Improvement of Planosol Solum: Part 4, Field Experiments with Prototype Roll-In and Drop-Down Ploughs

This paper deals with the design of prototype roll-in and drop-down ploughs and field tests on a planosol solum in China to obtain mixing of the second (Aw) and third (B) horizons to improve the planosol solum, leaving the first (Ap) horizon undisturbed.The field test results showed that the drop-down plough was effective for mixing of the Aw and B horizon of the planosol solum. However, a new problem was encountered, in that the ploughing of the Aw horizon produced large clods, and the Ap horizon crumbled and dropped down among the large clods and reached lower horizons. The roll-in plough was not effective for soil mixing because planosol solum was much harger than the pseudogley soil used in soil-bin tests in Japan. The roll-in plough bodies simply penetrated into the soil such as subsoilers do.     

Improvement of Planosol Solum: Part 1, Experimental Equipment,Methods and Preliminary Soil Bin Experiments with Ploughs

Based on field cultivation tests by Zhao and soil investigations by Araya, a one to one mixing of the second (Aw) and third (B) horizons was conducted to improve the planosol solum in China, leaving the first (Ap) horizon undisturbed. This paper deals with basic soil bin tests of soil mixing with half size model ploughs in Japan.The results showed that stand-up and drop-down ploughs produced good soil mixing and required low draught forces. On the other hand, a fold-up plough could not mix the soil and required a high draught force because the furrow slices did not move smoothly over the plough.     

The effect of degradation of phalaris + white clover pasture on soil water regimes of a Brown Chromosol on the Northern Tablelands of NSW,Australia

The soil water regimes of a Brown Chromosol in response to drying and wetting are reported under three pastures types that were grazed all year long. The study was conducted at the Big Ridge 2 site near Armidale, on the Northern Tablelands of New South Wales (NSW) between 1994 and 1998. The three pasture types were degraded pasture (dominated by annual species), a phalaris dominant pasture, and an improved pasture containing phalaris + white clover. This study was conducted to assess the hydrological implications of losing perennial pasture species from the high rainfall (summer dominant) zone of south-eastern Australia. Pasture active rooting depth, water use and extraction during drying periods, and the ability of the soil profile to store water during wet periods were evaluated for each pasture type.Pasture active rooting depth, which affects water use, varied with season and water availability. During a typical autumn drying period between 1 and 22 March 1996, the phalaris + white clover pasture with an active rooting depth down to 100 cm used 46 ± 3.9 mm of water, with 16% of this extracted from the 55–130 cm zone. In the same period, the degraded and the phalaris pasture with active rooting depths of ≤60 cm used 30.7 ± 5.2 and 23.6 ± 7.9 mm of water, respectively, all from the surface 0–55 cm zone. However, under extreme drought conditions such as those in spring and early summer 1997 and autumn 1998, no differences in water use were detected between pastures.Pasture water use during dry periods affects the amount of water that can be stored in the soil profile and the potential amount of water loss during subsequent wet periods. In any wet period, the increase in soil water storage was greater in the 0–55 cm depth than in the 55–130 cm zone. For example, between 24 January and 14 February 1997 with total rainfall of 203 mm, water storage in the 0–55 cm zone was increased by 104.4 ± 6.7 mm under the phalaris + white clover pasture compared with 86.4 ± 4.3 and 84.4 ± 3.3 mm for the degraded and the phalaris pastures, respectively. The water storage increase in the 55–130 cm zone was not different between pastures (<12 mm).It was concluded that without appropriate grazing management and the presence of the legume component, the phalaris based pasture became unstable and failed to persist. The decline in the phalaris pasture caused invasion of annual species and weeds resulting in low water use, similar to that of the degraded pasture. In contrast, the combination of white clover and phalaris pasture showed a greater potential to maintain the phalaris component and a greater total biomass, and so was able to extract more water and from deeper parts of the soil profile. The vigorous phalaris + white clover pasture has greater potential to store more water than the degraded pasture and the phalaris pasture without legumes in the summer dominant rainfall area of temperate Australia. Therefore, maintaining pasture in good condition should be the main objective for sustainability of a grazing system in this region.     

Evapotranspiration of flood-irrigated pecans

Pecan orchards require more irrigation water to maximize yield than any other crop grown in the Southwest US. This paper reports daily evapotranspiration (Et) measurements for 2001 and 2002 in a 5.1 ha, mature pecan orchard on the Rio Grande floodplain, 7 km south of Las Cruces, NM, USA. The 21-year-old stand had an average tree height of 12.8 m, diameter at breast height of 30 cm, and tree spacing of 9.7 m × 9.7 m. Additional pecan orchards surrounded the study orchard. When the tensiometer reached a suction of 65 kPa at the 45 cm depth, the orchard was flood-irrigated. Sparling meters were installed on the pumps and read before and after each irrigation. The total irrigation amount was 1940 mm in 2001 and 1870 mm in 2002. A walk-up tower was placed in the orchard’s center to support flux sensors at 16 m height. The instrument package included a net radiation (Rn), discs for soil heat flux (G), and two sets of one-propeller eddy covariance (OPEC) sensors. OPEC systems measure sensible heat flux (H) with a sensitive, vertically oriented propeller anemometer and a fine-wire thermocouple. Latent heat flux (LE) was obtained as a residual in the surface energy balance LE = Rn − G − H. The maximum daily evapotranspiration was 8 mm/day, and the yearly cumulative evapotranspiration averaged for 2 years was 1420 mm, resulting in a yearly average irrigation application efficiency of 79%. The crop coefficient (daily measured Et/reference Penman Et) ranged from 0.2 to 1.1. Increased evaporation due to irrigation was detected only for the April 9 irrigation in 2001. The seasonal water use was 4% lower in 2001 and 12% lower in 2002 than previously reported values.     

Response of safflower (Carthamus tinctorius L.) to saline soils and irrigation: I. Consumptive water use

Salt-tolerant crops can be grown with saline water from tile drains and shallow wells as a practical strategy to manage salts and sustain agricultural production in the San Joaquin Valley (SJV) of California. Safflower (Carthamus tinctorius L.) was grown in previously salinized plots that varied in average electrical conductivity (EC_e) from 1.8 to 7.2 dS m⁻¹ (0–2.7 m depth) and irrigated with either high quality (EC_i<1 dS m⁻¹) or saline (EC_i=6.7 dS m⁻¹) water. One response of safflower to increasing root zone salinity was decreased water use and root growth. Plants in less saline plots recovered more water on average (515 mm) and at a greater depth than in more salinized plots (435 mm). With greater effective salinity, drainage increased with equivalent water application rates. Seed yield was not correlated with consumptive water use over the range of 400–580 mm. Total biomass and plant height at harvest were proportional to water use over the same range. Safflower tolerated greater levels of salinity than previously reported. Low temperatures and higher than average relative humidity in spring likely moderated the water use of safflower grown under saline conditions.     

Determining water consumption in olive orchards using the water balance approach

Efficient irrigation regimes are becoming increasingly important in commercial orchards. Accurate measurements of the components of the water balance equation in olive orchards are required for optimising water management and for validating models related to the water balance in orchards and to crop water consumption. The aim of this work was to determine the components of the water balance in an olive orchard with mature ‘Manzanilla’ olive trees under three water treatments: treatment I, trees irrigated daily to supply crop water demand; treatment D, trees irrigated three times during the dry season, receiving a total of about 30% of the irrigation amount in treatment I; and treatment R, rainfed trees. The relationships between soil water content and soil hydraulic conductivity and between soil water content and soil matric potential were determined at different depths in situ at different locations in the orchard in order to estimate the rate of water lost by drainage. The average size and shape of the wet bulb under the dripper was simulated using the Philip’s theory. The results were validated for a 3 l h⁻¹ dripper in the orchard. The water amounts supplied to the I trees during the irrigation seasons of 1997 and 1998 were calculated based on the actual rainfall, the potential evapotranspiration in the area and the reduction coefficients determined previously for the particular orchard conditions. The calculated irrigation needs were 418 mm in 1997 and 389 mm in 1998. With these water supplies, the values of soil water content in the wet bulbs remained constant during the two dry seasons. The water losses by drainage estimated for the irrigation periods of 1997 and 1998 were 61 and 51 mm, respectively. These low values of water loss indicate that the irrigation amounts applied were adequate. For the hydrological year 1997–1998, the crop evapotranspiration was 653 mm in treatment I, 405 mm in treatment D and 378 mm in treatment R. Water losses by drainage were 119 mm in treatment I, 81 mm in treatment D and 4 mm in treatment R. The estimated water runoff was 345 mm in treatments I and R, and 348 mm in treatment D. These high values were due to heavy rainfall recorded in winter. The total rainfall during the hydrological year was 730 mm, about 1.4 times the average in the area. The simulated dimensions of the wet bulb given by the model based on the Philip’s theory showed a good agreement with the values measured. In a period in which the reference evapotranspiration was 7.9 mm per day, estimations of tree transpiration from sap flow measurements, and of evaporation from the soil surface from a relationship obtained for the orchard conditions, yielded an average daily evapotranspiration of 70 l for one I tree, and 48 l for one R tree.     

Performance of temperate aerobic rice under different water regimes in North China

Since the late 1990s, aerobic rice varieties have been released to farmers in the North China Plain to grow rice as a supplementary-irrigated upland crop to cope with water scarcity. Little is known about their yield potential, water use, water productivity (WP), and flood tolerance. In 2001–2002, experiments with aerobic rice varieties HD502 and HD297 and lowland rice variety JD305 were conducted under aerobic and flooded conditions. Under aerobic conditions, five irrigation treatments were implemented. Under flooded conditions, JD305 yielded up to 8.8 t ha⁻¹, HD502 up to 6.8 t ha⁻¹, and HD297 up to 5.4 t ha⁻¹. Under aerobic conditions, the aerobic varieties yielded higher than the lowland variety. HD502 produced 3–3.5 t ha⁻¹ with 450–500 mm total water input and 5.3–5.7 t ha⁻¹ with 650 mm water input and more. HD297 produced 3–3.5 t ha⁻¹ with 450–500 mm total water input and 4.7–5.3 t ha⁻¹ with 650 mm water input and more. The water productivity of aerobic rice under aerobic conditions was higher or on a par with that of the lowland variety under flooded conditions, reaching values of 0.6–0.8 g grain kg⁻¹ water. The relatively high yields of the aerobic varieties under aerobic soil conditions were obtained under “harsh” conditions for growing rice. The soil contained more than 80% sand, was permeable, and held water above field capacity for a few hours after irrigation only. The groundwater table was deeper than 20 m, the soil moisture content in the rootzone was mostly between 50 and 80% of saturation, and soil moisture tension went up to 90 kPa. We conclude that the aerobic rice varieties HD502 and HD297 are suitable for water-scarce environments, and can stand being periodically flooded.     

Modeling net water requirements for wetlands in semi-arid regions

Wetlands in arid and semi-arid regions often experience water shortage problems due to interrupted water supply. Rapid population growth and economic development have caused deterioration or total destruction of many wetlands in such regions. Protection or restoration of these wetlands require a good understanding of the relationship between water supply and the soil wetness. This paper presents a model simulation study of such a relationship based on weather and soil data from Xi’an, China. The study area has an average annual precipitation of 600 mm and evaporation of 1200 mm. The simulation results showed that, to produce a certain wet condition, the required amount of water supply varied with recharging time due to different evapotranspiration rates. To maintain a consecutive water table depth within 30 cm (1) for 5% of the growing season, water requirements varied from 7 cm to 16 cm for different recharging months; (2) for 12.5% of the growing season, water requirement varied from 9 cm to 20 cm; and (3) for 25% of the growing season, water requirements varied from 13 cm to 27 cm. The highest water requirement occurred in summer when the air temperature is the highest of the year. Simulation results also showed that the timing of recharge not only has an important effect on the threshold water requirement, but also on the overall soil wetness of a year. Recharging at earlier time of the growing season produced longer wet periods, but the overall water table remained low during the rest of the growing season. Later inflow only influenced the water table for a small portion of the growing season, but it maintained a generally high water table in winter months and the early part of the next growing season.     

Irrigation management in arid areas affected by surface crust

The effects of supplemental irrigation and irrigation practices on soil water storage and barley crop yield were studied for a crust-forming soil at the University of Jordan Research Station near Al-Muwaqqar village during the 1996/97 growing season. An amount of 0.0, 48.9, 73.3, 122.2 and 167 mm supplemental irrigation water were applied. The 48.9, 73.3 and 122.2 mm applications were applied through surface irrigation into furrows with blocked ends, and the 0.0 and 167 mm applications via sprinkler irrigation. The greatest water infiltration and subsequent soil storage was achieved with the 122.2 mm application followed by the 73.3 mm irrigation, both surface applied. Application efficiency (the fraction of applied water that infiltrated into the soil and stored in the 600 mm soil profile) and soil water storage associated with supplemental blocked furrow irrigation was significantly greater than with supplemental sprinkler irrigation. For arid zone soil, which has little or no structural stability, application of supplemental irrigation water via short, blocked-end furrows prevents runoff and increases the opportunity time for infiltration, thereby increasing the amount of applied water that is infiltrated into the soil and stored in the soil profile. Supplemental irrigation, applied by a low-rate sprinkler system, was not as effective because of the low infiltration rates that resulted from the development of a surface throttle due to dispersion of soil aggregates at the soil surface. The differences in stored water had a significant effect on grain and straw yields of barley. Without supplemental irrigation, barley grain and straw yields were zero in natural rainfall cultivation with a total rainfall of 136.5 mm. Barley yields in the control treatment, with a 167 mm supplemental sprinkler irrigation were low being 0.19 and 1.09 ton/ha of barley grain and straw, respectively. Supplemental irrigation through blocked-end furrows increased barley grain and straw yields significantly compared with supplemental sprinkler irrigation to a maximum of 0.59 and 1.8 ton/ha, respectively. The improvement coming from the increased water storage associated with furrows. Since irrigation water is very limited if available, farmers are encouraged to form such furrows for reducing runoff from rainfall thereby increasing the amount of water available for forage and field crop production.     

Water and nitrate movement in drip-irrigated onion under fertigation and irrigation treatments

Frequent fertigation of crops is often advocated in the technical and popular literature, but there is limited evidence of the benefits of high-frequency fertigation. Field experiments were conducted on an Indo-American Hybrid var., Creole Red, of onion crop during three winter seasons of 1999–2000 through 2001–2002 in coarse-textured soil of Delhi under the semi-arid region of India. Three irrigation levels of 60, 80 and 100% of the crop evapotranspiration (ET) and four fertigation frequencies of daily, alternate day, weekly and monthly comprised the fertigation treatment. Analysis of soil samples indicated considerable influence of fertigation frequency on NO₃-N distribution in soil profile. NO₃-N in lower soil profiles (30.0–60.0 cm soil depth) was marginally affected in daily, alternate day and weekly fertigation. However, fluctuations of NO₃-N content in 0.0–15.0, 15.0–30.0, 30.0–45.0 and 45.0–60.0 cm soil depth was more in monthly fertigation frequency. The level of soil NO₃-N after the crop season shows that more NO₃-N leached through the soil profile in monthly fertigation frequency. Amounts of irrigation water applied in three irrigation treatments proved to be too small to cause significant differences in the content of NO₃-N leached beyond rooting depth of onion. Yield of onion was not significantly affected in daily, alternate day and weekly fertigation, though there was a trend of lower yields with monthly fertigation. The highest yield was recorded in daily fertigation (28.74 t ha⁻¹) followed by alternate day fertigation (28.4 t ha⁻¹). Lowest yield was recorded in monthly fertigation frequency (21.4 t ha⁻¹). Application of 56.4 cm irrigation water and 3.4 kg ha⁻¹ urea per fertigation (daily) resulted in highest yield of onion with less leaching of NO₃-N.     

Evaluation of irrigation schemes for sugarcane in Sindh,Pakistan, using SWAP93

A computer simulation model, SWAP93, was used to simulate the soil water balance of sugarcane (Saccharum officinarum L.) over a period of 6 years, in order to develop an efficient irrigation scheduling scheme for Sindh, Pakistan. Given the limitations and inflexibility of the existing warabandi irrigation system, which does not allow on-demand irrigation, only irrigation depth and irrigation interval were varied in order to assess the best irrigation depth/interval combination for sugarcane production. Twelve irrigation treatments were simulated. These treatments were four irrigation amounts (900, 1200, 1650 and 1800 mm) and three irrigation frequencies (7, 10 and 15 days). Three seasons with rainfall totaling less than 20 mm were compared with three seasons of over 200 mm rainfall. Two approaches were used in assessing the irrigation schemes: yield parameters and water management response indicators. Treatment parameters (e.g. irrigation amounts, weather conditions, soil characteristics, etc.) served as input for SWAP93, actual transpiration was calculated and then used in a crop water production function to predict yield and water use efficiency. Additionally, water management response indicators were derived from model outputs, and used to assess the impact of the schemes on soil salinity and water logging. Both these indicators and the yield and water use efficiency indicated that a seasonal total of 1650 mm, applied at a 15-day interval was the best irrigation scheme for the region.     

A Geometrical Analysis of Inclined and Tilted Spherical Plough Discs

An analysis is presented of the effect of disc geometry in relation to its areas of contact with soil at the working depth, treating the disc as a segment of a thin spherical shell. In addition, the effects of the disc angle of tilt, the disc angle of inclination to its direction of motion and the working depth are examined in detail. Shallow and deep disc concavities are considered. A presentation is made of formulae derived for disc critical angle and depth and for bearing and pressure areas of contact with the soil on vertical and horizontal planes. In addition, a study was made of the overlapping of soil working areas when adjacent discs are working in a gang arrangement. The effects of inside and outside sharpening of the circumferential edge of the disc are also examined.For the practical range of tilt angle (15° to 25°) and disc angle (35° to 55°) it is shown that the bearing area of the rear spherical area of discs is zero, so there is no soil contact with the rear surface of the disc. The vertical pressure area is only slightly affected by tilt angle and there is little difference for the two disc concavities. Disc angle and working depth have significant effects on this area. The horizontal presssure area is not affected by disc angle over its practical range. It has larger values for the 81 mm concavity than for the 51 mm concavity and is significantly influenced by tilt angle and working depth. Discs working in a gang, have overlapping of the areas of soil cut for disc angles and spacings (180 to 300 mm) adopted in practice. The area cut by an individual disc is not markedly affected by disc angle. Inside and outside sharpening does not significantly affect the overall findings concerning critical disc parameters. The flat bevelled surface formed by outside sharpening will generally be in contact with soil over the practical range of tilt and disc angles.     

Rainwater,soil and nutrient conservation in rainfed rice lands in Eastern India

In rainfed rice ecosystem, conservation of rainwater to maximum extent can reduce the supplemental irrigation water requirement of the crop and drainage need of the catchment. The results of 3 years of experimental study on the above stated aspects in diked rice fields with various weir heights (6–30 cm at an interval of 4 cm) revealed that about 56.75% and 99.5% of the rainfall can be stored in 6 and 30 cm weir height plots, respectively. Sediment losses of 347.8 kg/ha and 3.3 kg/ha have been recorded in runoff water coming out of 6 cm and 30 cm weir height plots, respectively in a cropping season. Similarly, total Kjeldahl nitrogen (TKN) loss in runoff water from rice field ranged from 4.23 kg/ha (6 cm weir height plots) to 0.17 kg/ha (26 cm weir height plots) and available potassium loss ranged from 2.20 kg/ha (6 cm weir height plots) to 0.04 kg/ha (30 cm weir height plots). Conservation of rainwater in rice fields with various weir heights could not create any significant impact on grain yield differences, leaf area index and other biometric characters. Irrigation requirement of 18 cm and above weir height plots was found to be half of the requirement of 6 cm weir height plots. Keeping in view the aspects of conserving rainwater, sediment and nutrient and minimizing irrigation requirement, 22–26 cm of dike height is considered to be suitable for rice fields of Bhubaneswar region.     

Water use and lint yield response of drip irrigated cotton to the length of irrigation season

A 2-year experiment was conducted at Tal Amara Research Station in the Bekaa Valley of Lebanon to determine water use and lint yield response to the length of irrigation season of drip irrigated cotton (Gossypium hirsutum L.). Crop evapotranspiration (ET_crop) and reference evapotranspiration (ET_rye-grass) were directly measured at weekly basis during the 2001 growing period using crop and rye-grass drainage lysimeters. Crop coefficients (K_c) in the different growth stages were calculated as ET_crop/ET_rye-grass. Then, the calculated K_c values were used in the 2002 growing period to estimate evapotranspiration of cotton using the FAO method by multiplying the calculated K_c values by ET_rye-grass measured in 2002. The length of irrigation season was determined by terminating irrigation permanently at first open boll (S1), at early boll loading (S2), and at mid boll loading (S3). The three treatments were compared to a well-watered control (C) throughout the growing period. Lint yield was defined as a function of components including plant height at harvest, number of bolls per plant, and percentage of opened bolls per plant.Lysimeter-measured crop evapotranspiration (ET_crop) totaled 642 mm in 2001 for a total growing period of 134 days, while when estimated with the FAO method in 2002 it averaged 669 mm for a total growing period of 141 days from sowing to mature bolls. Average K_c values varied from 0.58 at initial growth stages (sowing to squaring), to 1.10 at mid growth stages (first bloom to first open boll), and 0.83 at late growth stages (early boll loading to mature bolls).Results showed that cotton lint yields were reduced as irrigation amounts increased. Average across years, the S1 treatment produced the highest yield of 639 kg ha⁻¹ from total irrigations of 549 mm, compared to the S2 and S3 treatments, which yielded 577 and 547 kg ha⁻¹ from total irrigations of 633 and 692 mm, respectively, while the control resulted in 457 kg ha⁻¹ of lint yield from 738 mm of irrigation water. Water use efficiency (WUE) was found to be higher in S1 treatment and averaged 1.3 kg ha⁻¹ mm⁻¹, followed by S2 (1.1 kg ha⁻¹ mm⁻¹), and S3 (1.0 kg ha⁻¹ mm⁻¹), while in the control WUE was 0.80 kg ha⁻¹ mm⁻¹. Lint yield was negatively correlated with plant height and the number of bolls per plant and positively correlated with the percentage of opened bolls. This study suggests that terminating irrigation at first open boll stage has been found to provide the highest cotton yield with maximum WUE under the semi-arid conditions of the Bekaa Valley of Lebanon.     

Effect of shallow groundwater table on crop water requirements and crop yields

Due to the increasing demand for food and fiber by its ever-increasing population, the pressure on fresh water resources of Pakistan is increasing. Optimum utilization of surface and groundwater resources has become extremely important to fill the gap between water demand and supply. At Lahore, Pakistan 18 lysimeters, each 3.05 m × 3.05 m × 6.1 m deep were constructed to investigate the effect of shallow water tables on crop water requirements. The lysimeters were connected to bottles with Marriotte siphons to maintain the water tables at the desired levels and tensiometers were installed to measure soil water potential. The crops studied included wheat, sugarcane, maize, sorghum, berseem and sunflower. The results of these studies showed that the contribution of groundwater in meeting the crop water requirements varied with the water-table depth. With the water table at 0.5 m depth, wheat met its entire water requirement from the groundwater and sunflower absorbed more than 80% of its required water from groundwater. Maize and sorghum were found to be waterlogging sensitive crops whose yields were reduced with higher water table. However, maximum sugarcane yield was obtained with the water table at or below 2.0 m depth. Generally, the water-table depth of 1.5–2.0 m was found to be optimum for all the crops studied. In areas where the water table is shallow, the present system of irrigation supplies and water allowance needs adjustments to avoid over irrigation and in-efficient use of water.     

Application of drag reducing commercial and purified guargum for reduction of energy requirement of sprinkler irrigation and percolation rate of the soil

Drag reducing polymers reduce the drag in a turbulent flow while increase the drag in a laminar flow, due to an increase in shear viscosity. This feature of drag-reducing polymers has been utilised in reducing the energy requirements of sprinkler irrigation system and increasing the area of coverage as well as reducing the percolation loss of water added with drag reducing polymers. Two types of polymers at various concentrations were studied at the Indian Institute of Technology, Kharagpur. Two different methods of injection of polymers were also tested to determine the effect of polymer on mode of injection. The concentrations of 100, 250, 300 and 450 ppm of commercial guargum and 50, 100 and 150 ppm of purified guargum were used for homogenous injection (T₁). The concentrations of 100, 300, 600, 1000 and 1500 ppm of commercial guargum and 50, 100, 200, 350 and 500 ppm of purified guargum were used for injection at the suction side of the pump(T₂). The infiltration characteristics of the soil with polymer added water was tested for 300, 1000 ppm of commercial guargum and 100, 500 ppm of purified guargum. For T₁, the maximum power reduction of 28% was obtained in case of 300 ppm commercial guargum and approximately the same percentage was obtained in case of 100 ppm purified guargum. The maximum drag reduction was 35.5% at 300 ppm of commercial guargum and 38% at 100 ppm of purified guargum. The maximum increase in the radius of coverage at 500 ppm commercial guargum was estimated to be 33 and 37% at 150 ppm of purified guargum. For T₂, the maximum power reduction at 1000 ppm of commercial guargum and at 500 ppm of purified guargum was 31.25%. The maximum increase in radius of coverage at 1000 ppm of commercial guargum and at 500 ppm of purified guargum was 37 and 38.2%, respectively. The maximum drag reduction of 40% was obtained at both 1000 ppm of commercial guargum and 500 ppm of purified guargum. The reduction in the infiltration rate at 1000 and 300 ppm of commercial guargum was found to be 56.52 and 27%, respectively, whereas the reduction in the infiltration rate at 100 and 500 ppm purified guargum is 15 and 22.8%, respectively. The study reveals great potential of using drag reducing polymers for irrigation water management.     

Modelling the water balance of effluent-irrigated trees

Irrigation of effluent is an increasingly popular treatment option due to concern about nutrient additions to rivers and coastal waters. Since some studies have shown that irrigation with waste water can lead to contamination of groundwater resources, there is need for a model to predict the fate of irrigated water, salt, and nitrogen that can be applied to a variety of different soils, climates, and crops. We present the development of the water balance part of such a model, APSIM for Effluent, and carry out a comparison against data obtained from an effluent-irrigated plantation of Eucalyptus grandis. Over 10 months, modelled tree water use was within 1.5% of that obtained by sap-flux measurements. When compared over 5 years of the experiment, modelled drainage lay above that estimated by a water balance technique, which was known a priori to underestimate drainage, and was close to that estimated by the chloride mass balance technique. Simulated chloride accumulated in the soil was within the scatter of the observations, although it was consistently at the lower end of the range of the data. There was good agreement between the model predictions and measured chloride concentration distribution with depth in the soil. A considerable amount of water was lost as deep drainage, even for the treatment that aimed to add only enough effluent to replace that lost by evaporation. During 5 years, of the 3370 mm rainfall and 4480 mm effluent received by that treatment, 6710 mm was lost by the various evaporative routes, and 1080 mm was lost by deep drainage.     

The auto-regulative capability of pitcher irrigation system

Among traditional irrigation systems, pitcher irrigation is one of the most efficient. Water seeps out of a buried pitcher due to the pressure head gradient across the wall of the pitcher directly into the root zone of the irrigated crop. The pressure gradient results from positive pressure head inside pitcher and negative pressure head at the outer surface of the pitcher which is in contact with soil. Laboratory experiments were conducted to quantify the influence of evaporation, which controls the negative pressure condition in the soil, on the seepage rate of pitchers and to explore the auto-regulative capability of the pitcher irrigation system. Two groups of pitchers, group A with high hydraulic conductivity and group B with low hydraulic conductivity, were filled with water and placed in a closed chamber at varying temperature and humidity. The pan evaporation rate (Ep) inside chambers ranged from 1 mm/d when the temperature and relative humidity were kept around 20 °C and 97% to 16 mm/d at 45 °C and relative humidity of 40%. A positive and significant correlation was found between seepage rate of pitchers and Ep with an average R² of 0.97. The average seepage rates of all pitchers at Ep values of 1 and 16 mm/d were 125 mL/d to as high as 1020 mL/d, respectively. Differences in the seepage rates in pitchers from group A and group B were high at low Ep and low at high Ep values. At low Ep, seepage rate was more affected by water pressure head inside pitchers and the hydraulic conductivity whereas at high Ep values the negative pressure head at the outer surface of pitchers were more important. The average seepage rate of type A pitchers was 200% higher than that of type B at Ep value of 1 mm/d but for the high Ep value of 16 mm/d the corresponding increase in the seepage rate of type A compared to type B was only 4%. Thus, evaporation rate controls seepage from the pitcher and the system is auto-regulative.     

Irrigation based on a nomogram using soil suction measurements

Pivot-irrigation was managed with tensiometers on a field cropped with sugarcane at Analaiva (west coast of Madagascar). The volumes of water delivered by the pivot were 20 mm or less (expressed in terms of equivalent rain), depending on the stage of the crop. These applications were made when suctions at certain depths attained fixed values: a single value of 500  hPa in the top layer of the soil in the installation and growth stages of the crop, double values (600 hPa at 50 cm, 400 hPa at 150 cm) at the ripening stage.To characterize water movement in the soil, lines of isoflux were calculated from the hydrodynamic properties of the soil, and plotted so that the flux of water determined by the Darcy–Buckingham law would be read on the profile of hydraulic head. Maps of two-dimensional water movement were drawn independently from the data collected by an array of 30 tensiometers crossing a cane row. Before the rainy season, the water consumed by the crop corresponds to the irrigation water. At the ripening stage, an upward flux from the deep layer of the soil (wetted during the previous months of rains) is obtained by the strict management of irrigation.     

On-farm sampling density and correction requirements for soil moisture determination in irrigated heavy clay soils in the Gezira,central Sudan

Using the neutron scattering technique, with separate calibration for each measuring depth and temperature corrections, an over-sampling experiment with a worst case analysis was conducted in tenant irrigated fields under arid conditions. The purpose was to better understand actual on-farm soil moisture distribution as well as to determine minimum sampling density requirements for water use efficiency calculations in the heavy cracking clay soils of the Gezira irrigation scheme, central Sudan, under inhomogeneous watering conditions. Results show that actual soil moisture inhomogeneities can seriously distort the moisture distribution and water use pictures if the sampling density is too low. In a 2.1 ha end field under Gezira conditions 20 equally spaced neutron probe samples had to be collected from the 30 cm soil depth if the total experimental errors were to be kept within 12.5% of the average moisture content being measured. Sampling density requirements increased to 24, 28 and 33 samples for worst case error limits of 10%, 7.5% and 5% at 30 cm depth. At the agronomically more important lower depths, at or below 70 cm, less than 10 samples only could be afforded with an error of 10% at 70 cm, of 15% at 50 cm and of 20% at 30 cm, the errors typically becoming smaller at larger depths throughout. Credible soil water averages were obtained with this sampling. Field moisture patterns were well recognized when averaging several days of measurements.