Rainwater Harvesting for Small-Holder Horticulture in Namibia: Design of Garden Variants and Assessment of Climate Change Impacts and Adaptation
2. Study Area
3. Methods and Data
3.1. Overview of Methodology
3.2. Modeling the Tank Balance
3.2.1. Modeling Tank Inflow
3.2.2. Modeling Tank Outflow
3.3. Climate Data and Scenarios
|Scenarios||Temperature (°C)||Precipitation (%)|
|Best future climate||minimum increase||maximum decrease|
|Medium future climate||median increase||median decrease|
|Worst future climate||maximum increase||minimum decrease|
4. Results and Discussion
4.1. Climate Analysis
4.2. Scenarios of Future Precipitation, Temperature and Reference Crop Evapotranspiration
|Scenarios||Temperature Change (°C) for 2050||Precipitation Change (%) for 2050|
|Minimum (Best Case)||Median (Medium Case)||Maximum (Worst Case)||Minimum (Worst Case)||Median (Medium Case)||Maximum (Best Case)|
|December, January, February||+1.2||+2.1||+3.1||−4||0||+7|
|March, April, May||+1.1||+2.1||+3.1||−17||0||+8|
|June, July, August||+1.3||+2.3||+3.2||−29||−15.3||−2|
|September, October, November||+1.4||+2.5||+3.3||−29||−8.7||+2|
4.3. Garden Variants
|Garden variants||Subsistence||Cash||Low Water||Super Cash|
|Description||Aimed for household consumption, with high nutritional value of crops combining a healthy diet with saved expenses for food.||Maximizes revenue from market sale, crops combine high yields and market with low water requirements.||Most water efficient crops, market prices are not considered.||Maximizes revenues, by planting the crop that achieves the highest market price through a high yield and low water requirement.|
|Crops||water melon, cucumber, cabbage, pepper, tomato, potato, 1 orange tree||cucumber, lettuce, water melon, 1 mango tree||cucumber, water melon, lettuce, onions, zucchini, 1 avocado tree||cucumber|
|Growing period||01 January–14 June||01 January–15 April||01 January–15 April||01 January–15 April|
|fruit tree all year||fruit tree all year||fruit tree all year||-|
|Garden area (m2)||66||94||100||104|
|Irrigation requirement (m3)||37||39||37||45|
|Irrigation requirement (m3/m2)||0.557||0.402||0.372||0.431|
|Crop production (kg/yr)||338||370||369||416|
|Revenue (€/yr) *||65||105||84||206|
|Water efficiency (kg/ m3)||9.2||9.8||9.9||9.3|
|Water productivity (€/m3)||1.8||2.8||2.3||4.6|
4.4. Analysis of Climate Change Impacts
4.5. Development of Adaptation Strategies to Climate Change
|Adaptation Measure||Effect||Complete Compensation of Worst Future Climate Scenario|
|Adaptation of garden area||Fully irrigate a smaller garden area||Yes. Disadvantage: Yield and revenue reduction.|
|Switch to garden variant with higher water efficiency||Rise in water efficiency of e.g., +8% when switching from the subsistence to the low water garden||Partly. Only possible for gardens with low water efficiency and by changing the strategic goal of a farmer.|
|Purchase of irrigation water from the grid||Garden can be continuously irrigated||Yes.|
|Adaptation of the roof catchment size||More rain can be harvested||Yes. With an extension of the roof by +10% (current: 100 m2 roof, 28.4 m3 harvested rainwater) 28.7 m3 can be harvested under the worst case climate scenario. Disadvantage: costs and practicability.|
|Observation of water quantity stored and early reaction in case water is not sufficient||Result was not significant||No.|
4.5.1. Adaptation of Garden Area
4.5.2. Switch to Garden Variant with Higher Water Efficiency
4.5.3. Purchase of Tap Water
4.5.4. Enlargement of Roof Size
4.5.5. Observation of Water Level and then Reduction of Garden Area
4.6. Applicability and Limitations of Results and Methodology
- Two important factors influencing the yield that can be achieved with rainwater harvesting based gardening are the household’s decision regarding a certain garden variant and future climate conditions.
- Except for the best-case climate scenario, climate change is projected to lead to considerable negative impacts on rainwater harvesting and gardening. The risk of water-related harvest losses would increase if garden sizes and, thus, revenues are not decreased.
- This study demonstrated a methodology to quantify the effects of climate change on rainwater harvesting based gardening. The actual values resulted in the study may not be accurate due to the assumptions made for climate projections in this study and may be refined with further studies.
- Viable measures to adapt rainwater harvesting and gardening to the prevailing climate situation in the future include an extension of the roof size, the purchase of additional tap water and a switch from low water efficient crops to high water efficient crops. When taking these measures, the negative impacts of climate change can be partly or completely compensated, however each tied to specific repercussions.
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Woltersdorf, L.; Liehr, S.; Döll, P. Rainwater Harvesting for Small-Holder Horticulture in Namibia: Design of Garden Variants and Assessment of Climate Change Impacts and Adaptation. Water 2015, 7, 1402-1421. https://doi.org/10.3390/w7041402
Woltersdorf L, Liehr S, Döll P. Rainwater Harvesting for Small-Holder Horticulture in Namibia: Design of Garden Variants and Assessment of Climate Change Impacts and Adaptation. Water. 2015; 7(4):1402-1421. https://doi.org/10.3390/w7041402Chicago/Turabian Style
Woltersdorf, Laura, Stefan Liehr, and Petra Döll. 2015. "Rainwater Harvesting for Small-Holder Horticulture in Namibia: Design of Garden Variants and Assessment of Climate Change Impacts and Adaptation" Water 7, no. 4: 1402-1421. https://doi.org/10.3390/w7041402