In situ rainwater harvesting has a long history in arid and semi-arid regions of the world
buffering water shortages for human consumption and agriculture. In the context of an
Integrated Water Resource Management (IWRM) in the Cuvelai Basin in northern
Namibia, roof top rainwater harvesting is being introduced to a rural community for the
irrigation of household scale gardens for the cultivation of horticulture products. This
study elaborates how harvested rainwater can be used for garden irrigation in a
sustainable manner evaluating ecologic, economic and social implications. Considering
local conditions eight cropping scenarios were designed, including different criteria as
well as one and two annual planting seasons. These schemes were tested under present
climate conditions and under three future climate change scenarios for 2050 with the
help of a tank model designed to model monthly tank inflows and outflows. Special
attention was laid on risk and uncertainty aspects of varying inter-annual and interseasonal
precipitation and future climate change. A framework for the assessment of
sustainability was adapted to the purposes of this study and indicators have been
developed in order to assess the cropping and irrigation schemes for sustainability.
The study found that with the given tank size of 30 m³, depending on crop scenario,
under optimized conditions a garden area of 60 to 90 m³ can be irrigated. The choice of
crops highly impacts water use efficiency and economic profitability, compared to the
considerably lower impact of amount of annual planting seasons and future climate
change. In the case of worsening future climate conditions, adaptation measures need to
be taken as especially the economic as well as the environmental situation are expected
to exacerbate due to expected decreases in yields and revenues. Already under present
conditions however, the economic dimension represents the most limiting factor to
sustainability, particularly due to the excessive investment costs of the rainwater
harvesting and gardening facility. Nonetheless, rainwater harvesting in combination
with gardening can be regarded as successful in securing household nutrition, providing
sufficient horticulture products for household consumption or market sale. At the same
time with the optimal choice of crops the investment costs can be recovered within the
end of the lifespan of the facility.