Rainwater harvesting to enhance water productivity of rainfed agriculture in the semi-arid Zimbabwe

TLDR: In this article, the impact of rainwater harvesting on the crop yield gap and the Transpirational Water Productivity (WPT) was investigated in six districts of the semi-arid Zimbabwe, where three parameters (water source, primary use and storage capacity) can help differentiate storage-type RWH systems from conventional dams.

Abstract: Zimbabwe’s poor are predominantly located in the semi-arid regions and rely on rainfed agriculture for their subsistence. Decline in productivity, scarcity of arable land, irrigation expansion limitations, erratic rainfall and frequent dry spells, among others cause food scarcity. The challenge faced by small-scale farmers is to enhance water productivity of rainfed agriculture by mitigating intra-seasonal dry spells (ISDS) through the adoption of new technologies such as rainwater harvesting (RWH). The paper analyses the agro-hydrological functions of RWH and assesses its impacts (at field scale) on the crop yield gap as well as the Transpirational Water Productivity (WPT). The survey in six districts of the semi-arid Zimbabwe suggests that three parameters (water source, primary use and storage capacity) can help differentiate storage-type-RWH systems from “conventional dams”. The Agricultural Production Simulator Model (APSIM) was used to simulate seven different treatments (Control, RWH, Manure, Manure + RWH, Inorganic Nitrogen and Inorganic Nitrogen + RWH) for 30 years on alfisol deep sand, assuming no fertiliser carry over effect from season to season. The combined use of inorganic fertiliser and RWH is the only treatment that closes the yield gap. Supplemental irrigation alone not only reduces the risks of complete crop failure (from 20% down to 7% on average) for all the treatments but also enhances WPT (from 1.75 kg m−3 up to 2.3 kg m−3 on average) by mitigating ISDS

Figures

Fig. 2. Zimbabwe natural region map.

Fig. 4. Rainfall and ISDS from July 99 to July 2000.

Fig. 3. Rainfall and ISDS from July 91 to July 92.

Table 4 Average yield gap, transpirational water productivity & risks of complete crop failure

Fig. 5. Cumulative crop yield.

Fig. 1. District map of Zimbabwe.

Frequently asked questions

Q1. What are the contributions in "Rainwater harvesting to enhance water productivity of rainfed agriculture in the semi-arid zimbabwe" ?

The paper analyses the agro-hydrological functions of RWH and assesses its impacts ( at field scale ) on the crop yield gap as well as the Transpirational Water Productivity ( WPT ). The survey in six districts of the semi-arid Zimbabwe suggests that three parameters ( water source, primary use and storage capacity ) can help differentiate storage-type-RWH systems from ‘ ‘ conventional dams ’ ’. 

Q2. What is the way to improve water productivity in sub-Saharan Africa?

An option for improving water productivity will be the reduction of non-productive soil evaporation (ES) in favour of productive plant transpiration (T). 

Q3. What is the common type of drought in sub-Saharan Africa?

In semiarid regions (SAR) the rainfall has extreme temporal and spatial variability and generally occurs as storms of high rainfall intensity, resulting in agricultural droughts and intra-seasonal dry spells (ISDS) that reduce the yield of rainfed agriculture. 

Q4. What is the key to the widespread of RWH?

An involvement of the government and the local water authorities who are the decision makers and the implementers of the national water resources plan is key to the widespread of RWH. 

Q5. What is the effect of supplemental irrigation on yield?

During such dry years, it was observed that inorganic nitrogen application (without supplemental irrigation) exacerbate water stress thereby resulting in total crop failure. 

Q6. How much of the population of Zimbabwe depends on agriculture for food and employment?

About 70% of the population of Zimbabwe depends on agriculture for food and employment but only 37% of the country receives adequate rainfall for agriculture (FAO, 2005). 

Q7. What is the main reason why the study was conducted?

The analysis conducted in this paper show the use of RWH for supplemental irrigation increases WPT and stabilises the crop yield. 

Q8. What is the main source of income in sub-Saharan Africa?

The majority of the population in sub-Saharan Africa make their living from rainfed agriculture (FAO, 1995), and largely depend on small-scale subsistence agriculture for their livelihood security (Rockström, 2000). 

Q9. What is the common method of irrigation in Zimbabwe?

Rock Catchments, an XRWH, are also common since, the granite areas of Zimbabwe that cover over 50% of the country are well supplied with domes (ruware), often of sufficient size to be utilised as water catchment areas (Dry Land Farming, 2006). 

Q10. What is the main purpose of the APSIM?

The Agricultural Production Systems SIMulator (APSIM) is a modelling environment that uses various component modules to simulate cropping systems (Keating et al., 2003). 

Q11. What are the main reasons for the lack of water in Zimbabwe?

In Zimbabwe there are quite a number of small dams which have been constructed to store water for mitigating local and temporary water shortages. 

Q12. What is the main reason for the low yield of rainfed agriculture?

Supplemental irrigation of rainfed crops by the use of Rainwater harvesting (RWH) is a likely viable option to increase water productivity at production system level (Oweis et al., 2001; SIWI, 2001). 

Q13. What is the common way to assess the yield of a crop?

Since the Crop growth is directly governed by transpiration, it is more appropriate to consider the transpirational WP given by:WP T ¼ Y Tð1ÞHowever, since it is difficult to separate Evaporation (E) and Transpiration (T), it is common to assess Evapotranspirational WP. 

Q14. What is the key to the widespread implementation of RWH in Zimbabwe?

Successful implementation of RWH in Zimbabwe requires an integrated approach where not only the technical aspect is considered but also the socio-economic and the institutional aspects. 

Q15. What are the main reasons for the lack of data on the extent of RWH in Zimbabwe?

In recent years, to mitigate the effects of ISDS and stabilise the crop yield, RWH techniques have been introduced and promoted by non-governmental organisations (NGOs). 

Q16. What is the main source of food and livelihood security in sub-Saharan Africa?

To make rainfed agriculture the main source of food and livelihood security for rural communities, the yield gap between the actual yield and the maximum yield must be reduced. 

Q17. What is the effect of supplemental irrigation on crop yield?

The study indicates a 13% reduction of the risks of total crop failure that occurs once out of 5 years because of ISDS (Fig. 5) when RWH is used for supplemental irrigation. 

Q18. What is the maximum yield of a crop?

The maximum yield (Ym) is the yield of a crop planted at the optimal plant density for agiven soil type and climatic conditions without nutrient limitation, pests, diseases, weeds, soil damage or other factors amenable to management control. 

Q19. What is the driest year in Zimbabwe?

July 1991 to July 1992 with an MAR of 109.7 mm is the driestyear recorded for Masvingo while July 1999 to July 2000 with an MAR of 1134.8 mm is the wettest year.