Directory UMM :Data Elmu:jurnal:A:Agricultural Water Management:Vol45.Issue3.Aug2000:
Agricultural Water Management 45 (2000) 101±125
Effect of tied-ridging on soil water status of
a maize crop under Malawi conditions
K.A. Wiyoa,*,1, Z.M. Kasomekerab,2, J. Feyena
a
Institute for Land & Water Management, 102 Vital Decosterstraat, B-3000 Leuven, Belgium
b
Agricultural Engineering Department, Bunda College of Agriculture,
University of Malawi, P.O. Box 219, Lilongwe, Malawi
Accepted 22 October 1999
Abstract
Tied-ridging is being promoted in Malawi as a rainwater harvesting technique to reduce drought
risk in maize (Zea mays L.) production. Before tied-ridging can be promoted to subsistence farmers
as a viable rainwater harvesting technique, there is need to evaluate the likely impact of tied-ridging
on soil water status and maize yield. A calibrated ®eld capacity-based water balance model
(TIEWBM) was used to assess the impact of tied-ridging on soil water status of a maize crop under
Malawi conditions. Effect of tied-ridging on soil water status was evaluated by simulating seasonal
(140 days) changes in retained rainwater, surface runoff, drainage, soil moisture storage (SMS),
waterlogging and actual evapotranspiration (ETa) for 5 soils and 12 rainfall regimes.
The simulation results indicate that tied-ridging reduced surface runoff and this increased
retained rainwater within the ®eld. Over 80% of the gained rainwater was lost as drainage while the
remainder increased SMS and ETa in ®ne-textured soils (clayey texture) but not in coarse-textured
soils (sandy texture). Tied-ridging is not likely to bene®t the maize crop in coarse-textured soils
regardless of seasonal rainfall amount. Tied-ridging, however, is likely to bene®t the maize crop in
®ne-textured soils and for seasonal rainfall between 500±900 mm (drought or dry years). Below
500 mm, the rainfall is not suf®cient to meet maize crop water requirements (CWR) with or without
tied-ridging. Above 900 mm (normal and wet years), rainfall is suf®cient to meet CWR without
tied-ridging making them unnecessary. Furthermore, in normal or wet years, tied-ridging is likely to
lead to waterlogging in ®ne but not coarse-textured soils. The results cast doubt on the bene®ts of
tied-ridging in coarse-textured soils. # 2000 Elsevier Science B.V. All rights reserved.
Keywords: Maize; Malawi; Rainwater harvesting; Ridge tillage; SADC; Subsistence farmers; Soil water; Tiedridges
*
Corresponding author. Present address: Bunda College of Agriculture, University of Malawi, P.O. Box 219,
Lilongwe, Malawi. Tel.: 265-277-222; fax: 265-277-364.
E-mail address: [email protected] (K.A. Wiyo)
1
Tel.: 32-16-23-9721; fax: 32-16-23-9760; E-mail: [email protected]
2
Tel.: 265-277-222; fax: 265-277-364.
0378-3774/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 8 - 3 7 7 4 ( 9 9 ) 0 0 1 0 3 - 1
102
K.A. Wiyo et al. / Agricultural Water Management 45 (2000) 101±125
1. Introduction
1.1. Problem situation
Agricultural production in the Southern Africa Development Community (SADC) is
largely rainfed. For most SADC countries including Malawi, maize is the staple food and
its production is critical for national food security. Food security in Malawi and the rest of
the SADC has been made worse by drought conditions in the 1990s. For example, one of
the worst drought in living memory occurred in the 1991/92 growing season with serious
consequences on food security and economic growth of several countries including
Malawi. To reduce drought risk in maize production, there have been calls to promote onfield rainwater harvesting technologies to subsistence farmers. Such technologies work
by retaining surface runoff within the field, thereby altering the soil water status within
the root zone.
For Malawi, tied-ridging is one such technique that is being promoted to conserve
rainwater in farmer's fields. Tied-ridging is known in other parts of the world as boxedridges, furrow dikes, furrow damming, basin listing, basin tillage and microbasin
tillage (Jones and Stewart, 1990). In tied-ridging, ridge furrows are blocked with
earth ties spaced a fixed distance apart to form a series of microcatchment basins in
the field (Fig. 1). The created basins retain surface runoff within the field. Tied-ridging
is not a new technology within Malawi and the rest of the SADC. It has been promoted
in the 1960s and 1970s for surface runoff-induced erosion control since tied-ridges
retain surface runoff within the field. Given erratic rainfall, the aim is no longer just
erosion control but also rainwater harvesting. The aim in such cases is to `harvest' the
limited rainwater and store it in the maize root zone for use during dryspell periods.
Before tied-ridging can be promoted to subsistence farmers as a viable rainwater
harvesting technique, there is need to evaluate the likely impact of tied-ridging on
soil water status and maize yield. Subsistence farmers are not likely to take up tiedridging unless it improves soil water status for the crop during dry or drought years
and will not lead to waterlogging, ridge destruction and excessive nutrient leaching
in a wet year.
1.2. Effect of tied-ridging on soil water status
Past and recent research in Botswana (Carter and Miller, 1991), Zimbabwe (Piha,
1993; Vogel, 1993), Burkina Faso (Hulugalle and Matlon, 1990) and USA (Krishna,
1989) have revealed that tied-ridging is effective in reducing surface runoff and
increasing soil water storage. Tied-ridging often leads to little or no surface runoff
during normal storms. In severe storms, however, other research in Africa as reviewed
by El-Swaify et al. (1985) has revealed that tied-ridging can lead to ridge overtopping,
ridge failure, waterlogging and total loss of the crop. Many studies on tied-ridging
have focused on the relationship between tied-ridging and crop yield. The yield has
been found to vary depending on the amount and distribution of rainfall, soil type
and the crop grown. Few studies have focused on the effect of tied-ridging on soil
water status under different soils, rainfall regimes and crops. Research on the effect
K.A. Wiyo et al. / Agricultural Water Management 45 (2000) 101±125
103
Fig. 1. Microcatchments formed by tied-ridging (top) and relationship of tied ridges with contour ridges in a
®eld (bottom). Ridges are spaced 0.9 m apart and earth cross ties are at 2 m and staggered. Maize is planted on
the ridge crests at 0.9 m apart. The ridges are 0.3 m in height while cross-ties are at 0.2 m high.
of tied-ridging on all water balance components is largely nonexistent in the literature.
This is partly due to the fact that to evaluate soil water status, extensive soil moisture and
surface runoff data need to be measured. This is often expensive, time consuming and not
easy to do.
Calibrated deterministic and functional water balance models can be used instead
to evaluate the likely impact of tied-ridging on soil water status at a fraction of cost
and time (Krishna, 1989). Deterministic models are process-based, and thus, accurate
but need detailed crop and soil input data not routinely available in developing
countries. Functional models, whilst less accurate than deterministic models, have
minimum input data requirements, thus suitable for conditions in developing countries.
In this study, the effect of tied-ridging on all water balance components (surface runoff,
drainage, soil moisture storage, actual evapotranspiration) under different soils and
rainfall regimes is evaluated using a simple field capacity-based water balance model
(TIEWBM).
104
K.A. Wiyo et al. / Agricultural Water Management 45 (2000) 101±125
2. Materials and methods
2.1. Description of the TIEWBM water balance model
The TIEWBM water balance model is based on the field capacity-based FAO model
(Doorenbos and Pruitt, 1977; Doorenbos and Kassam, 1979) but with slight
modifications. In the TIEWBM model, the soil is described as a single reservoir with
moisture storage varying on a daily basis depending on the balance of fluxes coming in
and out of the soil reservoir. All the water fluxes or quantities are expressed in mm and
are defined by daily inputs of rain (Ri), surface runoff (ROi), drainage (Di), actual
evapotranspiration (ETai) and available soil moisture storage (SMSi). Daily SMSi is found
by doing a water balance knowing Ri, ROi, Di, ETai and previous day's SMS as shown in
Eq. (1). Initial SMS is input at the start of simulations.
SMSi Ri ÿ ROi ÿ Di ÿ ETai SMSiÿ1
(1)
Tied-ridging was modelled by assuming that all surface runoff generated is retained
within the field while it is lost in ridging without ties and plain/flat cultivation. This
assumption is good, mostly with small and medium storms. In large severe storms, ridges
and ties may overtop leading to surface runoff loss out of the field. Surface runoff was
calculated from daily rainfall by a simple linear model (Eq. (2)) defined by the runoff
coefficient (COEF) and a rainfall threshold amount (Rw). COEF is the percentage of
rainfall converted to surface runoff and Rw, the minimum amount of rainfall required to
generate surface runoff. These two parameters depend on soil type and texture (Table 4).
Daily rainfall is taken from historical rainfall data.
If Ri > Rw ; ROi
COEF
Ri ÿ Rw and if Ri < Rw ; ROi 0
100
(2)
The soil reservoir can store the maximum amount of water (SMSmax) defined by
the product of the field capacity (FC) and a user defined non-varying rooting depth
(RD). SMS at wilting point (SMSwp) was similarly calculated. Drainage occurred before
the next day (within 24 h) whenever SMSi was greater than SMSmax. The depth of
drainage was given by the difference between SMSi and SMSmax. The root zone on
day i was close to waterlogging if SMSi was above 90% of SMS at saturation (arbitrary
set). The critical water storage in the soil reservoir (SMScrit) below which maize dry
stress begins to occur was defined by the maize depletion factor (p) set at 0.65
(Doorenbos and Pruitt, 1977).
Daily reference evapotranspiration (EToi) was calculated by the FAO recommended
method of Penman±Monteith (Allen et al., 1998) using historical daily climatic data for
each year of the simulation. Daily EToi was converted to maize potential ET (ETci) using
crop coefficients (stage I 0.25, II by linear interpolation, III 1.15 and IV by linear
interpolation between 1.15±0.30). Actual evapotranspiration (ETai) was calculated based
on previous day's soil moisture storage (SMSiÿ1) as follows: If the SMSiÿ1 was above
SMScrit, ETai was set equal to ETci (no water shortage). If the SMSiÿ1 was between
SMSwp and SMScrit (water shortage), ETai was calculated by linear proportion (Eq. (3)).
K.A. Wiyo et al. / Agricultural Water Management 45 (2000) 101±125
105
ETai was assumed zero below wilting point.
ETai ETci
SMSiÿ1 ÿ SMSwp
SMScrit ÿ SMSwp
(3)
The TIEWBM water balance model has several limitations. It assumes that water uptake
by roots is uniform throughout the root zone and does not allow for differential root water
uptake by depth. Secondly, it assumes a constant rooting depth throughout the season,
when in practice, rooting depth is small at the beginning and high at the end. Further, it
does not model water ¯ow within the root zone and ignores capillary rise. Lastly, the linear
model used to calculate daily surface runoff does not allow for the effect of slope, rainfall
intensity and antecedent moisture content on surface runoff generation. Despite these
limitations, TIEWBM has minimum data requirements and once calibrated (Section 2.3) can
be useful as a management tool in assessing the effect of tied-ridging on soil water status.
2.2. Site description and measurements
2.2.1. Field experimental setup
The research site was located at Bunda College Research Farm (latitude 148150 ,
longitude 338450 , altitude 1200 m) in the Lilongwe plains of the Central Region of
Malawi. The Central Region has predominantly red soils (FAO: Ferric luvisols) with a
clayey texture and deep water tables (>8 m). The organic carbon content is generally low
(80%) of the gained rainwater
due to tied-ridging was lost as drainage out of the root zone (Table 5 compare drainage
gain with surface runoff).
The potential of tied-ridging to result in waterlogging was assessed by counting the
number of days when simulated daily SMS was above 90% SMS at saturation. The results
are shown in Table 5. Fields with ridging without ties are not likely to be waterlogged
even in a wet year. Tied-ridging, however, will increase the likelihood of waterlogging in
fine-textured soils (clay, clayloam and loam) in normal and wet years (above 900 mm)
but not in dry or drought years (
Effect of tied-ridging on soil water status of
a maize crop under Malawi conditions
K.A. Wiyoa,*,1, Z.M. Kasomekerab,2, J. Feyena
a
Institute for Land & Water Management, 102 Vital Decosterstraat, B-3000 Leuven, Belgium
b
Agricultural Engineering Department, Bunda College of Agriculture,
University of Malawi, P.O. Box 219, Lilongwe, Malawi
Accepted 22 October 1999
Abstract
Tied-ridging is being promoted in Malawi as a rainwater harvesting technique to reduce drought
risk in maize (Zea mays L.) production. Before tied-ridging can be promoted to subsistence farmers
as a viable rainwater harvesting technique, there is need to evaluate the likely impact of tied-ridging
on soil water status and maize yield. A calibrated ®eld capacity-based water balance model
(TIEWBM) was used to assess the impact of tied-ridging on soil water status of a maize crop under
Malawi conditions. Effect of tied-ridging on soil water status was evaluated by simulating seasonal
(140 days) changes in retained rainwater, surface runoff, drainage, soil moisture storage (SMS),
waterlogging and actual evapotranspiration (ETa) for 5 soils and 12 rainfall regimes.
The simulation results indicate that tied-ridging reduced surface runoff and this increased
retained rainwater within the ®eld. Over 80% of the gained rainwater was lost as drainage while the
remainder increased SMS and ETa in ®ne-textured soils (clayey texture) but not in coarse-textured
soils (sandy texture). Tied-ridging is not likely to bene®t the maize crop in coarse-textured soils
regardless of seasonal rainfall amount. Tied-ridging, however, is likely to bene®t the maize crop in
®ne-textured soils and for seasonal rainfall between 500±900 mm (drought or dry years). Below
500 mm, the rainfall is not suf®cient to meet maize crop water requirements (CWR) with or without
tied-ridging. Above 900 mm (normal and wet years), rainfall is suf®cient to meet CWR without
tied-ridging making them unnecessary. Furthermore, in normal or wet years, tied-ridging is likely to
lead to waterlogging in ®ne but not coarse-textured soils. The results cast doubt on the bene®ts of
tied-ridging in coarse-textured soils. # 2000 Elsevier Science B.V. All rights reserved.
Keywords: Maize; Malawi; Rainwater harvesting; Ridge tillage; SADC; Subsistence farmers; Soil water; Tiedridges
*
Corresponding author. Present address: Bunda College of Agriculture, University of Malawi, P.O. Box 219,
Lilongwe, Malawi. Tel.: 265-277-222; fax: 265-277-364.
E-mail address: [email protected] (K.A. Wiyo)
1
Tel.: 32-16-23-9721; fax: 32-16-23-9760; E-mail: [email protected]
2
Tel.: 265-277-222; fax: 265-277-364.
0378-3774/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 8 - 3 7 7 4 ( 9 9 ) 0 0 1 0 3 - 1
102
K.A. Wiyo et al. / Agricultural Water Management 45 (2000) 101±125
1. Introduction
1.1. Problem situation
Agricultural production in the Southern Africa Development Community (SADC) is
largely rainfed. For most SADC countries including Malawi, maize is the staple food and
its production is critical for national food security. Food security in Malawi and the rest of
the SADC has been made worse by drought conditions in the 1990s. For example, one of
the worst drought in living memory occurred in the 1991/92 growing season with serious
consequences on food security and economic growth of several countries including
Malawi. To reduce drought risk in maize production, there have been calls to promote onfield rainwater harvesting technologies to subsistence farmers. Such technologies work
by retaining surface runoff within the field, thereby altering the soil water status within
the root zone.
For Malawi, tied-ridging is one such technique that is being promoted to conserve
rainwater in farmer's fields. Tied-ridging is known in other parts of the world as boxedridges, furrow dikes, furrow damming, basin listing, basin tillage and microbasin
tillage (Jones and Stewart, 1990). In tied-ridging, ridge furrows are blocked with
earth ties spaced a fixed distance apart to form a series of microcatchment basins in
the field (Fig. 1). The created basins retain surface runoff within the field. Tied-ridging
is not a new technology within Malawi and the rest of the SADC. It has been promoted
in the 1960s and 1970s for surface runoff-induced erosion control since tied-ridges
retain surface runoff within the field. Given erratic rainfall, the aim is no longer just
erosion control but also rainwater harvesting. The aim in such cases is to `harvest' the
limited rainwater and store it in the maize root zone for use during dryspell periods.
Before tied-ridging can be promoted to subsistence farmers as a viable rainwater
harvesting technique, there is need to evaluate the likely impact of tied-ridging on
soil water status and maize yield. Subsistence farmers are not likely to take up tiedridging unless it improves soil water status for the crop during dry or drought years
and will not lead to waterlogging, ridge destruction and excessive nutrient leaching
in a wet year.
1.2. Effect of tied-ridging on soil water status
Past and recent research in Botswana (Carter and Miller, 1991), Zimbabwe (Piha,
1993; Vogel, 1993), Burkina Faso (Hulugalle and Matlon, 1990) and USA (Krishna,
1989) have revealed that tied-ridging is effective in reducing surface runoff and
increasing soil water storage. Tied-ridging often leads to little or no surface runoff
during normal storms. In severe storms, however, other research in Africa as reviewed
by El-Swaify et al. (1985) has revealed that tied-ridging can lead to ridge overtopping,
ridge failure, waterlogging and total loss of the crop. Many studies on tied-ridging
have focused on the relationship between tied-ridging and crop yield. The yield has
been found to vary depending on the amount and distribution of rainfall, soil type
and the crop grown. Few studies have focused on the effect of tied-ridging on soil
water status under different soils, rainfall regimes and crops. Research on the effect
K.A. Wiyo et al. / Agricultural Water Management 45 (2000) 101±125
103
Fig. 1. Microcatchments formed by tied-ridging (top) and relationship of tied ridges with contour ridges in a
®eld (bottom). Ridges are spaced 0.9 m apart and earth cross ties are at 2 m and staggered. Maize is planted on
the ridge crests at 0.9 m apart. The ridges are 0.3 m in height while cross-ties are at 0.2 m high.
of tied-ridging on all water balance components is largely nonexistent in the literature.
This is partly due to the fact that to evaluate soil water status, extensive soil moisture and
surface runoff data need to be measured. This is often expensive, time consuming and not
easy to do.
Calibrated deterministic and functional water balance models can be used instead
to evaluate the likely impact of tied-ridging on soil water status at a fraction of cost
and time (Krishna, 1989). Deterministic models are process-based, and thus, accurate
but need detailed crop and soil input data not routinely available in developing
countries. Functional models, whilst less accurate than deterministic models, have
minimum input data requirements, thus suitable for conditions in developing countries.
In this study, the effect of tied-ridging on all water balance components (surface runoff,
drainage, soil moisture storage, actual evapotranspiration) under different soils and
rainfall regimes is evaluated using a simple field capacity-based water balance model
(TIEWBM).
104
K.A. Wiyo et al. / Agricultural Water Management 45 (2000) 101±125
2. Materials and methods
2.1. Description of the TIEWBM water balance model
The TIEWBM water balance model is based on the field capacity-based FAO model
(Doorenbos and Pruitt, 1977; Doorenbos and Kassam, 1979) but with slight
modifications. In the TIEWBM model, the soil is described as a single reservoir with
moisture storage varying on a daily basis depending on the balance of fluxes coming in
and out of the soil reservoir. All the water fluxes or quantities are expressed in mm and
are defined by daily inputs of rain (Ri), surface runoff (ROi), drainage (Di), actual
evapotranspiration (ETai) and available soil moisture storage (SMSi). Daily SMSi is found
by doing a water balance knowing Ri, ROi, Di, ETai and previous day's SMS as shown in
Eq. (1). Initial SMS is input at the start of simulations.
SMSi Ri ÿ ROi ÿ Di ÿ ETai SMSiÿ1
(1)
Tied-ridging was modelled by assuming that all surface runoff generated is retained
within the field while it is lost in ridging without ties and plain/flat cultivation. This
assumption is good, mostly with small and medium storms. In large severe storms, ridges
and ties may overtop leading to surface runoff loss out of the field. Surface runoff was
calculated from daily rainfall by a simple linear model (Eq. (2)) defined by the runoff
coefficient (COEF) and a rainfall threshold amount (Rw). COEF is the percentage of
rainfall converted to surface runoff and Rw, the minimum amount of rainfall required to
generate surface runoff. These two parameters depend on soil type and texture (Table 4).
Daily rainfall is taken from historical rainfall data.
If Ri > Rw ; ROi
COEF
Ri ÿ Rw and if Ri < Rw ; ROi 0
100
(2)
The soil reservoir can store the maximum amount of water (SMSmax) defined by
the product of the field capacity (FC) and a user defined non-varying rooting depth
(RD). SMS at wilting point (SMSwp) was similarly calculated. Drainage occurred before
the next day (within 24 h) whenever SMSi was greater than SMSmax. The depth of
drainage was given by the difference between SMSi and SMSmax. The root zone on
day i was close to waterlogging if SMSi was above 90% of SMS at saturation (arbitrary
set). The critical water storage in the soil reservoir (SMScrit) below which maize dry
stress begins to occur was defined by the maize depletion factor (p) set at 0.65
(Doorenbos and Pruitt, 1977).
Daily reference evapotranspiration (EToi) was calculated by the FAO recommended
method of Penman±Monteith (Allen et al., 1998) using historical daily climatic data for
each year of the simulation. Daily EToi was converted to maize potential ET (ETci) using
crop coefficients (stage I 0.25, II by linear interpolation, III 1.15 and IV by linear
interpolation between 1.15±0.30). Actual evapotranspiration (ETai) was calculated based
on previous day's soil moisture storage (SMSiÿ1) as follows: If the SMSiÿ1 was above
SMScrit, ETai was set equal to ETci (no water shortage). If the SMSiÿ1 was between
SMSwp and SMScrit (water shortage), ETai was calculated by linear proportion (Eq. (3)).
K.A. Wiyo et al. / Agricultural Water Management 45 (2000) 101±125
105
ETai was assumed zero below wilting point.
ETai ETci
SMSiÿ1 ÿ SMSwp
SMScrit ÿ SMSwp
(3)
The TIEWBM water balance model has several limitations. It assumes that water uptake
by roots is uniform throughout the root zone and does not allow for differential root water
uptake by depth. Secondly, it assumes a constant rooting depth throughout the season,
when in practice, rooting depth is small at the beginning and high at the end. Further, it
does not model water ¯ow within the root zone and ignores capillary rise. Lastly, the linear
model used to calculate daily surface runoff does not allow for the effect of slope, rainfall
intensity and antecedent moisture content on surface runoff generation. Despite these
limitations, TIEWBM has minimum data requirements and once calibrated (Section 2.3) can
be useful as a management tool in assessing the effect of tied-ridging on soil water status.
2.2. Site description and measurements
2.2.1. Field experimental setup
The research site was located at Bunda College Research Farm (latitude 148150 ,
longitude 338450 , altitude 1200 m) in the Lilongwe plains of the Central Region of
Malawi. The Central Region has predominantly red soils (FAO: Ferric luvisols) with a
clayey texture and deep water tables (>8 m). The organic carbon content is generally low
(80%) of the gained rainwater
due to tied-ridging was lost as drainage out of the root zone (Table 5 compare drainage
gain with surface runoff).
The potential of tied-ridging to result in waterlogging was assessed by counting the
number of days when simulated daily SMS was above 90% SMS at saturation. The results
are shown in Table 5. Fields with ridging without ties are not likely to be waterlogged
even in a wet year. Tied-ridging, however, will increase the likelihood of waterlogging in
fine-textured soils (clay, clayloam and loam) in normal and wet years (above 900 mm)
but not in dry or drought years (