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CHALLENGES AFFECTING IRRIGATION WATER SUPPLY AT NYANYADZI SMALLHOLDER IRRIGATION SCHEME IN ZIMBABWE

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Nyanyadzi irrigation scheme is characterised by problems of inadequate water supply. Decreasing baseflows during the winter season, siltation along the river channel and diversion canal contribute to poor irrigation water supply. The aim of this study is to determine effects of river flow variability on scheme water supply Hydrological data from 1968 to 2003 were obtained from the Zimbabwe National Water Authority (ZINWA) while meteorological data were gathered from the Department of Meteorological Services. The scheme's Block C was selected to analyse effects of flow variations on water demands during the bean cropping seasons. Values of crop water requirements were estimated using CROPWAT model. The study observed that between 1968 and 2000, the river significantly (p < 0.001) failed to satisfy the scheme's daily water allocation for 1 996 days although the 90 % reliability level was satisfied. Significant (p = 0.019) monthly water shortages occurred from May to June in Block C. Established linear equations to predict the magnitude of water deficit from river flows and crop yield reduction from water shortages were significant. The study recommends stakeholders to introduce water saving and harvesting measures to minimise problems of water scarcity.
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CHALLENGES AFFECTING IRRIGATION WATER SUPPLY AT
NYANYADZI SMALLHOLDER IRRIGATION SCHEME IN ZIMBABWE
1N. MUJERE and 2D. MAZVIMAVI
1Department of Geography and Environmental Science, University of Zimbabwe,
Box MP 167, MT Pleasant, Harare, Zimbabwe
mujere@arts.uz.ac.zw (corresponding author)
2Harry Oppenheimer Okavango Research Centre, University of Botswana
dmazvimavi@orc.ub.bw
Abstract
Nyanyadzi irrigation scheme is characterised by problems of inadequate water
supply. Decreasing baseflows during the winter season, siltation along the river
channel and diversion canal contribute to poor irrigation water supply. The aim of
this study is to determine effects of river flow variability on scheme water supply
Hydrological data from 1968 to 2003 were obtained from the Zimbabwe National
Water Authority (ZINWA) while meteorological data were gathered from the
Department of Meteorological Services. The scheme’s Block C was selected to
analyse effects of flow variations on water demands during the bean cropping
seasons. Values of crop water requirements were estimated using CROPWAT model.
The study observed that between 1968 and 2000, the river significantly (p < 0.001)
failed to satisfy the scheme’s daily water allocation for 1 996 days although the 90 %
reliability level was satisfied. Significant (p = 0.019) monthly water shortages
occurred from May to June in Block C. Established linear equations to predict the
magnitude of water deficit from river flows and crop yield reduction from water
shortages were significant. The study recommends stakeholders to introduce water
saving and harvesting measures to minimise problems of water scarcity.
Key Words
Challenges, river flow, relative water supply, CROPWAT, Nyanyadzi
INTRODUCTION
Several studies on smallholder irrigation schemes in developing countries have
observed that these schemes are under performing partly due to inadequate inputs and
inaccessible markets (Makadho, 1994; Rukuni, 1994; Chancellor and Hide, 1997;
FAO, 2000). In some cases government policies on land tenure and water allocation
do not create a conducive environment for successful operation of smallholder
irrigation schemes (Tafesse, 2003). Mpande (1984), Motsi et al. (2001) and
Manzungu (1999) ascribed the failure of the smallholder schemes in Africa to
substandard infrastructure, unclear irrigation scheduling and inefficient water use.
Inequitable water allocation was also identified as a cause of poor performance by
some farmers (Pazvakavambwa and Van der Zaag, 2000). Failure of smallholder
irrigation schemes is also partly due to environmental factors such as water scarcity,
poor soil fertility, pests and diseases adversely affecting crop production (Bembridge,
1
1980; Rukuni, 1994). Poor water management by farmers can lead to problems of
land degradation and soil salinisation in smallholder irrigation schemes.
Water management at the scheme and plot levels is a major factor influencing the
success of smallholder irrigation schemes (Motsi et al., 2001; Samakande, 2002).
Pazvakavambwa and Van der Zaag (2000) found on one irrigation scheme in
Zimbabwe that farmers close to water sources had crop yields twice those at tail ends.
Due to poor water management at the scheme, water delivery to tail-end farmers was
unpredictable, and thus adversely affecting their crop yields. At the plot level, some
farmers have been observed to apply 50-150 % more water than needed by the crops
(Early, 1980; Chancellor and Hide, 1997; FAO, 2000). Excess water causes water
logging hence depressed crop yields.
The studies reviewed above have focused on how water management within
smallholder irrigation schemes affect their performance. The success of smallholder
irrigation schemes is also partly influenced by the availability of water at source. In
the case where water is obtained from river flow abstractions, the variability of flows
will affect the availability of water at the plot level.
In Zimbabwe, water supply systems for smallholder irrigation schemes are generally
designed so that adequate water is available 90 % of the times (AREX, 1980).
Reliability levels of 80 to 90 % are sometimes used by large-scale commercial
farmers when deciding to develop their own irrigation systems (Mazvimavi, 1998).
This paper examines the reliability level attained in supplying irrigation water to a
smallholder irrigation scheme and to Block C from Nyanyadzi River. The paper
further examines the relationship between magnitudes of water deficits and crop
yields.
STUDY AREA
The study was undertaken at Nyanyadzi smallholder irrigation scheme located in the
Chimanimani District of Zimbabwe (Figure 1). This scheme was opened in 1934 to
reduce the vulnerability of peasants from crop failures associated with rainfed crop
production (Ministry of Water Development, 1975; Bolding, 1996; 1999). The
Department of Agricultural Research and Extension Services (AREX) manages
operations at this scheme including scheduling water delivery.
2
Figure 1. Location of Nyanyadzi irrigation scheme.
Nyanyadzi irrigation scheme is located at an altitude of 530 m above sea level within
the Save Valley and on the footslopes of the Eastern Highlands. Soils are of alluvial
origin comprising deep, well draining sand loams and clays underlain by coarse river
sand (Ministry of Water Development, 1975). Rainfall occurs during the mid-
November to mid-March period with the rest of the year being dry. The scheme
receives an average annual rainfall of 484 mm/year while the neighbouring Eastern
Highlands receive 1100 to 1400 mm/year. The average annual pan evaporation rate is
2000 mm/year.
The scheme supports about 578 plotholders on 418 hectares that are divided into four
irrigation blocks; A (137 ha), B (147), C (65 ha) and D (69 ha). The blocks obtain
their irrigation water supply from Nyanyadzi and Odzi Rivers (Figure 2).
3
Figure 2 Location of scheme blocks in relationship to sources of water supply.
Each block receives its irrigation water supply from 6 a.m. to 4 p.m. three times a
week. Water from Odzi River is pumped into a night storage dam which supplies
Blocks A, B, and D. Block C is supplied with water abstracted from the Nyanyadzi
River using a concrete weir and a gated off-take, and then conveyed to a night storage
dam through a lined canal. Flood irrigation method is practised using siphons. A water
permit granted in 1937 allows the irrigation scheme to abstract a maximum of 0.033
m3/s or 86 x 103 m3/month from Nyanyadzi River.
Farmers plant the same type of crop in each block every season. The general cropping
pattern is maize and groundnuts during summer (October-March), beans, wheat and
tomatoes in winter (April-September). Beans are planted in April and harvested in
July. This paper specifically examines the effects of water deficits that have been
experienced on Block C on production of beans.
4
METHODOLOGY
The amount of water available for abstraction to the Nyanyadzi irrigation scheme has
been estimated from records of the Zimbabwe National Water Authority (ZINWA)
flow measuring station, E119 on Nyanyadzi River. The station was located 20 km
upstream of the scheme, and there are no tributaries in between were bringing in
significant amounts of water. Station E119 was equipped with an autographic recorder
and had continuous data for the October 1968 to February 2000 period when it was
washed away by floods. Monthly E119 flows from 2000 t0 2003 were estimated from
E120 data. A Department of Meteorological Services rainfall station exists at the
scheme, and this has been used to estimate rainfall received by the irrigated land
during the 1968-2003 period.
Daily flow data for E119 Nyanyadzi River (1968-2000) was used to construct a flow
duration curve which gives the frequency with which flows of specified magnitudes
are exceeded (IH, 1980). The flow duration curve was used to determine the
frequency of having flows greater than the maximum permissible abstraction rate.
When the Nyanyadzi River has flows equal or greater than the maximum permissible
abstraction rate, then the demand for water by the irrigation scheme is satisfied.
The reliability level of the water supply system estimated at the river will differ from
that determined at the field level due to water losses occurring along the distribution
system. Irrigation water supply to the field should satisfy crop water requirements
(CWR) and losses. Pearce and Lewis (1988) and Pearce and Armstrong (1990)
established through measurements a relationship between flow rate (Qj) in Nyanyadzi
River and amount of water available for irrigation at the field level (Sj) in each month.
This relationship has been used to estimate the monthly amount of water available at
the field edge between 1969 and 2003.
To determine whether water supply was adequacy or not, a comparison was then
made each year of the monthly amount of water available at the field level (Sj) with
the monthly volume of water demand (Dj) for bean crop production during the April
to July period. This was accomplished by using the relative water supply (RWS)
index (Makadho, 1994):
D
S
RWS = (1)
where
RWS is relative water supply
S is water supply
D is water demand
The ratio provides the relative abundance or scarcity of water in the fields by
matching water available to the farmers with that which is actually needed by them.
Water supply, S comprises irrigation water supply (Sj) and effective rainfall (Pj) while
irrigation water demand (Dj) in each season was estimated as the sum of crop water
requirement (CWRj) and losses (L). It was assumed that the scheme still attains its
design irrigation efficiency of 70 % below the field gate. Thus, conveyance,
distribution, field application and percolation losses were estimated to be 30 % of
5
irrigation water supply (Wj). CROPWAT model version 5.7 was used to estimate bean
crop water requirements using Penman-Monteith equation (FAO, 1991).
Water deficit (Vj) was assumed to be experienced when Sj < Dj if RWS < 0.8. A RWS
figure of 80 % denotes the minimum requirements below which significant yield
reduction occur (Makadho, 1994; Chancellor and Hide, 1997). The frequency of
occurrence of water deficits was used to estimate the reliability level in each month.
Every bean cropping season received less than 400 mm of rainfall. Thus, rainfall in
each season was assumed to be effective for crop production as recommended by the
Department of Meteorological Services (1981).
RESULTS AND DISCUSSION
River flow and scheme water requirements
The flow duration curve of Nyanyadzi River reveals that flows are greater than the
maximum abstraction rate in 92 % of the days during the April to July period (Figure
3). Thus adequate water is available at source to satisfy the design criteria of the
scheme.
0.05 0.50 5.00 50.00
Exceedance probability (%)
0.005
0.050
0.500
5.000
50.000
Daily flows (m3 s-1)
Figure 3 Flow duration curve for daily flows on E119 Nyanyadzi River.
Considering that water supply was adequate with regards to water permit allocation,
insufficient water supply to satisfy water demand for Nyanyadzi irrigation scheme
cannot be due to the variability of flows on the river at the intake point, but rather
management problems within the scheme. However, such a situation pertains to water
permit abstraction requirements at the scheme level given that the other three
irrigation blocks also get water from Odzi River. The following sections examine
whether water supply reliability at scheme level implies adequate supply for Block C.
6
Relationship between river flows and water deficit in Block C
Periods of water shortages during different crop growth stages were expressed as
percentages of 35 cropping seasons from 1969 to 2003. Most frequent water shortages
were experienced in June while during crop establishment and vegetative periods in
April, irrigation water supply was always adequate. In this initial stage, the crop
requires least amount of water (Table 1).
Table 1 Monthly water shortages, CWR and crop growth stages.
Month % time of water
shortages Crop water
requirements
(mm)
Crop growth
stages
April 0 47 Initial
May 27 97 Development
June 42 94 Mid-season
July 18 83 Late
From Table 1, water supply significantly (p = 0.019) failed to satisfy demand and the
design criteria from May to July. Water shortages occur during the development stage
in May significantly reduce yields because it causes non-uniform crop growth, poor
flowering and pod setting. This is the critical period when the crop starts to bloom and
set fruit. The mid-season growth stage is when yield formation (pod development and
bean filling) occurs and insufficient water supply during gives rise to small, short
discoloured pods with malformed beans. Also, pods will have a higher fibre content
and seeds lose their tenderness. During the bean crop ripening period, Block C
received adequate irrigation water supply 72 % of the times. Inadequate supply in this
late stage crop growth reduces yield and seed health (FAO, 1991).
Considering the 35 cropping seasons, the average bean crop water requirement in
each season was estimated to be 80 x 103 m3. Significant (p = 0.001) water shortages
were experienced during 11 seasons while water supply was adequate 70 % of the
times, thus the 90 % reliability level was not satisfied (Figure 4).
7
0
80
160
240
320
1969
1971
1973
1975
1977
1979
1981
1983
1985
1987
1989
1991
1993
1995
1997
1999
2001
2003
Bean cropping seasons
RWS (%)
Threshold RWS
Figure 4 Relative water supply for the bean crop.
Crop yield records at the scheme showed that no bean crop was harvested in 1973,
1992, 1993, 1994 and 1995. This is attributed to persistent water shortages
experienced from May to July. An average of 0.38 t/ha was obtained when water
supply was inadequate as compared to 1.07 t/ha during adequate water supply.
Application of the Mann-Whitney test show that the difference between crop yields
obtained during adequate and inadequate water supply is significant (p = 0.001).
The magnitudes of water shortages vary over the 11 bean cropping seasons with mean
deficit volume being 48 x 106 m3. A least deficit volume of 15 x 106 m3 was
experienced in 1984, while the worst water shortage of 68 x 103 m3 occurred in 1992
when the river dried from 21 April to 30 September (Figure 5)
8
0
10
20
30
40
50
60
70
80
1970 1971 1973 1983 1984 1987 1991 1992 1993 1994 1995
Bean cropping seasons
Deficit volume (1000 m3)
Figure 5 Magnitude of water shortages.
The relationships between magnitudes of water shortage V (m3), river flow volumes Q
(m3) and bean crop yields Y (t/ha) were estimated for the 11 seasons which
experienced inadequate water supply. In order to establish such relationships, the type
of data distribution was determined using Kolmogorov-Smirnov test. The test
revealed normality of the three data sets; river flow (p = 0.761), water shortage data (p
= 0.856) and crop yield (0.354).
Significant models were derived to estimate magnitude of water deficit from river
flow and yield reduction from water deficit;
V = 63.2 -0.55Q (R2 = 0.64, p = 0. 003)
and Y = 1.08 – 0.02V (R2 = 0.58, p = 0. 011) (2)
9
CONCLUSION AND RECOMMENDATIONS
The study revealed that although flows significantly failed to supply the amount
entitled to farmers by the 1937 water permit for 1 996 days, the design criteria was
satisfied (Figure 3). In Block C irrigation water supply failed to meet the design
criteria for 11 bean cropping seasons when less than average yields were harvested.
Also, water supply was adequate in less than 90 % of the times in May, June and July.
This means that even though water supply was reliable as per permit requirements for
the whole scheme, the design criterion was not satisfied in Block C during crop
growth stages.
During seasons of no harvest, persistent water shortages occurred from May to July.
Block C experienced the worst water shortage in 1992 due to poor rains during the
previous summer season. Linear models were derived to predict water shortage from
river flow volume and reduction in crop yield from water deficit magnitudes are
significant (p < 0.05).
To minimise water shortages, supply can be improved by increasing the scheme water
permit allocation, having boreholes in the scheme environs, on Odzi and Nyanyadzi
riverbeds to augment river water supply. Also, the construction of a proposed dam on
Nyanyadzi River may offer a lasting solution to seasonal water shortages. In addition,
significant water savings can be realised by retrofitting the flood irrigation type (less
than 75 % efficient) with more efficient irrigation methods such as conventional (75
%) sprinkler centre pivot (80 %), micro-jet (85 %) and drip (90 %) irrigation methods
(FAO, 2000).
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AREX (1980). Irrigation handbook. AREX, Harare.
Bembridge, A. (1980). Problems and lessons from irrigation projects in less
developed countries in Africa. Development Southern Africa 3 (4): 606-618.
Bolding. A. (1996). Wielding water in unwilling works: Negotiated management in
water scarcity in Nyanyadzi irrigation scheme, winter 1995. pp. 69-101. In:
Manzungu, E. and Van Der Zaag, P. (Eds.) The practice of smallholder
irrigation: Case studies from Zimbabwe. University of Zimbabwe, Harare.
Bolding, A. (1999). Caught in the catchment: Past, present and future of Nyanyadzi
water management. pp. 123-152. In: Manzungu, E. Senzanje, A and Van Der
Zaag, P. (Eds.) Water for agriculture in Zimbabwe: Policy and management
options for the smallholder sector. University of Zimbabwe, Harare.
Chancellor, F. M. and Hide, J. M. (1997). Smallholder irrigation: Ways forward:
Guidelines for achieving appropriate scheme design. Volume 2. Summary of
case studies. Report OD 136, Hydraulics Research, Wallingford.
Department of Meteorological Services (1981). Climate handbook of Zimbabwe.
Department of Meteorological Services, Harare.
Early, A. C. (1980). An approach to solving irrigation system management
problems. pp. 83-113. In: IRRI (Ed.) A report of a planning workshop on
irrigation water management. IRRI, Philippines.
FAO (1991). CROPWAT version 5.7: Irrigation and management tool. FAO, Rome.
FAO (2000). Socio-economic impact of smallholder irrigation in Zimbabwe: Case
studies of ten irrigation schemes. FAO SAFR, Harare.
IH (1980). Low flow studies. Research Report I, Institute of Hydrology, Wallingford,
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Magadlela, D. (1996). Whose water? Interlocking relations over water in Nyamaropa
irrigation scheme. pp. 102-125. In: Manzungu, E. and Van der Zaag, P (Eds.)
The practice of smallholder irrigation: Case studies from Zimbabwe,
University of Zimbabwe Harare.
Makadho, J. M. (1994). Analysis of water management in smallholder irrigation
schemes in Zimbabwe. Unpublished D.Phil. Thesis. University of Zimbabwe,
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Manzungu, E. (1999). Strategies of smallholder irrigation management in Zimbabwe.
Unpublished Ph.D. Thesis, Wageningen, Netherlands.
Mazvimavi, D. (1998). Water availability and cultivation in Zimbabwe. Geographical
Journal of Zimbabwe 20: 42-52.
Ministry of Water Development (1975). Nyanyadzi irrigation scheme: Project
Report. Ministry of Water Development, Zimbabwe.
Motsi, K. E., Zirebwa, J. and Machingambi, L. (2001). Assessment of the smallholder
irrigation performance: Management of conveyance and distribution
infrastructure at Murara and Nyamatanda north-east of Zimbabwe. Journal of
Applied Science in Southern Africa 7 (1): 9-22.
Mpande, C. V. (1984). Smallholder irrigation schemes in Malawi. pp. 317-320. In:
Blackie, M. J. (Ed.) African regional symposium on smallholder irrigation.
University of Zimbabwe, Harare.
Rukuni, M. (1994). An analysis of the economic and institutional factors affecting
irrigation development in communal lands of Zimbabwe. Unpublished D.Phil.
Thesis. University of Zimbabwe, Harare.
Pazvakavambwa, G. T. and Van Der Zaag, P. (2000). The value of irrigation water in
Nyanyadzi smallholder irrigation scheme, Zimbabwe. Paper presented at the
1st WARFSA/WaterNet Symposium: Sustainable use of water resources.
Maputo, 1-2 November 2000. Maputo.
Pearce, G. R. and Armstrong, A. S. B. (1990). Small irrigation design, Nyanyadzi,
Zimbabwe: Summary report of studies on field-level water use and
distribution. Report ODI 98, Hydraulics Research, Wallingford, UK.
Pearce, G. R. and Lewis, N. S. (1988). Small irrigation design, Nyanyadzi,
Zimbabwe: Distribution of water supply. Report ODI 97, Hydraulics Research
Wallingford, UK.
Samakande, I. (2002). Improving the performance of smallholder surface irrigated
schemes. pp. 151-158. In: Manzungu E. (Ed.) The processes and dynamics of
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Tafesse, M. (2003). Small-scale irrigation for food security in Sub-Saharan Africa.
CTA Working Document No. 8031, CTA, Ethiopia.
11
... Furthermore, boreholes were suggested for supplementing irrigation water where nonsaline aquifers are available and contain sufficient water reserves. Mujere and Mazvimavi (2015) agree that boreholes are important because they provide a standby solution when the main water source is challenged, for instance, when the pump is broken or the river becomes dry, or even when canals are broken or blocked. Lastly, there is a need for assisting incapacitated farmers at Luvhada to develop fundable proposals. ...
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"Oct. 1984." Thesis (D. Phil.)--University of Zimbabwe, 1984. Includes bibliographical references (leaves 420-443). Photocopy.
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An analysis of the economic and institutional factors affecting irrigation development in communal lands of Zimbabwe. Unpublished D.Phil. Thesis. University of Zimbabwe The value of irrigation water in Nyanyadzi smallholder irrigation scheme, Zimbabwe
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Rukuni, M. (1994). An analysis of the economic and institutional factors affecting irrigation development in communal lands of Zimbabwe. Unpublished D.Phil. Thesis. University of Zimbabwe, Harare. Pazvakavambwa, G. T. and Van Der Zaag, P. (2000). The value of irrigation water in Nyanyadzi smallholder irrigation scheme, Zimbabwe. Paper presented at the 1 st WARFSA/WaterNet Symposium: Sustainable use of water resources.
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Pearce, G. R. and Lewis, N. S. (1988). Small irrigation design, Nyanyadzi, Zimbabwe: Distribution of water supply. Report ODI 97, Hydraulics Research Wallingford, UK.