Innovations in Dryland Agriculture

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quality is especially important when a structure provides water for domestic use.
Water quality is less relevant when used only for irrigation (Lasage and Verburg
2015 ).
There is sufficient freshwater available each year to fulfill the needs of the cur-
rent population on Earth. However, in certain regions and countries, the annual
renewable supply of water per person is less than 500 m^3 (Qadir et al. 2007 ). The
need for WH, as mentioned above, arises from many factors such as low rainfall and
uneven distribution, high losses due to evaporation and runoff, and increased
demand for water due to population growth (Abu-Awwad and Shatanawi 1997 ;
Prinz 2002 ). With a large proportion of the human race living in arid and semiarid
regions, the applicability of WH to increase water access in these areas should be
investigated.
As WH becomes an important strategy to deal with water scarcity or water stress,
it is important to consider the factors involved in selecting the appropriate WH
method to maximize hydrological returns. It is tempting to assume that a system
which works in one area will also work in another, seemingly similar zone. However,
there may be technical dissimilarities such as the availability of stone or intensity of
rainfall, and distinct socioeconomic differences (Critchley et al. 1991 ).
Several critical factors that need to be considered when selecting the appropriate
WH method are outlined below.

5.1 Rainfall

WH depends on limited and uncertain rainfall, thus understanding the dynamics of
precipitation within the environment will influence which WH method fits best in
each context (Qadir et al. 2007 ). Various factors should be taken into account
regarding precipitation including:

1. Number of days where the rain exceeds the threshold rainfall of the catchment,
   on a weekly or monthly basis.


2. Probability of occurrence (in years) of the mean monthly rainfall.


3. Probability and reoccurrence of the minimum and maximum monthly rainfall.


4. Frequency distribution of storms of different specific intensities (Prinz and Singh
   2000 ).
   Rainfall in arid and semiarid regions is highly erratic, and most rain falls as
   intensive, generally convective storms, with very high rainfall intensity and extreme
   spatial and temporal variability. The result is very high risk for annual droughts and
   intra-seasonal dry spells. The annual variation of rainfall can typically range from a
   low of one-third of the long-term average to a high of approximately double, mean-
   ing that a high rainfall year can have some six times higher rainfall than a dry year.
   Statistically, in a semiarid region, severe crop reductions caused by a dry spell occur
   once or twice in five years, and total crop failure by annual droughts occurs once
   every ten years (Rockström 2002 ). This means that poor distribution of rainfall over

Water Harvesting in Dry Environments