Article pubs.acs.org/est
Improving Crop Yield and Water Productivity by Ecological Sanitation and Water Harvesting in South Africa Jafet C. M. Andersson,†,‡,* Alexander J. B. Zehnder,§ Bernhard Wehrli,‡,∥ Graham P. W. Jewitt,⊥ Karim C. Abbaspour,† and Hong Yang† †
Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600 Dübendorf, Switzerland Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, 8092 Zurich, Switzerland § Alberta Innovates − Energy and Environment Solutions, Edmonton, Alberta, Canada, and Nanyang Technological University (NTU), Singapore ∥ Eawag, Swiss Federal Institute of Aquatic Science and Technology, 6047 Kastanienbaum, Switzerland ⊥ School of Bioresources Engineering and Environmental Hydrology, University of KwaZulu-Natal, Private Bag X01, Scottsville 3209, South Africa ‡
S Supporting Information *
ABSTRACT: This study quantifies the potential effects of a set of technologies to address water and fertility constraints in rainfed smallholder agriculture in South Africa, namely in situ water harvesting (WH), external WH, and ecological sanitation (Ecosan, fertilization with human urine). We used the Soil and Water Assessment Tool to model spatiotemporally differentiated effects on maize yield, river flow, evaporation, and transpiration. Ecosan met some of the plant nitrogen demands, which significantly increased maize yields by 12% and transpiration by 2% on average across South Africa. In situ and external WH did not significantly affect the yield, transpiration or river flow on the South Africa scale. However, external WH more than doubled the yields for specific seasons and locations. WH particularly increased the lowest yields. Significant water and nutrient demands remained even with WH and Ecosan management. Additional fertility enhancements raised the yield levels but also the yield variability, whereas soil moisture enhancements improved the yield stability. Hence, coupled policies addressing both constraints will likely be most effective for improving food security.
1. INTRODUCTION
simultaneously improve sanitation, prevent pollution and enhance soil fertility.7,8 One strategy to enhance the crop water availability, and thereby minimize the impact of dry-spells on smallholder food production, is to use water harvesting and conservation technologies (WH).9 The principal hydrological functions of WH are to reduce surface runoff in favor of enhanced infiltration and soil moisture, and to reduce soil evaporation in favor of enhanced crop transpiration.10 Agriculturally aimed WH can be classified as in situ WH and external WH. In situ WH refers to technologies that capture surface runoff and enhance infiltration on the agricultural fields themselves. External WH refers to technologies that capture runoff from uncultivated areas (e.g., roads and grasslands), store water in
How shall we make undernourishment history in a world of increasing human population, ecosystem degradation, and stress on water resources?1 A number of strategies exists and contextual variability calls for tailored solutions.2 Effective management of water and nutrients is often emphasized because of their key role in crop production and in biogeochemical cycles of the environment.3 The challenges of undernourishment, poverty, sanitation and water scarcity converge in Sub-Saharan Africa, where livelihoods primarily depend on smallholder rain-fed farming.4,5 High rainfall variability and low soil fertility are two critical challenges facing smallholder farmers in several parts of SubSaharan Africa (SSA). Lack of financial capacity typically constrains smallholder farmers from addressing the low soil fertility with conventional synthetic fertilizers.6 A potential alternative is low-cost ecological sanitation (Ecosan): the recycling of nutrients from human excreta to agriculture. By turning waste into a resource, the Ecosan strategy aims to © 2013 American Chemical Society
Received: Revised: Accepted: Published: 4341
November 9, 2012 March 2, 2013 March 26, 2013 March 26, 2013 dx.doi.org/10.1021/es304585p | Environ. Sci. Technol. 2013, 47, 4341−4348
Environmental Science & Technology
Article
(40% of the population25). In South Africa, the share of the population with inadequate food access is highest in the Free State (34%), KwaZulu-Natal (23%), Mpumalanga (22%), and Eastern Cape (21%) provinces.26 The adoption of WH and Ecosan technologies among smallholder farmers is currently negligible.6,27,28 2.2. Summary of Modeling Methodology. We used the agro-hydrological SWAT model29 (Soil and Water Assessment Tool) to analyze WH and Ecosan. The model was set up, calibrated and validated for the study area.30 All analyses were made within a parameter uncertainty framework,31 yielding results with a 95% prediction uncertainty range (95PPU) and an optimal parameter set (β) within the 95PPU. A number of scenarios representing different water and fertility management in smallholder agriculture were simulated (Table 1). Andersson
small-scale storage systems and provide supplementary irrigation to the fields.11 WH and Ecosan have been analyzed individually from a diverse set of perspectives including economic feasibility,12,13 perception and adoption,6,8,14 and potential impacts on water fluxes,14−16 crop yields,1,14,16−18 and human health.19,20 The field-scale research generally indicates that crop yields increase with utilization of WH. However, reported impacts vary considerably between different locations, seasons, technologies, and socio-economic contexts. In some cases, yields have even decreased by up to 25%.21,22 Given the diversity of impacts on field scale, it is not evident what effect these technologies may have on larger scales. It is not clear if their impacts will be substantial in all conditions, or if spatiotemporal variability in climate and soil type, for example, will constrain their impact. Conclusions from earlier work on river basin scale in different parts of South Africa have not been unanimous.23,24 This prompted the interest to analyze the consistency and spatial variability of impacts on country scale. Additionally, little is known about the potential of these technologies in relation to each other, or if combined. The aim of this study is to quantify and compare the potential effects of in situ WH, external WH, and Ecosan technologies in smallholder systems in South Africa; and to identify potentially suitable locations for these technologies in the region. We analyze effects on maize yields (the dominant food crop in the region), evaporation, transpiration, and river flow regimes; and include spatiotemporal dynamics and prediction uncertainty in the analysis.
Table 1. Management Characteristics on Smallholder Farmlands for Each Scenario, And the Simulated Median Maize Yield, Coefficient of Variation of the Yields in Time (cv,t) and in Space (cv,s) Calculated for All Time Steps, Locations and Parameterizationsa scenario baseline WHi WHe10 WHe50
2. MATERIALS AND METHODS 2.1. Study area. The watersheds of all rivers flowing through South Africa defined the geographic boundaries of this study (Figure 1). Agriculture is practiced in both smallholder systems (average farm size: 1.5 ha) and large-scale commercial systems (average farm size: >700 ha) in the region.17 Smallholder systems are predominantly rain-fed and rely on local cultivars, and low amounts of fertilizers and other inputs. The food security situation varies markedly. Zimbabwe and Mozambique have the highest prevalence of undernourishment
Ecosan EcosanWHi EcosanWHe50 I Ecosan-I F F-WHi F-WHe50 F-I
median yield (t ha−1)
median cv,t (-)
median cv,s (-)
0.98 a
0.33 a
0.54 a
0.98 a 0.98 a
0.33 a 0.32 b
0.54 a 0.53 b
0.98 a
0.32 b
0.53 b
1.10 b
0.33 c
0.54 c
1.10 b
0.33 c
0.54 c
Ecosan and WHe50
1.10 b
0.33 a
0.53 a
unlimited irrigation Ecosan and unlimited irrigation unlimited fertilization unlimited fertilization and WHi unlimited fertilization and WHe50 unlimited fertilization and irrigation
1.18 1.34 2.46 2.46 2.48
0.20 0.19 0.40 0.40 0.40
0.39 0.38 0.66 0.66 0.66
management characteristics dominant current smallholder management in situ water harvesting (WH) external WH draining 10% of the sub-basin area external WH draining 50% of the sub-basin area human urine fertilization (Ecosan) Ecosan and WHi
c d e e e
4.27 f
d e f f g
0.09 h
d e f f f
0.38 g
a
Rows with differing letters in each column are significantly different from each other (p-value
