Feasibility of water desalination for irrigation: the case of the coastal irrigated area of Dyiar-Al-Hujjej, Tunisia

The Mediterranean region is vulnerable to the consequences of climate change. According to the Inter-governmental Panel on Climate Change (Eckstein et al. 2019), North Africa is considered a vulnerable area to climate risks worldwide. For Tunisia, by 2050, average temperature could increase between 2 °C and 2.3 °C and precipitation could decrease by 1 to 14%. It becomes strategic for Tunisia to maintain food production for sustainable socio-economic development.

Tunisia is an arid country in two-thirds of its territory. Average annual rainfall is about 200 mm and around 15% of the working population works in agriculture (Chebbi et al. 2019). Tunisia suffers from structural droughts; 25% of Tunisian arable lands are saline (Hamrouni & Daghari 2010). The main source of soil degradation is water irrigation and sodium chloride is the most prevalent salt in Tunisian water resources (Slama 2004).

All these constraints have affected the irrigation sector. Indeed, irrigated areas represent only 10% of arable lands. Despite of these constraints, the irrigated sector accounts for over 35% of national agricultural production while the world average is about only 20%. Thereby, the role of irrigation in Tunisian development is certain but the main constraint is the lack of fresh water. Only 30% of mobilized water resources in Tunisia have salinity less than 2.3 dS/m.

For example, in Tunisian oases, the driving force of development for all of southern Tunisia, the average yield of irrigated date palms is 4.6 tonnes/ha. In Egypt, where these oases are also present in the arid zone, 36,000 hectares (El-Juhany 2010) produce 1.47 million tonnes of dates (Zafar 2020). This represents 41 tonnes/ha, or 9 times the yield in Tunisia. The date palm in Egypt is irrigated with water from the Nile.

In Sidi Bouzid governorate, economically based solely on irrigated agricultural activity, the aquifer is overexploited and we must expect that this aquifer will be exhausted within a few years, while many other aquifers with an electrical conductivity (EC) of more than 8 dS/m are available in the area. In this rural region, no other alternative for economic development is possible apart from agriculture.

This is confirmed by the allocated water volume for a Tunisian which does not exceed 500 m³/capita/year, far below the Food and Agriculture Organization (FAO) standard which is a minimum of 1,000 m³/capita/year.

Another big problem is dam water transfer in Tunisia. All the coastal areas are supplied by state networks either for irrigation, drinking water, industry or tourism by transferring water from other very distant regions localized in the interior of the country (even over a distance of more than 300 km). All these transfer systems are saturated today and water shortages have multiplied in recent years. After the Arab Spring which started in 2011, the local populations living in the regions where water is produced contest this transfer and they ask that this water be used for local development. These people live with money sent by family members working in other regions or abroad. Protests, brutal breakage of pipes and vandalized drilling have become common in these regions. By contrast, the coastal regions observe a high economic development (rural exodus). The rural population is forced to migrate to these coastal areas in search of employment leaving their families on the spot or living in popular neighborhoods with very high population densities. Also, several aquifers currently used for this transfer are overused. Tunisia no longer has sites where we can build dams with a capacity exceeding 100 million m³. The regions sheltering these saltwater resources are inhabited by peasants who have no source of living.

The search for sustainable agricultural development for all these regions with the potential for saltwater, drainage water, wastewater, or seawater is essential in Tunisia. Desalination can be an alternative for freshwater production, especially since Tunisia has great potential in desalination. Tunisia has commendable experience in the desalination of seawater and groundwater. Desalination of water started in the 1980s (Elfil 2018) to improve the quality of drinking water in all urban agglomerations in the south and in islands with a daily production of 210,000 m³ of drinking water produced throughout the country via 16 desalination plants. A further 3 large seawater desalination plants will be added to the existing stations for the citizen of Sousse (50,000 m³ to 100,000 m³ per day), Zarat (50,000 m³ to 100,000 m³ per day), and Sfax (100,000 m³ to 200,000 m³ per day). Indeed, policymakers in Tunisia are aware of the desalination potential offered by the Mediterranean Sea over 1,300 km of coastal shore.

Desalination plants have enormous potential to provide freshwater for irrigation in Tunisia. Indeed, many studies of water desalination costs appear regularly to promote this option for water supply intended for drinking water or irrigation.

Tunisia has experienced several field studies of desalinated water in irrigation. In the ‘fifth season’ private project in southern Tunisia, where cherry tomato is grown under greenhouse, desalinated water has been blended with saltwater for more than 20 years. The selling price of cherry tomato is 0.5 US $/kg at least and a great part of the production is intended for export.

According to Mhiri (2014), a 1.2 ha farm occupied with olive trees in southern Tunisia irrigated by reverse osmosis desalinated water using electrical energy saw its gross margin evolving from 57 US $/ha under rainfed conditions to 3,676, 3,869 and 3,819 US $/ha respectively for the years 2011/2012, 2012/2013 and 2013/2014 following a supplementary irrigation with desalinated water.

A reverse osmosis desalination project was realized in the Gounat area in Mahdia governorate in Tunisia to irrigate 3.5 ha (75 greenhouses with vegetable crops) during 2016. The farmer's profit should double at least (Ergaieg et al. 2018).

In our study area, a widespread salinization of the groundwater which was the main source of irrigation has been observed following a sea intrusion. The average sodium adsorption ratio (SAR) was 8.6^0.5 (meq L⁻¹) and the average EC was 6.6 dS/m (Mekni 2017). Thus, there was an abandonment of irrigated areas that had become unsuitable for agricultural production-land which was previously considered as the best tomato production area in Tunisia with the main processing factories. Some farms and wells were abandoned because of sea intrusion and general salinization. Following overexploitation, salinity levels of 19 dS/m were measured in some irrigation wells. To save this irrigated area considered vital for Tunisian agriculture, the Tunisian state carried out an expensive transfer (with many pumping plants) of fresh water over a distance exceeding 100 km from another watershed. But in recent years, this operation of transferring fresh water has been disputed by the populations of the area where the transferred water comes from. Furthermore, this transfer is not regular. While during the years 2015 and 2016, the volumes of water transferred were 1,714,603 and 1,714,492 m³, this volume was only 800,000 m³ in 2017. During the year 2018, no transfer of water was completed due to the lack of rains, which weakened the agricultural system and limited farmers’ choices.

The farmers adopted localized irrigation and blended the transferred fresh water and the aquifer saltwater, which made it possible to partially save this area. All farms very close to the sea were abandoned definitively because the salinity of the aquifer was very high. In addition to this operation of freshwater and saltwater blending, farmers' adaptation to this new situation has been observed by:

• the adoption of crop rotation by the application of successions between irrigated crops, rainfed crops and fallow by all farmers in order to reduce the soil salinity (Bani et al. 2020);

• the introduction of new and more profitable crops such as strawberries following this supply of fresh water and the practice of associated crops on the same site (strawberry-pepper combination) (Daghari et al. 2020a);

• the adoption of winter and spring crops with the aim of reducing water supply and taking advantage of leaching of salts by the rains.

The objective of this paper is to evaluate the economic feasibility of the use of desalinated seawater to irrigate this area. Several scenarios have been studied: (i) if all the irrigation water comes from a desalination plant, (ii) if the food-industry development of this region is exploited, which will allow an increase in the selling prices of agricultural products and (iii) if a blending between desalinated water and groundwater will be practiced (in reproduction of the current situation) except that the supply of desalinated water will become regular and safe.

In our work, to consider integrating a desalination plant as a matter of urgency to save an agricultural area threatened by salinity requires various information (Figure 1) concerning the specific considerations of the study site and also concerning the water supply of the area.

The irrigated area of Dyiar-Al-Hujjej, in Tunisia, the subject of our study, plays an important role in the production of tomato and pepper. The main tomato and pepper processing factories in the country are located in the peninsula of Cap Bon (Nabeul, Korba, Dyiar-Al-Hujjej, Menzel–Temime, Grombalia). It is also the main productive strawberry-growing area in Tunisia. This area has experienced seawater intrusion and aquifer salinization has been observed. Dyiar-Al-Hujjej has a coastal irrigated area of 800 hectares located inside the Delegation of Korba northwest part of Tunisia (Figure 2) having as latitude and longitude 36,617 N and 10,817 E, respectively. The selected location belongs to the Mediterranean semi-arid floor with warm winter. The average annual rainfall is 441 mm and it is very irregularly distributed with most rains from September until April and no rain during all the period May-August when the main crops (tomato and pepper) are grown, while the evapotranspiration (ETP) is about 1,167 mm (Table 1).

The irrigation water is currently managed by a farmers’ association called ‘Izdihar’. The selling price of one cubic meter of fresh water to farmers is around 0.05 US $; these are only expenses corresponding to the operating costs for the irrigation management. All mobilization costs (dam, pipes, and pumping stations) as well as all major repairs (broken pipes, maintenance of pumping stations) are not taken into account in the calculation of the sale price of irrigation water to farmers. Within this area, the farmers blend fresh water (60%) with salty pumped water (40%) by themselves, with a pumping cost of about 0.02 US $/m³.

To determine the percentage of desalinated water to be blended with saline water, the principle of mass conservation will be applied.

where:

• C_s: saline aquifer water salinity concentration (ppm)

• C_d: desalinated water salinity concentration (ppm)

• C: blended water salinity concentration (ppm)

• V_s: saline aquifer water volume (m³)

• V_d: desalinated water volume (m³)

• V: blended water volume (m³)

Yield (ton/ha), selling price ($/ton) and production cost ($/ha) are taken according to the Tunisian Ministry of Agriculture database and farmers associations of different irrigated areas in the region and verified by our surveys. In the production cost, the price of water is not included. The selling price of the agricultural products considered is the selling price at the farm level (MAREP 2019a).

CROPWAT 8.0 for Windows is a free downloadable computer program used to calculate potential evapotranspiration using the Penman-Monteith formula, the crop water requirements and the quantity of water essential for irrigation, taking into account the characteristics of the soil, crops, and data collected on the dominant climate for the crops cultivated in the study (Onyancha et al. 2017).

Current water desalination technologies are classified into two categories, according to the principle applied (Cabrera-Reina et al. 2019): (i) the processes using membranes called reverse osmosis (RO), (ii) thermal processes involving a phase change multi-effect distillation (MED).

RO and MED are technologies whose performance has been proven for water desalination. Indeed, these two processes are the most widespread in the world desalination market.

In our study, the desalination technique chosen is desalination by reverse osmosis which is a process for separating water and dissolved salts using semi-permeable membranes under the action of pressure. As the experience of Tunisia in the use RO goes back several years, this technique is well mastered. Recent studies have shown that it is more profitable to use Photovoltaic (PV) + RO technology which cheaper and cleaner by comparison to thermal desalination technologies. The Emirate of Abu Dhabi paid in excess of US $100 million in 2012 due to its choice of multi-stage flash (MSF) and MED over RO to fuel expenses for seawater desalination (Kaya et al. 2019).

The RO process operates at room temperature and does not involve a phase change. The polymer membranes used allow water molecules to pass through and do not allow particles, dissolved salts and organic molecules to pass through (Figure 3).

The irrigation water is currently managed by a farmers’ association called ‘Izdihar’ (prosperity). The selling price of one cubic meter of fresh water to farmers is around 0.05 US $ and is an average price for all Tunisian irrigated areas. These are only fees corresponding to the operating expenses. All mobilization costs (dam, pipes) as well as all major repairs (broken pipes, maintenance of pumping stations) are not taken into account in the calculation of the selling price of irrigation water to farmers. The saltwater pumping cost is 0.02 US $/m³. Within this area, the farmers blend fresh water (60%) with salty pumped water (40%) by themselves in concrete tanks or by injection directly in wells if no tank is available; the quantities of fresh water are insufficient to meet the total water needs of crops throughout the area. The prices of the desalinated water and brackish water were respectively 0.55 and 0.19 US $/m³ in the Campo de Níjar region (Aznar et al. 2017). Other costs must be considered such as those as a result of high boron amounts contained in sea water which can damage crops; its elimination costs at least 0.05–0.07 US $/m³ for a large system (Hilal et al. 2011).

The desalination can be used for water supply and it can improve the food security and the economic prosperity of these peasants. The expansion of irrigation in Tunisia will stabilize yields and develop several other regions with no other economic activity. Kang et al. (2015) concluded that it is significant to develop irrigated agriculture to mitigate climate change effects and yield variation. For the whole world, whose population will increase to 10 billion by 2050, the expansion of irrigation is imperative.

In the reverse osmosis solar desalination project in the Gounat area in the Mahdia governorate in Tunisia, the additional energy costs generated by the electromechanical equipment of the desalination plant induces a total cost, including the pumping from drilling and the discharge to the blending tank, of the order of 0.12 US $/m³ without taking into account the initial investment cost of US $140,000 taxes not included. The salty aquifer water price sold by the state is about 0.05 US $/m³ in this irrigated area (Dhahbi 2016).

The average solar desalinated water cost covering all expenses was 0.42 US $/m³ for a small private exploitation using aquifer water in a farm planted with olive trees in southern Tunisia irrigated by reverse osmosis desalinated water (Mhiri 2018). In our case, the use of groundwater is forbidden because the region already suffers from a sea intrusion which should not be accentuated. In a study done for all of south-eastern Spain, the cost of desalinated saltwater varies from 0.55 to 0.74 US $/m³ (Martínez-Alvarez et al. 2017). The barrier for desalinated water use is price (Aznar et al. 2017). According to SONEDE (Tunisian National Agency of Potable Water), dealing with the production and supply of drinking water throughout Tunisia and whose experience in reverse osmosis desalination dates back to the 1980s, energy accounts for 40% of the cost of desalination using electrical energy. The average desalination cost of the production of one potable cubic meter is about 0.5 US $/m³ for large desalination plants; investment cost is about 0.3 US $/m³ (Kamel 2017). In Tunisia, for all dams, irrigation networks, pumping and desalination plants, the state tries to make stakeholders pay only the operating costs and it fails. A seawater desalination cost equal to 0.5 US $/m³ will be retained for this study.

Before analyzing the feasibility of seawater desalination's added value in the irrigated area of Dyiar-Al-Hujjej, an assessment of the current situation was carried out.

The crust has a depth less than 20 cm sometimes, which limits the growing to only vegetable crops. The soil is inherently low in organic matter and high in sand (clay: 10%, silt: 36%, sand: 55%). Manure is always added.

Currently, only surface water is supplied to farmers and blended with salt water from the aquifer. The average well depth is about 20 m measured in 2011 and in 2019.

Following our field visits, we noted a 100% localized irrigation adoption ratio which was only 5% before the freshwater transfer in 1998. This water transfer operation allowed a resumption of agricultural activity. The network is in good condition with 90% irrigation uniformity (Daghari et al. 2020a). The area originally developed was 950 ha (Table 2). Many small owners do not exploit their farms regularly due to their unprofitable income. Some farms with high salinity are left fallow for a whole year while waiting for the salts to be washed off in the rain. The size of the farms is divided into two classes: 0–4 and 4–10 ha each occupying 435 ha with 185 farmers and 365 ha with 60 farmers, respectively. The intensification rate is 136 and 125%, respectively, for small and medium farms. The area actually exploited is around 750 ha. It is an irrigated area equipped with a large number of hydrants, which allows good proximity to farms and easier coordination between farmers. The irrigated area is divided into 29 districts. The number of hydrants is 260 for 621 subscribers. One hydrant serves about 3.5 hectares while the average in Tunisia is 1 hydrant for 5 hectares. The same hydrant is used by a small number of irrigators with an average of 2.4 subscribers per hydrant. Farmers highlight the commendable services of the farmers’ association. Irrigated agriculture was the only source of income in this region, and this is an activity passed on from one generation to the next. Water management is on demand and dispatchers (‘aiguadiers’) are responsible for opening and closing the irrigation valves.

Having an irrigation network in good condition is an advantage for our study because for this particular area, we may not include the cost of the irrigation network in the final desalinated cubic meter cost. Even if this irrigation network will be redone in a few years, the desalination operation will be well advanced and the cost of a new network can be borne by the farmers themselves in the event of significant net income.

For the year 2019, the percentage of land use observed is 23, 15, 17 and 16% respectively for the main crops: tomato, potato, pepper and strawberry (Table 3). The effective irrigated area varies from one year to another, according to the surface water volume supplied. At the beginning of the agricultural season (in September), the extension service indicates to farmers the approximate volumes of water available for the different seasons, and with the farmers, plans the areas to be reserved by each crop. The area occupied by tomato in this irrigated area decreased from 450 to 210 ha between 1998 and 2019 while a net spread of strawberry is observed. Strawberry is a crop with high added value and whose growth has become possible thanks to transferred fresh water and it is cultivated during the rainy period, so part of its water needs is met by very good quality rainwater.

Strawberry plants are imported with high prices. Transplanting of strawberry is done in late September, early October, and harvesting is done until late May. Strawberry is grown in association with pepper during two successive years. Strawberry, being very sensitive to salinity, is grown during the rainy season between September and May while pepper, which is less sensitive to salinity, is cultivated during the dry season between May and August. Farmers are very aware of the problem of salinity. In recent years, if heavy rain does not fall during the month of September to leach the soil salts, strawberry and pepper have been removed after one year and the land left fallow or used to cultivate rainfed or another winter crop resistant to salinity. An increasing salinity was observed when strawberry and pepper are grown in association in two consecutive years; measured soil EC was 1.1, 3.3 and 5.52 dS/m on July 2011, August 2012 and August 2013, respectively (Daghari et al. 2020a).

Crop rotation is systematic in this area and it is a successful key in the management of soil salinity. Electrical conductivities measured under irrigated tomato and fallow were respectively 5.6 and 1.4 dS/m (Daghari et al. 2020a). On the other hand, the areas occupied by rainfed crops have increased from 55 ha in 1998 to 209 ha in 2019. Tunisia is going through a drought cycle and the transferred amounts of water have been very disturbed in recent years. The dam from which freshwater is transferred has observed a very low filling rate in recent years. Note that during December 2019, January 2020 and February 2020, the rains were almost zero while this period is normally rainy. During 2016, fresh water supply was stopped from May because of general drought in Tunisia and only drinking water was satisfied. The arboreal sector received just 1/3 of its needs to preserve trees.

Crop water requirements were calculated using CROPWAT software for monthly average values of minimum and maximum temperature, relative humidity, wind speed and sunshine duration and solar radiation. The rain considered is the average monthly rain. The crop coefficients are taken according to Allen et al. (1998).

Average total water requirements calculated by CROPWAT model for this area are equal to 2,532,021 m³/year (Table 4) while the transferred fresh water volume is about only 1,660,321 m³ for the year 2019. The deficit, i.e. 871,700 m³, is pumped from the saline aquifer. The amounts of water distributed differ from one year to another depending mainly on the availability of surface water. The farmers' association estimate that generally the contribution of groundwater and surface water are 40 and 60%, respectively. The farmers' association provides insufficient fresh water (1.6 dS/m) amounts to be used for irrigation after blending with the aquifer saltwater (6 dS/m). After examination of the archives of the farmers’ association, these quantities were 1,569,467 and 1,700,603 m³, respectively, during the years 1998 and 2008, ten years later. The most important consumptions are recorded during May-June, July and August corresponding to the period of full growth of the main crops (strawberry, tomato and pepper). During the dry period (May, June, July and August), fresh surface water is allocated primarily to drinking water and the irrigated sector is only partially satisfied. For these 4 months, theoretical crop water requirement calculated by CROPWAT is 2,086,771 m³ while the amount of surface water supplied is only 1,169,377 m³ (about 50%). The farmers use aquifer saltwater to irrigate only tomato resistant to salinity but they accept low yields; pepper is not marketable, the pod is small. The reduction in yields as a function of salinity was 50 and 40% respectively for tomato and for pepper for an EC of irrigation water slightly exceeding 4.5 dS/m (CRUESI 1970). The farmers know that the rains in September wash away all the soil salt, especially since it is a light and thin soil. On the other hand, the quantities of surface water distributed to farmers during the months of January and February far exceed theoretical needs. During this rainy period, when transferred fresh water is available, the farmers over-irrigate to leach out the salts. So, when in May, the farmers are repeating tomatoes and peppers, the soil will have low salinity. Thereafter, the salinity will increase because saltwater is blended with fresh water wish is much lower than the crop water requirement.

In Tunisia, the average yields of tomato and pepper are respectively 70 and 18 t/ha while they are not more than 50 and 12 t/ha in our study area, due to excessive irrigation water salinity according to the questionnaires asked of farmers. When desalinated water is used, Tunisian average yields will be considered (MAREP 2019b). The strawberry grown only in this region during the rainy season has a good yield of about 70 t/ha. Potato is grown during the wet season also when fresh water is available; an average national yield of 22 t/ha will be considered.

Value of agricultural products and gross margin were calculated by using Equations (5) and (6). Except for strawberry, the gross margin calculated is very low and even negative for pepper (Table 5). The farmers cultivate pepper only in association with strawberry. The pepper benefits from the residual fertilizers left by the strawberry but it also benefits from an unsalted soil support since strawberry is irrigated only with fresh water and is grown during the rainy seasons of autumn, winter and spring (about 400 mm of rainfall) detailed in Table 1. When desalinated water is used, benefit is about 15,000 US $/ha for tomato and watermelon grown under greenhouses in Spain (Reca et al. 2018).

In other regions of Tunisia when fresh water is used, the tomato yield can reach 100 t/ha and a profit of 4,192 US $/ha can be reached.

Based on crops percentage occupation (23, 15, 17 and 16% respectively for tomato, potato, pepper and strawberry) observed in 2019 and as the size of the farms is divided into two classes 0–4 ha and 4–10 ha, the average gross margin is 4,300 US $ and 15,000 US $, respectively, for average farms of 2 and 7 ha for these two classes. These incomes are low especially for the first class which counts 75% of the farmers with average families of 5 to 6 people. Several smallholders have abandoned their land or are renting it out to other farmers and are looking for work elsewhere. Even those who stay on their land practice other activities in parallel which leaves them unavailable entirely for their farm.

During the dry years, when the amounts of water transferred are reduced, the strawberry crop occupies a very small area and even the crops supporting salinity are cultivated in proportions not exceeding 60%, and the farmers see their incomes fall unfortunately. This weakens the system and also explains the non-use of the entire equipped area. If the amount of water transferred is insufficient, priority is given to potable water by the Tunisian government. In our study area, for the dry agricultural season 2016–2017, water transfer for irrigation was stopped because of lack of rain for all the dry season. Only 0.8 × 10⁶ m³ was supplied during 2016/2017 while this amount is about 1.7 × 10⁶ m³ for a normal year. In Tunisia, the state hopes to settle these populations on their farms. It relies heavily on agriculture for the reduction of unemployment especially since agriculture is the main economic sector which can be the engine of development of all regions of Tunisia. Tunisia is very close to Europe in order to be able to export mainly early vegetables. Neighboring countries have demand for these agricultural products. Agriculture is the largest employer in Tunisia (more than 15% of the working population). So, every effort should be made to extend irrigation to other areas even when only salt water is available; can desalination be a solution? Alternative options such as desalination of seawater or wastewater are increasingly appearing as possible solutions in Tunisia to be a source of water for irrigation.

As the problem of sea intrusion is observed and aquifer salinity has continued to increase, an analysis of the scenario using only desalinated seawater deserves to be studied. In the Canary Islands Archipelago (Spain), seawater is used for desalination because an over-exploitation of coastal groundwater and a sea intrusion were observed after several years of operation (Monterrey-Viña et al. 2020).

The irrigation water amounts will be equal to the crop water requirements during the dry season given the absence of rain, while for the rainy season we will subtract the actual rain (equal to 70% of the average recorded rain). Crop growing during the rainy season is an advantage when desalinated water is used; low amounts of desalinated water will be supplied and therefore at lower cost.

Normally, in a project where desalination is introduced, we must seek maximum profitability and we must exploit the entire area of 800 ha. The double objective is to focus on specialized production with high added value such as strawberry, tomato, potato, pepper.

But depending on many surveys with farmers and extension service, in the case of available desalinated water, they are very clear:

• all non-profitable irrigated crops will be abandoned, such as marrow, cabbage;

• only high value-added crops will be irrigated: pepper, potato, tomato and strawberry;

• it was considered useful that 25% of the total area, i.e. 200 ha, would be kept fallow or used to cultivate rainfed crops (barley, etc.) for the feeding of their animals. These 200 ha will give farmers some flexibility to manage crop rotation for the entire area. If desalinated water is available enough, supplementary irrigation can be done for these rainfed crops.

Fallow and rainfed crops must be kept to manage crop rotation due to its positive effect on crops nutrition, soil biological activity and to encourage the introduction of small breeding. Crop rotation breaks the cycle of harmful organisms affecting crops by restricting pathogens and weeds (Leteinturier et al. 2007).

They suggest also, that even if the strawberry is very profitable, it must not exceed 25% of the total area, about 200 ha, for several reasons. The cost of producing strawberries is quite high (6,000 US $/ha) compared to tomatoes (1,808 US $/ha) and other crops grown in the region (detailed in Table 5). The strawberry market is limited compared to tomatoes, peppers and potatoes. The strawberry damages quickly in 2 to 3 days. Also, if desalination is spreading to other regions, several other farmers who belong to other areas in the region will practice strawberries and we can witness a decline in prices unless a strawberry processing industry and an export policy develops.

On the other hand, for tomato, potato and pepper, the Tunisian market is very demanding in these products. Tunisian cuisine uses tomato paste in all meals. Demand for export to neighbouring countries is also high. For the potato, Tunisia imports quantities many times every year. Tomato and potato can be grown on 400 ha. The tomato is currently grown between May and August. This region is home to more than 2/3 of the tomato processing plants in Tunisia. The potato is grown between November and March taking advantage of the fall and winter rains. The cropping intensity is 200% (Table 6).

The strawberry being very sensitive to salinity, must be cultivated following a rainfed crop or fallow (Table 7). Only a single plot of potato-tomato will be occupied by the same crops two successive years. But for these 200 ha, tomato ends in August and potato is planted in November; in addition, it is a rainy period which promotes the leaching of salts.

Given the high prices and high yields obtained with greenhouse crops, the use of greenhouse farming will be analyzed under desalination, especially given that the surrounding areas are known for greenhouse crops. For crops grown in a greenhouse, during the rainy season, the irrigation water requirements are taken equal to the crop water requirements because these crops will not benefit from the rain given the presence of greenhouses and the rain does not reach crops and soils. Crop water requirement under a greenhouse is equal approximately to 70% of crop water requirements in open-field. In recent years, experiments with movable greenhouses allowing the rains to arrive inside the greenhouses are being conducted but they are very limited.

Regarding the selling price, tomatoes and peppers grown in greenhouses on small areas observe a high price compared to their prices when these crops are grown during the dry season as open-field crops in many regions in Tunisia and when abundant production is observed. The transformation of pepper and tomato into concentrate is only done during the abundance of production. During winter, both for pepper and tomato, they are consumed in Tunisian cuisine in fresh form or exported.

The case of potato is different. It is consumed continuously throughout the year. To stabilize prices, the state often uses potato imports throughout the year. The potato is grown in the open-field. We will retain the extra-early crop which grows between December and March when only few areas are favourable for growing this crop in Tunisia and when the rains are present, which reduces the irrigation water amounts. However, seasonal cultivation between March and June requires more irrigation water because the months of May and June are dry in Tunisia. As an example, water requirements of open-field extra-early potato cropped in the wet season and open-field potato grown in the dry season are respectively 1,345 and 4,000 mm.

Strawberry is cultivated only during the rainy season so a good part of the water needs will be satisfied by the rain and the selling price is quite high because it is rare and can only be cultivated in regions where very fresh water is present. More than 95% of the Tunisian strawberry production is grown in the irrigated area of Dyiar-Al-Hujjej and in nearby irrigated areas.

In calculating the final net revenue, two situations will be analyzed for all these crops (i) case when the cost of the irrigation network is not payable by farmers and (ii) case when the cost of the irrigation network is due to farmers.

In Tunisia, the irrigation network is carried out by the state and this cost is not charged to farmers. The state does not seek to recover the initial investment and expects social and economic development that will generate wealth and taxes for the state. All major repairs such as broken pipes, breakdowns in pumping stations, and cleaning of the drainage network are always the responsibility of the state. In our irrigated area, the irrigation network already exists and it is in good condition.

For all open-field crops grown in dry season having a high-water demand, where all water needs have to be met by desalinated water, the use of desalination water is not profitable; their net revenues are negative (−2,457, −1567 and −842 US $ respectively for pepper, potato and tomato) (Table 8). The price of desalinated water is a barrier for use (Aznar et al. 2017). The use of desalinated water for irrigation is profitable for (i) crops grown during the rainy season (extra-early potato in wet season and strawberry) when a part of its water requirement is satisfied by rain, and (ii) greenhouse crops where both yields and sale prices are higher compared to the same dry season crops and there is a positive income, 7,413 US $/ha and 1,657 U$/ha respectively for tomato and pepper. But when crops are grown under greenhouse, farmers cannot manage really more than 2,000 m² (about 4 greenhouses or less by family); it requires a lot of care.

For the strawberry cultivated during the rainy season, the irrigation water amounts supplied are low (3,600 m³/ha) and the sale price is high, and it presents the highest net income. The strawberry cannot be generalized everywhere in Tunisia even if fresh water is present for several reasons including its production cost that is very high compared to other crops; this cost is beyond the reach of the majority of Tunisian farmers.

Already and without taking into account other production costs, for open-field pepper, the solar desalination cost (3,000 US $/ha) by itself is higher than the value of agricultural products (1,944 US $/ha). They are almost equal for open-field potato (Table 8). In this calculated net income, the cost of the irrigation network was not covered. But for a complete vision, the feasibility of desalination in the case when the costs of this network will be charged to farmers also deserves to be analyzed.

Any seasonal crops that are unprofitable when the cost of the irrigation system is not taken into account will be not considered because they will be more unprofitable. Only greenhouse crops (tomato and pepper), extra-early potato and strawberry will be considered.

In Tunisia, the cost of setting up an irrigation network for all the irrigated area is 7,000 US $/ha for a lifespan of approximately 25 years with an interest rate of 5% given the state incentives for agriculture in Tunisia, an additional amount of 948 US $/ha/year should be expected as a minimum.

To calculate the net revenue taking into account all the charges (water cost, production cost, network cost and irrigation equipment), the amount 948 US $ will be deducted from the income obtained without taking into account the irrigation network cost.

Taking into account the cost of the irrigation network, only crops with a significant income either because the rain contributes to their water needs (strawberries) or because they have a high yield and high selling price, have a positive final net income. For tomato and pepper under greenhouses, their selling prices and yields are superior compared to those of open-field seasonal crops (Table 9); selling price can even reach 5 times the price during the dry season. Open-field extra-early potato grown in wet season observes negative net revenue mainly because of low selling price that does not change during all the year in Tunisia.

In southeast Spain, net margin with 1.95 and 3.39 US $/m³, is obtained under greenhouse for pepper and tomato respectively. Intermediate net margins of 1.34, 0.82, 0.70 and 0.60 US $/m³ are observed in the case of watermelon, melon, peach and apricot crops respectively (Ministerio de Agricultura, Alimentación y Medio Ambiente, 2016). In our case, net margins are low and they are 0.17 and 1.43 US $/m³ respectively for pepper and tomato. The selling prices are low in Tunisia. Chaibi & Bourouni (2007), by analyzing the financial balance sheet of a desalination application for the irrigation of greenhouse crops, found that this balance sheet is positive only for crops with high added value (floriculture). In the Sahel region in Tunisia, extra-early pepper and tomato are grown under greenhouse with drinking water with a price of 0.4 US $/m³. Yields and selling prices are high because of small quantities of tomato and pepper available in the Tunisian market during winter and spring. When desalinated water is blended with aquifer saltwater, profit of the farmers growing greenhouse tomato and pepper has been doubled in the Gounat irrigated area (Ergaieg et al. 2018).

Pepper, potato and tomato grown during the summer dry season as open-field crops show a net negative income with current yields and prices if desalinated water is used. For most open-field crops, there would be zero revenue but we can expect an improvement in crop yields when desalinated water with low salinity is supplied. For hydro-ponic pepper, the yield obtained was 100,000 kg/ha (López-Marín et al. 2017) while it is less than 20,000 kg/ha in our area. The banana highest yield observed with fresh water (1.5 dS/m) was obtained in the case of desalinated water (0.3 dS/m) with almost half the amount of water (Silber et al. 2015). We must also look simultaneously for other ways such as agro-industry.

The agro-transformation concerns only the open-field season tomato and pepper grown between May and August and when an over-production is observed. When these crops are cultivated as early-crops under greenhouses, the quantities produced are low and the selling prices are high. For potato, no agro-industry is available in Tunisia and the selling price is constant during all the year.

In terms of agro-industry, this region has an important advantage: it is the main region of Tunisia where we proceed to the transformation of tomato and pepper into concentrate and harissa and it is the hub of the majority of processing plants. The agro-industry (tomato and pepper concentrate) can also be a way of developing desalinated water. We can expect an increase in the selling price for food processing, by at least 50% more, compared to the selling prices for everyday consumption.

Besides the agro-industry, the use of desalinated water will improve crop yield. In areas where fresh water is used, tomato yields easily exceed 100 tons/ha. An increase in water productivity with desalinated water was observed for all four staple crops; a statistically significant yield increase was observed for sorghum (+10%), (Ghermandi et al. 2013). Reduction in yield of 23% and of only 7% were observed respectively when reclaimed water and desalinated water are used to irrigate almond under deficit irrigation by comparison to full irrigation (Vivaldi et al. 2019).

For pepper, even for a 50% increase in yields and selling prices, the net revenue is negative (Table 10). This crop is to be excluded from irrigation with desalinated water. On the other hand, for tomatoes, even a 10% increase in selling prices and yields results in almost zero net income. An increase of 25% in both selling prices and yields results in a net income of 1,521 US $/ha. An increase in yield of 25% is possible especially since tomatoes are a well-controlled crop and high yields are achievable. For the 25% increase in prices, negotiations with the state, agro-industrialists and the farmers’ association are necessary because the demand for tomatoes is high both for processing and for export. High value-added crops show positive profit justifying the use of desalination for agriculture (Kaner et al. 2017).

We can also think of introducing other crops with a high added value such as condiments or medicinal plants which have a high added value but the Tunisian experience in this field remains very limited. Unfortunately for the farmers, the crust is present everywhere in this area even at a depth of 20 cm, preventing any tree cultivation.

In order to reduce the cost of irrigation water, the scenario consisting of a mixture between desalinated water and aquifer saltwater deserves to be studied, especially since the blending of groundwater and fresh surface water is currently practiced. The blending of desalinated water and other types of water is usually done by farmers with the aim to improve the final water quality, according to a farmers survey (Monterrey-Viña et al. 2020). The use of waste water for irrigation can led to an accumulation of chloride, sodium, and boron which damage soils, cause phytotoxicity to crops and reduce yields (Maestre-Valero et al. 2019). The water blending of different water sources with better quality is recommended for a sustainable irrigation, after Maestre-Valero et al. (2019). The research also points out that soil degradation can be reduced and that the qualities of irrigation water can be improved when blending desalinated water with groundwater or wastewater. A similar growth of Buxus and Pistacia was observed when well water and blended water were used for irrigation (Gori et al. 2008). In the case of water blending, the interval between the de-salinated and saline water intake must be reduced. A complete water blending before irrigation is recommended. The longer the interval between salt water inflow and freshwater inflow, the higher the observed salinity peak (Daghari et al. 2020b).

As this aquifer is quite salty and a marine intrusion is observed, the pumping of water from the aquifer must be very limited. Only the main field crops (peppers, potatoes and tomatoes) grown during the dry season will be taken into account. Also, when a negative net income is observed if only desalinated water is used, it will be taken into account. Analysis will focus on the net income obtained when desalinated and saline water are mixed for these crops. In this case, the desalinated water supply is regular throughout the year, in contrast to the transferred surface water whose supply is very irregular. The average salinity of salt aquifer water and desalinated water is respectively 6 and 0.5 dS/m. The following percentages will be considered:

• 100% desalinated water

• 25% of desalinated water and 75% of saltwater

• 50% of desalinated water and 50% of saltwater

• 75% of desalinated water and 25% of saltwater

• only aquifer saltwater

The cost of pumping groundwater is 0.02 US $/m³. The total pumping cost is almost 129 US $/ha even if the entire crop water requirement is pumped, negligible compared to water desalination costs. The highest crop water requirement is observed in the case of tomato (Table 8). Average tomato, pepper and potato normal yields are respectively 70 tons/ha, 18 tons/ha and 22 t/ha (average Tunisian yields).

Crops yield will vary with water blended salinities and the accepted yield is the product of average yield and percentage of yield which depends on water salinity and is calculated by using Equations (4) and (5).

Even if a blending of desalinated water and saltwater is done, the net revenue is again negative whatever the percentage of mixture, due mainly to yield decrease and/or high cost of desalinated water (Table 11). In Tunisia, the cost of irrigation water never exceeds 25% of other production costs in irrigated areas using water from the state network. For private areas using pumped water, this percentage is less than 10%. When blended water is used, the cost of desalinated water reaches 3,234, 2,425 and 1,617 US $/ha representing almost 200, 150 and 100% of the other production costs respectively for desalinated water percentages of 100, 75 and 50% (Table 11). In the case when the percentage of desalinated water is less than 25%, the final salinity is high because the salinity of the aquifer is already high (6 dS/m) and a drop in yield of at least 50% is observed. The value of agriculture production barely covers the other production costs (Table 11). Low or negative incomes of −368, 22 and −172 US $/ha were observed when fresh water, desalted seawater and mixed water, respectively, were used to irrigate mandarins during Oct-2017 to Sept-2018 (Maestre-Valero et al. 2020).

In fact, the tomato is currently profitable when fresh surface water and saltwater are blended, thanks to the low selling price of water (0.05 US $/m³) (Table 5). A study in progress on the water sector in Tunisia (Water 2050) indicates an overall economic cost for surface water of about 0.55 US $/m³. Irrigation and drinking water are heavily subsidized in Tunisia.

The net revenue is also negative for pepper even if a blending of desalinated water and saltwater is done (Table 12). Blending between desalinated water and saltwater results in catastrophic net negative returns of −2,457, −1,929 and −1,401 US $/ha for the respective percentages of desalinated water of 100, 50 and 0% (Table 12).

Compared to tomatoes, its yield per hectare is much lower. The yield drops by half if the percentage of desalinated water is less than 50%. The value of agriculture production is generally low and it covers production costs only if the percentage of desalinated water is close to 75% or more. For 50%, the value of agriculture production is 972 US $/ha while the production cost is already 1,401 US $/ha without taking into account water costs (750 US $/ha). Pepper is cultivated only in association with strawberry.

For the open-field potato grown during the dry season, net revenue is also negative whatever the percentage of desalinated water used. If the desalinated water percentage is less than 75%, the value of agriculture production does not cover even the production cost, not considering the water cost (Table 13). The desalinated water cost is very high compared to the current network water cost. If only desalinated water is used, the water cost is 2,000 US $/ha while the current cost is only 200 US $/ha (0.05 US $/ha * 4,000 m³/ha), i.e. 10%.

Full dry season crops for which all of their water needs must be satisfied by irrigation water are to be excluded from this desalinated water use in irrigation; they have net negative income (Table 14). The blending of desalinated and salted water is unprofitable. High cost of desalinated water limits its use in crop irrigation (Martínez-Alvarez et al. 2016). The best economic feasibility due to the higher yield in proportion to the increase in the cost of water is observed in the case of treatment using mixed water compared to the two treatments using fresh water or desalinated seawater when young mandarins were irrigated (Maestre-Valero et al. 2020). In our case, the only problem posed is the supply of fresh surface water is not regular; it varies from year to year depending on the amount of rainfall.

For agricultural products with current prices considered low in Tunisia, desalination should be excluded. It may be necessary to apply it to the arboricultural sector, but in the form of supplementary irrigation. The gross margin of olive trees increased from US $ 57/ha in rainfed conditions to US $ 676/ha, US $ 3,869/ha and US $ 3,819/ha respectively for the years 2011/2012, 2012/2013 and 2013/2014 following a supplementary irrigation with desalinated water (Mhiri 2014).

Martínez-Alvarez et al. (2019) indicated that compared to other water resources, desalinated water requires high energy needs and associated costs which limit its widespread use for agriculture. In the case of corn and pepper, a negative benefit is observed whatever the salinity of the mixed water and the volume of water supplied. For grapes and dates, if the mixed water irrigation volume is less than 1,000 m³/ha, negative profits are observed if desalinated water and saltwater are blended (Kaner et al. 2017). More energy is required when desalinated water is used in irrigation due to the ionic concentration standards required for agricultural irrigation water. The mean net margin of farmers was about zero when desalinated seawater was used in many regions in South East Spain. The farmers ask to have simultaneously desalinated seawater and other kinds of water with lower prices. A blending can be done leading to an affordable final water price (Kumar et al. 2018). In Tunisia, currently, the blending of desalinated water and groundwater is applied to crops with high selling prices such as greenhouse crops, crops intended for export (cherry tomato, condiment plants, etc.). The selling price of one kilogram of cherry tomato is at least three times the cost of 1 m³ of desalinated water and a water productivity of more than 3 kg/m³ is reached. Several foreign promoters come to settle in Tunisia and cultivate crops with high added value mixing desalinated water and saltwater.

Tunisia has poor quality water resources. With the increase in population and standard of living, use of poor-quality water is very common in Tunisia. Unfortunately, after about twenty years of use of these waters, drops in yields are observed, and a drop in groundwater levels and sea intrusion are recorded. The desalination of water for irrigation which consumes more than 80% of Tunisian water resources is starting to emerge but the cost of desalinated water constitutes the main handicap to its use for irrigation, especially since the selling prices of agricultural products are very low in Tunisia.

The purpose of seawater desalination to save the irrigated area of Dyiar-Al-Hujjej has shown its limits. The use of desalination for irrigation is profitable only for crops with high added value and whose water needs are partially met by rain. Only the strawberry crop whose water needs is partially satisfied by rain and whose net income is high is of interest when desalinated water is used. But this crop cannot be generalized given the limited market.

For the dry season crops (pepper, potato and tomato), constituting the main activity of the irrigated area of Dyiar-Al-Hujjej and where all their water requirements must be met by irrigation water, the desalination application is not recommended. For these crops, even if a mixture of desalinated water and aquifer saltwater is practiced, they remain unprofitable given the drop in yield observed in the case of the water blending.

For greenhouse crops (pepper and tomato), the use of desalinated water is beneficial because of the high selling price given the areas equipped with greenhouses are very low. Irrigation of greenhouse crops with desalinated water is already practiced but in very few regions of Tunisia.

Even in the case of agro-industrial development and higher yields, irrigation with desalinated water is not profitable for open-field crops except for tomatoes.

If water desalination is used for irrigation in the Dyiar-Al-Hujjej region, the recommended development scheme will be: (i) the use of strawberries and greenhouse crops in the wet season; (ii) the use of field irrigated tomatoes with blended water in the dry season. The acceptance of desalinated water by farmers, the effects on soils and on crops must be considered and analyzed in the future if desalinated water is used. Solar and wind power could be alternative sources of energy, especially in this region that is known for its strong wind speeds and high irradiation and costs of desalination will decline over the years.

All relevant data are included in the paper or its Supplementary Information.