are shown below in Data Sheet 2. ### 3.4.3 Determining the irrigation need for a rice-based cropping pattern

As was explained in Section 4.4 of Training Manual 3, Part II, the net irrigation needs of paddy rice consist of the crop evapotranspiration, ETrice, minus the effective rainfall, Pe, if any. Moreover, additional irrigation water is needed to compensate for the continuous percolation losses, PERC. At the start of paddy rice cultivation, a certain amount of water must be supplied to saturate the rootzone and prepare the wet soil, called puddling. This amount of water, SAT, is about 200 mm. Finally, because rice grows well only on a submerged field, a water layer, WL, measuring about 50 mm deep must be established on the field.

The formula to determine the net irrigation need of paddy rice is written as follows:

INnet rice = ETrice + SAT + PERC + WL - Pe (mm/month)

In the same way as for other crops, one has to calculate the INnet rice of the earliest and the latest plantings to find the irrigation requirement for a rice crop with a staggered cultivation. The irrigation needs of a second crop after rice during the dry season, beans for instance, are determined as explained in Section 3.4.1.

 SAMPLE PROBLEM: Rice is grown on an irrigated scheme of 150 ha during the wet season. Determine the scheme irrigation water need, including the saturation requirement, SAT, the establishment of a water layer, WL, and the supply needed to make up for percolation, PERC. ASSUMPTIONS: Planting period: 16 June (early) - 1 August (late) Growing period: 130 days Crop factors: Kc Day 0-60 1.1 Mid-season (40 days) 1.15 Last 30 days 1.0 SAT: 200 mm Early 16 May - 15 June Late 1 July - 31 July PERC: 6 mm/day =180 mm/month WL: 100 mm Early 16 June - 15 July Late 1 August - 30 August Climatic data are the same as the data used in 4.4 in Manual 3. CALCULATION: Start with Data Sheet 3 which is a modified version of Data Sheet 7 in Training Manual 3. The blank sample of this sheet is found in Annex III.3. Step 1: Draw the cropping pattern diagram with SAT and WL, and fill ET0. Step 2: Calculate the INe (mm/month) according to the method explained in 3.4. Step 3: Calculate the INl, (mm/month) in the same way as Step 2. Step 4: Calculate the average monthly irrigation need INav (mm/month). INav mm/mon 49 149 146 205 203 296 171 57 Step 5: Determine SINnet (l/s) for each month. SINnet l/s 28 86 84 118 117 171 99 33 Note: When cultivation does not start on the first of a month or when cultivation periods do not coincide with full months, the water requirement should be divided over the two months according to the number of days that fall in each month. For example, in establishing a water layer of 100 mm from 16 June to 15 July, i.e. during 15 days in June and 15 days in July, note that 15/30 x 100 = 50 mm is to be given in June, and 15/30 x 100 = 50 mm in July.

## 3.5 Converting from SINnet to SINop

The preceding sections of the present manual illustrated the calculations of the irrigation need for a simple cropping pattern. In Section 3.4.2, we calculated the net irrigation need for the entire scheme in litres per second, SINnet, by adding up the irrigation needs for each crop. Now, these irrigation needs have to be converted into the gross scheme irrigation need, SINgross, and the operational scheme irrigation need, SINop. This process is explained in section 3.2, and is repeated here with the following four-step process.

Step 1: Estimate the overall irrigation efficiency, e.

e = econveyance x eapplication / 100

Step 2: Determine the gross Scheme Irrigation Need, SINgross

SINgross = SINnet x 100 / e

Step 3: Establish the operational criteria or the total operation time Top.

Top = (d/7) x (h/24)

Where:

d = days of operation per 7 days of the week
h = hours of operation per 24 hours of the day.

Step 4: Determine the operational Scheme Irrigation Need, SINop. SAMPLE PROBLEM: Determine the operational flow for the cropping pattern on page 28. The following data is assumed: SINnetIrrigated area is 150 haConveyance efficiency is 85 %Application efficiency is 60%Hours of irrigation per day: 12Days of irrigation per week: 6 CALCULATION: Step 1: Estimate the scheme irrigation efficiency. e = 85 x 60 /100 » 50 % Step 2: Determine the gross Scheme Irrigation Need SINgross in litres per second for April. SINgross = SINnet x 100 / e = 11 x 100 / 50 » 22 l/s. Calculate the same for the months May, June, July, August, and September in the following schedule. Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec SINgross (l/s) 22 58 78 50 32 30 Step 3: Establish the operational criteria, or the total operation time, Top. Top = days of operation per week times hours of operation per day.Top = (6/7) x (12/24) » 0.43 Step 4: Determine operational scheme irrigation need, SINop, for April. SINop = SINgross / Top = 22 / 0.43 » 51 l/s Calculate the same for the months May, June, July, August, and September in the following schedule. Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec SINop (ls) 51 135 181 116 74 70