Abstract
Evidence suggests that, in general, subjective rather than objective irrigation scheduling decisions are adopted by farmers. Irrigators have ‘calibrated’ themselves with years of experience to irrigate subjectively according to perceived crop water requirements. This study aimed to determine the associated benefits of objective versus subjective scheduling of two shallow groundwater cropping systems. Weekly measurements included rainfall and irrigation amounts, soil water content, groundwater table depth, artificial drainage volumes, and electrical conductivity of irrigation water, groundwater and drainage water. Simulations of evaporation and transpiration were done with the SWAMP model. Based on soil water and salinity status, matric and osmotic stress during the four cropping seasons is considered unlikely. When rainfall-plusirrigation was compared to evapotranspiration, objective scheduling resulted in an under-supply of 15%, and rainfall and shallow groundwater served as supplementary water sources. Subjective scheduling did not use rainfall efficiently as a source of water and resulted in an over-supply of 10%. Approximately 50% less salt was leached with objective compared to subjective irrigation scheduling. Under shallow groundwater conditions, irrigating subjectively according to crop water requirement results in excessive irrigation, salt addition and leaching compared to objective scheduling. Farmers can address some of the environmental problems associated with irrigation by adopting objective scheduling and reducing the leaching fraction (< 0.15) of shallow groundwater cropping systems. Keywords : evapotranspiration, leaching, salinity, water conservation, water degradation
Highlights
On-farm water and salt management must be continually evaluated and improved
Irrigation, soil water content and groundwater table depth for Case Studies 1 and 2 during the four cropping seasons are shown in Figures 3 and 4, respectively
During the early part of the second wheat season (Week 1, 2008), the groundwater table level rose sharply to 1 500 mm, because of high rainfall (115 mm) that fell during the drying phase of maize grown during the first season (2007) when evapotranspiration was low (Fig. 3)
Summary
On-farm water and salt management must be continually evaluated and improved. Overirrigation may deteriorate the quality of water resources, because of salt pollution resulting from excessive drainage and leaching (United States Salinity Laboratory Staff, 1954; Van Schilfgaarde, 1990; Letey, 1994; Rhoades, 1997; Hillel, 2000; Oster and Wichelns, 2003; Hillel and Vlek, 2005; Kijne, 2006; Le Roux et al, 2007; Van Rensburg et al, 2008; Van Rensburg et al, 2011). Farmers are under increasing pressure, to prevent the degradation of water resources, and to produce higher yields with less water (Hillel and Vlek, 2005; Pott et al, 2009; Kijne, 2011). Advocates for a more sustainable irrigation sector attempt to empower farmers and encourage them to continually evaluate and improve on-farm water and salt management (Kijne, 2006)
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