Water Management in Horticultural Crops (1+1)

Factors affecting frequency of irrigation

• The factors which affect the ET have been dealt in the earlier schedule (Lecture No.5). The effect on affecting the frequency of irrigation is briefly explained hereunder.
• The two main consideration namely, water need of crops and the availability of irrigation water decide the irrigation frequency. Once these two are known, the frequency of irrigation is influenced mainly by climate, soil characteristics, crop characteristics and management practices.

i. Climate


• Climate is responsible for causing variations in consumptive use rate and frequency of irrigation. High temperature, low humidity, high wind velocity and greater solar radiation in a place emphasize the need to irrigate crops more frequently as evapotranspiration takes place at a higher rate due to greater evaporative demand of the atmosphere.
  
• This is particularly evident in arid regions and during summer season. On the other hand, higher rainfall and greater relative humidity during the rainy season reduce the irrigation requirement of crops and irrigations may be applied at longer interval, if it becomes necessary.


ii. Soil characteristics


• Water retentive capacity of soil is considered as the most important soil factor deciding the frequency and interval of irrigation. A soil with greater water retentive capacity serves as a bigger water reservoir for crops and can supply water for longer duration. Consequently, frequency of irrigation is lower and interval of irrigation is longer. On the other hand, the frequency is higher in porous sandy soils with coarse texture, poor structure and low organic matter content. Retention of greater amount of available water is considered more important than total quantity of water retained by a soil.
• Depth of soil is another factor that influences the frequency of irrigation. A shallow soil cannot hold enough water to meet the crop demand for a longer period. Necessarily, frequent irrigations are required with smaller depth of water each time. Irrigations at longer interval is applied to deep soil that has a greater water storage capacity. Such a soil can supply water for longer duration particularly when the root system is quite deep and extensive.

iii. Crop characteristics


• Crops vary in their consumptive use of water, sensitivity to water stress, water extraction capacity and optimum water regime. Frequency of irrigation thus varies with crops. Crops like vegetables, onion and sugar beet that require a higher level of water to be maintained in the soil need frequent irrigations than other field crops.
• Many crops have varieties that are either sensitive or tolerant to drought conditions. Varieties sensitive to drought conditions require frequent irrigations compared to tolerant varieties.
• Rooting characteristics of crops such as shallow or deep, fibrous or tapering, vertically or laterally extensive root systems decide the frequency of irrigation. When the root system is shallow and fibrous, crops are not able to utilize water from deeper soil layers and are frequently irrigated with smaller depth of water to wet only the upper soil layers. Crops with deeper and extensive root system command a greater depth of soil and water reserve and require irrigations at longer interval. Sometimes, they may get water from water table which is not deep enough. Shallower water table reduces the irrigation requirements and help to increase the interval between irrigations. Besides, the concentration and relative proportion of the root mass in different soil layers decides the water extraction capacity. They represent the extraction capacity of crops from different depths of soils.
• Irrigation frequency varies with stages of crop growth. The consumptive use rate, sensitivity to water stress and rooting characteristics of crops change at different stages. A crop when young and delicate needs frequent irrigations. Subsequently, the consumptive use rate gradually increases and at the same time the root system also develops. Irrigations can then be applied at longer interval, as roots are able to draw water from greater volume of soils. When a crop approaches maturity, the demand for water greatly declines because of steep fall in consumptive use rate.

iv. Management practices


• Soil water conservation practices such as artificial or soil mulching and crop cultural practices like weeding and hoeing help to reduce the evaporation loss and conserve more soil water for crops use. Thus, there is a reduction in irrigation requirement of crops. Method of irrigation, depth of water applied each time and the water distribution efficiency influence the frequency of irrigation.

v. Irrigation period


• Irrigation period is the time, usually expressed in days, that can be allowed for applying one irrigation to a given design crop area during the peak consumptive use period of the crop. It is a function of the peak-period consumptive use rate. It is considered for designing the irrigation system capacity and equipment. The irrigation system must be so designed that the irrigation period is not greater than the irrigation interval.