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7 Supplemental Irrigation and Water Harvesting
In dry areas, it is water, not land, which is the most limiting resource for improved
agricultural production. Maximizing water productivity, not yield per unit of land, is a
better strategy for dry farming systems. Under such conditions, more efficient water
management techniques must be adopted. Supplemental irrigation (SI) is a highly-
efficient practice with great potential for increasing agricultural production and
improving livelihoods in dry rainfed areas. In drier environments, most rainwater is
lost by evaporation such that rainwater productivity is extremely low. Water harvesting
can improve agriculture by directing and concentrating rainwater through runoff to the
plants and other beneficial uses. Over 50 % of lost water can be recovered at very little
cost. However, socioeconomic and environmental benefits of this practice are far more
important than increasing agricultural water productivity (Oweis and Hachum 2006 ).
Supplemental irrigation is defined as the application of a limited amount of water
to the crop when rainfall fails to provide sufficient water for plant growth to increase
and stabilize yields. According to Oweis et al. ( 1999 ), the characteristics of SI in
rainfed areas include: (1) water is applied to rainfed crops which are normally pro-
duced without irrigation, (2) water is applied only when rainfall is inadequate
because rainfall is a prime source of water for rainfed crops, (3) the amount and
timing of SI are not meant to provide water stress conditions over the growing sea-
son, but to provide enough water during the critical stages of crop growth to ensure
optimal yield in terms of yield per unit of water.
SI during dry spells with microcatchment rainwater harvesting could improve
the soil water content in the rooting zone by up to 30 % (Biazin et al. 2012 ).
Harvested water from a small pond increased sorghum harvests by 41 % and, when
combined with added fertilizer, by 180 % (Fox and Rockström 2000 ).
SI systems are affordable for small-scale farmers (Fox et al. 2005 ). However,
policy frameworks, institutional structures and human capacities similar to those for
full irrigation infrastructure are required to successfully apply SI in rainfed agricul-
ture. Rainfed agriculture has traditionally been managed at the field scale. SI sys-
tems, with storage capacities generally in ranging from 100–10,000 m^3 , are small in
comparison to irrigation storage but require planning and management at the catch-
ment scale as capturing local runoff may impact other water users and ecosystems.
Legal frameworks and water rights pertaining to the collection of local surface run-
off are required, as are human capacities for planning, constructing and maintaining
the storage systems for SI. Moreover, farmers must take responsibility for the opera-
tion and management of the system. SI systems also can be used in small vegetable
gardens during dry seasons to produce fully-irrigated cash crops. SI is a key strat-
egy, which is underused, for unlocking rainfed yield potential and water productiv-
ity (Oweis et al. 2001 ; Rockström et al. 2010 ).
A. Yazar and A. Ali