Crop Water Requirements and Traditional Farming

Abdullah Al-Ghafri

Professor of the UNESCO Chair on Aflaj Studies-Archaeohydrology

Plants need air to grow to take up carbon dioxide and oxygen (CO2 and O2). Plants also need nutrients, water, light, energy, and mechanical support. In geoponics (traditional farming in soil), plants get water, nutrients, and mechanical support from the soil. However in soilless-agriculture, hydroponics and aeroponics, plants take the nutrients directly from water. In this article, we will consider only geoponics agriculture, where the soil is the growth medium. In this method, the nutrients are dissolved in water and the plant absorbs the dissolved nutrients from the soil through the roots and mobilizes them to other parts of its body. Plants need water for photosynthesis in daytime, where they take carbon dioxide from air and with light, to synthesize their food and release oxygen and water. This process is called transpiration, where plants lose more than 80% to 90% of their water (Fig. 1). Water is also lost from the soil and plants’ bodies through evaporation. The plants’ requirements of water are the results of combining both evaporation and transpiration and that is called: evapotranspiration.

Although all plants need water to grow, the amount of water they need tremendously differs with different factors. The main factor is the plant species and its growth stage and health, and another factor relates to environmental conditions. We can classify plants into three groups according to their water requirements:

1. Plants that need high amounts of water: these are called hydrophytes, such as lilies, water lettuce and other aquatic plants.
2. Plants that need a moderate amount of water: these are called mesophytes, such as most agricultural crops like dates, lemon, mangos, roses etc.
3. Plants that need small amounts of water: these are called xerophytes such as desert plants like aloe vera and pineapple.

Most of Oman’s plants are categorized as mesophytes and xerophytes, but there may be some hydrophytes  in the south.

In tropical and high rainfall lands, we do not worry about the water requirements of plants. However, in arid and semi-arid lands such as Oman, we must apply water artificially to plants and that is called irrigation. To get the optimum production of crops in agriculture, we need to apply the right amount and quality of water to the plants at the right time. If we supply the plants with less water than they need, they will suffer and the production will be less in quantity and quality, but it does not mean that we can increase the production by increasing the irrigation applied to plants to an unlimited level. If we exceed the crop water requirements, production will deteriorate, and the plants will suffer and may die from rotting or suffocation of the roots; the efficiency of irrigation will be less and water is wasted; however, too little water can lead to salinization of the soil, again lowering production. Therefore, understanding how to estimate crop water requirements is so important to save water and get the optimal growth and production of plants. In simple words, the amount of irrigation water should be equal to or slightly more than the crop water requirement.

Now we know that for optimal agricultural production in traditional agriculture, calculating, estimating, or at least understanding the crop water requirement is so important. As well as the plant type that determines the crop coefficient, other environmental factors play an important role in determining the crop water requirement and need to be included in the calculation or estimation of the plants’ water needs. The main environmental factors contributing to the crop water requirement are:

1. Sunlight: Sunlight is important for plants’ photosynthesis; both the intensity and quality of light are important for plants as well as the duration of the light. The higher the intensity and duration of light received by the plants’ leaves, the higher the evapotranspiration, which means a higher crop water requirement.
2. Temperature: As the temperature increases, plants need more water, therefore in summer the plants’ requirements for water are higher than in winter. In Oman, we can see in afternoon of hot summer days that some plants are wilting, due to the excessive loss of water from leaves without enough compensatory intake from the roots, this is called temporary wilting.
3. Winds speed: When the wind speed increases, it leads to more evaporation, like an increase in temperature that makes plants demand more water.
4. Air humidity: The humidity of air has an inverse effect on crop water requirements because if the air is humid, evaporation will be less leading to a decrease in the crop water requirement.

The crop water requirement can be obtained physically by lysimeters or by calculation. Lysimeters are devices that measure the exact loss of water from plants grown in the soil which can be obtained by subtracting the water lost from evapotranspiration from the amount of irrigation and rain. These devices are used mainly for research purposes. However, in calculation, all the factors of plant type, evaporation, temperature, sunlight, wind speed, temperature and humidity are considered.

The successful irrigation system is the system that provides the crop water requirements in an efficient way without wasting water or making the plants suffer. The irrigation water requirements must consider the crop water requirement plus the type of irrigation system (surface, sprinkler, drip, subsurface, etc.), the soil type, soil depth and water quality. The main soil (edaphic) factors can be summarized as follow:

1. Soil type: The two most important characteristics of agricultural soil related to water are infiltration rate and soil-water holding capacity. Both are dependent on the size of soil particles; clay soil has a low infiltration rate and high-water holding capacity, on the other hand, sandy soil has a high infiltration rate and low soil-water holding capacity. Agricultural soil is a combination of both that results in high infiltration rate and high-water holding capacity.
2. Soil depth will determine the total volume of water that can be stored in it. The soil depth should match the size of the plants. Vegetables and grasses do not need deep soil, However, trees like date palms and mangos need a relatively deep soil to accommodate their roots.
3. Water quality also affects the irrigation requirements: if the water is quite saline, we need to leach the accumulated salts from the soil surface, time after time, especially in an arid hot region such as Oman.

Let us see what happens when we apply water to soil until it is saturated:

1. The excessive water that the soil cannot hold seeps down below the plants’ root zone, which is called gravitational water. Although the plant does not benefit from this water, we need to saturate the soil to wash its salts away in hot arid climates from time to time, especially during the summer.
2. After the gravitational water moves away from soil, the soil still wet and the water left in the soil is called capillary water. This water is held in the small spaces between soil particles by the capillary force, but the force is weak enough such that plants roots can take up the water.
3. Hygroscopic water occurs as a thin film of water around each soil particle after plants remove the capillary water by suction. This water is held around the soil particles with a force too strong for plants to obtain. Therefore, this water has no benefit to plants.

Now, the irrigation system must be designed considering providing water to soil to keep it at, or a bit above, the capillary water level, not dry and not too wet so the plants can get their water requirements easily and some soil voids do not contain water so that the roots can breathe (Fig. 2).

In the threadional irrigation systems of  Oman, aflaj (falaj is the singular of aflaj), older Omani farmers know these factors by practice, and they designed their irrigation system in such a way to be sustainable for the crops that they depend on all year round. The irrigated land is divided into two main parts, upstream called “dhawahi” and downstream called “awabi”. In the dhawahi area they grow permanent crops, mainly date palms, citrus and other fruits, and in the awabi area they grow vegetables, forage, cereals, and short rooted plants.  They put their fruit and economically valuable trees close to the water supply and the vegetables and short rooted crops in the tail. In summer, they cut irrigation from the tail of the system to make sure that they have enough water to for their trees to flourish,  mainly date palms because they are economically so important (Fig. 3). They also design their irrigation schedule according to the soil type: the aflaj that have heavy soil are given longer irrigation times while on the other hand, aflaj with light soils are given short periods of irrigation. To ensure higher irrigation efficiency in aflaj with a low flow rate, farmers constructed irrigation cisterns to store the water so that it can be released at a higher flow rate, albeit for a shorter period.

Note: All figures are drawn by the author.