By Kudzai F. Ndidzano Limpopo Basin Development Challenge (LBDC) Hydrological assessment of up-scaling rainwater harvesting techniques in Upper Mzingwane sub-catchment, Zimbabwe. By Kudzai F. Ndidzano Limpopo Basin Development Challenge (LBDC)

Research Background Mzingwane catchment characterized by Smallholder communal farming area Frequent droughts and long mid season dry spells Water shortages in the root zone often lead to crop yield reductions, SSA maize yield <1ton/ha Limited blue water resources Isolated successes in rainwater harvesting (RWH) interventions Need to assess the effect of wide scale adoption across a sub-basin level

Study Area 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 Methodology Identifying RWH techniques in practice Literature review Key Informant interviews Reconnaissance survey Investigating reasons for RWH adoption Administered structured questionnaire Focus group discussion Determining upscaling RWH effect on runoff RWH scenario development Soil and Water Assessment Tool (SWAT) modeling

Modeling Scenarios’ Roughness Coefficients Land surface characteristic Median Range Conventional Farmers Practice (CP) 0.09 0.008-0.012 Min Tillage + No residue 0.07 0.04-0.100 Min Tillage + 0.5-1 ton/ha residue 0.12 0.070-0.170 Min Tillage + 2-9 ton/ha residue 0.3 0.17-0.470 The median Mannings’ roughness coefficients were substituted for the Overland flow roughness coefficient (OV_N) for the agriculture land use in the model’s Hydrological Response Unit (HRU) file to simulate each scenario. (Arnold et al., 2011) Simulation results analyzed using the Paired Sample T-test for Comparison of Means using P = 0.05 significance level 5

Results

Obj. 1: Identified RWH practices 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

Obj. 2. Reasons behind adoption of Rainwater harvesting- Conservation Agriculture . Rainwater capture, improved yields and draught power shortages and are the main drivers for CA practice

Objective 2 Cont… CA was found to reduce runoff, soil erosion and draught power requirement while increasing yield, labour requirement

Objective 2 Cont… Labour, weed control and crop residue management are the major challenges to CA adoption

Obj. 3. CA Scenarios Modeling results Scenario Combination Paired Differences t df Sig. 95% Confidence Interval of the Difference Mean Std. Deviation Std. Error Mean Lower Upper Current v CP .143 .356 .051 .0397 .246 2.78 47 .008 CP v Min Tillage + Min Residue .006 .001 .0037 5.38 Current v Min Tillage + Min Residue .149 .358 .052 .0452 .253 2.89 Current v Min Tillage + Max Resdue .153 .364 .053 .0469 .258 2.91 Min Tillage + Max Residue v Min Till + Min Residue -.004 .021 .003 -.0097 -1.14 Significant (P<0.05) runoff reductions between the CP and Current scenario against CA scenarios 11

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

Conclusion and Recommendation Conclusions CA 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 Recommendations The 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.

Expected Impact Increased evidence based CA adoption Promotion of Mechanized CA techniques Farmer capacity building

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