Cultivating Sustainable Green Belts with ADW and RWH in Iraq's Arid Zones

2.1 The study limitations

The soil salinity is within the acceptable values. The rainfall depth in the study area is within the isohyet lines for Iraq that are expressed in Figure 2. The interpolation of isohyet map for the area around the MOD is about 120 mm as an average. There is no significant variation in MOD quality and quantity, the annual average discharge is 150 m³ s^-1, the salinity average is 4.4 dS m^-1.

2.2 Irrigation water requirements

To sustain the irrigated area around the MOD path, in this study, the irrigation periods was taken from (15/4 to 15/9), so the average discharge for these months were taken to estimate the validity of irrigation water quantities. In other period from (15/9-15/4), the rainwater harvesting technique is used to irrigate orchards, if there are low quantities of rainfall in any year, other MOD water will be discharged to cover the water requirements in that time.

The design discharge of MOD syphon at Theyqar is about 200 m³ s^-1. As there are no sufficient available data about the stream flow, the average MOD discharge is assumed to be 75% of the design flow, and recommend to not use greater than 50% of this discharge. Table 1 explains crop water requirements for the selected trees (palm and olives) data were taken from Ministry of Water Resource for the year 2019 [19].

2.jpg

Figure 2. Map of isohyet lines

Table 1. Net irrigation depth in (mm)

Total water requirements depth from 15 April to 15 September: Palm = 1.21 m, Olive = 0.88 m.

Q_average = 75% of design discharge of the siphon structure:

Qaverage $=200 * \frac{75}{100}=150 \mathrm{~m}^3 / \mathrm{s}$            (1)

$A_{i=} V_{M O D} / D$          (2)

where,

A_i = Irrigated area;

D = depth of irrigation water requirements.

2.3 Rainfall harvesting

The average rainfall depth in the MOD watershed can be forecasted by using the isohyet line map (Figure 2). The map of isohyet lines shows that the study region is located between lines 100 mm and 150 mm. The annual rainfall depth in this watershed is about 120 mm [20].

Figure 3 explains the negarim shape of the rainwater harvesting method. The area of the negarim estimated as an area can collect a sufficient rainwater during rainfall season. This area depends on annual rainfall depth. The annual rainfall depth in this watershed is about 120 mm. Although the greatest challenge is Iraq, one of the five countries that is the most affected by climate change in Asia, according to United Nation recommendations, but in general, the average annual rainfall can be considered as a constant in quantity but fluctuating as annual depth [21].

Considering the distance between the trees is 6 m, the area of the planting unit is 36 m². As for the area achieved using rainwater harvesting, it reached 267 m² for palms and 164 m² for olives, so the cultivated area using MOD water is as follows:

The water harvesting in arid areas means the collection of rainwater from a wide area named as Catchment Area (CTA), to convey it in a small area that surrounds the irrigated tree named as Cultivated Area (CA) (see Figure 3 and Figure 4).

3.png

Figure 3. Negarim micro-catchments for tree crops [22, 23]

4.png

Figure 4. CAT and CA

$\frac{C T A}{C A}=\frac{I_d-R f_d}{R F_d * R O_c * A c_c}$              (3)

where,

$I_d$: Irrigation water depth;

$R f_d$: Rainfall depth;

$R O_c$: Runoff coefficient;

$A c_c$: Activity coefficient.

To calculate the CA for palm and/or olive, the planted area assumed to be a circle of 2 m diameter, so CA was calculated as 3.14 m². As we consider that the runoff coefficient is 0.3 and the activity coefficient is 0.5, the catchment unit area can be calculated according to the following equation [21]. The irrigated area under rainwater harvesting technique is estimated according to Eq. (4).

$A_h=\frac{C T A}{U A} * A_a$               (4)

where,

$A_h$: The area irrigated by rainwater harvesting;

$A_a$: The actual irrigated area without harvesting;

$U A$: The square area of four adjacent trees in the orchard, it is assumed to be 6×6 m² in this study.

2.4 Forecasting soil salinity

To forecast the steady state condition of the soil salinity after using MOD water in irrigation, the WATSUIT program was applied. Table 2 explains the salinity of ADW in the study area (MOD) for many years of the period 1994-2021.

Table 2. EC, cations and anions for the MOD in Abu-Ghraib for period 1994-2021 [22-24]

The amount of anions and cations in MOD is shown in Table 4. The annual rainfall depth is about 120 mm, which is about 10% of the water requirements of olive and palm trees. Assuming that the salinity of rainwater is zero, so the average water salinity is 90% of the salinity of MOD water according to the salt balance equation [17]. The anions, cations leaching fraction, and amendments are the WATSUIT program inputs [26].

Table 4. Cations and anions in MOD stream (me L^-1)

Applying the WATSUIT program using this data as input, the face of the model input data is shown in Figure 5, the result of the salinity distribution in the soil column as a quarter of root zone depth is seen in Figure 6. The values of average EC and the EC of the upper root zone half of all LF scenarios contained in the WATSUIT model (0.05, 0.1, 0.2, 0.3 and 0.4) are listed in Table 5. These LF value were proposed according to the water salt balance equations in order to mitigate the soil water against the salinity effects of irrigation water, as the irrigation water salinity increased, the required leaching (LF) also increased. The LF must be not exceed 40% of crop water requirement [17]. The results explain that the increase of LF levels decreases EC values very little in the half of the upper root zone, but notable reductions were observed in the average EC values. The trend of EC decreasing is started very sharp, (from 20.19 to 14.77 dS m^-1 in first step) then be smoothly decreasing as shown in the last step (from 9.81 to 7.78 dS m^-1). These results were consistent with the findings of previously studied researchers of optimal leaching fraction values [27]. The rainwater reduces the salinity effects of MOD water as the average annual rainfall depth (about 10% of crop water requirements) [28]. That’s why the rainfall was assumed as the leaching value (0.1). This study recommend to use LF value of 0.1 because the steady state condition of the soil salinity is within the acceptable values recommended to the palm and olive trees salinity tolerance, it is also suitable for high tolerance winter crops like wheat and barley.

As the salinity of these areas is high, so the irrigation using moderate saline water and rainwater harvesting may decrease soil salinity values. We recommend using this technique to expand orchards in this region [29].

The farmers, engineers and decision-makers have to observe the weather conditions, especially annual rainfall depth, humidity and evaporation losses. An advanced infrastructure such as an advanced weather station network must be designed and applied in this region [30].

Table 5 provides a summary of the WATSUIT model’s primary input and output parameters.

Table 5. The effect of leaching fraction (LF) ratios on EC values

5.png

Figure 5. Input data (selecting LF=0.1 in this stage)

6.png

Figure 6. Result of WATSUIT, LF=0.1

The authors recommend dividing the proposed green belt area into ten locations, 52.5 km for each one, and planting as a five stages in five years, planting 10.5 km every year.

Planting some areas among the palm and olive orchards by high tolerance crops as barley, and irrigate it by river water to maintain the soil salinity within the acceptable range for planting crops.