Dori is a town in northeastern Burkina Faso, the capital of the department of the same name, in Séno Province and in the Liptako Region. The municipality of Dori covers an area of approximately 2532 km², with a population of 46,521 inhabitants [17]. The Dori site corresponds to the following geographical coordinates: 0˚30 west longitude and 14˚03 north latitude.

The climate of the Sahel is characterized by two seasons: a dry season lasting 8 to 9 months (from November to June) and a wet season lasting 3 to 4 months (from June to September) [18]. The dry season is subdivided into two sub-seasons: dry and cold, and dry and hot. This Sahelian-type climate is hot and dry from March to June, rainy from July to October, and cold and dry from November to February. It is characterized by long periods of drought [19]. During the year, the temperature generally ranges from 16 to 41˚C and is rarely below 13˚C or above 44˚C. Rainfall is irregular and lasts only about 3 months. It is approximately 567 mm/year, with strong evapotranspiration approaching 3 meters [20]. The average daily incident solar radiation in Dori is mainly constant, at around 5.7 kWh/m²/day [21]. Four soil types exist in Dori: soils on aeolian sands, deep clayey soils or eutrophic brown soils, deep alluvial soils, and medium- and shallow-depth soils [22].

The data used in this study were provided by the National Directorate of Meteorology of Burkina Faso [5]. They consist of observational records of air temperature, relative humidity, wind speed, sunshine duration, and rainfall collected at the Dori meteorological station over a 10-year period, from 2004 to 2013. The measurements were recorded daily at 3-hour intervals.

The climatic data entered into the software are the values presented correspond to the monthly averages calculated from the provided data which summarized in Table 1.

Table 1.Climatic data for the Dori site.

It can be seen that in Dori, the highest minimum temperature is observed in May, at 29˚C. The maximum temperature of 41˚C is recorded in April and May. As for relative air humidity, the highest value (99%) is measured in August. The highest wind speed (17 km/day) is obtained in January and February. The highest number of sunshine hours is 10 hours at the Dori site, recorded in July.

The date palm is scientifically known as Phoenix dactylifera. It is a monocotyledonous plant of the Arecaceae family, originally from humid tropical regions [15], but over time some species have adapted to hot, semi-arid, or arid climates. The date palm is a single-stem species composed of a stipe surmounted by a crown of fronds. Figure 1 shows the date palm variety selected in this study.

Figure 1.Photograph of a variety of young date palms.

The root system of the date palm is arranged in bundles of roots, sometimes with branches. Figure 2 shows the root system of the date palm.

Figure 2.Types of date palm roots [2].

The root system of young date palms consists of four types of roots. The respiratory roots extend from 0 to 20 cm below the soil surface; this zone contains pneumatodes, small mealy or wart-like outgrowths that are essential for plant aeration. The nutrient roots are located between 20 and 100 cm depth, where a high density of fine roots is found. The water absorption roots extend from 100 to 200 cm depth and are responsible for absorbing moisture and mineral nutrients. Finally, the anchorage roots penetrate deeper into the soil, sometimes exceeding 6 m depending on soil conditions, and form a dense root network that provides structural support and stability to the palm [2]. The roots may extend up to 25 m around the palm tree and up to 17 m deep if water requirements are not met at the level of the nutritional roots. Root density and depth allow the date palm to withstand water stress and drought conditions.

Water consumption for date palm irrigation varies considerably depending on the palm variety, climatic conditions, seasons, and soil types [23]. The Penman model, based on knowledge of evapotranspiration, is adopted for estimating date palm water requirements.

For the calculation of irrigation water requirements of the date palm at the Dori site, the Cropwat 8.0 software developed by the FAO is used. This software makes it possible to calculate crop water requirements and irrigation requirements from climatic and crop data. These data can be entered directly into Cropwat or imported from other applications. The program also makes it possible to establish irrigation schedules for different management conditions and to calculate the water supply of irrigated perimeters for various cropping patterns [24].

The most important factors for model parameterization are the water flux, consisting of evapotranspiration and precipitation, as well as the crop coefficient of the plant (K_c) [25]. The meteorological data required for model parameterization are air temperature, air humidity, wind speed, precipitation, and evapotranspiration. In addition, Cropwat determines date palm water requirements based on the duration of each phase and the period of the year.

The results displayed by the software include the climatic table, the date palm water requirements table, and the irrigation schedule table. The climatic table gathers all climatic data that can be entered into the software. It should be noted that only one value per month is calculated. The date palm water requirements table includes all crop- and rainfall-related data. In this table, the values of reference evapotranspiration, the proportion of the area occupied by the crop type, and the evolution of K_c as a function of date can also be recorded. The irrigation schedule table gathers all soil characteristics: total available water, readily available water, rainfall, actual evapotranspiration of the plant, etc.

The estimation of irrigation water requirement takes into account date palm evapotranspiration, soil, and rainfall at the cultivation site [26].

2.5.1. Calculation of Date Palm Evapotranspiration

Evapotranspiration is the quantity of water vapor transferred to the atmosphere through transpiration of the date palm via its stomata and through evaporation from the soil surface [27]. For the date palm, transpiration represents 32% of the total received radiation and 53% of the net radiation [28]. Two concepts should be clearly identified in this regard: reference evapotranspiration and maximum evapotranspiration [29].

Reference evapotranspiration corresponds to a characterization of the evaporative power of the atmosphere at a given place and at a specific time of the year and does not take into account crop characteristics or soil factors [27]. There are many empirical formulas for estimating reference evapotranspiration. The Penman-Monteith method is recommended by the FAO as the reference model because of its performance under different climatic conditions [30]. It is given by relation (1):

where: E T 0 is daily reference evapotranspiration, R n is net radiation at the crop surface, G is soil heat flux, T is the mean air temperature measured at 2 m above the ground, U 2 is wind speed at 2 m above the ground, e 0 and e a are the saturated and actual vapor pressures of the air at 2 m above the ground, respectively, Δ is the slope of the vapor pressure curve, and γ is the psychrometric constant.

2.5.2. Date Palm Crop Coefficient

The crop coefficient (K_c) is the ratio of the maximum evapotranspiration of a crop during a given period of its vegetative cycle, that is, under optimal conditions, to the reference evapotranspiration. The crop coefficient is given by expression (2):

where: K c is the date palm crop coefficient, K c b is a crop coefficient, K e is an evaporation coefficient, and K s is a water stress reduction coefficient.

In Burkina Faso, the crop coefficients (K_c) of date palm follow the standard values established for Sahelian conditions and vary according to the four main growth stages of the crop. During the initial growth stage (young palms/establishment phase), the crop coefficient ranges from 0.30 to 0.50 [31]. These average values should be adjusted to account for the hot, dry, and windy climatic conditions prevailing in Burkina Faso.

Maximum evapotranspiration can be estimated from reference evapotranspiration corrected by a coefficient called the crop coefficient at different stages of palm development [15]. Date palm water consumption is given by Equation (3) [23].

where: E T M is maximum evapotranspiration, K c is the date palm crop coefficient, and E T 0 is daily reference evapotranspiration.

The irrigation requirement is the volume of water needed to compensate for the deficit between, on the one hand, potential evaporation and, on the other hand, effective rainfall during the period of palm growth and the change in soil water content. The Dori site is located in a livestock-producing area where sheep and cattle constitute the predominant livestock. The sandy soil is amended with cattle manure, which modifies soil water dynamics, particularly the rate at which water infiltrates into the soil. According to [32], the application of organic amendments (sheep and cattle manure) has a significant effect on most soil properties, especially its water-holding capacity. According [33], the total porosity of the amended sandy soil is 43.7%.

Knowledge of irrigation requirement makes it possible to determine the irrigation dose and frequency by adjusting requirements to local conditions. Water requirements are obtained from a water balance according to relation (4):

Several methods are available in Cropwat for estimating effective rainfall. These include the Fixed Percentage method, which assumes that effective rainfall is a constante fraction of total rainfall (typically about 80%); the FAO Formula method, which is based on a water balance approach using a two-part linear equation according to the amount of rainfall ; and the Rainfall Probability method, a probabilistic approach that is particularly suitable for arid and semi-arid regions [34]. These methods estimate the proportion of rainfall that is effectively stored within the crop root zone and is therefore available for crop use, excluding losses due to surface runoff and deep percolation.

By default, Cropwat applies the empirical method developed by the USDA Soil Conservation Service (USDA-SCS), in which effective rainfall (P_eff) is automatically estimated from the measured total rainfall (P_tot) using two different equations [35].

For monthly rainfall less than or equal to 250 mm, effective rainfall is calculated using Equation (5):

For monthly rainfall greater than 250 mm, effective rainfall is calculated using Equation (6):

An empirical relationship exists between the quantities of water required for surface irrigation and those to be applied in the case of localized irrigation, expressed by Equation (7) [36]:

where: Q 1 is the water volume for localized irrigation, Q g is the water volume for surface irrigation, p is the fraction of soil covered by the plant canopy, and k 0 is the oasis effect coefficient.

According to the FAO-56 methodology, the fraction of soil covered by the canopy (p) during the initial growth stage (young plants) ranges from 0.10 to 0.30, while the oasis coefficient (k₀) typically varies between 1.05 and 1.25. Under the Sahelian climatic conditions and drip irrigation system considered in this study, the canopy cover fraction (p) was set to 0.20, and the oasis effect was incorporated into the adjustment of the crop coefficient (K_c).

The values of solar radiation and reference evapotranspiration are given in Table 2.

Table 2.Solar radiation and evapotranspiration.

The month of June records the highest daily average reference evapotranspiration (ET₀) value (5.55 millimeters per day), with solar radiation of 23.7 MJ/m²/day. The lowest reference evapotranspiration value (1.68 millimeters per day) is obtained in December, with solar radiation of 19.0 MJ/m²/day. It is observed that the daily average values of reference evapotranspiration and solar radiation in November, December, January, and February are very close. These four months constitute the cold dry season of the year. From March onward, the daily average values of reference evapotranspiration increase very rapidly to reach their maximum value in June. During this period, the solar radiation value remains practically constant. The reference evapotranspiration values then decrease to reach their minimum value in December.

Figure 3 shows the evolution of the daily average value of solar radiation and reference evapotranspiration as a function of the months of the year at the Dori site.

This curve shows that monthly reference evapotranspiration in Dori is practically equal to 2 mm for the months of November, December, January, and February. It increases during March, April, May, and June, reaching a maximum of 5.55 mm in June. It then decreases during July, August, September, and October.

As rainfall data are loaded into Cropwat 8.0, effective rainfall is automatically calculated. Table 3 gives the values of rainfall recorded at the site and the values of effective rainfall calculated by Cropwat.

Figure 3.Curve showing the evolution of solar radiation and reference evapotranspiration in Dori.

Table 3.Daily average rainfall at the Dori site.

Figure 4 shows the evolution of rainfall depths and effective rainfall at the Dori site.

It can be observed that the amount of rainfall received has practically the same values as effective rainfall during the months from January to May and in October. From June to September, the gap between rainfall recorded at the site and effective rainfall calculated becomes large. The greatest difference is obtained in August, with a value of 28.3 millimeters/day (Table 3).

Figure 4.Curve showing the evolution of rainfall depths and effective rainfall at the Dori site.

Table 4 gives the values of date palm crop coefficients provided by Cropwat.

Table 4.Date palm crop coefficient values.

The value of the date palm crop coefficients at the Dori site is 0.4 for the months of November, December, January, and February, corresponding to the cool dry season, 0.50 for the months of March, April, May, and June, corresponding to the hot dry season, and 0.45 for the remaining months, corresponding to the rainy season. These crop coefficient values are consistent because this is the initial stage of date palm cultivation.

The values of date palm water consumption, irrigation requirement, and the quantity of water to be supplied at the Dori site for a one-hectare plantation comprising 120 date palms are summarized in Table 5.

Table 5.Water consumption and water requirements of date palms.

Figure 5 shows the evolution of date palm water requirements at the Dori site.

Figure 5.Curve showing the evolution of date palm water requirements.

According to the results in Table 5, water consumption increases in the same way as reference evapotranspiration. This increase is observed particularly during the period from March to September. During this period, the date palm becomes active, and this period coincides with the hot season. Total date palm water consumption at the Dori site exceeds 2559.936 millimeters per year. The lowest monthly irrigation requirement occurs in December, at 82.656 millimeters per month. It increases from January to June and reaches its maximum value in June, with a requirement of 297.025 millimeters of water. Thereafter, the irrigation requirement decreases to reach its minimum value of 82.656 millimeters per month in December. The quantity of water to be supplied to date palms increases in the same way as the irrigation requirements. The quantity of water to be supplied to date palms reaches 2343.09 millimeters per year.

Table 6 gives the monthly quantities of water to be pumped for drip irrigation of one (1) hectare of date palms.

Table 6.Quantities of water to be pumped.

The minimum quantity of water to be pumped at the Dori site occurs in December, with a value of 95.70 mm/month. The highest value is recorded in June, with a value of 343.92 mm/month. The average quantity of water to be pumped is 214.783 mm/month. The quantity of water to be pumped increases from January to June, when it reaches its maximum value. It then decreases to reach its minimum value in December. The total quantity of water to be pumped is 23,196 cubic meters per year. The quantity of water to be pumped is rather high, since the Dori site is located in a Sahelian zone where evapotranspiration is very high.