1. GA Post-16 National Conference, London, 20 June 2011
Surviving Hot Deserts!
Dr Jenny Hill
UWE, Bristol
GA Post-16 National Conference, London, 20 June 2011

2. How would you define an arid environment / hot desert?

3. ‘no single value can define deserts’
What are the characteristics of an arid environment that make it extreme?
1. Precipitation
areas receiving <250mm precipitation pa
but average annual rainfall is simplistic:
‘no single value can define deserts’
(Mabbutt, 1977)
areas of annual water deficit: evapotranspiration exceeds precipitation

4. uneven distribution of rainfall over space:
- cloud-free sky & ground heating leads to localised low
pressure
- adiabatic cooling to dew point, condensation, rain
- spatially confined high magnitude-low frequency
convective storms
uneven distribution time:
- Erkowit, Red Sea Hills, E. Sudan:
mm mm

5. 2. Temperature
average summer temps. in tropical deserts: 35oC - 40oC
wide temperature variations diurnally: up to 40oC
3. Soils
skeletal sandy/rocky soils, lacking in organic matter
(humus), so easily drained

6. So, where are they found … and why?

7. Introducing water management for agriculture
due to annual deficit, water must be collected and stored to allow agriculture throughout the year
this tends to occur i) below the surface in the soil/groundwater or ii) at the surface as reservoirs
but how do we manage water use sustainably … and what are the threats to sustainable management?

8. Rainwater harvesting versus dams in Tunisia
distinct climate zones & contrasting water management techniques
south: traditional rainwater harvesting via terraced slope systems
centre: modern dam irrigation

9. Study areas
Matmata Plateau
falls just within arid zone
negative annual water balance of 200mm-300mm
loess soils; sparse vegetation
rainwater harvesting
Zeroud Basin
negative annual water balance of 300mm-400mm
runoff collected quickly by wadis
dam irrigation

10. Rainwater harvesting (r.w.h.) (Matmata Plateau)
climate + topography + soils = favourable for r.w.h.
The r.w.h. system

11. Agroforestry is practiced in the fields

12. fields are sited progressively downslope through valleys

14. Tier 1
Tier 2
Water trapped behind the bunds creates a local water supply

15. cultivation largely subsistence, limited surplus sold at local markets
sites managed on collective basis following local custom
systems utilise indigenous technical knowledge on a small scale …

16. Traditional rainwater harvesting & sustainability
awareness of relationship between surface water and groundwater
r.w.h. promotes soil conservation & aquifer recharge

17. Dam irrigation (Zeroud Basin)
integration of farming into world markets since Independence (1957)
community management replaced by centralised control
e.g. Kairouan Programme (1975)
replaced small barrages & 30 local dams with two large dams (Sidi Saad (1982) and El Haoureb (1990))

18. Original barrages & small dams:
irrigated 30,000 ha
replenished aquifers
cost £3 million
employed 40,000 locals
Sidi Saad Dam:
originally irrigated 4,000 ha, now 1,000 ha
cost £30 million

19. Dam irrigation & sustainability
spatially and temporally unpredictable sediment input  dam siltation
life expectancy of Sidi Saad Dam is 87 yrs
if the dam had been constructed prior to 1969 autumn floods it would have been filled completely with sediment!
dams are outside limits of climatic viability: high evaporative losses
groundwater depletion in upper catchment
over-irrigation of agricultural land  salinization

20. to combat salinisation at field level, farmers have moved from furrow irrigation to trickle/drip irrigation
perforated plastic pipes following crop lines administer water directly to plant roots
minimises losses via surface runoff, evaporation and deep percolation
maximizes delivery to root zone

23. Conclusions
Rainwater harvesting
transforms hazardous env. into one of relative security
maximises long-term resource potential of landscape
carrying capacity delimited by nature
exemplifies flexible adaptation to dynamic/extreme envs.
decentralised, allowing community autonomy
Modern dams
neglect long-term resource potential of landscape: can lead to insidious env. degradation
carrying capacity delimited by society
rigid structures in a dynamic environment
centralised control & decision-making

24. The future for sustainable water management
balance large- & small-scale developments
will allow flexibility under climate change:
projections suggest less precipitation will occur in
dry months
mix of traditional & modern methods will be the
foundation to sustainable water use in Tunisia ...
and possibly elsewhere