Water Scarcity and Drought

In many areas of the world, available irrigation water supplies are either very low or are being depleted rapidly. When water supplies diminish and population increases and municipal and industrial water demands increase, the optimization of irrigation water use becomes increasingly imperative. Droughts in many parts of the world have raised this question.

According to WMO drought is defined as a sustained, extended deficiency in precipitation [149]. Aridity is a permanent climatic feature of a region, resulting from low average rainfall [150]. Drought, by contrast, is a short-term lack of precipitation, a temporary feature of climate [151]. It is necessary to differentiate drought, aridity, and water shortage [152]:

• Drought is a natural temporary imbalance of water availability, consisting of persistent lower- than-average precipitation of uncertain frequency, duration, and severity, of unpredictable occurrence, with overall diminished water resources and carrying capacity of the ecosystem.

• Aridity is also a natural phenomenon but it is a permanent imbalance in the availability of water: The average annual precipitation is low. The spatial and temporal variation of precipitation is high, with overall low moisture, extreme temperature variations, and a low carrying capacity.

• Water shortage is a human-induced temporary water imbalance and results in groundwater overdraft, reduced reservoir capacities, disturbed and reduced land use, and an altered carrying capacity.

Droughts create temporary water-stressed environments similar to aridity. Thus, responses to aridity are partly applicable to droughts. Agricultural water shortages induced by droughts have much in common with human-induced shortages [140].

Drought is generally extensive in space and time. For irrigated agriculture, drought does not necessarily begin with the cessation of rain, but rather when available irrigation water falls below normal for a large period of time, when water supply is not sufficient to meet normal demands. Drought may require both managerial and technological adjustments in farming operations, depending upon its length and severity. Because of knowledge in prediction and forecasting of droughts, which could help in the development and application of nonstructural water management measures, is insufficient, structural measures to improve supply are preferred [152].

Drought periods can produce important negative impacts on agriculture. The severity of these impacts depends on the farmer's ability to adapt production systems to water availability. Drought can affect the irrigation-water demand by farmers. In these cases, farmers in rainfed agriculture resort to irrigation for supplementing natural rainfall for crop production.

In establishing an agricultural production system, several factors must be carefully studied (e.g., crop species, management strategies, irrigation technologies). All of these factors are based mainly on expected regional weather patterns. Depending on drought strength and duration, farmers must react rapidly to the reduced supplies and increased demands for water.

The increase in hydrometeorological information makes it possible to better use stochastic analysis for prevention of reservoir inflow and for reservoir management, to use demand forecasting models for planning releases, as well as to use optimization and simulation models to help in decision making.

Drought also affects irrigation water quality in several ways. There is less leaching of mineral nutrients and pesticides toward aquifers and, on the other hand, less transport of sediments and certain chemicals to surface streams.

Haas [153] described several general strategies for managing water under drought, although they are not specific to irrigation. Under limited water availability, it is necessary to look for new, specific solutions for the following [151]:

• Farmers must select and optimize cropping patterns as a function of water availability. The application of techniques of dynamic, linear programming, as well as the use irrigation-scheduling simulation models are tools of farmers and experts for achieving these goals.

• When designing irrigation-scheduling techniques, farmers and experts must take into account not only the water-availability limitation, but also the irrigation system used for applying water to the crops. Irrigation-scheduling simulation models and the implementation of irrigation-scheduling programs can be useful tools.

• A factor that interacts with irrigation adequacy and affects irrigation efficiency is irrigation uniformity. With any irrigation system, uniformity is strongly affected by irrigation management. Programs for field evaluation of irrigation practices and technical assistance for adoption of improved water application techniques are necessary.

• Farmers should be advised on the limitations of the irrigation system. They should be trained for collaboration in the operation and management of the system, which improves the interaction between off-farm and on-farm demand management.

• Methods of demand forecasting, aiming at rational planning of reservoir releases and deliveries, should be enhanced.

• Water pricing policies should be used as an incentive to farmers for using water-saving techniques related to both irrigation management and water application, and penalties should be imposed on farmers for wasting water or not using appropriate crop patterns.

The process of technology transfer must be accelerated to achieve this goal, so that farmers can benefit from the efforts of research to enhance WUE.

The study of the deficit irrigation strategies is highly relevant to the preceding discussion. These strategies are capable of reducing the amount of water applied with minimum impact on production. Deficit irrigation is an optimizing strategy under which crops are deliberately allowed to sustain some degree of water deficit and yield depletion [154]. Deficit irrigation may provide higher economic return per unit of surface than returns attained with irrigation for achieving maximum production [155].

Much information on crop response to deficit irrigation is available, which concludes that water deficit reduces yields. However, these irrigation strategies may have a favorable effect on yields when reducing the incidence of diseases and on crop quality. Late-season deficits that develop as temperature declines also can enhance the conditioning of tree crops for winter dormancy.

In a strategy known as high-frequency deficit irrigation, water is supplied throughout the crop cycle below demand levels, but at a frequency high enough to avoid situations of significant stress.

Although high-frequency deficit irrigation represents a clear alternative in several circumstances, it has important limitations, such as the significance of the water deficit to the phenological stage. Thus, a new concept has been introduced, called regulated deficit irrigation [156], which is based on reducing water supplies in the phenological stages where the water deficit does not affect either production and harvest quality. At the same time, plant requirements during the sensitive periods of the crop cycle are addressed.

Irrigation during noncritical periods is one of the essential aspects of regulated deficit irrigation. The water application must be estimated by means of tests to select the most suitable conditions as a function of the impact on harvest and on environment. When there is insufficient water availability, the possibility of fully covering plant water requirements during the critical stages must be considered.

Deficit irrigation in the Columbia Basin is typically practiced on silt loam and fine sandy loam soils. With the introduction of center-pivot systems, deficit irrigation also has been used on sandy soils [154]. Nevertheless, the nearly continuous irrigation required even for deficit irrigation of such soils reduces because of WUE the nearly constant surface evaporation that results.

Deficit irrigation can involve modification of some cultural practices [154], including use of moderate plant densities, reduced application of fertilizers and other farm chemicals, use of fallowing when crops are grown in rotations and when a fallow interval is desired for precipitation storage, and flexible planting dates and associated use of shorter-maturity-length cultivars.

Scheduling for deficit irrigation is potentially more challenging than for full irrigation. Ideally, the decision maker should evaluate not only the amount of water remaining in the soil profile but also the level of stress that the crop is experiencing and how that stress will affect yields.

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