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Irrigation with saline water using low-cost drip-irrigation systems in sub-Saharan Africa
KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering.
2005 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

In the scope of future population support, agricultural productivity, in particular in sub-Saharan Africa, has to increase drastically to meet the UN’s millennium development goals of eradicating extreme poverty and hunger by 2015. Water availability in the root-zone limits crop production in large parts of the developing world. As competition for fresh water increases, water of lower quality, for example saline or polluted water, is often used for irrigation. Low-cost drip systems are suitable for saline water irrigation because they effectuate a minimisation of salt accumulation, leaf burn and peaks in salt concentration. Nonetheless, all types of saline water irrigation contain the risk for causing soil salinisation. Thus, in order to achieve long-term sustainability of these systems, appropriate management strategies are needed. The choice of management practices may be influenced by local conditions such as climate, soil and irrigation water salinity. A litera-ture review showed that there is a potential for saline water irrigation in sub-Saharan Africa in water scarce areas. Low-cost drip irrigation with saline water (6 dS m-1) was successfully used to irrigate two consecutive crops of tomato in semi-arid South Africa. An integrated ecosystems model was developed to simulate long-term yield and salt accumulation in a drip-irrigated agricultural system for a range of salinities, climates and management techniques. Crop, salt and water balance data from two field experiments conducted in Israel and South Africa, respectively, were used to parameterise and test the model. Emphasis was placed on testing the usability of the model as a tool for evaluating the importance of certain plausible management options of low-cost, drip-irrigation systems. Therefore, particular focus was directed towards correctly describing soil salinity stress on plant growth and soil evaporation from a distributed (wetted and dry) surface. In addition, the model was developed to function for different climates without having to change any other parameters or variables except for the actual climatic data. Simulations were subsequently run over a 30-year period to study long-term yield and salt accumulation in the soil profile for two sites in South Africa, demonstrating the applicability of the model. Model simulations showed that high soil salinities reduced crop growth and thus increased both drainage and soil evaporation. Further, covering the soil with a plastic sheet led to a reduction of soil evaporation and a subsequent increase in both transpiration and drainage. Rainfall was crucial for the leaching of salts from the soil, and thus in regions with low levels of rainfall, a higher leaching fraction of supplied saline irrigation water has to compensate for the lack of rain. However, a high leaching fraction also causes large amounts of salt leaching, which could potentially pollute underlying groundwater and downstream ecosystems. This risk can be mitigated using mulching, which minimises non-productive water losses, thereby lowering irrigation water needs. The choice of irrigation water salinity, frequency of irrigation and soil coverage may differ between the farmer and the regional water manager due to different preferences. Furthermore, the study highlighted how environmental variables such as water use efficiency and radiation use efficiency can be used as indicators of system performance. Whereas the latter is first and foremost a general stress indicator, water use efficiency more precisely describes specific factors such as plant size, allocation patterns and evaporative demand, which will affect the exchange of carbon dioxide and water through the stomata.

Place, publisher, year, edition, pages
Stockholm: KTH , 2005. , x, 26 p.
Series
Trita-LWR. PHD, ISSN 1650-8602 ; 1020
Keyword [en]
Ecology, Water use efficiency; Radiation use efficiency, Management techniques, Modelling, Tomato
Keyword [sv]
Ekologi
National Category
Ecology
Identifiers
URN: urn:nbn:se:kth:diva-209ISBN: 91-7178-063-7 (print)OAI: oai:DiVA.org:kth-209DiVA: diva2:7922
Public defence
2005-05-27, D3, Lindstedtsvägen 5, KTH, 10:00
Opponent
Supervisors
Note
QC 20101102Available from: 2005-05-18 Created: 2005-05-18 Last updated: 2010-11-02Bibliographically approved
List of papers
1. Exploring potentials and constraints of low-cost drip irrigation with saline water in sub-Saharan Africa
Open this publication in new window or tab >>Exploring potentials and constraints of low-cost drip irrigation with saline water in sub-Saharan Africa
2004 (English)In: Physics and Chemistry of the Earth, ISSN 1474-7065, E-ISSN 1873-5193, Vol. 29, no 15-18, 1035-1042 p.Article in journal (Refereed) Published
Abstract [en]

Irrigation with saline water could provide an interesting opportunity to meet increasing food demands without competing with other pressing needs for fresh water such as domestic and industrial water use in water scarce regions. In sub-Saharan Africa, saline groundwater could be a plentiful and under-utilised resource; however, there is a lack of data to confirm this assumption. Saline water is deliberately and successfully used for irrigation of field and garden crops in several countries. The water saving characteristics and the distribution patterns of water in the soil under drip irrigation make this water application technique suitable for use in combination with saline water. Low-cost drip irrigation has already been successfully implemented in sub-Saharan Africa. It is suggested that low-cost drip irrigation with saline groundwater for the cultivation of horticultural crops can be a feasible option under conditions of water shortage, and has the potential to contribute to improved and sustainable crop production for smallholder farmers.

Keyword
saline water, drip irrigation, sub-Saharan Africa, South Africa, small-scale farmers, horticultural crops
National Category
Ecology
Identifiers
urn:nbn:se:kth:diva-5131 (URN)10.1016/j.pce.2004.08.004 (DOI)000224928800003 ()2-s2.0-5144235523 (Scopus ID)
Note
QC 20101101. 4th WaterNet/Warfsa Symposium. Gaborone, BOTSWANA. OCT 15-17, 2003 Available from: 2005-05-18 Created: 2005-05-18 Last updated: 2017-12-04Bibliographically approved
2. Low-cost drip irrigation of tomatoes using saline water: a suitable technology for southern Africa?
Open this publication in new window or tab >>Low-cost drip irrigation of tomatoes using saline water: a suitable technology for southern Africa?
2007 (English)In: Agricultural Water Management, ISSN 0378-3774, E-ISSN 1873-2283, Vol. 89, no 1-2, 59-70 p.Article in journal (Refereed) Published
Abstract [en]

Using saline water for. irrigation increases water productivity by freeing up fresh water that can be allocated to domestic or other uses. Drip irrigation is widely regarded as the most promising irrigation system in combination with saline water. Simple drip irrigation kits that are affordable for smallholder farmers have successfully been implemented for irrigation of vegetable gardens in several countries in sub-Saharan Africa. The possibility of using low-cost drip irrigation with saline water to successfully irrigate a common garden crop, tomatoes, was tested in this study. Two low-cost drip irrigation systems with different emitter discharge rates (0.2 and 2.5 1 h(-2)) were used to irrigate tomatoes (Lycopersicon esculentum Mill. cv. "Daniella") with water of three different salinity levels (0, 3 and 6 dS m(-1)). In addition, plastic mulch to minimise soil evaporation was also compared to a "bare soil" or uncovered treatment. Two consecutive tomato crops (spring and autumn) were produced during two growing seasons, starting from September 2003 and ending in April 2004, at the Hatfield Experimental Farm in Pretoria, South Africa. An average yield of 75 Mg ha(-1) was recorded for all treatments and seasons, which can be compared with the average marketable yield for South Africa of approximately 31.4 Mg ha(-1). Even at the highest irrigation water salinity (6 dS m(-1)), a yield above the average marketable yield was achieved, indicating that low-cost drip irrigation works well in combination with saline water. Furthermore, the study showed that the choice of drip irrigation system with regard to discharge rate is of minor importance when irrigating with saline water. However, combining low-cost drip irrigation with plastic mulch increased the yield by on average 10 Mg ha(-1) for all treatments. For the bare soil treatments, rainfall had an important role in the leaching of salts from the soils. Finally, the study showed that specific leaf area was higher at high irrigation water salinities, which is contrary to results from other studies. To be able to generalise the promising findings from this study, there is a need to mechanistically model the impact of different climates, soils and irrigation management practices, as well as the long-term sustainability of these systems.

Keyword
smallholder farmers, water productivity, discharge rate, plastic mulch, specific leaf area, micro irrigation
National Category
Ecology
Identifiers
urn:nbn:se:kth:diva-5132 (URN)10.1016/j.agwat.2006.12.011 (DOI)000245579500006 ()
Note
Uppdaterad från submitted till published: 20101101. QC 20101101Available from: 2005-05-18 Created: 2005-05-18 Last updated: 2017-12-04Bibliographically approved
3. Modelling transpiration and growth in salinity-stressed tomato under different climatic conditions
Open this publication in new window or tab >>Modelling transpiration and growth in salinity-stressed tomato under different climatic conditions
2006 (English)In: Ecological Modelling, ISSN 0304-3800, E-ISSN 1872-7026, Vol. 190, no 1-2, 15-40 p.Article in journal (Refereed) Published
Abstract [en]

Models aiming to simulate growth under salinity stress and varied climatic conditions must rely on accurate methods for predicting transpiration and photosynthesis. Traditionally, models have described salinity stress as a decrease in water uptake caused by a low osmotic potential in the soil; however, many physiological studies suggest that reduced plant growth observed under saline conditions could be caused by increased respiration. Explicit calculation of photosynthesis and respiration enables both approaches to be tested and compared in a simulation model. We used an integrated ecosystems model (the CoupModel) to simulate photosynthesis and transpiration over a range of salinities. The model was calibrated and tested on two sets of data (two different seasons) on saline water, drip-irrigated tomato from lysimeter trials in the Arava Valley, Israel. Yields for the spring season were significantly lower than during the first autumn season even though transpiration was higher. As a result, water use efficiency differed by a factor of two between seasons. The model was successful in capturing this large variation, which was caused primarily by high levels of radiation and vapour pressure deficits during spring. For autumn the salinity stress approach in which water uptake was reduced performed well, whereas during spring the increased respiration approach correlated better with measurements. The concept of water use efficiency was found to be a useful tool for interpreting the accumulated effects of climatic and environmental conditions on particular agricultural systems. An attempt to simulate tomatoes grown in production beds indicated that the model set-up was also able to describe conventional cropping systems

Keyword
drip-irrigation, WUE, photosynthesis modelling, ion toxicity, osmotic effect
National Category
Ecology
Identifiers
urn:nbn:se:kth:diva-5133 (URN)10.1016/j.ecolmodel.2005.04.015 (DOI)000233859500002 ()2-s2.0-27844524536 (Scopus ID)
Note
Uppdaterad från "Accepted" till published: 20101101. QC 20101101Available from: 2005-05-18 Created: 2005-05-18 Last updated: 2017-12-04Bibliographically approved
4. Model-based evaluation of low-cost drip-irrigation systems and management strategies using saline water
Open this publication in new window or tab >>Model-based evaluation of low-cost drip-irrigation systems and management strategies using saline water
2007 (English)In: Irrigation science, ISSN 0342-7188, E-ISSN 1432-1319, Vol. 25, no 4, 387-399 p.Article in journal (Refereed) Published
Abstract [en]

A drip-irrigation module was developed and included in an ecosystem model and tested on two independent datasets, spring and autumn, on field-grown tomato. Simulated soil evaporation correlated well with measurements for spring (2.62 mm d(-1) compared to 2.60 mm d(-1)). Changes in soil water content were less well portrayed by the model (spring r(2) = 0.27; autumn r(2) = 0.45). More independent data is needed for further model testing in combination with developments of the spatial representation of below-ground variables. In a fresh-water drip-irrigated system, about 30% of the incoming water was transpired, 40% was lost as non-productive evaporative flows, and the remainder left the system as surface runoff or drainage. Simulations showed that saline water irrigation (6 dS m(-1)) caused reduced transpiration, which led to higher drainage and soil evaporation, compared with fresh water. Covering the soil with plastic mulch resulted in an increase in yield and transpiration. Finally, two different drip-irrigation discharge rates (0.2 and 2.5 l h(-1)) were compared; however the simulations indicated that the discharge rate did not have any impact on the partitioning of the incoming water to the system. The model proved to be a useful tool for evaluating the importance of specific management options.

Keyword
transpiration, evaporation, sprinkler
National Category
Ecology
Identifiers
urn:nbn:se:kth:diva-5134 (URN)10.1007/s00271-006-0054-8 (DOI)000247785100007 ()2-s2.0-34347396379 (Scopus ID)
Note

QC 20101102. Updated from submitted to published 20120327. Previous title: Modelling management strategies for low-cost drip-irrigation systems using saline water

Available from: 2005-05-18 Created: 2005-05-18 Last updated: 2017-12-04Bibliographically approved
5. Long-term impact of different management strategies for low-cost drip-irrigation systems using saline water: modelling two hypothetical agricultural systems in South Africa
Open this publication in new window or tab >>Long-term impact of different management strategies for low-cost drip-irrigation systems using saline water: modelling two hypothetical agricultural systems in South Africa
Show others...
(English)In: Agricultural and Forest Meteorology, ISSN 0168-1923, E-ISSN 1873-2240Article in journal (Other academic) Submitted
National Category
Ecology
Identifiers
urn:nbn:se:kth:diva-5135 (URN)
Note
QS 20120327Available from: 2005-05-18 Created: 2005-05-18 Last updated: 2017-12-04Bibliographically approved

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