872007 ), did not differentiate between areas under rainwater harvested water supply
and areas under other types of water supply for irrigation. This lack of global
information on where and how much rainwater harvesting is currently in use makes
it impossible to say how many people actually benefit from rainwater harvesting
today. It also becomes challenging to summarize the global and/or regional benefits
and costs in specific locations, countries or regions using rainwater harvesting for
human wellbeing or ecosystem impacts arising from rainwater harvesting (UNEP-
SEI 2009 ).
Introducing rainwater harvesting to improve soil ecosystem productivity in rain-
fed agriculture promises large social, economic and environmental paybacks, par-
ticularly in poverty reduction and economic development. Rainwater harvesting
presents a low-cost approach for mediating dry spell impacts in rainfed agriculture.
Remarkable success has been witnessed in poverty-stricken and drought-prone
areas in India and Africa. In sub-Saharan Africa, the future of more than 90 % of
rainfed farmers depends heavily on improved water security. In South Asia, about
70 % of agriculture is rainfed and some good work has been done in the design and
successful demonstration of a range of water harvesting structures, for both drink-
ing water supply and irrigation. In several other countries in the Middle East, Latin
America and South East Asia, rainwater harvesting is a traditional practice in cer-
tain regions, but the transferability of these models and practices has been limited.
One of the main problems is that the local institutions needed often are ineffective
and inconsistent with the predominant governmental structures and institutional
arrangements and policies prevailing in these countries (Samra 2005 ).
10 Uncertainties in Water Harvesting and Coping
Mechanisms
Water harvesting largely depends on rainfall which is variable in space and time,
amount and intensity, and its potential for runoff generation. These variabilities
along with the impacts of climate change induce uncertainty and constrain farmers’
crop production planning and investment. Frequent drought in some dry areas can
also prohibit effective water harvesting operations. Variability of annual and mean
monthly rainfall at Matrouh in northwestern Egypt is shown in Fig. 16 a, b. Soil
moisture also varies with location and time and can constrain crop production.
Among other factors, soil moisture depends on the rainwater harvesting potential of
the catchment and the properties of soil as a storage medium vis-à-vis the evapo-
transpiration needs of the crop.
Weather forecasting and dissemination of weather parameters (rainfall, tempera-
ture, wind, humidity) can help with the planning and timely actions of farmers and
users of water harvesting technology. Technologies such as weather radars, weather
forecasting models, and mobile communication can be used to disseminate infor-
mation to the users of water harvesting.
Water Harvesting in Dry Environments