Humid regions are characterized by the fact that rainfall equals, or even exceeds, ET. Many humid areas have sufficient annual precipitation for crop production but do not have the proper seasonal distribution. The spatial distribution and intensity of rainfall can vary greatly within a season and from year to year.
Water in these areas is plentiful, generally of good quality, without inducing salinity problems.
In humid regions, amount and distribution of precipitation are largely beyond the control of people, but soil management may have a major influence on how effectively the available precipitation, or even irrigation, is used for crop production. One of the foremost requirements for more efficiently using precipitation is retaining the water-storage reservoir.
Irrigation provides predictability and stability of water supplies, enables farmers to obtain higher yields by using modern agricultural technology and optimal levels of production inputs, and increases intensity of cropping. Most irrigation in humid areas is supplemental to reduce crop stress caused by short- duration rainfall deficits. The purpose of irrigation in these areas thus is to increase ET but with minimal losses from runoff, poor drainage, and leaching of fertilizers and pesticides. Scheduling under conditions of rainfall thus is the major goal of irrigation in humid areas. Irrigation systems should be operated mostly to partially refill the profile. Problems arise when irrigating soils with low water-holding capacities, which do not maximize the effects of rainfall.
In humid areas, irrigation systems frequently are designed with a smaller capacity than required for extended dry periods, or the systems are moved from one field to another. These factors complicate the water management when rainfall is low and evaporative demand is high.
Irrigation scheduling used in arid, semiarid, and subhumid areas should not be used in humid areas. One of the most common methods for irrigation scheduling is based on field soil water balance. The effective rainfall must be considering in computations, but its determination is very difficult. Rainfall of high intensities or in large amounts may produce significant runoff, and only part of the rainwater can be considered to be effective. Similarly, rainfall on a wet soil profile will produce losses through drainage.
Field observation of soil water status or plant water potentials or other plant parameters, and the combination of these, can be used to determine irrigation timing.
The water balance can be based on real-time meteorological data or average climatic data. Various forms of computer-based, water-balance scheduling methods have been developed and evaluated for humid-area conditions. Several of these include a short-term forecast of rainfall.
The most widespread system is gravity irrigation, especially in areas where water availability is not a limiting factor and the water price is low. Furrow or flood irrigation is used in humid areas for sugar cane or rice, respectively. Frequent low-volume irrigation often is practiced in humid areas, which permits chemigation with little change in irrigation schedules. Most frost protection in humid areas is accomplished with overhead sprinkler irrigation. Although rainfall is often sufficient for crop establishment, irrigation can ensure uniform germination and emergence. Most sprinkler systems are well adapted for this purpose.
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