A generally accepted definition of arid or semiarid lands are those lands where the crop water requirements exceed the plant water availability (growing season precipitation plus soil water stored in the root zone) by a significant amount . At least one-third of the world's land surface is taken up by arid or semiarid lands.
The ratio between precipitation (P) and reference evapotranspiration roughly delineates the severity of water deficit in various zones. Various aridity indices are defined as follows :
• Hyperaridzones, P/ET0 < 0.03, yearly rainfall under 100 mm. The zones include true deserts with virtually no rainfall over periods of one to several years.
• Arid zones, 0.03 < P/ET0 < 0.20, annual rainfall of 100 to 200 mm. Zones of pastoral nomadism and irrigated farming include sparce and scanty vegetation of perennials and annuals.
• Semiarid zones, 0.2 < P/ET0 < 0.5, annual rainfall of 200 to 400 mm. Zones of pastoral nomadism and irrigated and rainfed agriculture include semidesert or tropical shrublands with an intermittent grass cover.
• Subhumid zones, 0.5 < P/ET0, yearly total rainfall of 400 to 800 mm. Zones of traditional rainfed agriculture include Mediterranean communities of the maquis and chaparral types.
With precipitation being such a major limitation to crop production, its efficient use is an important consideration in any sustainable system. Dry farming can be an excellent means to cultivate regions of low productivity. A small amount of rainfall can be sufficient to grow drought-resistant crops. With extremely low rainfall rates, irrigation water is required to create possibilities for agricultural practices. However, in many arid and semiarid regions, the availability of either surface or groundwater is limited. Water in deep aquifers often has collected over many years or even centuries. If not replenished at the rate of utilization, groundwater ultimately will be a finite resource. Runoff farming in dry environments is still practiced in modern agriculture, albeit on a different scale. Systems that supplement rainfall can be sustainable in the long term. Irrigation management to prevent overwatering, soils impermeable to deep percolation, and crops that utilize water efficiently all contribute to long-term sustainability. Likewise, runoff farming systems need to be balanced carefully to utilize all incoming rainfall without creating excesses during wet years or shortages and crop failures during dry years.
Dryland farming should be designed to cope with the negative impact of highly variable rainfall timing and amounts on crop production. Dryland farming has to increase crop-available soil water by enhancing water infiltration into the profile, by increasing its storage, by reducing evaporation, and by minimizing water losses.
Dryland farming techniques to improve infiltration include tillage operations that leave a coarse tilth on the soil surface, create layers that cause the soil to self-mulch; and leave a mat of crop residues on the surface to protect the surface tilth and to reduce evaporation. Conservation of water stored in the root zone from one rainy season to the next under fallow requires weed control and minimizing surface evaporation and deep percolation.
Disturbing crusts during periods between rains to increase infiltration probably constitutes an early example of soil surface modifications. Other approaches to increase infiltration retain residue on the surface by means of reduced-tillage or conservation-tillage practices. The average rainfall holding from six conservation-tillage systems was 9% higher than that from four conventional tillage systems . Improvement in rainfall holding was attributed to a buildup of surface residue and improved soil tilth in the surface horizon, especially protection of the surface from raindrop impact. This reduces the sealing of the surface against infiltration. Residues also shade the soil surface, which reduces the surface temperature and soil evaporation. Increased residue on the soil surface also has been associated with an increased organic-matter content of soils or altered distribution in the profile, thereby affecting infiltration.
Because infiltration is to be kept as high as possible, any restriction caused by shallow plow pans or a clay layer requires correction by plowing, chiseling, or sweep plows. Although the primary objective of subsoiling is to create zones acceptable for rooting in lower horizons, clearly the operations also increase infiltration. Burrows of earthworms and other soil fauna provide channels for water infiltration.
Subsoiling, deep tillage, the use of gypsum to produce stable aggregates, and breeding programs that select for stronger rooting cultivars are traditional methods to improve storage capacity. Where such methods increase water penetration, root growth, and water use by the crop, they will have a major impact on sustainability.
Cover crops provide an effective method for improving water quality because they accumulate and retain plant nutrients as well as reduce soil erosion.
High levels of fertility in soil lead to both optimal crop growth and enhancement of root-system spread, thus helping the crop to endure water deficits .
Tillage systems designed to conserve water and eradicate weeds also must contribute to pest and disease control. Crop rotations maintain a change in crop succession to minimize carryover of pests, diseases, and weeds, and attempt to maintain soil fertility. Many factors are involved in selecting a crop rotation, including soil water-storage capacity; amount and variability of rainfall; crops and their profitability; pressure of weeds, pests, and diseases; and requirements and cost of tillage operations .
Every arid, semiarid, and subhumid region has developed appropriate water conservation techniques, some of them quite particular. In North Africa, among others are biological stabilization of gullies, small earth dams, meskat, loose-rock belts, and broad-base level terraces .
Water harvesting can significantly increase plant production in drought-prone areas by concentrating the rainfall/runoff in parts of the total area . The goals of water harvesting are restoring the productivity of land that suffers from inadequate rainfall; increasing yield of rainfed farming; minimizing the risk in drought-prone areas; combating desertification by tree cultivation; and supplying drinking water for animals. Of the great number of forms in existence with various names, six forms generally are recognized : roof top harvesting; water harvesting for animal consumption; inter-low water harvesting; microcatchment water harvesting; medium-sized-catchment water harvesting; and large-catchment water harvesting.
Bucks  discussed several technologies for improved water management and conservation of irrigated agriculture in arid and semiarid regions. Although no single
Table 5.20. Demand management for water conservation in irrigated agriculture
Reduce water delivery Increase irrigation efficiency and water application uniformity.
Irrigation scheduling and control based on monitoring the soil, the plants, and/or the microclimate.
Reduce water evaporation from lakes, reservoirs, or other water surfaces.
Reduce evaporation of water from soil surfaces.
Reduce water use by non-economic and phreatophyte vegetation.
Reduce evapotranspiration Limit irrigation by applying less water than maximum ET demand.
Limit irrigated cropland acreages by converting irrigated cropland in water-short areas to dryland farming.
Change crops by introducing those with lower water requirements.
Crop selection and modification for drought-resistant strains that can withstand dry periods.
Decision-making models and systems for irrigation scheduling and crop simulation, and using expert systems.
Source: Adapted from .
technology can solve all water quantity and quality problems confronting irrigated agriculture, he did indicate that advanced irrigation scheduling, increased irrigation efficiency, limited irrigation, soil moisture management, and wastewater irrigation can be used more effectively in the future.
Table 5.20 lists the primary technologies available for demand management for water conservation in irrigated agriculture. Demand management objectives include reducing water delivery (nonbeneficial ET) and reducing water requirements (beneficial ET) .
Supply management objectives include storing runoff water, increasing water yield, capturing precipitation, and adding to available water supply. Table 5.21 lists the major technologies available for supply management for water conservation in irrigated agriculture for arid areas.
Because of a combination of low application efficiencies and inadequate drainage, irrigation has led to large-scale waterlogging and salinization of irrigated land. In semiarid and arid regions, prevention of irrigation-induced salinization is the main concern. Drainage requirements can be reduced by improving the efficiency and management of irrigation. Thus drainage for irrigated land must be treated as a component of the water management system and its design should depend on the design and management of other components. The primary design and operational objectives of water-table management systems in the arid, semiarid, and subhumid zones are to provide trafficable or workable conditions for farming operations, to reduce crop stresses caused by waterlogging to control salinity and alkalinity, to minimize harmful offsite environmental impacts, and to conserve water supplied by rainfall, thus minimizing irrigation water requirements. Modern research emphasizes the importance of controlling the water table by using a combination of drainage and irrigation.
As the demand for water increases, wastewater reclamation and reuse have become an increasingly important source for meeting some of this demand. The level of wastewater treatment required for agricultural and landscape irrigation uses depends on the soil
Table 5.21. Supply management for water conservation in irrigated agriculture
Increase storage runoff water Small reservoirs to catch and retain floodwater for release during droughts
Groundwater recharge by conveying or confining surplus runoff to recharge areas to increase water storage. Increase water yield Water harvesting by constructing an impermeable surface to reduce infiltration and store runoff. Vegetative management by manipulating vegetative cover to increase or decrease runoff for improved groundwater recharge storage. Capture and retain precipitation Snow management.
Soil moisture management by cultural and mechanical practices to decrease runoff and evaporation, thereby increasing soil moisture storage.
Increased crop rooting depths by breaking up hardpans and selecting crop species and cultivars that root more deeply to expand soil moisture extraction. Add to available water supply Inter- and intrabasin transfers.
Wastewater irrigation, using moderately saline drainage waters and renovated wastewater effluents for irrigation.
Source: Adapted from .
characteristics, the crop irrigated, the type of distribution and application systems, and the degree of worker and public exposure.
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