Management of cracked soils for water saving during
land preparation for rice cultivation
Romeo J. Cabangon, T.P. Tuong
*Soil and Water Sciences Division, The International Rice Research Institute, MC P.O. Box 3127, Makati City 1271, Philippines
Abstract
High water loss during land preparation of soils for rice (Oryza sativaL.) production results from bypass ¯ow through cracks. It was hypothesized that the losses can be reduced by measures that minimize crack development during the soil drying period or impede the ¯ow of water through these cracks. The effect of straw mulching and shallow surface tillage on crack formation during the fallow period, and on water ¯ow components during land preparation was investigated in ®eld experiments on an Epiaqualf and a Pellustert in the Philippines. Cracks did not completely close upon rewetting, resulting in high loss (152± 235 mm of water) during land preparation of the control (i.e. no soil management treatment) plots. Straw mulching helped conserve moisture in the soil pro®le, and reduced the mean crack width by 32% of the control. Mulching did not signi®cantly reduce mean crack depth and the amount of water used in land preparation. Shallow tillage formed small soil aggregates which made the crack water ¯ow discontinuous and impeded groundwater recharge from the water ¯ow through cracks, reduced total water input for land preparation by 31±34%, equivalent to about 120 mm of water. The average surface irrigation water ¯ow advanced faster and less time was needed for land preparation in the shallow tillage plots compared to the control. Shallow tillage offers a practical means for improving water-use ef®ciency of irrigation systems. In rainfed areas, it may facilitate early crop establishment and, thus, reduce the risk of late-season drought.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Bypass ¯ow; Irrigation; Shallow tillage; Mulching; Water-use ef®ciency
1. Introduction
Over the next 30 years, rice production must increase by 70% from the present production to avoid rice shortage (IRRI, 1995). Rice is known to be less water ef®cient than many other crops, and water for irrigation is becoming increasingly scarce because of escalating demand for non-agricultural uses. Improv-ing water-use ef®ciency of rice culture is a pre-requi-site for food security in Asia.
The ®rst step in lowland rice production is land preparation. Water is applied to rice ®elds until the topsoil is saturated and a ponding water layer of 10± 50 mm depth is maintained (land soaking) for 2 days or more on the ®eld. Land soaking is followed by plowing and harrowing several times under saturated condition to puddle the topsoil to a depth of 10±20 cm. After transplanting or direct seeding, the ®eld is kept ¯ooded to a depth of about 50 mm throughout the growing season. To facilitate harvesting, irrigation is often stopped 2 weeks before the crop reaches matur-ity (De Datta, 1981). Fields often are left fallow and allowed to dry before the next crop.
Drying of a puddled soil usually results in soil shrinkage and cracking. Cracks are especially promi-*Corresponding author. Tel.:63-2-8450563;
fax:63-2-8911292.
E-mail address: t.tuong@cgiar.org (T.P. Tuong).
nent if expanding clay minerals are present, but they may also be clearly noticeable in kaolinitic soils and can reach depths of 20±65 cm (Moorman and van Breemen, 1978; Ishiguro, 1992; Wopereis et al., 1994; Ringrose-Voase and Sanidad, 1996; Tuong et al., 1996). Irrigation for land preparation of the next rice crop, thus, involves water application to cracked soils and results in bypass ¯ow losses (water that ¯ows through cracks to the subsoil). Tuong et al. (1996) reported that bypass ¯ow accounted for 41± 57% (equivalent to about 100 mm of water) of the total water applied in the ®eld during land soaking. Water loss throughout the period of land preparation may be much greater than this, because cracks may not close after rewetting (Moorman and van Breemen, 1978; Ishiguro, 1992; Wopereis et al., 1994; Tuong et al., 1996), and bypass ¯ow may continue until soil is repuddled. This might explain the very high percola-tion losses during land preparapercola-tion, accounting for up to 40% of the total water supplied for growing a rice crop (Wickham and Sen, 1978). Reducing these losses will contribute greatly to improving water-use ef®-ciency of rice.
Tuong et al. (1996) quanti®ed the ¯ow process when ¯ood irrigation is applied to cracked soils. Irrigation water moves rapidly in the crack networks, ahead of the surface water front. Part of this crack water in®ltrates into the subsoil, bypassing the topsoil, thus, recharging the groundwater. When the subsoil is permeable, about 70% of this bypass ¯ow may be lost to the surroundings through lateral drainage. Since the ¯ow processes are dominated by water ¯ow in cracks, measures that in¯uence crack geometry or the water ¯ow in the cracks may affect the amount of water loss. Straw mulching, by reducing evaporation from the soil surface (Hundal and Tomar, 1985) can minimize soil shrinkage, lessen crack development during the fallow period before land soaking and, therefore, may reduce bypass ¯ow losses. Shallow tillage can also reduce evaporation loss. It may also form soil aggregates which block the cracks and impede water ¯ow into them. Wopereis et al. (1994) showed that shallow tillage reduced the crack bypass ¯ow in undisturbed cores by 45±60%. Tillage effectiveness in reducing water loss in the ®eld conditions has not been tested. This study was carried out to assess straw mulching and shallow tillage, as possible measures to reduce water loss during land preparation of dry, cracked rice
soils. The processes by which these measures affects crack development during the fallow period and water balance components during the land preparation were quanti®ed in ®eld experiments.
2. Methodology
2.1. Experimental sites
The study was conducted in rice ®elds (i) at the International Rice Research Institute (IRRI), Los Banos, Laguna (148300N, 121
8150E) during the 1993 and 1994 dry seasons; (ii) in the Angat River Irrigation System, Bulacan (148470N, 120
8550E) dur-ing the 1993 wet season and (iii) in Munoz, Nueva Ecija (158400N, 1208500E), during the 1995 dry sea-son. All sites have two distinct seasons, i.e. wet from June to November and dry from December to May. According to soil taxonomy (Soil Survey Staff, 1992), the soil at IRRI was classi®ed as an Aquandic Epia-qualf, at Bulacan Typic Epiaqualf (Tuong et al., 1996) and at Nueva Ecija Entic Pellustert (Raymundo et al., 1989). Some major soil characteristics of the sites are presented in Tables 1 and 2. At the time of land soaking, water table depths were 0.85 m (1993) and 1.1 m (1994) at the IRRI ®elds, 0.5 m at the Bulacan site, and 1.1 m at Nueva Ecija.
2.2. Treatments
2.2.1. At IRRI
The experiments were conducted in 6 m11 m plots surrounded by bunds (30 cm width and 25 cm height). The plots were hydraulically isolated by polyethylene sheets installed to 0.7 m depth along the center of the bunds. At the start of the experiment (18±20 April 1993 and 23±25 January 1994), all plots were ¯ooded, plowed, puddled to depth of 10 cm and leveled to attain similar conditions before the treat-ments were applied. Surface water was drained 1 day after land leveling. All ®elds were allowed to dry under the sun during the fallow period until irrigation for land soaking for the next rice crop was carried out on 31 May 1993 and 17 May 1994.
Straw mulching. Dry rice straw (5 Mg haÿ1) was broadcast over the plot 1 day after drainage (DAD) and remained on the soil surface during the fallow period.
Shallow surface tillage. Plots were rototilled to 5± 10 cm depth by two passes of a IRRI-manufactured rototiller at 18 DAD in 1993 and 14 DAD in 1994. The schedule of rototilling was decided by the operator based on the adequacy of soil bearing capacity to support the implement. Shallow tillage produced top-soil aggregates of about 1±5 cm diameter.
Control. No treatment was applied during the fallow period.
In the 1994 experiment, the straw mulching treat-ment was not included.
2.2.2. In Bulacan
The amount of water used in six farmers' ®elds (measured 80±90 m120±165 m) were monitored from 17 June until land preparation activities were completed. After harvest of the previous rice crop on second week of March 1993, three ®elds were left
Table 1
Texture and vertical saturated hydraulic conductivity of the experimental ®elds at IRRI, Bulacan and Nueva Ecijaa
Depth (m) Clay (g kgÿ1) Silt (g kgÿ1) Sand (g kgÿ1) Saturated conductivity
(m per day)
IRRIb
0±0.2 420 440 140 ±
0.2±0.5 490 370 140 0.5
Farmers' fields, Bulacanb
0±0.2 290 (3, 9) 560 (3, 2) 150 (3, 7) ±
0.2±0.3 400 (3, 9) 440 (3, 2) 150 (3, 2) ±
0.3±0.5 400 (3, 9) 420 (3, 3) 170 (3, 5) 0.1 (3, 004) Farmers' fields, Nueva Ecija
0±0.2 530 (10, 36) 340 (10, 44) 130 (10, 41) ± 0.2±0.4 560 (10, 39) 300 (10, 48) 140 (10, 44) ±
0.4±0.6 550 (10, 38) 310 (10, 57) 140 (10, 40) 0.007 (6, 0004)
aNumber of observations and standard deviations are indicated in parentheses. bSource: Tuong et al. (1996).
Table 2
Bulk density and soil water content at saturation and at different dates in farmers' ®elds; Bulacan and Nueva Ecijaa
Depth (m) Bulk density (mg mÿ3) Soil moisture content (mÿ3mÿ3)
Control Shallow tillage Saturated Start of monitoringb Start of land soakingc
Control Shallow tillage
Control Shallow tillage
Control Shallow tillage
Farmers' fields, Bulacan
0±0.2 0.99 (4, 0.05) 0.90 (4, 0.04) 0.63 (4, 0.03) 0.65 (4, 0.03) 0.42 (9, 0.02) 0.40 (9, 0.01) 0.52 (9, 0.02) 0.48 (9, 0.03) 0.2±0.3 1.06 (9, 0.04) 1.06 (9, 0.04) 0.61 (9, 0.01) 0.61 (9, 0.03) 0.49 (9, 0.01) 0.51 (9, 0.01) 0.57 (9, 0.02) 0.59 (9, 0.01) 0.3±0.5 1.04 (9, 0.04) 1.04 (9, 0.04) 0.61 (9, 0.02) 0.61 (9, 0.05) 0.50 (9, 0.01) 0.51 (9, 0.01) 0.56 (9, 0.02) 0.57 (9, 0.01) Farmers' fields, Nueva Ecija
0±0.2 1.24 (4, 0.01) 1.17 (4, 0.02) 0.52 (4, 0.02) 0.55 (4, 0.12) 0.26 (4, 0.05) 0.28 (4, 0.02) 0.2±0.4 1.32 (4, 0.04) 1.32 (4, 0.04) 0.50 (4, 0.01) 0.50 (4, 0.01) 0.41 (4, 0.03) 0.43 (4, 0.04) 0.4±0.6 1.35 (4, 0.05) 1.35 (4, 0.05) 0.48 (4, 0.01) 0.48 (4, 0.01) 0.46 (4, 0.02) 0.46 (4, 0.01)
aNumber of observations and standard deviations are indicated in parentheses. b17 June 1993 in Bulacan.
fallow until land soaking was carried out on 10±11 July. Land was prepared and ®nal land leveling com-pleted on 15±20 July. In the other three ®elds, farmers used tractor-powered rototillers to rototill the land to a depth of about 10 cm at the onset of the rainy season (12, 20, and 29 May 1993). In these ®elds, land soaking was carried out on 28 June, 6 and 10 July and land leveling accomplished on 4±17 July.
2.2.3. In Nueva Ecija
An experiment was conducted in eight farmers' ®elds (measured 16±27 m220±610 m) from Decem-ber 1994 to 15 January 1995. The previous rice crop was harvested on the second week of November 1994. Two tillage treatments were imposed during the land preparation period in a randomized complete block design with four replications. In each replication, the dimensions of the ®elds were almost similar. The treatments were:
Control. Fields were kept fallow until land soaking on 20±26 December 1994, plowed on 21±28 Decem-ber and harrowed using comb-tooth harrow on 24±29 December. Final leveling was completed on 3±12 January 1995.
Shallow surface tillage. Fields were dry rototilled to a depth of 10 cm on 15±16 December 1994, using four-wheel tractor drawn rototillers. Land soaking was carried out from 21±26 December 1994. The ®elds were harrowed using comb-tooth harrows and ®nal leveling was completed on 4±13 January 1995.
2.3. Measurements of soil water content and physical properties
All measurements in the IRRI plots were taken from walk-boards installed in each plot to minimize the disturbance to the soil and crack formation. Samples, collected using 100 cm3 cylinders, for bulk density and volumetric moisture content of the puddled and/or the tilled layer (approximately 0.1 m thick), and layers at depths 0.1±0.2, 0.2±0.3 and 0.3±0.5 m were taken at 2±3 days intervals during the fallow and land soaking periods. Similar samples were used to determine the saturated water content (Tuong et al., 1996). Vertical saturated conductivity of the 0.3±0.5 m layer at the sites were determined using constant head method and encased soil columns 0.25 high and 0.20 m in diameter (Wopereis et al., 1994).
In Bulacan and Nueva Ecija soil moisture contents were monitored by the same method as at IRRI, at the start of the monitoring program before and after land soaking at four stations along the center transect of each ®eld. Sampling depths varied depending on the soil pro®le of each site: 0±0.1 m (tilled layer), 0.1±0.2, 0.2±0.3, and at 0.3±0.5 m in Bulacan and 0±0.1 m (tilled layer), 0.1±0.2, 0.2±0.4, and 0.4±0.6 m in Nueva Ecija.
For simplicity and for the presentation purpose, the above depths (and else where in this paper) refer to distances from the original soil surface. The puddled or tilled layer, however, might change its thickness due to shrinking and swelling during the fallow and land soaking periods. To ensure that the same layers were sampled at various times, all subsoil samplings used the bottom of the tilled layer (0.1 m from the original soil surface) as the datum. For example, for the layer at depth 0.2±0.3 m, the sample was taken from 0.1±0.2 m below the bottom of the tilled layer.
2.4. Measurement of crack dimensions
At the IRRI ®elds, crack depth and width were monitored at 1±2 days intervals during the fallow period in a 1 m1 m subplot in each of the control and mulched plots. In the mulched plots, straw was removed before and replaced after each measurement. Crack dimensions of the shallow tillage plots before the shallow tillage were assumed to be the same as those in the control plot. In the farmers' ®elds, crack dimensions were monitored in two 1 m1 m subplots per ®eld 2±3 days before land soaking. Methods of measuring and computing crack depth, width, volume and the surface area of soil islands (soil masses distinctively separated by cracks) are presented in Tuong et al. (1996).
2.5. Water application and monitoring
Nueva Ecija. The applied water was spread uniformly across the width of the ®eld by a distribution channel as described by Tuong et al. (1996).
In Nueva Ecija, the distance traveled by the advan-cing surface irrigation water front along the center longitudinal transect of the plots during the land soaking process was monitored at approximately 1 h interval. Groundwater table tubes were installed at 10 m apart along the same transects. The water table at each location was monitored from the start of land soaking until the water table reached the soil surface, following the procedures in Tuong et al. (1996).
2.6. Computation of water ¯ow components during land preparation
Water balance calculation was carried out during land soaking at the IRRI ®elds. In Bulacan and Nueva Ecija, water balance during land preparation was divided into two phases, namely, land soaking phase (from ®rst water application to ®rst harrowing) and harrowing phase (from ®rst harrowing to ®nal level-ing). In Bulacan, water balance prior to land soaking (from start of the monitoring, 17 June 1993 to land soaking irrigation) was also carried out to take into account the rainfall at the beginning of the wet season. For each period, the water balance for the topsoil (0± 0.2 m depth), expressed in mm of water over each ®eld, can be quanti®ed with the following equation (Tuong et al., 1996):
IRSsScAEL (1)
whereIis the irrigation water,R the rainfall, Ss the surface water storage, Sc the crack storage, i.e. the amount of water that ®lls the cracks in the ®eld,Athe water absorbed in the soil layer under consideration,E the evaporation from the ®eld, L the losses, i.e. the amount of water that goes beyond the topsoil, rechar-ging the groundwater.
Irrigation water was calculated from the total volume of water applied (integral of ¯ow discharge over time) divided by the ®eld area and Ss was the change in depth of surface water. R was monitored with rain gauges installed at the experimental sites. The Sc was the volume of cracks under the surface expressed in mm of water depth. TheAwas computed from the difference in soil moisture contents at the beginning and end of the study period. After land
soaking, soil became saturated, there was no more increase in crack storage and absorbed water. The E from the start of the monitoring period to land soaking, when the topsoil water content in the ®elds changed from dry to saturated, were estimated by multiplying open water evaporation (measured by Class A pan) by 0.5 (Tuong et al., 1996). Evaporation after land soak-ing was estimated by open water evaporation. The losses were derived from the difference between the sum of inputs (IR) and the sum of soil surface storage, crack storage, soil absorption, and evapora-tion losses SsScAE:
The water balance was computed separately for the tilled layer (approximately 0.1 m thick) and for the 0.1 m layer immediately below the bottom of the tilled layer, before being summed up for the topsoil under consideration (0±0.2 m depth of the original soil).
3. Results and discussions
3.1. Soil moisture content
Soil moisture contents at different depths at IRRI are presented in Fig. 1. Fluctuations in soil moisture content from 10 DAD (1993) and from 44 DAD (1994) were due to intermittent rains. In the 1993 experiment, soil moisture content of the mulched plots was con-sistently higher than the control and the shallow tilled plots for depths 0±0.1 and 0.1±0.2 m; and for depth 0.2±0.3 m until about 18 DAD. The difference among treatments at the later stage, being affected by inter-mittent rains, was not signi®cant. Moisture content at 0.3±0.5 m depth did not differ among treatments (data not shown). The results con®rmed previous ®ndings that straw mulch was effective in reducing soil surface evaporation (Hundal and Tomar, 1985).
(Bulacan) and 0.2±0.4 m (Nueva Ecija) depths were slightly higher than that of the control.
3.2. Crack dimensions
Mean crack width and depth of the control and the mulched plots at IRRI are presented in Fig. 2. Initial increase in crack depth and width on the ®rst 8±10 DAD corresponded to the rapid loss of moisture from the surface layer (Fig. 1). Both crack width and depth increased more rapidly in the control plots than in the mulched plots. This corresponded to the slower rate of soil drying in the mulched plots (Fig. 1) and resulted to a lower crack volume in the mulched compared to the control. In the control treatment, a slower rate of increase in crack depth and width in the 1994 experi-ment compared to the 1993 experiexperi-ment conformed with a slower rate of decrease in moisture content in
soil layers in 1994. Ringrose-Voase and Sanidad (1996) reported similar rates of crack development in fallow rice ®elds.
In the 1993 experiment, the mean crack depth in the control treatment reached maximum value of about 115 mm and width about 40 mm at 19 DAD. Both mean crack width and depth did not change signi®-cantly afterwards. At the end of the fallow period, the crack width of the mulch treatment was signi®cantly lower than that in the control treatment (Fig. 2). This corresponds to wide differences in the ®nal moisture content of the soil surface layer (Fig. 1) in the two treatments. The ®nal mean crack depth in the mulch treatment was also less, but not signi®cantly than that in the control treatment (Fig. 2). The formation of cracks at lower depths was in¯uenced by the soil moisture in the subsoil. The non-signi®cant differ-ences in soil moisture at deeper soil layers from 20 DAD between two treatments might have resulted in only slightly different crack depth. It was likely that at the day of shallow tillage (at 18 DAD), crack depth of the shallow tillage plots are similar to those of the control plots. In the 1993 experiment, this depth was about 110 mm (Fig. 2), i.e. about the same as the ®nal crack depth of the mulched plots (106 mm).
Crack width in the control plot in the 1993 and 1994 experiments were of about the same size (Fig. 2). The zero shrinkage portion of the shrinkage characteristic curve of the same puddled soil began at a soil moisture content of 0.25 m3mÿ3 (Wopereis, 1993). This implies that upon further drying, no more shrinkage will take place. Soil moisture of the topsoil layer in both years, was below 0.25 m3mÿ3, implying max-imum shrinkage had taken place in this layer. Crack depth was, however, greater in the 1994 experiment, reaching a mean value of about 130 mm (Fig. 2b). This corresponded to a lower soil moisture at the deeper soil layers in the 1994 experiment.
The crack dimensions prior to land soaking at IRRI, Bulacan and Nueva Ecija are shown in Table 3. The average widths and depths of cracks were similar to results of Ishiguro (1992), Wopereis et al. (1994), and Tuong et al. (1996). The wider and deeper crack at the Nueva Ecija site was probably due to the higher clay content in the Nueva Ecija ®elds. Intermittent rains during the fallow period at IRRI and in Bulacan probably caused some swelling and closure of cracks.
3.3. Overland and subsurface ¯ow
During land soaking of the control treatment in Nueva Ecija, water moved in the crack networks and on the soil surface. Water ¯ow in the cracks advanced faster than overland ¯ow of irrigation water. As a result, the water table rose close to the
soil surface at a distance of about 10 m ahead of the advancing surface water front. No signi®cant rise of the water table was observed ahead of the surface water front in the shallow tillage treat-ment (Fig. 3). The surface water front advanced faster in the shallow tillage than in the control plots (Fig. 4).
Table 3
Dimensions of crack and soil islands (soil masses distinctively separated by cracks) prior to land soaking in IRRI, Bulacan and Nueva Ecija
Location and treatment Crack Soil island
Width (mm)a
Depth (mm)a
Volume (m3mÿ2)b
Surface area (m2mÿ2)b,c
Peripheral surface area (m2mÿ2)b
IRRI 1993
Mulched 2719 10531 0.009 0.840.04 1.5
Control 4016 11029 0.013 0.760.03 1.9
IRRI 1994
Control 2815 11338 0.020 0.680.01 1.8
Bulacan
Control 3415 14253 0.010 0.800.03 3.8
Nueva Ecija
Control 3620 17856 0.025 0.710.03 3.4
aMeanS.D. of 120±139 observations at IRRI, 148 in Bulacan, and 352 in Nueva Ecija. bPer unit ®eld surface area.
cMeanS.D. of six sampling subplots in IRRI, six in Bulacan, and 12 in Nueva Ecija.
Observations in the control plot con®rmed ®ndings by Tuong et al. (1996) and indicated that part of water that moved in the cracks bypassed the topsoil, in®l-trated into the subsoil, and recharged the groundwater. Small soil aggregates in the shallow tillage treatment blocked the cracks, making them discontinuous and impeded water ¯ow in the cracks and reduced recharge to the groundwater. This conformed with Wopereis et al. (1994) who reported that small soil aggregates reduced 45±60% of the bypass loss through cracks in large undisturbed soil columns. Less loss to the subsoil meant more water was available for the surface water ¯ow, and resulted in a faster rate of advance of the
surface water front (Bassett et al., 1983) in the shallow tillage plots. Shallow tillage can, thus, help reduce time for land soaking. In Muda Irrigation scheme, Malaysia, it is credited with the bene®ts of timely crop establishment (Ho et al., 1993).
3.4. Water ¯ow components during land preparation
Table 4 shows the water balance components for different treatments during the 1993 and 1994 experi-ments at the IRRI ®elds. The amount of irrigation water needed for land soaking ranged from 93 to 272 mm. The increase in water needed for land soak-ing in the control plots in 1994 compared to 1993 was caused mainly by increased water loss. This corre-sponded to deeper cracks and deeper water table in 1994.
In the 1993 experiment, compared to the control plots, straw mulching did not reduce signi®cantly the amount of irrigation water for land soaking at IRRI ®elds (Table 4). By reducing the evaporation loss during the fallow period, mulching reduced the amount of absorbed water needed to saturate the surface soil layer (27 mm compared to 53±56 mm in other treatments). This reduction was a very small portion of the water input and did not result in total water savings.
In both years at the IRRI ®elds, the shallow tilled plots used signi®cantly less water for land soaking
Fig. 4. Distance travelled by the advancing surface water front in the control and shallow tillage plots, Nueva Ecija, 1995 dry season.
Table 4
Water balance components during land soaking as affected by surface soil management treatments at the IRRI ®elds in 1993 and 1994
Componenta Year Treatment (mm)
Mulch Shallow tillage Control
Irrigation water 1993 109 abb 93 b 130 a
1994 172 b 272 a
Surface storage 1993 20 20 20
1994 25 21
Crack storage 1993 9 0 13
1994 0 20
Water absorbed in the 0±0.2 m layer 1993 27 53 56
1994 48 26
Losses 1993 53 a 20 b 41 a
1994 99 b 205 a
aThere was no rain and evaporation was neglected during land soaking.
than the control plots. Losses from the shallow tilled plots (20 mm in 1993 and 99 mm in 1994) were about 50% of those from the control plots (41 mm in 1993 and 205 mm in 1994). In 1993, the amounts of irrigation water for land soaking and water loss in shallow tilled plots were signi®cantly less than those in the mulched plots, though cracks in shallow tilled were at least as deep as those in the mulched treatment plots. This highlighted the role of small soil aggregates in blocking and impeding water ¯ow through the cracks.
In farmers' ®elds, shallow tillage reduced the total water input for land soaking by 54±58% of the amount needed in the control plots (69 mm compared to 150 mm in Bulacan, Table 5; and 95 mm to 227 mm in Nueva Ecija, Table 6). Most of the savings in the water input for land soaking came from the reduced losses in the shallow tillage plots compared to the control. Findings in farmers' ®elds, thus, con-®rmed those at IRRI.
Shallow surface tillage reduced the total water input for land preparation by 31% in Bulacan (238 mm
Table 5
Water balance components during land preparation as affected by tillage treatments in farmers' ®elds, Bulacan, 1993
Component Prior to land soaking Land soaking stage Harrowing stage Total
Control With shallow tillage
Control With shallow tillage
Control With shallow tillage
Control With shallow tillage
Total water input 170 139 150 a* 69 b 26 a 30 a 346 a 238 b
Irrigation water 0 0 89 57 17 0 106 57
Rainfall 170 139 61 12 9 30 240 181
Evaporation 43 32 11 8 12 9 66 49
Surface storage 0 0 1 20 ÿ5 ÿ1 ÿ4 19
Crack storage 0 0 10 0 0 0 10 0
Absorbed in 0±0.2 m layer 28 19 11 11 0 0 39 30
Losses 99 88 117 a 30 b 19 a 22 a 235 a 140 b
Duration (day) 23.7 17.7 4.6 2.3 2.7 3.3 31 23.3
Loss rate (mm per day) 25 a 13 b 7 a 7 a 8 6
*All water components are expressed in mm of water over the area of the ®eld. Total water inputs are sum of irrigation water and rainfall.
In the same row and land preparation stage, treatment means followed by a common letter are not signi®cantly different at 5% level by DMRT.
Table 6
Water balance components during land preparation as affected by tillage treatments in farmers' ®elds, Nueva Ecija, 1995
Component Land soaking Stage Harrowing stage Total
Control With shallow tillage
Control With shallow tillage
Control With shallow tillage
Total water input (227) a* (95) b (122) a (137) a (349) a (232) b
Irrigation water 224 91 121 136 345 228
Rainfall 3 3 1 1 4 4
Evaporation 26 4 58 57 84 60
Change in storage 43 30 ÿ1 20 42 50
Crack storage 25 0 ± ± 25 0
Water absorbed in 0±0.2 m layer 46 41 ± ± 46 41
Losses 87 a 19 b 65 a 60 a 152 a 79 b
Duration (day) 6.0 1.9 10.6 13.1 16.5 14.9
Loss rate (mm per day) 14.5 10.0 6.1 4.6 9.2 5.3
*All water components are expressed in mm of water over the area of the ®eld. Total water inputs (values in brackets) are sum of irrigation
versus 346 mm, Table 5); and by 34% in Nueva Ecija (232 mm versus 349 mm, Table 6) of the amount needed for the control plots. Much of the differences in the amount of water-use between the two treatments occurred during the land soaking phase. High loss rate sustained in the control plots during this phase indi-cated that water loss through cracks continued until the ®rst harrowing was carried out. Harrowing reduced the loss rate of the control plots considerably and made them equivalent to those of the shallow tillage plots. The ®ndings supported the observation that cracks did not completely close upon rewetting (Moorman and van Breemen, 1978; Ishiguro, 1992; Wopereis et al., 1994; Tuong et al., 1996). Harrowing broke soils in the control plots into aggregates, which sealed the cracks and produced the puddling effects which reduced soil permeability (De Datta, 1981). Where dry shallow tillage can not be carried out, shortening the duration between land soaking and the ®rst harrowing may be an important measure to reduce water loss during land preparation.
In the above computations, it was assumed that the same soil was taken into consideration before and after land soaking. Since the whole puddled layer (control treatment) and the tilled layer (shallow tillage treat-ment) were included in the water balance, changes in their thickness during land soaking did not cause any error in the computation. Soaking decreased the bulk density of the 0.1 m stratum below the tilled layer by about 10% (e.g. from 1.15 to 1.04 at IRRI, data not shown). Assuming an isometric swelling in the stra-tum, the corresponding change in thickness of the stratum would be about 3%. Thus, error due to neglecting the effect of soil swelling in the computa-tion was negligible.
4. Conclusion
Straw mulching helped conserve moisture in the soil pro®le, reduced crack development during the fallow period but did not reduce the bypass loss during land preparation. Shallow tillage formed small soil aggregates, which blocked and impeded water ¯ow in the cracks and reduced the amount of water that recharged the groundwater via the bottom of the cracks and crack faces. Water was, therefore, retained better in the topsoil. Shallow surface tillage could
reduce about 31±34% of the water input for land preparation, equivalent to a saving of 108±117 mm of water depth and shortened time required for land preparation. Water savings during land preparation may increase the service area of an irrigation system. In rainfed areas, shallow surface tillage may also lead to earlier crop establishment and, thus, reduce the risk of late-season drought. This kind of tillage does not necessarily require high-powered tractors. Further more, tractors/rototillers are becoming more accessi-ble to small farmers for custom hiring, offering better opportunities for incorporating shallow surface tillage practice in the rice production system.
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