Rainwater Harvesting (RWH) in Limpopo Basin
- Challenges
CP 17 Project
4th Scientific Workshop, JHB 15th to 18th June 2009
Limpopo River Basin
Limpopo Basin - Rainfall
Rainfall Distribution and Waterbalance
Xai-Xaiaverage annual rainfall: 953.0 mm
Pafuriaverage annual rainfall: 380.0 mm
Chókwè: rainfall variability
Crop Failure Hazard (very high)
Options in Rainfed Agriculture
poverty trap
what to doto reverse trends
Climate: rainfall harvest rainfall storage
Crops: length tolerance root
Soil: soil structure soil management
Potential for Rainfall Water Harvest
Conceptual Model
conceptualization
modelling
Low Cost Interventions:• explore rainfall pattern;• crop (ETc);• soil storage.
Other possible interventions:• rainfall harvest and storage-
supplementary irrigation;• medium to large dams and irrigation.
ET crop = Kc x ETo (Kc = Kcb + Ke)
Decrease Crop Failure Hazard: mulching to decrease soil evaporation; increase rainfall collecting area; soil/root water storage reservoir
Rainfall Water Harvest Techniques
Rainfall Water Harvest Techniques
Micro catchment Macro catchment
Barreira trapezoidal
Barreira semi circular
Flood water catchment
Barreira semi circular
Barreira de pedra
Represa de terra
Rainfall Water Harvest Techniques
Rainfall Water Harvest Techniques
Rainfall Water Harvest Techniques
Computer Models to Assess Potential RWH
• Visual Water Balance Processes for In-Field RainWater Harvesting – VISWBP-IRWH)- animation program to explain farmers and extension officers
• CROPWAT model• SWAT model
• PARCHED THIRST (PT) - process-based model, which combines the simulation of hydrology with growth and yield of a crop.
Decision support tool that addresses challenges of low and unreliable crop production in semi arid areas
Limitations of some computer Models for RWH
Features
MODELS
CROPWAT SWAT PARCHED THIRST
Partition of Evaporation and Transpiration
No Yes Yes
Ratio catchment area : production area
No No Yes
Interaction with Soil fertility
No No Yes
Economic component
No No No
Spatial variation No Yes Yes
Rainwater Harvesting (RWH) in Limpopo basin - Challenges
UEM-FAEF-SUTA
Conceptual Model
conceptualization
modelling
• Crop growth model: water and soil fertility issues• Integrated Pest Management• Economical aspects
Economical Feasibility of RWH Technology
• Comparison of the performance of good RWH for maize, paddy and vegetable enterprises over a period of 5 years
Source: Hatibu et al (2004)
Rainfall Water Harvest Techniques
• Plastic-covered ridge and furrow rainfall harvesting for corn production
*Except labor cost
Source: Yan Li et al (2001)
Season Yield increase (%) Economic benefit *(US$)/ha
Dry/Average years 60 – 95 100 - 125
Wet years 70 - 90 135
Very wet years 20 - 30 6 - 12
Remarks
• Adoption of RWH improve the yields and reduce crop failure.
• Economic benefits of RWH are greater when coupled with high
value crops associated to other agricultural inputs (fertilizers,
IPM).
• Successful water harvesting projects are often based on farmers’
experience and trial and error rather than on scientifically well
established techniques. There are still big challenges in
hydrological and crop modelling associated to economical
analysis in different environments.
Remarks
• Although RWH can increase the water availability, the
climatic risks still existing.
• Planting dates are crucial, but should be based in rainfall
events and other factors (soil, pests) versus risk
management (e.g. two consecutive rainy days totalizing 35
mm and planting several periods in a year).
Rainwater Harvesting (RWH) in Limpopo basin - Challenges
CP 17 Project
4th Scientific Workshop, JHB 15th to 18th June 2009
Thank you