IS LARGE-SCALE PUBLIC IRRIGATION THE WAYFORWARD FOR SUB-SAHARAN AFRICA? - Sustainable Irrigation Development in the White Volta Sub-Basin - page 100

Agricultural production is achieved through the combination of inputs (land,
water, capital, nutrients, energy and labour) to produce agricultural outputs.
An analysis of the productivity of a single input factor enables assessment of
opportunities for maximizing returns from the use of this factor. Since the
provision of water is the main responsibility of the irrigation scheme for
agricultural production, the water productivity of the irrigation scheme is
worth knowing.
Table 4.6: Range of Global water productivity values for selected crops.

Crop Rice Maize Groundnut Tomatoes Onions Cowpea

Water
Productivity
(kg/m 3 )

0.15-
1.6

0.3-
2.0

0.1-0.4 5-20 3-10 0.3-0.8

Source: Molden et al., 2007, Renault and Wallender, 2000

Data used for the analysis of the water productivity included records of food
production and irrigation water use during the dry season. Water
productivity is a partial-factor productivity that measures how the systems
convert water into food (Molden et al., 2003). Its generic equation is:

Output Derived from Water use

Water productivity (kg/m 3 ) = (Equation

Gross Water Input

4.2)

Figure 4.11b shows that Vea has higher water productivity than Tono which
contradicts the irrigation efficiency. The reason is that in Vea the cultivated
area of tomatoes is higher than rice and the opposite occurs in Tono. The
yield of tomato is higher than that of rice. Meanwhile, rice consumes more
water than tomatoes. Therefore the relatively high production of tomatoes in
Vea results in higher crop yield at Vea which translates into high productiv-
ity as compared to Tono.
The highest irrigation efficiency experienced in 2002 was due to a measure
by the management of Vea to reduce the amount of waste water getting to
the downstream end of the irrigation field so as to compel the farmers using
the wastewater to pay water levies as well even though they were farming
outside the project area. This also resulted in high water productivity that
year. From this account it implies that, despite the weak structures, the
amount of wastage in irrigation can be reduced by management, namely if
management allows downstream users outside the scheme to benefit from the
return flows. The excess water used by users outside the scheme is included