**2. Gravity-driven or surface irrigation**

Gravity-driven irrigation is conventional and has been in use since time immemorial. This approach does not use pumps and relies on the ability of water to move through resistance. The irrigation system is efficient on plain topography for even distribution of water. It has three phases which are the advance, storage, and recession.

The advanced stage is the period of water introduction to the field. Water flows over the field to the end of the field with the help of gravity until the field becomes flooded. However, the storage period is the time frame for water to infiltrate the soil; whereas, the recession period begins after the source of the water is cut off. The water infiltrates the soil more and dries up as a result of evaporation and the closing of the water source. The success of surface irrigation depends on the water holding capacity of the soil, field slope, soil surface roughness, and the shape of the flow cross-section. Examples of surface irrigation include continuous flooding and furrow irrigation (**Figure 1**).

#### **2.1 Continuous flooding**

Continuous flooding is the process of artificially submerging a leveled land under water. It is a system predominantly used for rice cultivation in many regions of the world. Among continents, Asia is ranked the largest producer of rice, and it is responsible for 75% of the total global production. Rice is an aquatic plant but can survive under different soil conditions. However, the introduction of water-saving techniques and the release of drought-resistant varieties continuously prove that flooding is dispensable for rice production. In paddy, the field is irrigated until the water level reaches 5–6 cm above the ground level and is continuously maintained throughout the cropping season or drained two weeks before harvest when the rice plant is at physiological maturity. The soil condition under a continuous flooding system is anaerobic and the degree of anaerobicity depends on the level of water and oxygen availability [1].

#### *Fundamentals of Irrigation Methods and Their Impact on Crop Production DOI: http://dx.doi.org/10.5772/intechopen.105501*

Paddy fields account for about 40% of global irrigation [2] and it uses 2 to 3 times the volume of water required by other cereals such as wheat and maize to produce 1 kg of rice grains [3]. More than half of the water needed for irrigation in Asia is utilized in rice fields; however, most of this water is lost through unproductive water outflows such as evaporation, lateral seepage, deep percolation, and runoff. Apart from its excessive loss of irrigation water, the continuous flooding system is a major source of greenhouse gases such as methane and carbon dioxide thereby contributing negatively to the environment.

As a result of the decline in freshwater, more water-saving irrigation practices such as alternative wetting and drying (AWD), System of rice intensification (SRI), Ground cover production system (GCRPS), Drip irrigation with film mulch (DIP) had been introduced.

#### **2.2 Furrow irrigation**

This involves supplying water to the field along the furrow. The furrows are usually small parallel channels that serve as reservoirs of water on the field. The water gets to the plant root through lateral seepage.

The gravity-driven irrigation method requires minimal capital to construct and the energy required for it to work is obtained from free-flowing gravity. The system is easily controlled and does not require high technical know-how. Surface irrigation can be used on sloppy land. However, the irrigation method can affect plant growth and development due to the reduction in plant respiration caused by flooding. It could also increase the loss of water through deep percolation, runoff, infiltration, and evaporation.

#### **3. Pressure-driven irrigation**

The increase in global population resulting in rapid urbanization and industrialization have intensified competition for available water resources resulting in the decrease of fresh water available for crop production. Freshwater resources are becoming increasingly scarce and droughts are becoming more common as a result of climate change. Despite moderate rainfall in some regions of the world, over 50% of irrigation demands for crop production are met by pumping from underground aquifers, thereby depleting aquifers at an alarming rate. Therefore achieving food security requires high yields with efficient use of water resources [4]. Water-saving irrigation techniques that involve the use of pressure rather than gravity have been developed to help cope with water deficits and ensure maximum food production per unit drop of water.

A pressurized irrigation system involves the supply of water with the effort of pressure. This system is designed to achieve higher efficiency than the conventional method. The techniques help in quantifying the exact amount of water or nutrient to be supplied at a particular point in time. The choice of a pressurized irrigation system depends on the knowledge of the plant type, soil type, landscape characteristics, required flow rate, operating pressure, and cost. Examples of pressure-driven irrigation system are drip irrigation and sprinkler irrigation.

#### **3.1 Drip irrigation system**

A drip irrigation is an efficient irrigation system used for row cropping. In this system, water is directly supplied to the soil surrounding the root region with the of

**Figure 2.** *Drip irrigation set up: surface drip irrigation (A) and subsurface drip irrigation system (B).*

the drip tubes laid on the soil surface (surface drip) or that are buried few centimeters below the ground level (subsurface) (**Figure 2**). The advantages and efficiency of drip irrigation has increase it acceptance and use by agriculturists around the globe, most especially in the arid and semi-arid regions where there is limited freshwater availability. The precise application of water to the root region of crop without wetting the entire farm plots makes drip irrigation an efficient water-saving technique compared with others. In a drip irrigation system, only a fraction (between 15% and 60%) of the soil surface is wet [5]. The drop by drop sequence of watering reduces surface runoff and percolation; hence, providing better disease management and salinity control [6]. Other benefits of drip irrigation include improved crop quality, efficient fertilizer and other chemical usages, limited weed growth, and improved agronomic practices [7].

In a drip irrigation system, emitter spacing is necessary to ensure precise delivery of irrigation water. This largely depends on the planting distance or vice versa. The effectiveness and efficiency of a drip emitter is an important factor that affects water distribution and the performance of a drip irrigation system. The rate of water delivery by emitters varies and their use is based on the soil types and the water-use efficiency of the crop. Emitter clogging is mostly related to the quality of irrigation water. The turbidity of water as a result of physical (sand particles), biological (bacteria), and chemical (inorganic fertilizer, salts) composition results in emitter clogging. The compounds gradually settle around the water passage until the clusters could not allow further passage of water. Clogging affects the productivity of the crops around the affected emitters and in turn reduces yield outcome. However, to prevent emitter clogging, water could be treated and made less turbid before application. The combination of strategies, such as installation of a filtration system, the use of sedimentation tank and tube settlers, frequent flushing of the irrigation system, and chlorination of the irrigation system, could mitigate emitter clogging.
