Water scarcity and abundance, water productivity and their relation to poverty - Water Scarcity, Livelihoods and Food Security - page 21

viewpoint we are representing. In the end, water is scarce for someone in some
way nearly everywhere.
The right amount means that many crops have specific needs: while paddy
rice needs to be flooded, in contrast many crops are sensitive to waterlogging.
The purpose that water will be used for determines what the right quality is,
for example, some crops can tolerate more salinity than others so that the
salinity of irrigation water determines which crops can be grown with it.
Other aspects of quality include maintaining sediments and other pollutants at
acceptable levels. We can define the right purpose either narrowly in terms of
crop, agricultural system or landscape management, or broadly in terms of
water allocation across a wide range of ecosystem services. We use the right
time to take account of seasonality, and how seasonal patterns change over
time. The right people means equitable allocation between alternative groups
of water users, including people who benefit from water-related ecosystem
services, and sometimes water managers or polluters.
Several of these factors often come together to create scarcity where water
seems abundant. Water appears plentiful in coastal Bangladesh for most of the
year, but it is a water-scarce environment for some purposes. There is not
enough water of the right quality (salinity < 2 g/L) available at the right time
(end of wet season and throughout the dry season) for the specific purpose of
completing wet-season rice crops, followed by dry-season crops/aquaculture.
When the wet season ends with decreasing river flows, sea water intrusion
affects the quality of the river water surrounding the polders, so that quality,
timing and purpose together create scarcity (PN10 4 ) (Tuong and Hoanh,
2009).
Rainfall in the Ethiopian highlands is at least 1300 mm/yr and ET is modest,
yet water is often scarce for crops and livestock. The causes are sloping
landscapes that give high rates of runoff, soils with low water-holding capacity
and poor infrastructure for water-storage (Block, 2008; Awulachew et al.,
2010).
We now discuss water scarcity in terms of aridity, seasonal unreliability,
quality, excess and access using examples from CPWF projects.
Aridity
The water scarcity of arid lands, which have no rainy season, is physical scarcity
caused by low precipitation and high atmospheric demand. The ratio of mean
annual precipitation to PET—the aridity index (Middleton and Thomas,
1997)— ranges from zero to less than 0.20 in those regions. Water quality and
allocation are not part of this index.
Arid areas can only sustain agriculture with irrigation. Elsewhere they are
rangelands used at low intensity for ruminant production, often with nomadic
herders ranging 1000 km or more in their yearly transhumance. The herders
require access to crop residues and watering points, which is a critical
component that is coming under threat in the northern Sahel (PN64) (Clanet