Irrigation of Crops:
Pakistan has a cultivated area of 22.04 mha out of which 18 mha is irrigated area and the rest relies on rainfall which is quite variable and unexpected. The rainfall is not evenly distributed and Pakistan receives about 60% of its total rainfall in monsoon season alone. Crops require adequate and timely supply of water for proper growth and output. The staple crops of Pakistan like rice, wheat, sugarcane and cotton require high amount of water at their critical stages of growth.
Responsible Factors for Water Requirement:
Irrigation water requirement for a crop varies with following factors:
The water requirement for a crop varies with variety of crop, stage of growth, duration of the crop, plant population or density, and growing season of the crop. The variety with longer duration obviously requires more water for completion of the life cycle. Similarly, water requirement at the time of flowering of the plant is more than that seeding. Evapo-transpiration differs with the height and leaf area of the plant.
The water requirement also varies with texture, structure, depth and topography of the soil which affects hydraulic conductivity, reflectivity and thermal conductivity of the soil. For example, evaporation of water is higher in coursed textured soils and thus requires high amount of irrigation. High aggregates of soil reduce evaporation. Similarly, colour of soil and formation of ridges and furrows also influence evaporation of water from soil surface.
Evapo-transpiration is influenced by solar radiation, temperature, relative humidity and mind velocity. Hot and dry area around the irrigated area increases evaporation.
Crop Management Practices:
All the management practices that provide favourable environment to the crop increase the leaf area and thus increase evapo-transpiration. Frequent irrigation results in higher proportion of evaporation. Similarly, mudding reduces evaporation and evapotranspiration considerably.
Methods Of Irrigation:
There are several methods of irrigation being employed according to the crop, culture and other management practices. The most important irrigation methods are as follows.
Water is applied to the land surface in the form of a moving sheet.
Water is applied to the land surface without any restriction in its flow and movement. This method of irrigation is practiced in areas having abundant and cheap water supply and uneven land surface. Application losses are very high.
Water is applied in terms of blocks of land. This method of irrigation is applied where land, labour and water is expensive. In controlled flooding, water is applied in the following ways;
Border Strip Flooding:
Land is divides into a number of stops of few acres/hectares with the help of temporary ridges. Water is supplied to these strips along which it flows slowly towards the lower end, irrigating the soil as it advances.
In check flooding method, comparatively large streams of water is applied to relatively leveled plots surrounded by levees. This method is well suited for very permeable soils which must be quickly covered with water to prevent losses through percolation.
This method is used to irrigate garden plants. Basin is made for each plant which is filled and the rest of the field remains dry. This method has high application efficiency.
In this method, small furrows are made in the field. The lengths of the furrows vary from 100 feet in gardens to 1500 feet in crops. The spacing of furrows for irrigation depends upon the spacing of plant rows e.g., 2-3 ft. This method reduces erosion of soil and evaporation. It is often difficult to supply uniform amount of water to all furrows. The slow movement of water in furrows result in percolation fast movement brings erosion.
In this method, irrigation water is applied in shallow ditches about 3 feet wide and spaced 100-200 feet apart. Water sinks into the soil, causing the water table to rise high enough to provide moisture to the plant roots through capillary action. Water is provided to the soil slowly through underground pipes. This method of irrigation has several advantages. For example, once pipes placed no further expenditures are needed. The amount of evaporation is minimum. Deep root system is encouraged. On the other hand, this method of irrigation requires high initial capital and pipes are often choked with rest or other forces.
Overhead Spray or Sprinkling:
Sprinkling is the application of irrigation water in the form of spray over the crop. Water is passed through pipes under a pressure of different degrees and water is allowed to come out of the nozzles in the form of small droplets. This method of irrigation is favourable for porous and shallow soil or for steep slope where other methods of irrigation are not possible. Small amount of water is used efficiently without creating any interference for the farm operations. However, this method is quite costly and harmful for certain crops like cotton. It does not work properly when mind velocity is higher and is also energy dependent.
This method involves dripping water into the soil at very low rates from a system of small diameter plastic pipes filled with outlets called “Drippers” or “Emitters”. This method has high water use efficiency due to small evaporation and percolation of water. It is highly suitable for orchards and other row crops like vegetables etc. It is highly successful in saline and desert areas. However, it has a high initial cost which discourages its use.