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Agricultural Water Management 45 (2000) 275±296
Nutritional water productivity and diets
a,*,1,2 b
D. Renault , Professor W.W. Wallender
a
Irrigation Engineer, International Water Management Institute, P.O. Box 2075, Colombo, Sri Lanka
b
Departments of Land, Air and Water Resources (Hydrology Program) and Biological and Agricultural
Engineering, University of California, Davis, CA 95616, USA
Accepted 26 October 1999
Abstract
The increase in water productivity is likely to play a vital role in coping with the additional
requirement for food production and the growth of the uses of water other than in agriculture in the
coming century consistent with the shift from productivity per unit land to productivity per unit
water, the nutritional productivity of water is calculated as energy, protein, calcium, fat, Vitamin A,
iron output per unit water input.
Nutritional productivity is estimated in the agricultural context of California for the main crops
and food products. In general vegetal products are much more productive than animal products.
Four crops emerge as highly productive for one or several key nutrients: potato, groundnut, onion
and carrot. A balanced diet based on these four crops requires a consumption of water
(evapotranspired) of 1000 l per capita per day, while the current needs for the diet in the USA is
5400 l, and 4000 l for developed countries.
Onthebasisofnutritional productivity analysis it is further demonstrated that a signi®cant part of
the additional water resource to produce food for the next century population can be generated by
changes in food habits. A reduction of 25% of all animal products in the developed countries' diet
generates approximately 22% of the additional water requirements expected by the year 2025.
#2000Elsevier Science B.V. All rights reserved.
Keywords: Water productivity; Nutrition; Diet; Food Production; Water requirements
*Corresponding author. Tel.: 33-38824-8224; fax: 33-383388-248284.
E-mail addresses: d.renault@engees.u-strasbg.fr, d.renault@cgiar.org (D. Renault), wwwallender@ucdavis.edu
(W.W. Wallender)
1Tel.: 94-1-867404; fax: 94-1-866854.
2ENGREF. Montprllier, France, for the early stages of this study.
0378-3774/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S0378-3774(99)00107-9
276 D. Renault, W.W. Wallender/Agricultural Water Management 45 (2000) 275±296
1. Introduction
Attheturn of the third millennium there is a growing awareness that water is one of the
crucial limiting factors for increased food and fiber production to supply an ever growing
number of people under increasing competition with other users of water (municipal,
industrial, environmental, etc.). The fundamental question, which underlies current
debates in many forums is: how many people can the planet sustain, given our limited
availability of natural resources?
Theanswerwill obviously depend to a large extent on the availability of water for both
rainfed and irrigated agriculture, the size of the human population and ultimately the
water requirements to grow crops and produce food. The irrigated areas contribute a
major fraction of the global food supply. However, the possibility of expanding the
irrigated areas is becoming rare and costly (Carruthers et al., 1997), therefore, improving
productivity within the existing irrigated areas and within the rainfed agriculture is
crucial. The concept of productivity has, in recent decades shifted from `Crop per unit
area' to `Crop per unit volume of water'. The step sustaining the human population is
nutrition per water volume.
In this paper, nutrition per water volume is quantified in the context of improving
human food production given our limited water resources and modified diets are
evaluated. Water productivity is expressed in kg/m3 whereas nutritional water
productivity is expressed in nutritional units/m3 (nutritional units being energy, protein,
calcium).
2. Water productivity
The concept of productivity, i.e. production per unit input, focuses on limiting factors
or constraints. In the mid-70s, for example, the petroleum crisis highlighted the
importance of energy in agriculture and the productivity of energy became popular. In
areas where labor is constrained, due to rural migration, the concept of labor productivity
is used. Water is also a limiting resource and various productivity measures have been
suggested.
The concept of water productivity is certainly not new. There is a long history of the
development of efficient techniques for managing scarce water in arid areas. Even the
case of the Indus basin development in the 19th century, relies on the concept of water
productivity. In this case, the water delivery was purposely designed to meet only 1/3 of
the command area water requirements because the operational goal was to reach as many
farmers as possible within the available water resource. The productivity indicator of the
development was then the number of farmholdings per unit of water.
The development of large projects after World War II, temporarily led to the illusion
that water is limitless. During the 1970s, the world community again realized that water
resources are limited. It was at this time that, for example, breeders and geneticists
developed a better understanding of the water use during photosynthesis (Stone, 1975).
The difference in water use between C3 and C4 plants and the consequences on total
water use were documented. A C3 plant (wheat, barley, rice, potato) produces 1 tonne of
D. Renault, W.W. Wallender/Agricultural Water Management 45 (2000) 275±296 277
dry matter with 600 tonnes of water, while a C4 plant (maize, sorghum, sugarcane)
requires only 300 tonnes (Tinus, 1975). The ratio of the photosynthesis and the
transpiration expresses the water-use efficiency of the crop. This ratio is related to both
the gradient of CO2 at the leaf surface between the inside and outside, and the resistance
of the mesophyll for carbon dioxyde. C4 plants have a higher gradient and a lower re-
sistance than C3 plants, and therefore, a much better water-use efficiency (Feddes, 1988).
Agronomists evaluate the productivity of water through water use efficiency (WUE),
the ratio of yield to water consumed (kg/m3) by the crop through evapotranspiration at the
field scale (Doorenbos and Kassam, 1979) or as the yield per unit depth of water depth
per area (kg/ha/mm) (Gregory, 1991). Biomass yield may also include straw and roots
when the latter have an economic value (Gregory, 1991).
Water use efficiency concepts have been applied in diverse contexts for both rainfed
and irrigated agricultures (Shalhevet et al., 1992). Water productivity in irrigation was
debated during the late 70s and early 80s in India (Sundar and Rao, 1984; Chambers,
1985). More recently, studies on water efficiency and productivity have expanded to
include `real' or `virtual' water savings, and the necessity to analyze the problem at the
water basin level (Seckler, 1996) as well as advocating a consistent approach of water
accounting (Molden, 1997; Young and Wallender, 1999).
In a water scarce country such as Israel, water productivity has significantly increased
from 1.60 kg/m3 in 1949 to 2.32 in 1989 (Stanhill, 1992). This has been made possible by
increases in the water application efficiency at the field scale. Stanhill then identifies
plant breeding as the main avenue to further increase water productivity.
2.1. Models
Productivity may be estimated as the ratio of the output of an economic unit and the
inputs:
PRODUCTIVITYOUTPUTS (1)
INPUTS
Herein, assume water is the limiting input and calculate output. Water productivity is
based on the ratio of mass produced (actual yield, Y ) to the water consumed (actual
a
evapotranspiration ET ). This productivity is often expressed in kg/m3 and is increasingly
a
used to measure performance for irrigation systems. A more comprehensive approach for
productivity (Molden et al., 1998) introduces the economical value of the agricultural
production ($/unit of water). Performance comparisons among irrigation systems
producing different crops in different environments are thus possible.
2.2. Average and marginal productivity
Productivity is estimated as an average value for the whole cropping season, i.e. actual
yield (Y ) divided by actual water evapotranspired (ET ) as follows:
a a
AverageProductivity Ya (2)
ETa
278 D. Renault, W.W. Wallender/Agricultural Water Management 45 (2000) 275±296
Although the average productivity facilitates a comparison between crops and products, it
is not suf®cient to fully express the yield response to water. The marginal productivity, in
contrast, re¯ects the productivity of an additional unit of water, as follows:
MarginalProductivity dYa (3)
dET
a
The marginal productivity of water is crucial in determining the optimal allocation of
scarce water. In a rainfed system, an increment of water can be applied either as
supplemental preventative irrigation or as an emergency irrigation to avoid crop failure.
In an irrigated system when shortages occur, more sensitive crops and yield sensitive
periods have ®rst priority.
2.3. Water input
In general water productivity is a function of water applied which depends on space
scale and generally increases from small plots to large agricultural domains at a basin
scale because applied water is recycled and reused. Herein, the domain under
consideration is the field scale. We consider water supply through direct precipitation
and/or through irrigation and we are interested in the fraction of applied water which is
consumed by evapotranspiration. We assume crop transpiration and direct soil
evaporation as the water input of the process. Other components such as runoff and
percolation, or losses along the water delivery infrastructure are not accounted for.
2.4. Yield
The yield response to water is highly dependent on the yield response factor (k )
y
linking evapotranspiration to yield. The relationship between relative yield decline and
relative evapotranspiration deficit is linear for a range of deficits which do not lead to
crop failure. This relationship is (Doorenbos and Kassam, 1979):
1ÿYa k 1ÿETa (4)
Y y ET
m c
where Y is the actual harvested yield, Y the maximumharvested yield, k yield response
a m y
factor, ET the actual evapotranspiration, and ET the potential crop evapotranspiration.
a c
Theyield response factor (k ) varies from one crop to another, and from one vegetative
y
period to another. Doorenbos and Kassam (1979) states that maize is much more sensitive
to water stress (k 1.25) than groundnut (k 0.7). Therefore, in the case of water
y y
shortages priorities for water distribution should be based on the yield response factor
along with other considerations such as market prices. For crops having a yield response
factor below unity, the maximum water productivity is obtained for a water supply and a
yield less than the maximum values, as recorded by Maozheng and Wang (1992) for a
winter wheat in north China. For most crops the yield response factor reaches a peak
during the flowering period.
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