Effect of tied-ridging on soil water status of
a maize crop under Malawi conditions
K.A. Wiyo
a,*,1, Z.M. Kasomekera
b,2, J. Feyen
a aInstitute for Land & Water Management, 102 Vital Decosterstraat, B-3000 Leuven, BelgiumbAgricultural 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 ETain ®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; Tied-ridges
*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: bundalibrary@malawi.net (K.A. Wiyo)
1Tel.:32-16-23-9721; fax:32-16-23-9760; E-mail: kenneth.wiyo@agr.kuleuven.ac.be
2Tel.:265-277-222; fax:265-277-364.
0378-3774/00/$ ± see front matter#2000 Elsevier Science B.V. All rights reserved.
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 on-field 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 boxed-ridges, 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 tied-ridging 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
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).
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.
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 SMSiis found by doing a water balance knowingRi, ROi,Di, ETaiand previous day's SMS as shown in Eq. (1). Initial SMS is input at the start of simulations.
SMSiRiÿROiÿDiÿETaiSMSiÿ1 (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 andRw, 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.
IfRi>Rw; ROi
COEF
100 RiÿRwand ifRi<Rw; ROi0 (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 dayiwas close to waterlogging if SMSiwas 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 EToiwas converted to maize potential ET (ETci) using crop coefficients (stage I0.25, II by linear interpolation, III1.15 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 SMSiÿ1was above SMScrit, ETai was set equal to ETci (no water shortage). If the SMSiÿ1 was between SMSwpand SMScrit(water shortage), ETaiwas calculated by linear proportion (Eq. (3)).
ETaiwas assumed zero below wilting point.
ETaiETci
SMSiÿ1ÿSMSwp SMScritÿSMSwp
(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 (<2.2%). With adequate additions of fertilizers, ferric luvisols are highly productive and the Central Region produces most of the maize in Malawi. In Malawi, subsistence farmers grow most of the maize. The farms are small (<3 ha) and the inputs are low. Maize is grown on ridges constructed across the field slope using a hand-hoe. The use of ox-drawn or tractor-mounted implements is rare.
A factorial maize field trial in a randomized complete block design (RCBD) was used to evaluate the four tillage systems. The experimental layout of the site is shown in Fig. 2. The research site had 48 experimental plots comprising two blocks, four tillage treatments, and two maize varieties and replicated three times. The experimental plot was 12 m12 m to allow for border effects resulting in a net harvesting plot of 10 m10 m. The four tillage systems evaluated were ridging without ties; the 2 m staggered tied-ridge (recommended by government extension service); the 4 m non-staggered tied-ridge derived by the authors and plain/flat cultivation with no ridges and no ties.
The 2 m tied-ridge had 0.2 m high ties spaced 2 m apart brick-work staggered throughout the experimental plot while the 4 m tied-ridge had 0.2 m high ties spaced 4 m apart but not staggered in order to reduce tie construction labour. The aim was to see if the 4 m non-staggered tied ridge would perform just as well as the 2 m staggered tied-ridge. Our measurements at the site showed that construction of the 4 m non-staggered ties required additional 61 person-hours haÿ1while the 2 m staggered tied-ridge required 147 person-hours haÿ1. Ridges were tied before planting and reinforced at second weeding (ridge banking). The ridge spacing was 0.9 m while within-row maize spacing was 0.9 m with three maize plants per station based on MOAL (1996). This gave a maize population of 37,000 haÿ1or plant density of 3.7 plants mÿ2. The ridges were constructed about 0.3 m high using a hand hoe commonly used by subsistence farmers in Malawi.
Fig. 2. Experimental layout of the ®eld: total ®eld area120 m80 m; gross plot area12 m12 m (net area10 m10 m); runoffsurface runoff plots;
oTDR access tube (1 m depth); ®eld down slope1.3%; clayloam soil.
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Local maize (flint, day to maturity 140 days, maximum leaf area index 5.38, maximum yield 2000 kg haÿ1) and MH18 hybrid maize (semi-flint, day to maturity 125 days, maximum leaf area index 5.0, maximum yield 7000 kg haÿ1) were selected to compare and contrast the effect of the four tillage treatments under subsistence conditions. In Malawi, low-yielding local maize varieties are still popular because they pound and keep well in storage. About 65% of farmers grow local maize varieties and only 35% grow hybrid maize (Wiyo et al., 1997). All cultural practices were the same for all experimental plots as recommended by government extension service (MOAL, 1996). These included timely planting, secondary tillage (scarification) and ridge banking. The objective was to mirror the subsistence farmers' cultural practices and tillage conditions as closely as possible. Subsistence farmers' fertilizer application rates are lower than the recommended rates of 96 and 40 kg haÿ1 N and P respectively (Mughogho, 1989). Consequently, a revised, site-specific lower fertilizer rate was used in the experimental plots (35, 10 and 2 kg haÿ1N, P and S respectively) split into basal and top dressings (MOAL, 1996). This rate is close to the fertilizer application rates commonly encountered in subsistence farmers' fields (3045 kg haÿ1N).
2.2.2. Collection of climatic and soil data
Rainfall and other climatic data for the 1996/97 growing season were collected from Bunda College weather station located about 1 km from the research site. The data included rainfall, temperature, wind speed, sunshine hours, relative humidity and incoming radiation. Rainfall characteristics (depth, duration and intensity) during the growing season were measured using a recording rain gauge. Physical and hydraulic properties of the soil were measured at 3 soil depths (2 samples per tillage treatment4 tillage treatments3 soil depths24 samples) according to field and laboratory procedures outlined in Klute (1986). These included particle-size distribution and texture, infiltration rate, bulk density, field capacity (FC), permanent wilting point (WP) and saturated hydraulic conductivity. The results are shown in Table 1.
2.2.3. Collection of surface runoff data
Surface runoff was collected from one experimental plot for each tillage treatment as shown in Fig. 2. Exact details on surface runoff collection are found in Wiyo et al. (1997) but here we will just review the main features. Surface runoff from a fixed area runoff plot was channeled through plastic pipes to prior-calibrated sunken concentric and interconnected storage drums (each drum maximum 220 l). This was to accommodate storms of different sizes. Three steps were involved: (1) measure the depth of runoff collected in the drum on a per storm basis; (2) convert depth to surface runoff volume using a calibration curve for each drum and (3) calculate runoff depth by dividing surface runoff volume by runoff plot area.
The runoff plot area was dictated by rainfall characteristics of the area, tillage treatment and available storage drums. For the plain/flat cultivation, the runoff plot was 4.6 m5.1 m (23.46 m2) demarcated by iron sheets buried in the ground. For ridging without ties, the runoff plot was defined by 2 ridges spaced 0.9 m apart and 11.7 m in length (10.53 m2). For the two tied-ridge treatments, rainwater was retained between the ties. Surface runoff occurred when retained rainwater within the tied-ridge basin
exceeded the height of the ties. Since the likelihood of this occurring was low, surface runoff leaving the whole experimental plot (12 m12 m144 m2) was channeled to storage drums without fear of storage capacity being exceeded.
Table 2 shows observed surface runoff by storm size for the 1996/97 growing season. The seasonal rainfall was 1038.9 mm against a historical seasonal mean rainfall for Table 1
Measured soil physical parameters of the Bunda Research Farm ®eld (1996/97)
Soil parameter measured Plain/flat Ridges
In®ltration rate 10/1/97 16.0 24.0 30.0 28.0 24.5 25.0
Steady-state (mm hÿ1) 17±20/3/97 16.0 30.0 12.0 16.0 18.5 43.0
Bulk density (mg mÿ3) 0±0.2 m 1.52 1.40 1.60 1.36 1.47 8.0
0.2±0.6 m 1.54 1.49 1.32 1.48 1.46 7.0
>0.6 1.27 1.50 1.34 1.37 1.37 7.0
Field capacity (FC) (% vol) 0±0.2 m 37.85 39.02 39.0 33.73 37.4 7.0
0.2±0.6 m 39.59 37.25 31.65 36.85 36.3 9.0
>0.6 m 36.96 43.22 38.06 39.54 39.5 7.0
Permanent wilting point (WP) (% vol)
0±0.2 m 23.97 19.75 15.60 16.93 19.1 19.0
0.2±0.6 m 26.30 23.36 22.81 24.26 24.2 6.0
>0.6 m 23.42 25.62 23.26 24.70 24.3 5.0
Sat. hydraulic conductivity
(K) (mm hÿ1)
0±0.2 m 28.3 36.9 34.7 56.9 39.1 81.0
0.2±0.6 0.85 1.1 0.85 0.80 0.90 37.0
>0.6 m 32.3 63.9 18.1 14.5 32.2 102.0
Texture, pH and organic matter content
Soil depth (cm) Soil texture
0.2±0.6 m 36.7 (22) 25.0 (26) 38.3 (18) Clayloam 1.15 5.2
>0.6 m 39.2 (15) 25.0 (26) 35.8 (18) Sand
clayloam
0.69 5.2
Overall 36.7 (15) 26.9 (22) 36.7 (21) Clayloam 1.17 5.3
Soil taxonomy
aSource: Wiyo et al., 1997.
bSource: Lowole, 1983.
cSource: (23)CV (coef®cient of variation, %).
Bunda of 921.5 mm. The 80% dependable rainfall (exceeded 4 out of 5 years) for Bunda station is 720.4 mm while the 20% dependable rainfall (exceeded 1 out of 5 years) is 1125.3 mm. Consequently, the 1996/97 growing season was slightly wet. This was mainly due to above-average rainfalls in December, January and February. The mean rainfall intensity was high 38.9 mm hÿ1(CV116%) and it varied greatly from storm to Table 2
Observed surface runoff depth by storm size (1996/97 season, clayloam soil, ®eld slope1.3%)a
Storm size and intensity Observed surface runoff depth (mm)
Storm
8 31-12-96 71.2 178.0 37.5 38.3 0.0 0.0
9 10-1-97 61.0 76.3 30.1 33.1 0.0 0.0
24 05-2-97 30.1 109.1 20.5 21.3 0.0 0.0
25 06-2-97 11.0 2.6 2.5 2.0 0.0 0.0
Seasonal totals 981.6 N/A 311.2 348.5 0.0 0.0
Seasonal runoff coef. N/A N/A 0.317 0.355 0.0 0.0
Mean depth 22.0 38.9 9.1 10.6 0.0 0.0
Max storm measured 71.2 178.0 37.5 38.3 0.0 0.0
a±missing values; N/Anot applicable.
storm (range 2.6±178 mm hÿ1). No surface runoff left the two tied-ridged plots even during a high intensity (178 mm hÿ1) severe storm of 71.2 mm (maximum historical storm is 79 mm). It did not lead to ridge overtopping, ridge failure or soil erosion within the tied-ridged plots. It only led to waterlogging. The seasonal surface runoff for the plain/flat cultivation was 311.2 mm (runoff coefficient 0.317) and that of ridging without ties was 348.5 mm (runoff coefficient 0.355). The lower runoff coefficient of the plain/ flat cultivation is likely due to increased micro topography of plain/flat cultivation compared to ridging without ties (Kayira, 1994).
2.2.4. Monitoring daily soil moisture content
The daily soil moisture content at 3 depths (0±0.2 m, 0.2±0.6 m and 0.6±1.0 m) were monitored using Time Domain Reflectometry (TRIME TDR) moisture probe (Eijkelk-amp, 1996). TDR measurement gives a point measurement, it is fast and accurate (Topp et al., 1994). Gravimetric method was, however, used in a limited way for TDR moisture probe calibration. Specially designed glass-fibre TDR access tubes (Eijkelkamp, 1996) were randomly installed on 32 of the 48 experimental plots on the crest of the ridge to the depth of 1 m using specially designed soil auger. Measurements of the maize rooting depth at the research site showed that the roots reached a maximum depth of 0.65 m. To reduce measurement errors, precautions were taken to make sure that the TDR access tubes were installed vertically down and that the interface between the tube and the soil was not filled with air or loose soil (Eijkelkamp, 1996). Location of the access tubes in the experimental plots is shown on Fig. 2.
TDR measurements at three depths on 32 access tubes were taken daily in the morning (7±9 a.m.) for 171 days. This produced 96 soil moisture measurements every day and 16,416 measurements for 171 days. The comprehensive monitoring started on 5 December 1996 and ended on 25 May 1997 (maize growing season). However, prior to the rains (1 December), initial soil moisture levels were also measured as reference starting values. TDR TRIME soil moisture probe was calibrated by comparing TDR volumetric soil moisture values with gravimetric values converted to volumetric basis using corresponding bulk densities (Klute, 1986). Soil samples were collected at the same depths within 0.5 m of the access tube and at the same time soil moisture content was measured by TDR probe. Fig. 3 shows TDR calibration results. TRIME TDR consistently under-predicted soil moisture values. Regression analyses gave a coefficient of determination (R2) of 0.69 and a standard error (s.e.) of 0.035 cm3cmÿ3. The standard error is within the expected error range for TDR measurements (0.02±0.035 cm3cmÿ3) while theR2value is slightly low. The lowR2is likely due to the high clay content of the soil which may have affected bulk density and bound water; factors shown to influence TDR accuracy (Topp et al., 1994). The measured soil moisture values (TDRfield) were corrected prior to data analyses using a regression Eq. (4) with R20.69 and s.e.0.045 cm3cmÿ3.
THETA oven calibrated 1:055TDRfield0:055 (4)
There was no significant difference in the block means of THETA. THETA values of the two blocks were thus pooled (4 access tubes). This gave eight moisture time series (4 tillage treatments2 maize varieties) per soil layer (3). For each soil layer, we first
found the daily mean THETA and coefficient of variation (CV). Daily soil moisture storage (SMS) at each soil layer of the eight moisture time series was found by multi-plying the daily mean soil moisture at that depth with the layer depth in mm. Daily SMS in the root zone was the summation of the soil water held at each of the three soil layers. In line with observed rooting depth (0.65 m), all our SMS analyses were limited to 0.7 m.
2.3. Model calibration and validation
The TIEWBM water balance model was calibrated and validated by using daily surface runoff (RO) and soil moisture storage (SMS) as system parameters. Soil and crop data collected at the research site for the clayloam soil on local maize plots (Section 2.2, Table 4) were used as inputs for the calibration. Local maize data was used in the calibration given majority of subsistence farmers in Malawi are growing local maize. Initially, data from hybrid experimental plots was also tested and the trends in the results were similar. Ridging without ties and tied-ridging were separately calibrated to form a basis for comparison of simulation results. Ridging without ties is a good benchmark to compare with tied-ridging, because it is universally used in maize fields of subsistence farmers in Malawi.
Ridging without ties runoff and SMS data was used in calibration while independent plain/flat cultivation data was used in its validation. There was little difference between SMS of 2 m tied-ridging plots and 4 m tied-ridging plots, thus we recommended the 4 m tied-ridging given its low labour requirements. Consequently, runoff and SMS data from 4 m tied-ridging was used in the calibration while independent 2 m tied-ridging data was used in its validation. The other water balance components like drainage and ETawere not calibrated due to lack of field data. During calibration and validation, the model was run daily from 1 December to 15 April (133 days) equal to the maize growing season. Fig. 3. Time domain re¯ectometry (TDR TRIME) moisture probe calibration Soil type: clayloam (Local
ferruginous latosols, Nathenje series; FAO: Ferric Luvisols); coef®cient of determination R20.69;
s.e.0.035 cm3cmÿ3.
Fig. 4. Calibration of the daily water balance model using soil moisture storage in the root zone (SMS) and daily surface runoff: (a,b) SMS calibration ridges only; (c,d) SMS calibration 4 m tied-ridge; (e) 1996/97 seasonal rainfall (981.6 mm); (f) surface runoff calibration.
The results of the calibration and validation of surface runoff and SMS depth are shown in Fig. 4. The resulting calibration and validation parameters (ME, RMSE,R2, slope b and intercept a) are shown in Table 3. Overall, TIEWBM fitted SMS and runoff data reasonably well and within the level of accuracy of water balance models reported in
Fig. 4. (Continued).
the literature (e.g. Chopart and Vauclin, 1990). The ME and RMSE for SMS are less than 12 % andR2was between 0.68 and 0.77. The runoff linear model under-predicted large runoff events. RMSE for runoff was high (43.4%) and the slope was low (0.58). Despite this, seasonal predicted surface runoff for ridging without ties (347.0 mm) was nearly equal to observed surface runoff (348.5 mm).
After calibration and validation, the model was run separately for 4 m tied-ridging and ridging without ties for 140 days (26 November±23April). This was done on five soil textures (clay, clayloam, loam, sandyloam and sand) corresponding to Malawi major soil types and 12 rainfall regimes covering drought, dry, normal and wet years of Malawi (438.1±1540 mm). For Malawi's climate (1965±1992), the mean return period (T) for a drought year was 1 out of 14.1 years, dry year was 1 out of 12.3 years, normal year was 10 out of 16 years and wet year was 1 out of 5.2 years (Wiyo et al., 1997). Soil and crop parameters used in model simulations are shown in Table 4.
Table 3
Calibration and validation parameters of the water balance model (clayloam, local maize, 1.3% ®eld slope)a
Water balance component Calibration parameters
Optimum value 0.0 0.0 1.00 1.00 0.0
Calibration
SMS (ridges only) 7.3 8.8 0.77 0.79 47.1
SMS (4 m tie-ridge) 8.0 10.1 0.68 0.65 84.0
Surface runoff ÿ5.1 43.4 0.72 0.58 1.1
Validation
SMS (plain/¯at) 9.4 11.7 0.77 0.92 35.4
SMS (2 m tie-ridge) 8.4 10.6 0.68 0.62 87.0
aMEmean error (%mean), RMSEroot mean square error (%mean),R2coef®cient of determination,
SMSsoil moisture storage in the root zone (700 mm).
Table 4
Physical properties of the soils used in tied-ridging model simulations
Soil parametersa Soil texture
Clay Clayloam Loam Sandyloam Sand
Saturated content (Os) (cm3cmÿ3) 0.505 0.477 0.463 0.389 0.375
Field capacity (FC) (cm3cmÿ3) 0.396 0.380 0.270 0.207 0.140
Wilting point (WP) (cm3cmÿ3) 0.272 0.225 0.117 0.095 0.044
Residual content (Or) (cm3cmÿ3) 0.090 0.075 0.052 0.041 0.033
Maize depletion factor (p) 0.65 0.65 0.65 0.65 0.65
In®ltration rate (cm day-1) 7.5 13.5 50.0 165.0 1100
Runoff coef®cient (% of rain) 51.6 46.2 38.0 28.0 25.0
Threshold rainfall (Rw) (mm) 2.8 3.3 4.9 7.5 10.0
Maize rooting depth (m) 0.70 0.70 0.70 0.70 0.70
avalues obtained from literature (mainly Schaap and Leij, 1998).
3. Results
The effect of tied-ridging on water balance components (surface runoff, drainage, SMS, actual evapotranspiration) and waterlogging is shown in Fig. 5 and Tables 5 and 6. With a few exceptions, the modelling water balance error (as % of seasonal rainfall) was mostly less than 5% (Table 5) indicating a reasonable accuracy of the simulated results. Tables 5 and 6 only show data for clay, clayloam, sandyloam and sandy soils but not loam soil to save table space. The influence of soil texture and rainfall depth and distribution on the water balance components is discussed including the fate of the extra rainwater retained by tied-ridging.
3.1. Effect of tied-ridging on surface runoff and retained rainwater
From the simulation results (Table 5), surface runoff was lost from fields with ridging without ties and none was lost from tied-ridged fields. Retained rainwater is the difference between rainfall and lost surface runoff and represents the amount of rainwater infiltrating into the soil. Tied-ridging decreased surface runoff from the field and increased retained rainwater within the field. As expected, the amount of surface runoff (and hence retained rainwater) increased with seasonal rainfall depth. The amount of rainwater retained within the field due to tied-ridging was largest in fine-textured soils (clay, clayloam and loam) and smallest in coarse-textured soils (sandyloam and sand).
3.2. Effect of tied-ridging on drainage and waterlogging
The effect of tied-ridging on drainage loss out of the root zone is shown in Table 5. As expected, drainage loss from fields with ridging without ties was largest in coarse-textured soils (sandy) compared to fine-texture soils (clay). Also, drainage increased with seasonal rainfall depth. Compared to ridging without ties, tied-ridging increased the amount of drainage out of the root zone. There was a gain in drainage with tied-ridging (Table 5) and this gain was largest in fine-textured soils (clay, clayloam, loam) compared to coarse-textured soils (sand, sandyloam). In fact, most (>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 (<900 mm). Between 1965±1992 (27 years), normal rainfall years occurred, on average, 63.1% of the time, wet years 19.1% of the time, dry years 10.8% of the time and drought years 7.1% of the time (Wiyo et al., 1997). Thus, tied-ridged fields in fine-textured soils are likely to be waterlogged in 82.2% of the time (63.119.1%) under Malawi climate. In coarse-textured soils (sandyloam and sandy), there is no danger of waterlogging in tied-ridged fields regardless of seasonal rainfall amount.
Fig. 5. Effect of tied-ridging on soil moisture storage in the root zone of a maize crop. (a) clay; (b) clayloam; (c) loam; (d) sandyloam; (e) sand; (f) rainfall distribution (1991/92 dry year, 647.5 mm).
Fig. 5. (Continued).
Table 5
Effect of tied-ridging on seasonal surface runoff, drainage loss and waterlogging by soil and rainfall (140 days)
Soil texture Rainfall Surface runoff Drainage loss Water balance error Days when SMS above 90%
Continued
a438.1 mm seasonal rainfall but uniformly distributed throughout the growing season (no dryspells).
Table 6
Effect of tie-ridging on mean soil moisture storage and actual seasonal maize evapotranspiration (140 days)
Soil texture Rainfall Mean soil moisture storage (SMS) Actual evapotranspiration ETa Seasonal ETa/ETcratio
140 days
Clay 438.1 Ua 205.7 223.2 17.5 309.8 30.8 38.2 7.4 0.10 0.12
438.1 184.2 208.8 24.6 309.8 30.8 34.7 3.9 0.10 0.11
619.1 230.4 241.6 11.2 339.5 33.8 299.4 265.6 0.10 0.88
647.5 236.9 245.7 8.8 354.9 110.6 180.7 70.1 0.31 0.51
682.1 208.9 222.5 13.6 325.2 200.8 243.2 42.4 0.62 0.75
771.0 229.9 243.7 13.8 340.7 230.2 310.3 80.1 0.68 0.91
791.2 229.2 240.0 10.8 336.2 170.2 251.9 81.7 0.51 0.75
911.0 240.7 255.5 14.8 303.7 281.6 287.1 5.5 0.93 0.95
919.9 231.1 249.1 18.0 280.7 211.4 246.9 35.5 0.75 0.88
981.6 233.0 247.8 14.8 289.1 254.5 271.6 17.1 0.88 0.94
993.7 233.7 248.5 14.8 293.8 248.0 264.7 16.7 0.84 0.90
1540.2 238.3 252.9 14.6 318.4 271.6 289.9 18.3 0.85 0.91
Clayloam 438.1 U 206.8 218.2 11.4 309.8 30.8 50.0 19.2 0.10 0.16
438.1 187.0 203.8 16.8 309.8 30.8 44.5 13.7 0.10 0.14
619.1 223.0 233.3 10.3 339.5 123.9 303.2 179.3 0.36 0.89
647.5 213.4 231.5 18.1 354.9 191.7 203.5 11.8 0.54 0.57
682.1 201.3 211.1 9.8 325.2 249.3 273.9 24.6 0.77 0.84
771.0 218.9 234.9 16.0 340.7 310.0 314.4 4.4 0.91 0.92
791.2 213.6 225.8 12.2 336.2 220.1 286.0 65.9 0.65 0.85
911.0 235.6 245.9 10.3 303.7 284.3 287.5 3.2 0.94 0.95
919.9 227.2 240.1 12.9 280.7 232.5 258.2 25.7 0.83 0.92
981.6 225.4 237.8 12.4 289.1 269.9 273.8 3.9 0.93 0.95
993.7 228.8 240.1 11.3 293.8 259.9 272.8 12.9 0.88 0.93
1540.2 232.5 244.1 11.6 318.4 279.5 295.1 15.6 0.88 0.93
Continued
Sandyloam 438.1 U 128.2 130.0 1.8 309.8 308.4 308.4 0.0 1.00 1.00
438.1 122.0 122.8 0.8 309.8 214.5 256.0 41.5 0.69 0.83
619.1 134.9 137.2 2.3 339.5 337.6 337.6 0.0 0.99 0.99
647.5 128.2 129.9 1.7 354.9 187.0 187.0 0.0 0.53 0.53
682.1 119.9 121.0 1.1 325.2 277.4 277.3 ÿ0.1 0.85 0.85
771.0 134.8 136.4 1.6 340.7 338.0 338.0 0.0 0.99 0.99
791.2 127.7 127.9 0.2 336.2 225.2 247.4 22.2 0.67 0.74
911.0 136.7 138.4 1.7 303.7 301.6 301.6 0.0 0.99 0.99
919.9 142.0 143.5 1.5 280.7 279.7 279.9 0.2 1.00 1.00
981.6 132.3 134.5 2.2 289.1 281.5 282.1 0.6 0.97 0.98
993.7 142.2 143.8 1.6 293.8 291.6 291.6 0.0 0.99 0.99
1540.2 142.4 144.9 2.5 318.4 316.1 316.1 0.0 0.99 0.99
Sandy 438.1 U 90.5 90.5 0.0 309.8 308.4 308.4 0.0 1.00 1.00
438.1 86.4 86.4 0.0 309.8 236.1 252.2 16.1 0.76 0.81
619.1 91.7 92.6 0.9 339.5 337.6 337.6 0.0 0.99 0.99
647.5 84.2 85.0 0.8 354.9 184.5 184.5 0.0 0.52 0.52
682.1 80.7 81.5 0.8 325.2 274.6 274.6 0.0 0.84 0.84
771.0 92.4 93.5 1.1 340.7 338.0 338.0 0.0 0.99 0.99
791.2 85.4 85.6 0.2 336.2 224.8 239.3 14.5 0.67 0.71
911.0 94.8 95.8 1.0 303.7 301.8 301.8 0.0 0.99 0.99
919.9 97.5 98.3 0.8 280.7 279.7 279.7 0.0 1.00 1.00
981.6 89.9 91.2 1.3 289.1 279.1 279.3 0.2 0.97 0.97
993.7 98.3 99.2 0.9 293.8 291.6 291.6 0.0 0.99 0.99
1540.2 101.5 103.3 1.8 318.4 316.1 316.1 0.0 0.99 0.99
a438.1 mm seasonal rainfall but uniformly distributed throughout the growing season (no dryspells).
3.3. Effect of tied-ridging on soil moisture storage in the root zone
The effect of tied-ridging on soil moisture storage in the root zone (700 mm) is shown in Fig. 5 and Table 6. Fig. 5 plots the daily SMS of ridging without ties and tied-ridging for the dry year (647.5 mm). Table 6 shows the mean seasonal SMS (140 days) and the mean SMS gain due to tied-ridging. The trends from Fig. 5 and Table 6 are similar. Tied-ridging increased SMS in the root zone of fine-textured soils (clay, clayloam and loam) and hardly changed SMS in coarse-textured soils (sandyloam and sand). The mean gain in SMS (Table 6) was largest in fine-textured soils and smallest in coarse-textured soils. This was true regardless of seasonal rainfall amount and distribution. It was the same trend whether the year was a drought, dry, normal or wet year. In the interest of space, the rest of the SMS plots for the plots are not shown.
3.4. Effect of tied-ridging on actual evapotranspiration
Table 6 shows the simulated results of the effect of tied-ridging on actual seasonal evapotranspiration (ETa). The beneficial effect of tied-ridging to the maize crop is measured by the gain in ETa due to tied-ridging. In the absence of waterlogging, the seasonal ETais a better measure of the likely maize yield since ETa is correlated with maize yield (Doorenbos and Kassam, 1979). Thus, the ratio of ETato potential ETcof maize gives an indication of how well maize crop water requirements (CWR) were met for the season. A seasonal ratio of ETa/ETcof one means that all the CWR were met but in practice any ratio above 0.90 is good. A ratio of less than 0.5 indicate serious maize water shortage likely to lead to crop failure.
From Table 6, tied-ridging increased seasonal ETaof the maize crop in fine-textured soils (clay, clayloam and loam) and hardly affected ETa of the maize crop in coarse-textured soils (sandyloam and sand). The mean seasonal ETa gain was largest in fine-textured soils and smallest in coarse-fine-textured soils. The seasonal ETa/ETcratio followed a similar trend. With a few exceptions, this trend was true regardless of seasonal rainfall amount and distribution. This means that in coarse-textured soils, tied-ridging will not benefit the crop regardless of rainfall amount or distribution. For fine-textured soils, the simulation results seem to indicate that below 500 mm rainfall, the maize CWR will not be met (ETa/ETc< 0.5) with or without tied-ridging (crop failure). Above 900 mm rainfall, the maize CWR is largely met without need for tied-ridging (ETa/ETcratio near 0.90). These results seem to suggest that tied-ridging will benefit the crop in fine-textured soils when the seasonal rainfall is between 500±900 mm. For fine-textured soils, the results cast doubt on the crop benefits of tied-ridging outside this rainfall range.
3.5. Sensitivity analyses of the results
The TIEWBM water balance model is sensitive to number of parameters, some already considered by including different soil textures and rainfall regimes. For example, surface runoff depends on runoff coefficient and the threshold rainfall amount (function of soil type). Likewise, soil parameters FC,WP and MAD were sensitive but these also depend on the soil type. For the crop, the crop coefficient (kc) was sensitive and depends on the
maize variety used (local versus hybrid). Increasing kc values, increased ETc that increased ETa. ETcdepends on reference ETothat, in turn, depends on daily weather. We investigated whether seasonal rainfall of about the same depth but different seasonal distribution will result in the same trends in surface runoff, retained rainwater, drainage, waterlogging, SMS and ETa (438.1 U versus. 438.1 mm, 771 versus 791.2 mm, 911 versus 919.9 mm, 981.6 versus 993.7 mm). The sensitivity results indicate that the magnitude of the various water balance components changed with rainfall depth and distribution but the underlying trends found in this study did not change. Also, increasing the rooting depth (0.5, 0.7, 1.0 and 1.3 m), increased the magnitude of SMS and ETaand decreased drainage but did not change the trends found earlier.
4. Discussion
The results above agree with the results of Vogel (1993) and Piha (1993) in Zimbabwe on sandy soils, Carter and Miller (1991) in Botswana on sandy soils, Hulugalle and Matlon (1990) in Burkina Faso on sandyloam and sandy clay soils and Krishna (1989) in the USA on different soils and crops. In a wet year, tied-ridging can result in waterlogging which can lead to maize yield reduction (El-Swaify et al., 1985; Wiyo et al., 1997). The results found in this study are due to the fact that tied-ridges retain some rainwater within the field that would otherwise be lost as surface runoff. Tied-ridges alter the soil water dynamics by decreasing the surface runoff component while increasing other water balance components like drainage, SMS and actual evapotranspiration.
Differences between fine and textured soils are due to the fact that coarse-textured soils have high infiltration rates. This reduces the amount of surface runoff that is generated and retained within the field by tied-ridging. Further, sandy soils have a low water holding capacity (small FC and WP) and macropore flow is dominant. Thus, the surface runoff that is retained by tied-ridging in coarse-textured soils is quickly lost through drainage, hardly increasing SMS and ETa. Fine-textured soils are the opposite resulting in opposite conclusions. The increased drainage with tied-ridging is a concern since it may increase leaching of plant nutrients out of the root zone reducing maize yield. Further, excessive leaching of nutrients out of the root zone has environmental implications.
5. Conclusions
A calibrated field 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 moisture status was evaluated by simulating seasonal (140 days) changes in retained rainwater, surface runoff, drainage, soil moisture storage, waterlogging and actual evapotranspiration. The simulation results show that tied-ridging affects the soil water status of a maize crop under Malawi conditions. Tied-ridging is likely to increase retained rainwater within the field. The gained rainwater due to tied-ridging will mostly be lost as drainage out of the root zone (over 80%). The remainder
will increase SMS and ETa only in fine-textured and not in coarse-textured soils. Increasing rooting depth will decrease drainage loss while increasing SMS and ETa.
The results seem to indicate that tied-ridging is not suitable for coarse-textured soils regardless of the seasonal rainfall amount and distribution. For fine-textured soils, tied ridging will benefit the crop only within the rainfall band of 500±900 mm. Below 500 mm (severe drought), rainfall is insufficient to meet maize CWR with or without tied-ridging. Above 900 mm (normal and wet years), rainfall is sufficient to meet maize CWR without tied-ridging. Even worse in normal and wet years (82.2% of the years in Malawi), tied-ridging is likely to result in waterlogging leading to maize yield decline.
Acknowledgements
This study was made possible by a research grant from the International Development Research Centre (IDRC), Canada and a scholarship to the first author from the Belgian Administration for Development Cooperation (BADC/ABOS) at the Institute for Land & Water Management of Katholieke University of Leuven, Belgium. In Malawi, we thank messrs Ignatious Majamanda, Dick Sosola and Nkhondo Kasisi for assisting in data collection and input.
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