Acknowledgements for funding to: Murray & Roberts South Africa Rainwater Harvesting Potential for Semi Arid Smallholder Farming PhD Research Presentation

Acknowledgements for funding to:

Murray & Roberts South Africa

Rainwater Harvesting Potential for Semi Arid Smallholder Farming

PhD Research Presentation to WaterNet Challenge Programme Final Scientific Workshop

15-18 June 2009

By

Alexander Mhizha

Witwatersrand University

Supervision

Dr. J. Ndiritu (Wits University)

Dr. I. Nyagumbo (University of Zimbabwe)

Introduction

• Most smallholder farmers in Southern Africa rely on rain fed cropping;– (e.g., Malawi, 90 %; Botswana, 76 %; Kenya, 85 %; and

Zimbabwe, 70 –80 %, of the population) (Rockström, 2000).

• Average yields are low (0.5-1 t/ha) compared to on station yields (5-6 t/ha.)

• Rainwater harvesting can be used to improve water availability to crops


Definition

• Rainwater harvesting is defined as a method for inducing, collecting, storing, and conserving local surface runoff (Boers and Ben-Asher 1982 quoted in Boers 1994)


Introduction

• Various types of rainwater harvesting (RWH) strategies exist– In-situ RWH, e.g. terracing, contour ridges (bunds)– Off field e.g. runoff catchments (check dams, rock

outcrops etc), roof catchments, dams or reservoirs on natural streams /rivers.

• Growing interest in the use of contour ridges for water harvesting.

• Appropriate rainwater harvesting strategies suited to local environments are critical for effectiveness.


The Problem• Knowledge on hydrological functioning of

contour ridges is limited to soil erosion control & not on runoff harvesting.

• Current designs geared to safely dispose off excess runoff and prevent rill & gulley erosion


Justification

• Limited knowledge on the effectiveness of contour ridges in improving water availability to crops limits the design options available for them and their adaptability by farmers.

• Potential to channel runoff harvested from adjacent catchments into cropped areas.


Objective of the study

• To quantify the rainwater harvesting potential of contour ridges in a semi arid catchment and develop a model for guiding improved contour ridge design incorporating water conservation.


Specific Objectives 1. To establish the hydrological processes

controlling water conservation from contour ridges.

2. To compare water conservation in fields with different contour ridge designs and that with no contour ridges.

3. To evaluate the water conservation benefit of contour ridges.

4. To develop a model for guiding the design of contour ridges for water conservation.


Location of Study Area


Bulawayo

Filabusi

Beitbridge

Zhulube Catchment

Mzingwane Catchment

Study methods

1. On farm field trials

2. Modeling water movement in a field with contour ridges


Continuity equation for storage in ridge

1EquationLDEQPRdt

dSccccc

c

Where

dt

dSc

= the change in contour storage during the time dt; R = the runoff into the contour ridge during the time dt;

cP = the direct precipitation on the contour ridge;

cQ = The surface discharge from the contour ridge;

cE = direct evaporation from the contour ridge during the time dt;

cD = infiltration and percolation from the contour ridge during the time dt;

cL = lateral seepage flow from contour ridge during the time dt;

On farm field trials plot arrangement


Field experimental plots arrangement

Graded Contour

Graded Contour

Graded Contour

Dead Level Contour

No contour plot

Dead Level Contour

Soil moisture measuring point

Runoff plot

Rain gauge

Dead Level Contour

Instrumentation on one of trial fields


Soil moisture measurement

Preliminary Results Objective 1


• Increased infiltration in the field

• Groundwater seepage from the water that infiltrates into the ridge

15 Jan 2009 22 Jan 2009

Hydrological Processes controlling water availability

Comparison DLC versus NC

Soil moisture variation within a DLC plot

Soil moisture variation within a non C plot

Experimental challenges

Comparison DLC versus GC

Soil moisture variation within a DLC plot

Soil moisture variation within a GC plot

Preliminary Observations


Crops close to contour ridges tend to be healthier

Generally farmers neglect weeds in areas around contour ridges

Outstanding Work


Evaluation of water conservation benefits from contour ridges

Developing a model for designing contour ridges for water conservation

Work in all objectives is continuing but major works still to start on:

Acknowledgements for funding to:


I Thank You

Documents

Acknowledgements for funding to: Murray & Roberts South Africa Rainwater Harvesting Potential for Semi Arid Smallholder Farming PhD Research Presentation