The discussion above describes standard practices that can be done with current in- stitutional and technological systems. The next step is to look into the future with all of its uncertainty. To aid in that effort, the National Research Council (1996) devel- oped this list of likely future directions for irrigation in the U.S.:
Irrigation will continue to play an important role in the U.S. and the world for the foreseeable future, although there will certainly be changes in its character, methods, and scope.
The total irrigated area will likely decline, but the value of irrigated agriculture will remain about the same because of shifts to crops of higher value.
The amount of water dedicated to irrigated agriculture will decline as societal values change and competition for water increases.
A major factor in the sustainability of U.S. irrigation will be determined by our ability to compete in global markets.
Under-financed irrigation operations or those with less-skilled managers will tend to decline in number.
Previously, irrigation meant irrigation for agriculture. During the past 25 years irrigation has become an important part of the turf industry, and irrigation for urban landscaping and golf courses is growing steadily as urban populations in- crease.
With time, increasing amounts of water will be removed from agriculture to sat- isfy environmental goals. In conjunction with this, there will be increasing pres- sures to reduce environmental degradation associated with irrigation.
Substantial research and educational challenges must still be addressed regarding water availability, quantity, and quality, water use, and water institutions (National Research Council, 2001, 2004). Changes in policy and incentives will clearly become necessary. The following sections examine some potential issues and solutions to agri- cultural water security issues. Urban water users will have a similar set of challenges to reduce and modify water consumption.
1.5.1 Need for Innovation
Irrigation has been practiced for more than 6000 years, but more innovation has oc- curred in this arena in the last 100 years than in all of the preceding centuries. Almost every aspect of irrigation has seen significant innovation: diversion works, pumping, filtration, conveyance, distribution, application methods, drainage, power sources, scheduling, fertigation, chemigation, erosion control, land grading, soil water meas- urement, and water conservation.
Major future improvements in water saving will be realized through innovative de- sign and operation of integrated irrigation systems for both agricultural and urban set-
tings. It is obvious that all these technologies will have to continue to be improved and implemented to better manage energy, water, and soil resources. Novel irrigation techniques and management systems will be necessary to increase the cost- effectiveness of crop production, improve water quality, improve water reuse capabili- ties, reduce soil erosion, and reduce energy requirements while enhancing and sustain- ing crop production and water use efficiency. In addition, innovative water polices and institutional structures must evolve and foster emerging irrigation technologies.
Irrigation is a valuable technology, rooted in ancient tradition, and has proven to be dynamic and flexible. However, new and improved strategies and practices are needed to reduce surface and groundwater contamination from agricultural lands, conserve water and energy, and sustain food production for strategic, economic, and social benefits. Systems must be designed and managed to minimize health hazards due to chemical applications of fertilizers and pesticides as well as to minimize insect infesta- tion and parasitic diseases, such as the West Nile virus and malaria. The effects of water conservation and reuse technologies on recreation, tourism, wetlands, and aquatic ecosystems must be assessed and balanced with other societal needs.
Future irrigators will often be operating under various managed crop water deficit scenarios. Increasing crop productivity while reducing the amount of applied water implies that producers will often be managing irrigations under severe to moderate soil water deficit conditions during part or all of the growing season. Techniques such as partial root zone drying and regulated deficit irrigation will be more and more com- mon on tree and vine crops as well as many annual crops (Chalmers et al., 1986; Fer- eres et al, 2003). Techniques such as fallowing of irrigated fields in alternate years to conserve water need to be investigated as to potential water savings and reduced agro- chemical use.
Water reuse and treatment of impaired waters will be part of agricultural water se- curity. Innovative approaches to groundwater recharge using treated and excess sur- face waters for later withdrawals by a multitude of users will be an essential part of future water resource programs..
The following brief sections present more details on some of the issues that agricul- ture will have to implement to address water security issues. These measures will in- clude: modernizing irrigation delivery systems and on-farm systems, improved levels of management, strategies for local water supply enhancement, and biotechnological advances in crop breeding and selection.
1.5.2 Modernizing Delivery and On-Farm Systems
Traditional approaches to modernizing irrigation projects have focused on minimiz- ing water loss during delivery and maximizing field application efficiencies. These are necessary first steps, but future water delivery systems and application techniques must be modified to enhance grower flexibility in managing rates, irrigation frequen- cies, and durations, as well as reduce water evaporation and other losses. Small, dis- tributed internal regulation reservoirs, closed-conduit systems to reduce evaporation and leave unused water in the distribution system, extensive automated water-level controls, accurate automated flow measurement, and improved ways to reduce weed growth on canal and lateral banks to minimize non-beneficial ET are all potential means of improving water delivery efficiencies. Some of these features are just now beginning to be implemented in a few modernized irrigation projects.
To maximize the potential of existing and emerging technologies, irrigators must have the flexibility to manage rate, frequency, and duration of their water supplies. Thus, the delivery system and the farm must be considered as one integrated unit with two parts rather than two independent systems. With the imperative need to implement the agreements and mandates discussed previously, the Imperial Valley in California will be a proving ground for these concepts over the next couple of decades, and there is much to learn. In this case, the delivery system will provide irrigation water to sat- isfy the specific field condition (i.e., rate, frequency, and duration) that is calibrated for each crop and irrigation condition. This will require extensive canal automation and on-farm monitoring as well as economic incentives for achieving better water use productivity. Positive payments for achieving a certain target efficiency or tailwater levels rather than solely raising water costs are anticipated.
The following identifies some of the potential areas where innovation is likely to improve delivery and on-farm systems in the 21st century:
Computers and wireless control systems will play an ever-expanding role. Cell phone and satellite communications and internet technologies will likely play an increasingly major part in management of irrigation systems. Feedback control technologies for automating canal operations, surface and pressurized systems, and drainage systems must be developed, tested, and supported by incentives. On-farm systems will benefit from advanced technologies, such as precision ir-
rigation, site-specific management, remote sensing, within-field real-time sensor systems, and decision support systems, which collectively have great potential to facilitate reduction of water quantity and quality problems in irrigated agriculture. The use of real-time irrigation scheduling techniques (sensor-based) and site-
specific precision applications of water through center pivot machines and mi- croirrigation are the next steps in the evolution of those technologies.
It will be necessary to expand modern crop production technologies to less pro- ductive rainfed and irrigated lands characterized by poor soils, low and unstable rainfall, steep slopes, and short growing seasons to increase food production and stimulate economic growth. Novel approaches will be needed to address these areas.
Microirrigation with its many variations must be made less expensive before most growers will be able to adopt and utilize these technologies, especially in developing countries. Some localized efforts are ongoing and one company is manufacturing tubing at relatively low cost, but these innovative efforts and technologies need to be extended to other areas.
For developing countries, innovative research and extension education is needed to provide and implement simple but efficient low-cost methods of irrigation (e.g., pitcher irrigation) to make them easy to operate, suitable for the crop, and acceptable to growers. There is also a huge need for low-lift pumps that are in- expensive to buy and operate in these areas. Some of this is already being done on relatively small scales, but there is much room for innovation.
Surface irrigation methods can be made more efficient using surge flow, dead- level basins, and other techniques for more uniform infiltration along the length of the field. Properly designed and operated level basins eliminate runoff, can be quite efficient and uniform, and are relatively inexpensive to construct and oper- ate. However, considerable investment for delivery system improvements, as
well as sensor feedback controls and automation for both the delivery and appli- cation systems, is needed to fully realize the potential water savings.
Urban and agricultural irrigators will be the primary users of degraded waters. New approaches and techniques will be required to safely minimize detrimental effects while maintaining production goals.
Farm- and district-level drainage systems will require improved design, evalua- tion and simulation models defining the physical limits. Automated control sys- tems will assist in providing a more uniform soil water environment for plant growth to improve productivity and minimize the volumes of drainage waters requiring treatment, especially in arid areas.
Water table elevations can be managed to permit subirrigation, if the groundwa- ter is relatively shallow and of suitable quality, by controlling water tables or in- ducing water tables with irrigation applications. Subirrigation has been practiced successfully in climate regions ranging from humid to arid. Using the effluent from deeper subsurface drainage systems as a source of irrigation water has proven effective in many regions of the world. The biggest concern is the quality (i.e., salinity) of the drainage system effluent.
There is still no reliable, inexpensive electronic soil water sensor that matches or exceeds the accuracy and repeatability of neutron scattering devices. Innovative development of such sensors is essential for water management, particularly un- der deficit conditions.
Many of the needed and evolving technologies will require stand-alone, spatially distributed electrical power to be feasible. Controllers, monitoring equipment, and communications devices must be low power consumers. Photovoltaic, wind turbine, and storage systems will need to be developed and implemented at low cost at the farm or field level.
Economies of scale have led to large field sizes for irrigated production in many areas, and engineers have been very successful in designing pressurized irriga- tion systems that apply water quite uniformly over these fields. However, the challenge over the next 50 years for the irrigation industry and designers is to develop highly efficient systems that are also suitable for small-scale farms and provide the necessary extension education to equip the farmers with the skills to run them.
Although widely variable, it is estimated that 1% of the global water storage ca- pacity in reservoirs is lost each year to sedimentation (Palimeri, 1998), decreas- ing the ability to store water. Innovative methods to reduce erosion at the water- shed and basin scale will be needed to increase the life of reservoirs for storage, flood control, and recreation uses.
Innovation will be required to enable adoption of more efficient irrigation methods. For example, high-frequency drip irrigation and other microirrigation methods have been shown to increase the yield and quality of fruit and vegetable crops through re- duced water and nutrient stresses. Tied to an effective soil water monitoring program, good design, and appropriate management practices, microirrigation can be 95% effi- cient or better. A modification of center pivot irrigation called “low energy precision application” or LEPA has been found to be 95% efficient as well. However, microirri- gation and LEPA irrigation are being used on less than 1% of irrigated lands world- wide.
1.5.3 Management
Inherent in the evolution of on-farm management discussed above will be the inte- gration of irrigation, fertilizer, and pest management strategies into systems that opti- mize total management practices for temporal and spatial variability. This should re- sult in substantial labor, water, and energy savings and minimize losses to the groundwater.
Improved irrigation technologies usually result in reduced labor requirements, but require expanded management. However, most producers who irrigate do not have sufficient time to properly manage all their critical inputs. At certain times of the year, a producer’s time is extremely valuable in terms of net returns from the crop, and irri- gation water management is often not as high a priority as other concurrent cultural factors. Thus, decision support aids must also be developed that improve the pro- ducer’s ability to implement decisions quickly and easily because climatic variations and pest outbreaks require precisely timed water and chemical applications on a daily and seasonal basis. The decision support process must also provide accurate predic- tions of application efficiencies and uniformities to improve management flexibility.
Ensuring the success of irrigated farming enterprises will require the development of reliable and more-timely information on field and plant status to support the deci- sion-making processes. Current plant models capable of predicting the physiological needs of a crop over space and time tend to be complex and impractical for real-time on-farm management. Furthermore, most of these models are point models that are not sensitive enough to adequately predict site-specific plant needs across a field in a timely fashion. Simpler, more appropriate models might be used but will likely need frequent updating via automated, field-based sensor systems to readjust model vari- ables to ensure reasonable tracking and spatial predictions of field conditions.
A more focused approach will be required for the development of spatial and tem- poral management strategies that address site-specific crop water, nutrient, and pest management requirements, and irrigation scheduling in real time. The ultimate goal should be to integrate controls, sensor systems, plant and physical models, and other techniques to provide workable solutions that reduce time requirements for busy deci- sion makers while improving their management capacity.
Self-propelled irrigation systems, such as center pivots and linear moves, are par- ticularly amenable to site-specific approaches because of their current level of automa- tion and large area coverage with a single pipe lateral. These technologies hold much promise for spatially varying water and agrochemical applications to match differ- ences in irrigation, nutrient, and pesticide requirements throughout the field. This could potentially increase productivity and minimize adverse water quality impacts. By aligning irrigation water applications with variable water requirements in the field, total water use may be reduced, decreasing deep percolation and surface runoff. This also suggests that water-soluble fertilizers and other agrochemicals can be effectively applied spatially through the irrigation system to match changing conditions across a field. However, there is a need to develop more efficient methods of applying crop amendments (e.g., nutrients, pesticides) with irrigation systems that will reduce agro- chemical usage, improve profit margins, and reduce environmental impacts. In some cases, it may be necessary to schedule and irrigate on a plant-by-plant basis. This technology is available today, but is not yet economical or practical. Research is
needed to give farmers confidence that the use of these technologies is practical and potentially valuable in improving irrigated crop production.
Much effort has gone into ET research in the past several decades, and it is one of the most accurate estimates available to irrigation delivery and on-farm managers as long as plants are at relatively low stress levels. However, much additional informa- tion is still required on the yield and crop quality affect at various ET levels at differ- ent growth stages of both major and minor crops that are in managed soil water deficit conditions, perhaps in combination with increased soil salinity levels. The long term effect of deficits on perennial crops also needs more research to ensure sustainability. Critical information is lacking on actual ET under irrigation systems that are being used for environmental modification to protect the crop from cold temperatures or excessive heat. In addition, research on irrigation management and water use require- ments is lacking for intercropping production systems. More research is also needed on how to truly address spatial variability of ET across large fields. This information will be extremely critical to future determinations of both agricultural and urban water rights, water banking, and governmental allocations.
Remote sensing of plant and soil status using integrated satellite, aerial, and field level plant- and soil-based sensor systems is another way of providing information, but it also needs further development to improve spatial-temporal modeling and on-farm management as well as irrigation district operations. Better systems and methods ca- pable of precisely measuring specific plant parameters (e.g., nutrient status, water status, disease, and competing weeds) on a timely basis are needed to improve crop modeling and thus improve within-season management.
Real-time, on-the-go irrigation scheduling could be very effective in improving wa- ter management when based on distributed networks of farm-level microclimate and soil water sensor stations that feed into a microprocessor control system to manage irrigations based on rules established by the producer. This effort must be supported by expanded agricultural weather networks with a greater spatial density and grower- friendly information delivery systems for scheduling irrigations combined with pest management and marketing information. Input from distributed weather networks must be integrated with other information sources to effectively contribute to on-farm and irrigation district decision support processes.
In addition to irrigation, self-propelled irrigation systems also provide an out- standing platform on which to mount sensors for real-time monitoring of plant and soil conditions, which in turn provide input to the control system for optimal environ- mental benefits. Similar sensor technologies tied to a global positioning system can also be mounted on-farm equipment so that site-specific information is collected each time the grower is in the field. Early detection of diseases, weeds, insects, and even nutrient deficiencies would allow more economical spot treatments of small areas within a field.
1.5.4 Strategies for Local Water Supply Enhancement
Techniques to capture more water in either surface or subsurface impoundments and aquifers will only be briefly mentioned because they are beyond the scope of this