FIGURE 4.3. Water use efficiency of irrigated and rainfed cereal crops (Source: Reprinted from Agricultural and Forest Meteorology, 103, M. Smith, The application of climatic data for planning and management of sustainable rainfed and irrigated crop production, pp. 99-108,2000, with permission from Elsevier Science.)
and introduction of irrigation scheduling strategies are analysis of the climatic conditions of the region and use of weather forecast information. Stochastically determined variability of rainfall and evapotranspiration is required for simulation of expected yield improvements and options for water storage (Smith, 2000).
Computerized procedures are available (Joshi, Murthy, and Shah, 1995; Smith, 2000) that greatly facilitate the estimation of crop water requirements from climatic data. The FAO CROPWAT program incorporates procedures for reference evapotranspiration and crop water requirements and allows the simulation of crop water use under various climate, crop, and soil conditions (Smith, 1992). As a decision support tool, there are several functions of the CROPWAT. These are (1) the calculation of reference evapotranspiration according to the FAO Penman-Monteith formula; (2) crop water requirements using crop coefficients and crop growth periods; (3) effective rainfall and irrigation requirements; (4) irrigation supply scheme to a given cropping pattern; and (5) daily water use computation.
The water balance procedure in CROPWAT allows the development of irrigation schedules and evaluation of irrigation practices in terms of water-use efficiency and the impact of water stress on crop yield. The system also allows the assessment of impact of rainfall, dry spells, and drought on crop production (Smith, 2000).
A short-range weather forecast (up to a lead time of five days) would be of significant value to farmers, particularly in surface irrigation. This value comes from such advantages as:
• Better ability to manage waterlogging, particularly in surface irrigation: This could mean the difference between scheduling an irrigation event or not. By chosing not to irrigate, farmers may reduce both waterlogging and irrigation-associated costs.
• Better ability to manage soil moisture and plant stress: In viticulture and horticulture this could have significant implications for the quality of the crop, with significant financial implications as well.
• Better time management (when to apply) of sprays for disease and pest control: Again, such information could have important financial and environmental rewards.
• Water-use savings and the associated cost savings: These include cost of water; labor; fuel; access to the crop, especially at harvest; soil compaction; less water table accessions; management time, etc.
Nonetheless, more predictive information on likely weather adds another string to the bow for improving water-use efficiency. There are a number of reasons for this. A forecast information is a race with time, which makes it more likely to be acted upon. Farmers are already accustomed to using weather forecasts. Use of a new service would take time as farmers learn, from experience, how to use the information and the degree of trust they can put in it. Adoption is also likely to be high because it is also driven by the other potential gains in farm management and profitability.
Water-use savings with seasonable predictions can also allow farmers to determine their need for storage and the associated economic costs.
Considerable potential exists to optimize the use of water for crop production, but strategies for more efficient usage are different for rainfed and irrigated agriculture (Smith, 2000).
Rainfed crop production is subject to frequent fluctuations in precipitation. Failing rains result in droughts and yield deficits, while excessive rains cause flooding and crop losses. Yields and water-use efficiency will therefore remain low even in periods with ample water supply or increased fertility levels. Crop water use needs to be optimized through more effective use and conservation of rainwater.
Extensive literature is available on technologies that can help conserve soil moisture and reduce evaporation and seepage from soil (Bos et al., 1994; Ventrella et al., 1996; Jalota and Prihar, 1998; Kazinja, 1999; Boldt et al., 1999; Mando and Stroosnijder, 1999). Most of the technologies have proved successful in achieving their objectives.
Petrochemicals have shown promising results in reducing water movement and evaporation from bare soil. In Saudi Arabia, petrochemical soil conditioner (Hydrogrow 400) has been tested for reducing water losses from a sandy soil. The application of the conditioner resulted in an increase of stored water under saturated conditions due to the minimization of both gravitational and evaporation losses. An application rate of 0.75 percent of Hydrogrow 400 was found to be the optimum for water conservation and the maintenance of an adequate supply of water for plant growth. This rate reduced water movement under saturated conditions by 79.2 percent and loss of water from evaporation by 30 percent (Sabrah, 1994).
Strip cropping, contour plowing, and terracing are used for reducing runoff, resulting in significantly increased soil moisture content. The protection provided by vegetation is also a major factor in runoff control. Plants intercept part of the rainfall and reduce the velocity of raindrops. They also slow down the movement of water on the soil's surface. Mulches of straw or crop residues, shredded bark, and wood chips break the impact of the raindrops and markedly improve infiltration and check evaporation (Smith, 1992).
The effect of tillage methods on crop growth and yields is to a large degree attributable to an increased soil moisture reservoir. This is achieved by creating soil conditions that favor root growth and penetration and improved infiltration and conservation of water. Tillage can be effective in reducing surface runoff if it is carried out according to soil conservation practices. By sacrificing a crop, moisture is conserved from one season to the next so that the combined precipitation of two seasons is sufficient for one crop.
An integrated watershed development approach was successfully used in peninsular India (Raoa Mohan Rama et al., 1996). Measures adopted were (1) diversion drains and staggered contour trenches in nonarable land, (2) terraces of trapezoidal cross section with a graded channel on the upstream side (locally termed a graded bund) and stone checks in arable lands and rockfill dams, and (3) archweir (a curved barrier) and earthen embankment across a gully. Hydrological analysis revealed that integrated measures consistently improved the groundwater regime. Surface runoff from the treated forest and agricultural catchments were only 27.4 and 57.4 percent, respectively, of the untreated agricultural catchment, reflecting high infiltration of rainwater due to enhanced opportunity time. Consequently, water levels in the open wells rose by 0.5 to 1.0 m, thereby increasing the area irrigated by the wells by 172 percent when compared to the pre-project period, which in turn improved crop yields by 70 percent.
A concept of temporal and spatial management of soil water (TSMSW) as a means to ensure effective use of soil water was developed by Jin and colleagues (1999) in North China. Four aspects were studied: readjusting crop structures and rotations to fit changes in soil water; increasing soil water resources; reducing soil water evaporation; and managing soil water to meet temporal and spatial crop water demand. Field experiments showed that temporal and spatial management of soil water can significantly increase water-use efficiency. For cotton, adopting an integration of microto-pography and plastic mulch increased WUE from 0.49 to 0.76 to 0.86 kg-m-3; stalk mulch with manure for winter wheat reached 2.41 kg-m-3; and straw mulch with deep furrows (microtopography) for summer maize increased it from 2.06 to 2.34 kg-m-3.
Water spreading schemes are applicable at specific sites to (1) assist in the control of erosion of susceptible soils and (2) increase the infiltration of water into the soil following rainfall (Wheeler, 1994). The additional soil moisture increases the yield of crops and pasture and may contribute to a general improvement in the soil condition.
An effective way to increase the benefit of high-intensity rainfall on pasture and crops is the construction of small spreading banks. Banks are usually from 100 to 120 m long and store a depth of water up to 400 mm. A gap of approximately 10 m between adjacent banks is left to allow for access during and after construction and to allow a passage for outflows when they occur. These outflows lead into the next bank downstream until they reach a natural stream or gully. Provision is made for water to be distributed downfield via sill boards, pipes, or open sections in the bank. Runoff from adjacent areas such as roads and rocky ridges or in nearby gullies is sometimes diverted to water-spreading schemes to supplement local rainfall.
Two levels of deficit irrigation strategies (irrigation limited to a certain growing period) for maize crop were evaluated by Boldt and colleagues (1999) in Nebraska. The first limited irrigation period started when the crop had accumulated 560 growing-degree days and ended when 1,220 GDDs were accumulated. On average, this is a five-week irrigation season. The second limited irrigation period started when 720 GDDs had accumulated and ended when 1,110 GDDs were accumulated, representing approximately a 3.5-week irrigation season.
The five-week irrigation season resulted in little yield reduction; however, applied water was reduced by 19 percent compared to irrigating for maximum yield. Limiting the irrigation season to 3.5 weeks decreased applied water depths further but had a more noticeable impact on grain yields. The yield was reduced by about 15 percent as compared to the maximum yields. Applied water decreased by 39 percent.
The Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia conducted successful field trials (Anonymous, 2000) into partial root-zone drying (PRD) to make horticultural crops more water efficient. This system involves the irrigation of only one side of the root zone. This causes a biochemical change in the roots that in turn leads to a reduced water loss from plant foliage. Fruit yield remains unaffected. Trials were done with drip-irrigated citrus fruit trees and grapevines. Results suggest that using the PRD system could provide farmers with water allocation savings of up to 50 percent.
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