Advances in Crop Science and Technology
Open Access

Crop water Requirements; Irrigation Efficiency; Irrigation scheduling; Climate change; Cropping pattern

Introduction

The UAE lies in the southeastern part of the Arabian Peninsula between latitudes 22° 40’ and 26° 00’ N, and longitudes 51° 00’ and 56° 00’ E. It is located within the arid region, with very high summer temperature and less than 100 mm annual average rainfall. Abu Dhabi Emirate is the largest Emirate in the UAE, accounting for approximately 87% of its total area. Abu Dhabi has two mains Agroecological zones; (a) the coastal areas have a hot and humid climate in the summer season and mild winter. (b)The interior southern desert region has hot summers with temperatures rising to about 50°C and cool [1].

The United Arab Emirates (UAE) is among the most water-scarce countries in the world. This water challenge involves different factors including the severe scarcity of groundwater reserves, groundwater salinization, high cost of desalinated water for domestic and agriculture use, limited re-use of treated water [2]. Additionally, UAE has one of the highest per capita water usages of globally. The average per capita water consumption rate in the UAE is 500 Liters per person per day which is more than double the world national average of 180 liters per person per day [3].Furthermore, rapid population growth, socioeconomic development, and the increase in the living standards had led to dramatic increase on water demands for different uses [4].

On the other hand, agriculture in the UAE totally depends on irrigation and consumes around two-thirds of water consumption in the country. The irrigational water uses for the three main crop types: date palm and fruit trees, field crops, and vegetable crops. Domestic and industrial demands are covered by desalinated water, whereas agriculture demands are met mainly by groundwater [5].

So securing a safe and sustainable water supply in Abu Dhabi consider a challenge to fulfill the Agriculture, domestic and industrial needs. Accordingly a range of essential strategic initiatives are already being adapted in Abu Dhabi continuous pursuit in order to increase the irrigation efficiency and narrow the gap between the agricultural water supply and the need without affecting the agricultural food production.

Efficient water resources use requires a good development, and operating system with considering the vegetation pattern of the region. Accurate estimation or determining the actual crop water requirement and amount of irrigation water for the plants in the crop pattern enable to develop suitable irrigation programs and can also prevent irrigationinduced yield reductions. The main purpose of irrigation is to ensure stability in actual crop water requirement and to deliver sufficient moisture in the root zone of the crops. Since water is an important input for crop production, it should be well-planned and distributed effectively. The expected benefit from irrigation can be increased and irrigation efficiencies can be improved with the appropriate modifications of conventional irrigation systems [6].

Under these circumstances, it is important to adopt water saving strategy in agriculture, which intends to raise the water utilization rates and water use efficiency resulting in higher economic yield on irrigated farm land with a minimum input of water. It is comprehensive exercise using every drop of water for lint production including the use of natural precipitation as well as efficient management of irrigation network through a suitable irrigation method. A better understanding of the complicated relations between climate, water and crop growth needs to be a priority area in the UAE. Here comes the importance of developing the scientific crop water requirements for enhancing efficient irrigation scheduling, water balance, canal design capacities, regional drainage, water resources planning, reservoir operation studies, and to assess the potential for crop production

FAO Penman-Monteith method [7]is used for determining reference crop evapotranspiration (ET0) and provide values that are very consistent with actual crop water use data worldwide [8-10]. The irrigation schedule recommendations for various crops should be location-specific, considering the soil types and agro-ecological conditions. The right estimation of the crop evapotranspiration (Etc) is essential in the management and improvement of water resources. The scarcity of freshwater resources or the mismanagement of water in agriculture has made the effective and valuable use of water even more vital.

Water resources should be run at an optimum level in terms of irrigation parameters. The calculation of crop water requirement is very important in local balancing of water resource and correct planning and operation of water resources, determination of water demands that may occur in the future and maintaining water supply–demand balance. crop water requirement values vary depending on climate, planting area, planting time, soil type [11] and can be estimated by using direct or indirect measurement methods and meteorological data. The direct measuring method of Etc is often complicated and expensive. Therefore, the potential evapotranspiration can be calculated using the Penman-Monteith equation in the windows-based CROPWAT 8.0 software developed by Food and Agriculture Organization (FAO) of the World. Factor crop coefficient is used to calculate actual from potential evapotranspiration. In addition, estimations of irrigation water requirements and yield reduces in response to critical reductions in irrigation water allow improvement of irrigation programs [12] The irrigation water requirement for different crop patterns can be calculated with this model (using soil, plant and climate data), and reliable estimates can be carried out.

The FAO-CROPWAT model has been effectively used in many parts of the world for various crop patterns under irrigated conditions [13].The CROPWAT 8.0 software has been widely used in different geographical regions and climatic conditions to calculate crop evapotranspiration and irrigation water requirements vegetables [14]. Sustainable agricultural development is based on a balanced framework for developing and implementing suitable farming systems and good agricultural practices. Increasing production efficiency through modern techniques and combining high value crops and animal wealth are essential. Since the agricultural sector is one of the UAE largest water consumers, and this is a reflection of the management of agricultural lands and soil treatment. And as CropWat considered as ideal software due to reliable results reported in studies conducted in semi-arid climatic conditions, without causing a decrease in yield. This review article is aiming to provide irrigation requirements as well as the irrigation scheduling, by determining the actual crop water requirements the most common open field crop grown in Abu Dhabi Emirate, to offer the Abu Dhabi agriculture sector as estimated tool to guide towards environmentally sustainable consumption of water

Materials and Method

This study is carried out in Abu Dhabi Emirate, UAE, the three main agro climatic regions were considered in this study: Al Ain, Al Dhafra and Ab Dhabi. For calculating a daily potential evapotranspiration (ET0), a long term (around 30 years) climatic parameters were analyzed for estimating the monthly averages and calculating the ET0 using FAO Penman Monteith equation. Some of the collected data were provided from the weather stations. Other parameters are related to commonly measured data and can be derived with the help of direct or empirical equations. As soon as the ET0 has been determined, a crop coefficient approach was applied to adjust ETo value for the type of crop being irrigated. The crop coefficient takes into account the crop type and crop growth development stage to adjust the ETo for corresponding crop. Crop coefficients values were obtained from the documented tabulated records for many different types of crops developed by FAO.

Use of a single crop coefficient combines soil evaporation and crop transpiration into one value, Kc.

A survey for the most common grown vegetables with their possible growing season in Abu Dhabi has been done and used to calculate the seasonal needed irrigation requirements. Some of those vegetables’ crops are growing in one season and others are growing in two seasons per year.

Results and Discussion

Climatic Indicator

Abu Dhabi has a hot arid climate. The temperature varies from 13°C to 43°C and is rarely below 10°C or above 46°C, August is the hottest month. The warm season started from May 15 till September 26 with an average daily high temperature above 40°C. While the cold season lasts from December 4 to March 4 with an average daily high temperature below 26°C and January if the coldest month. Relative humidity usually ranges from 20% (dry) to 90% (very humid) over the year, rarely dropping below 9% (very dry) and reaching as high as 100% (very humid).

The precipitation varies throughout the year with variation in the amount, intensity and frequency. The highest Precipitation amount occur in December with almost no rainfall likely occur during the summer months. Wind speeds vary from 2.5 m/s to 7.8 m/s (light air to moderate breeze), rarely exceeding 10 m/s (fresh breeze). The day length varies over the year, with shortest day is December 21st with 10:37 hours of daylight; the longest day is June 20 with 13:39 hours of daylight. Sunshine and blue skies can be expected almost every day in Abu Dhabi. The city experiences a really hot and humid climate in the months from April to September when the maximum temperatures average above 40 °C (104 °F). During this period, unpredictable sandstorms occur in the city and sometimes visibility gets down to a few meters.

A long-term average for the climatic parameters is shown in tables 1,2 and 3 for Abu Dhabi, Alain and Al Dhafra regions.

Table 1: summarizes the geographical and climatic parameters for Abu Dhabi Metrological Station average monthly measurement between 1980 to 2016.

Table 2: summarizes the geographical and climatic parameters for Alain Metrological Station average monthly measurement between 1980 to 2016.

Table 3: summarizes the geographical and climatic parameters for Al Dafra Metrological Station average monthly measurement between 1980 to 2016.

Potential Evapotranspiration in Abu Dhabi Agroclimatic Zones

The crop water requirement is the total quantity of water required from sowing time up to the harvest. Different crops have different water requirements at different regions in Abu Dhabi Emirate, depending upon the climate parameter, type of soil, and method of cultivation.

Climate change is projected to have significant impacts on agricultural production. Therefore, understanding the regional impacts of climate change on irrigation demand for crop production is important for managers and agricultural producers to understand for effective water resources management.

The amount of water needed for irrigation depends on many different factors. A reasonably accurate estimate of the amount of irrigation water needed can be made using potential evapotranspiration data. on other hand, to study the possible impact of the climate change on the irrigation requirements in Abu Dhabi CROPWAT model (FAO, 2009) have been used. Figure 1 showed the average monthly of potential evapotranspiration between the years 2003-2016. The results showed that Al Ain region needs the highest irrigation requirements as compared with Abu Dhabi and Al Dhafra regions except in January when Abu Dhabi region required a slightly more water compared to Al Ain. The potential evapotranspiration (ET0) is the summation of the monthly ETo for each region, this reflects the relative irrigation amount variation in the three of Abu Dhabi regions.

Figure 1: The average monthly evapotranspiration for the three main regions in Abu Dhabi for the period 2003-2016.

Table 4 shows the percentage of the monthly potential evapotranspiration for Abu Dhabi and Al Dhafra regions. The results showed the potential evapotranspiration for both regions over the whole year is less than that for Al Ain (< 100%) except for Abu Dhabi in January which showed almost the same evapotranspiration as Al Ain.

Table 4: The percentage of the average monthly evapotranspiration for Abu Dhabi and Al Dhafra as compared with Al Ain region.

Since the potential evapotranspiration is closely affected by the temperature and other climatic parameters, the monthly percentage ratio of the total annual evapotranspiration for each region is increased in the summer months and decreased in the winter months. Table 5 shows the monthly percentage ratio of the annual potential evapotranspiration for the three regions in Abu Dhabi Emirate. The monthly percentage ratios showed the potential evapotranspiration in the three regions is about one tenth of the total annual evapotranspiration for the months May, June, July, August and September. The ratio of evapotranspiration to the rainfall range from 11 to more than 3741 times (Table 6). With an average ratio of 41 times for the months between November to April and to about 1100 for the period between May to October.

Table 5: The monthly percentage of the annual potential evapotranspiration for the three Abu Dhabi main regions.

Table 6: The monthly ratio of the evapotranspiration to rainfall the three agricultural location in Abu Dhabi Emirate.

Due to climate change, there is probable increase in temperature and decrease in the relative humidity therefore it is essential for long-term water resources development and planning to estimate the agricultural water demand in the changing environment.To estimate this probable increase in the irrigation requirements two scenarios been analyzed to indicate the possible impact of the climate change.

Scenario 1: increase of the average annual temperature by 1 °C and reduction of the relative humidity by 1%

Scenario 2: increase of the average annual temperature by 2 °C and reduction of the relative humidity by 2%. For both scenarios with the temperature rises and relative humidity declines, it is expected that the amount of ground water will extracted more rapidly to fulfill the irrigation requirements which might not be sufficient (in term of quantity and quality) in the future to retain the common agricultural pattern. The results of the above suggested scenarios are obtained using CROPWAT irrigation model (Table 7).The results show the monthly percentage increase in the water demand for the three locations in Abu Dhabi Emirate as compared with the current status. The average annual increases in the irrigation water demand in the three locations are about 3% and 6% for the scenarios 1 and 2, respectively. The annual increase of 3% in the irrigation water demand indicates that the water demand will be doubles after 34 years (Scenario 1). While the 2nd scenarios only 17 years will lead to double the annual irrigation water requirements.

  Table 7: monthly annual percentage increase in the irrigation water demand for the three locations of Abu Dhabi Emirate per the two expected climatic scenarios.

Agriculture and Natural Resources Indicator

Water used for irrigation purpose in Abu Dhabi are classified to three subsectors; Agricultural farm, amenity and forest. The discussion focus on the agricultural farm as the category for producing food. The major crops which are planted in Abu Dhabi could come under four classes; Fruits (included Date palm), Field crops and vegetables either in the open field or protected greenhouses. Statistics for the area of the cultivated agricultural land in Abu Dhabi Emirates showed less than 5% reduction between 2005 till 2007 and followed by 6.5% increment in 2015 as compared with 2007, with no change after 2015 reaching to 2019. Groundwater accounts for 94 % water consumption in the agriculture sector in the Emirate. However, the current usage of groundwater reservoirs is about 15 times more than the natural recharge rates, according to the Environment Agency-Abu Dhabi [15- 20].

Fruit trees occupy the largest proportion of 36.3% of the total plant holdings area. Palm trees are the main type of fruit trees in terms of cultivated area and total production. The date palm production in 2015 was 93,075 tons which is less by 0.6% compared with 2014. Field crops occupy 6.4% of the total plant holdings area, a significant decrease of 18.6% compared with 2014. The total production of field crops in 2015 was 148,195 tons. Vegetable crops grown in open fields and under greenhouses constituted 2.6% of the total plant holdings area. Cucumber accounted for 22.2% of the total area of vegetable crops, followed by tomatoes at 19.1%. Cabbage, onion, accounted for 10.0%, 6.8% respectively, meanwhile rest of the crops formed 41.9% of the total area of vegetable crops in 2015. Forest trees and windbreaks occupied 2.2% of the total plant holdings area. The current fallow area represented 37.5% and buildings area represented 1.7%. Data shows that the potentially productive area accounted for 13.2% of the total plant holdings in 2015.

Crop Calendar for Abu Dhabi

UAE is a hot country,that creates unfavourable conditions for growing vegetables in open fields all year around. The Crop calendar provides timely information on suitable planting and harvesting periods of specific crops. In conjugation with irrigation management knowledge and optimum agricultural practices can help to overcome the climate challenges. A large range of vegetables is growing in Abu Dhabi such as tomato, cucumber, eggplant; onion, squash and even pumpkin are suitable. The production of those vegetables in open field is low due to lack of knowledge about optimal agricultural practices such as irrigation. The water scarcity and salinity are a pressing challenge on this production system. Most area of open field growing in Abu Dhabi is drought types that grown alongside, those vegetables ate love the warm nights and moderate daylight. According to local observation, table 8 shows the most common winter and spring open field cultivated vegetables growing in Abu Dhabi, as Vegetables planting dates affect yield and water consumption, the information in the table chosen the best date/duration to start sowing vegetables to avoid most probably heat shock and to maximize yield and minimize water use which increases the water use efficiency.

Table 1: Cropping pattern and calendar for the open field vegetables growing in Abu Dhabi.

Crop Water Requirements- Open Field Vegetables

Consideration of crop water requirement (CWR) is essential for better irrigation use efficiencies, and irrigation scheduling. This study estimates the reference evapotranspiration (ET0) using FAO modified Penman Monteith equation, based on the average climatic parameters in the three Abu Dhabi Emirate main regions, Crop water and net irrigation requirements is estimated using FAO crop coefficients (Table 9, 10, 11).

Table 9: Estimation of daily Irrigation crop water requirements for open field vegetables grown in Al Dhafra- Abu Dhabi based on the period of the local growing season.

Table 10: Estimation of daily Irrigation crop water requirements for open field vegetables grown in Al Ain- Abu Dhabi based on the period of the local growing season.

Table 11: Estimation of daily Irrigation crop water requirements for open field vegetables grown in Abu Dhabi based on the period of the local growing season.

Conclusions

This study carried out to estimate the crop water requirements of major crops in different Abu Dhabi Emirate agro-ecological zones using CROPWAT 8.0 model of FAO and showing the current scenario and the future impact on crop water requirements under two different scenarios of climate change. The major grown vegetable crops. Study results showed that ET0 varied from 5 mm during the winter session to 13 mm/day for summer months. There is a higher water demand for crops during the dry seasons (summer and autumn) and a lower demand during the wet seasons (winter and spring).

Study results enhance our understanding of the water requirements of some major crops in Abu Dhabi Emirate, which will consequently help improve the management of water resources and the yield through strategies based on these outcomes, and providing a help for water resource planners for future planning, so helping to save water in matching the CWRs, and can be used as a guide for farmers to select the amount and frequency of irrigation for the crops being studied. Study also showed the impact of two climate change scenarios on crop water demand.

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