Hydrological assessment of up-Hydrological assessment of up-scaling rainwater harvesting scaling rainwater harvesting

techniques in Upper Mzingwane sub-techniques in Upper Mzingwane sub-catchment, Zimbabwe.catchment, Zimbabwe.

By

Kudzai F. Ndidzano Limpopo Basin Development Challenge (LBDC)

Research BackgroundResearch BackgroundMzingwane catchment characterized by

Smallholder communal farming areaFrequent droughts and long mid season

dry spellsWater shortages in the root zone often

lead to crop yield reductions, SSA maize yield <1ton/ha

Limited blue water resourcesIsolated successes in rainwater

harvesting (RWH) interventionsNeed to assess the effect of wide scale

adoption across a sub-basin level2

Study AreaStudy Area

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•Land Area = 7 700km2

•Average Annual rainfall = 450mm•Mean Annual Runoff = 48mm (CV =118%)•Evaporation = 1800-2000mm•Mean Annual T°C : Min = 5°C and Max = 30°C (ZINWA, 2009)•Moderately shallow kaolinitic sands, very shallow to mod shallow sandy loams and clays and v shallow sands from basalts.

Objectives and Objectives and MethodologyMethodology

1. Identifying RWH techniques in practice◦Literature review◦Key Informant interviews◦Reconnaissance survey

2. Investigating reasons for RWH adoption◦Administered structured questionnaire◦Focus group discussion

3. Determining upscaling RWH effect on runoff◦ RWH scenario development◦ Soil and Water Assessment Tool (SWAT) modeling

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Modeling Scenarios’ Modeling Scenarios’ Roughness CoefficientsRoughness Coefficients

Land surface characteristic Median

Range

Conventional Farmers Practice (CP)

0.09 0.008-0.012

Min Tillage + No residue 0.07 0.04-0.100Min Tillage + 0.5-1 ton/ha

residue0.12 0.070-0.170

Min Tillage + 2-9 ton/ha residue 0.3 0.17-0.470

•The median Mannings’ roughness coefficients were The median Mannings’ roughness coefficients were substituted for the Overland flow roughness coefficient substituted for the Overland flow roughness coefficient (OV_N) for the agriculture land use in the model’s (OV_N) for the agriculture land use in the model’s Hydrological Response Unit (HRU) file to simulate each Hydrological Response Unit (HRU) file to simulate each scenario. (Arnold scenario. (Arnold et al., et al., 2011)2011)

•Simulation results analyzed using the Paired Sample Simulation results analyzed using the Paired Sample T-T-test for Comparison of Means using P = 0.05 significance test for Comparison of Means using P = 0.05 significance levellevel

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ResultsResults

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Obj. 1: Identified RWH Obj. 1: Identified RWH practicespractices Mainly Conservation

Agriculture (CA) fertilizer micro-

dosing small plots

averaging 0.5 ha Isolated dead level

contours with infiltration pits

Proportion of RWH to conventional farmers’ practice (CP) still fairly low (<35% of ward population).

CA Umzingwane Ward 13

Fertilizer microdosing

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Obj. 2. Reasons behind adoption of Obj. 2. Reasons behind adoption of Rainwater harvesting- Conservation Rainwater harvesting- Conservation Agriculture .Agriculture .

•Rainwater capture, improved yields and Rainwater capture, improved yields and draught power shortages and are the main draught power shortages and are the main drivers for CA practicedrivers for CA practice

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Objective 2 Cont…Objective 2 Cont…

• CA was found to reduce runoff, soil erosion and CA was found to reduce runoff, soil erosion and draught power requirement while increasing yield, draught power requirement while increasing yield, labour requirementlabour requirement

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Objective 2 Cont…Objective 2 Cont…

•Labour, weed control and crop residue management Labour, weed control and crop residue management are the major challenges to CA adoption are the major challenges to CA adoption 1

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Obj. 3. CA Scenarios Modeling Obj. 3. CA Scenarios Modeling resultsresults

Scenario Combination

Paired Differences

t df Sig.

95% Confidence

Interval of the Difference

Mean Std. Deviatio

n

Std. Error Mean

Lower Upper

Current v CP .143 .356 .051 .0397 .2462.7

847

.008

CP v Min Tillage + Min Residue

.006 .008 .001 .0037 .0085.38

47.001

Current v Min Tillage + Min Residue

.149 .358 .052 .0452 .2532.89

47.006

Current v Min Tillage + Max Resdue

.153 .364 .053 .0469 .2582.91

47.006

Min Tillage + Max Residue v Min Till + Min Residue

-.004 .021 .003 -.0097 .003-

1.14

47.258

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•Significant (P<0.05) runoff reductions between Significant (P<0.05) runoff reductions between the CP and Current scenario against CA scenariosthe CP and Current scenario against CA scenarios

Runoff Comparison under CA Runoff Comparison under CA ScenariosScenarios

•The simulated CA scenarios show significant The simulated CA scenarios show significant (P<0.05) runoff reduction compared to the (P<0.05) runoff reduction compared to the current scenariocurrent scenario•Mutiga Mutiga et al., et al., (2011) also noticed moderate (2011) also noticed moderate runoff reduction from RWH intensification in the runoff reduction from RWH intensification in the Upper Ewaso Ngiro basin, KenyaUpper Ewaso Ngiro basin, Kenya 12

Conclusion and Conclusion and RecommendationRecommendation

ConclusionsCA is the main RWH practice in Upper Mzingwane

mainly to capture rainwater, improve soil moisture and crop yields.

Modeling results showed that up-scaling CA practices significantly (P< 0.05) reduces runoff

RecommendationsThe study recommends physical field verification of

runoff to confirm modelling results and further study on the effect up-scaling RWH on the downstream water requirements.

It is also recommended to capacitate farmers in overcoming CA challenges through mechanized systems, improved weed and pest management.

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Expected ImpactExpected ImpactIncreased

evidence based CA adoption

Promotion of Mechanized CA techniques

Farmer capacity building

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Thank you!!!Thank you!!!

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