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