CHAPTER VII: DATE PALM IRRIGATION

By P.J. Liebenberg and A. Zaid
Date Production Support Programme

This chapter describes date palm irrigation and aims to calculate water requirements of this species as well as schedule irrigation to ensure that the date palm gets the necessary quantity of water when needed.

1. Introduction

Like any other fruit tree, date palm needs suffi cient water of acceptable quality to reach its potential yield. In Table 48 quantities of water made available to date palm around the world can be seen. It is worth mentioning that all these countries use fl ood irrigation, except for Israel, which uses drip irrigation.

Table 48
Date palm irrigation around the world

Table 49 shows differences in summer and winter requirements in Tunisia. Summer water requirements (July, August and September) are about 7,154 m³/ha, while only 4,372 m³/ha are needed for the winter period (December, January and February). Summer requirements are almost double the winter ones and constitute ¹/₃ of the total annual consumption. Note these values are made available to the trees through fl ood irrigation.

Differences in water requirements between different regions of the same country are common as illustrated in the case of Algeria (Table 50). The date growing area of the Sahara needs approximately 34,190 m³/ha/year,while Ziran region needs only 15,000.

Table 49
Water quantity consumed per ha of Deglet Nour date palm at Tozeur (Tunisia)

Table 50
Approximate water requirements of date palm at different regions of Algeria

2. Factors infl uencing water requirements

It is necessary to take certain aspects into consideration in order to calculate the volume of water required by a palm. The following aspects play a major role in this calculation:

a. Soil salinity: If the soil is saline, more water must be given to enable a leaching process for clearing the salt from the soil.

b. Temperature: The higher the temperature, the higher the rate of evaporation and the more water the plant needs.

c. Humidity: The lower the humidity level, the more water needed.

d. Wind (speed and occurrence): Higher constant wind speeds cause higher evaporation and thus higher water demands.

e. Cloud cover: More water is required during periods of less cloud cover.

It is worth mentioning that all above factors infl uence evapotranspiration, which strongly determines the water requirements.

Irrigation

Irrigation is the timely application of water to a crop in need of water. Any water applied when not necessary, is a waste of a precious commodity. For example: if water is applied too late in the season, then it is useless because the crop is already dead or the production suffered so much that there will be no fruit, even though defi cient water is then applied over the growing period. The opposite is also true; if too much water is applied, the plant may also suffer. The crop may die due to waterlogging. Usually date palms do not suffer from too much water although, as illustrated, i t is possible in uncontrolled fl ow from artesian wells at Qatif, Saudi Arabia (Dowson, 1982). It, will however, still be waste of water, as the farmer could use this water to irrigate other palms or crops.

Irrigation must take place where the roots of the plant can easily reach it. It is of no use to the plant if water is applied where the roots cannot reach it. Let us look at the root development of a date palm tree. If the soil is divided into four layers of equal depth from top to bottom, 40 % of all roots can be found in the top layer, 30 % in the second layer, 20 % in the third layer and the remaining 10 % in the last layer. The same percentages apply in concentric rings around the plant (Figure 62). The same percentage of water will also be extracted from the soil in the different layers due to the presence of the roots in these respective layers.

For mature date palms, the depth is about 5 m,and 3 m radius around the trunk. Thus, it is seen that for dates 40 % of all water is extracted from the first 50 cm, 70 % is from the first 100 cm, 90 % is from the top 150 cm and only 10 % is from the last layer or 150 to 200 cm and deeper. For young date plantlets this depth can vary from 25 to 50 cm and the radius from 10 to 30 cm, depending on the size of the plant. This means that the irrigation water must be applied within these boundaries to enable the plant to reach it. However, it is important to apply water be applied in such a way that it does not reach the deeper soil levels in order to ensure proper root development of the date palms.

Localised irrigation (e.g. drip and micro) will therefore be more effi cient than non- localised one (e.g. fl ood irrigation).

After planting small tissue culture-derived date palms, the volume of soil from which it can extract water is very small. If a person is not careful, suffi cient water may be applied, but not enough will be available to the plant for optimum growth. It is thus necessary to ensure that enough water reaches the area where the roots are. Irrigation must preferably be done by basin, micro or drip methods. Due to the shallow root depth at this stage, frequent irrigation is also necessary to ensure that the palm does not suffer from water deficiency. Even more care should be given if the palm is planted in a very sandy soil.

3. Different irrigation techniques

Different irrigation techniques are available to irrigate crops, but not all of them are suitable for date palm irrigation. The following methods are of importance and each has its own advantages and disadvantages:

a. Flood irrigation

This irrigation method is the oldest method known, and is also the method most widely used in date palm culture. It has, however, advantages as well as disadvantages which are outlined below:

i. Advantages:

(1) running costs are low;
(2) easy to apply; and
(3) initial costs are low if the area is fairly flat.

ii. Disadvantages:

(1) diffi cult to achieve a high effi ciency rate;
(2) labour intensive;
(3) irrigates areas in between where no palms are planted; and
(4) not well suited for sandy soils.

b. Furrow and basin irrigation

It is basically a redesign of fl ood irrigation to eliminate some of the disadvantages listed above and thus make it more effi cient.

i. Advantages:

(1) running costs are low;
(2) easy to apply; and
(3) initial costs are low if the area is fairly flat,

ii. Disadvantages:

(1) labour intensive; and
(2) interferes with mechanical operations.

c. Sprinkler irrigation

This is the oldest modern irrigation method and was introduced to enhance effi ciency and to enable automation.

i. Advantages:

(1) more effi cient use of water is possible;
(2) easy to schedule - manage;
(3) less labour is needed; and
(4) tpography is not a limitation.

ii. Disadvantages:

(1) expensive (installation);

(2) running costs are high;

(3) heavily influenced by wind and temperature (spray pattern and evaporation);

(4) not well suited for small palms because water can enter from above into the growth point of the palm.

d. Micro irrigation

This method was more recently introduced and was developed in South Africa to irrigate mine dumps to prevent the wind from blowing the sand away. It was then adapted for irrigation of trees and other crops.

i. Advantages:

(1) more effi cient use of water is possible;

(2) running costs are lower than sprinkler irrigation (lower pressure needed);

(3) easy to schedule - manage;

(4) only areas that need water are irrigated;

(5) topography is not a limitation;

(6) It is easy to automate;

(7) It is not labour intensive; and

(8) several spray patterns are available to suit date palms (e.g. gaps in the spray pattern so as not to wet the growth point or the trunk of the palm.)

ii. Disadvantages:

(1) Installation costs are high;
(2) needs clean water; and
(3) infl uenced by wind and temperature (spray pattern and evaporation).

e. Drip irrigation

This is the latest irrigation method introduced and was developed in Israel where there is scarcity of water (Figure 62).

i. Advantages:

(1) more effi cient use of water;
(2) running costs are low;
(3) easy to schedule/manage;
(4) topography is not a limitation;
(5) only the water needed by the palm is applied;
(6) not infl uenced by wind;
(7) easy to automate; and
(8) not labour intensive.

ii. Disadvantages:

(1) expensive (Installation);

(2) requires very clean water; and

(3) sometimes difficult to determine if the correct amount of water has been applied by the system, and when it becomes clear that it is too little, it may be too late.

4. Methods for calculating date palm water requirements

From the earliest times, different methods were used to calculate the water requirements of different crops. As a result, numerous methods have been developed and adopted for date palms. Some of these methods are more accurate than others and some more convenient to use than others, because of the availability of information for the site where the date trees will be planted. The following are a few of the methods available:

- Evapotranspiration/Class A Pan Method;
- Penman's Equation;
- Blaney-Criddle Equation; and
- Solomon and Kodama's Equation.

a. ETP Class A Pan

In Israel, USA and Southern Africa, the evapotranspiration/Class A Pan Method is frequently used because the needed information, is readily available.

Where:

AWR = Amount of water required during period under observation.
ET_pan = Evaporation for period in mm as measured with Class A Pan.
CF_pan = Crop Factor for that period.
h = Efficiency of irrigation system (in decimal).

Table 51 shows in more detail the calculations done to forecast water requirements of the palms for the 12 months of the year and using different irrigation methods for Naute - Namibia. (Note that this is for the Southern Hemisphere harvesting period is March to April)

b. Revised Penman-Monteith Method

The Penman method is widely accepted as the most accurate method of calculating water requirements for crops. This method makes use of daily climatic information (e.g. maximum and minimum temperatures, wind velocity, humidity and radiation per day) to calculate the reference evaporation ETo. Due to the relative complexity of the formula, it is best used with the help of a computer program. The reference crop evaporation (Eto) is first determined and then the water requirement is calculated using the following formula:

Where:

kc = Crop Factor
Et_o = Reference Evaporation mm/day
Et_crop = Crop Evapotranspiration mm/day

ET_crop = kc * ET₀ [mm/day]

In Tables 52, 53 and 54, calculations done with Cropwat 7 can be seen. Cropwat 7 is a computer programme based on the revised Penman-Monteith method, to calculate crop water requrements (Smith, 1992)

TABLE 51
Water requirements for date palm at Naute, Namibia

Flood Irrigation ® h = 60%
Micro Irrigation ® h = 85%
Drip Irrigation ® h = 90%
i - This is an estimate according to some desk study by the authors of this chapter - 1989.
ii - Use this crop factor only with class A evaporation pan fi gures.

TABLE 52
Monthly reference evapotranspiration (revised Penman Montheith)

TABLE 53
Crop data

TABLE 54
Crop evapotranspiration and irrigation requirements

From tables 51 & 54 it is clear that the date palms at Naute (Namibia) need between 2,157 and 2,419 mm Nett irrigation per annum to fulfi l their needs.

5. Leaching

As mentioned earlier, the date palm needs suffi cient water of acceptable quality to enable it to reach its full yield potential. To reach this aim, if all agricultural practices are catered for, (except water), then the average electric conductivity of the soil (ECe) must not exceed 4 dS/m (Ayers and Westcot, 1985), and that of the water (Ecw) not 2.7 dS/m. If situations occur where these values are exceeded then leaching must be practised to overcome this problem. However, due to the scarcity of water or the high cost of water, it will not always be viable to meet the leaching requirements. In such a case it may be viable to opt for a lower yield which may be more economical. In Table 55, ECe and ECw values corresponding to % of yield for date palm are shown.

TABLE 55
ECe and ECw values corresponding to yield percentage

However, to calculate the quantity of water needed for leaching, the following formula is used:

Where:

LR = Leaching Requirement (fraction).
Ecw = Electric conductivity of the water (dS/m).
Ece = Electric conductivity of the soil at % yield to be obtained (dS/m).

This quantity of water is over and above the nett irrigation required by the crop during the season. The total annual requirement is then calculated from the following formula:

Where:

AW = Depth of water supply (mm/yr).
ET = Total annual water demand (mm/yr).
LR = Leaching requirement.

6. Scheduling

Once it is known how much water to apply, it is also important to know when to apply it. To determine this, knowledge of the type of soil and how deep it is, is required. This gives an indication of how much water is in the soil and how much is available for the palm. This information, combined with the daily usage of water by the palm, enables the determination of when the next irrigation cycle is due.

The following fi gures are mean values of available water for the three major soil types:

The best approach is to determine, through laboratory tests, the water holding capacity of the specifi c soil under consideration and then to establish an effective scheduling program.

To ensure that the palm will not be put under water stress, it is the normal practice to allow for only a fraction of the available water to be extracted. For date palm, as illustrated below, this fraction equals 0.4 or 40 % of the available soil water.

EXAMPLE

The water usage of date palm for a certain period is 8.7 mm/day. Table 56 shows that the available water for the soil is 140 mm/m depth. The rooting depth of a full grown date palm is 2 m. Thus:

In Tables 57 and 58, an example of a fi xed scheduling programme can be seen for date palm at Naute (Namibia) as done by Cropwat 7. For this example, note that no rainfall is taken into consideration.

TABLE 56
Soil data

TABLE 57
Irrigation scheduling

CROPWAT 7.0 (The information in the last column is only valid for fl ood irrigation.)

TABLE 58
Water requirement using cropWat 7

7. Layout of date palm orchard and irrigation

The spacing between date palms differs worldwide. This can be ascribed to differences in variety as well as climatic conditions. In Namibia, the trend is to a 10 × 8 m spacing, 10 m between rows and 8 m in the rows. Some private farmers also use a 8 × 8 m spacing but, it is not advisable to use a narrower spacing.

The usage of micro irrigation is recommended due to the sandy soils where date palm is commonly grown, and the efficiency of this type of irrigation. Care should however be taken that no water is sprayed into the crown of the small palm. To this effect, micro's with a 300° - 320° spray pattern should be used. Furthermore, to optimise the efficient usage of water it was decided to change the type of micro's during the initial growing period of the date palm to ensure 100 % coverage of the drip area (rooting area). As stated before, due to shallower rooting in the first years of development, a more frequent irrigation schedule is recommended during these years than in the later ones. From planting to year (4) the area covered is about 12 m² and the flow rate 96 l/h/palm, from year (5) to year (10) the area covered = 18 m² and the flow rate 104 l/h/palm and from year ten the area covered = 28 m² and the flow rate 156 l/h/palm (Figure 63). This bigger area covered in the initial years (0 -3 and 5 - 8) will lead to waste of water, but on the other hand it will serve as a leaching operation that will benefit the date palm as a whole. Due to shallower rooting in the first years of development a more frequent irrigation schedule is required in those years

Figure 62. Drip area of adult date palm tree and root distribution

Figure 63. Wetting pattern of Micro's