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.     

Physical and hydraulic properties of synthetic envelopes for subsurface drainage in Pakistan

Three imported and four local synthetic envelope materials were tested in the laboratory with upward flow permeameters to determine their physical properties and hydraulic performance. The base soil was taken from the site of the Fourth Drainage Project, Faisalabad, Pakistan. The performance parameters included ratios of gradient and hydraulic conductivity of the interface of envelope material and base soil and the discharge through the permeameter, which were evaluated against established particle-retention and filter criteria.The study concluded that each of the tested materials satisfied the particle-retention criteria because the ratio of characteristic diameters of envelope (O₉₀) and soil (d₉₀) remained less than 2.5. The filter criteria, however, could not be met satisfactorily, particularly during receding hydraulic gradients, when the ratio of envelope and base soil hydraulic conductivities (Ke/Ks) dropped below 1, except in the case of one imported material. Blocking of the soil-envelope interface occurred due to the internal movement of soil particles, particularly during receding gradients.     

Discharge rates,salinities, and the performance of subsurface collector drains in Egypt

To evaluate the hydraulic performance of subsurface collector drains and to study the relationships between discharge rates, crop patterns, and the salinity of drainage water, subsurface drains were monitored in different parts of the Nile Delta and Valley.Actual discharges were much smaller than design discharges. Also, overpressure in the pipes occurred frequently, indicating too small a capacity in the design. From research in one pilot area, it was concluded that if construction methods and materials are not improved, the roughness factor in the design should be increased by 100% to allow sufficient capacity.The cultivation of rice increases discharges. Salinity of drainage water is higher in winter than in summer, and higher in the north of the Delta than in the south.     

Changes in specific properties of PVC drains under various storage conditions

Corrugated PVC drains should meet minimum requirements to prevent breakage during installation and trench backfilling. Storage can alter specific properties of drains so that the minimum requirements are not met anymore. To investigate the influence of storage on the properties of corrugated PVC drains, tubes from five manufacturers were stored for 5 years in three different ways: (1) outdoors, (2) indoors uncovered, and (3) indoors covered. Specific mass and elongation were determined yearly, while impact strength and brittleness were only determined at delivery and at the end of the 5 years storage period. Specific mass and elongation did not undergo noticeable changes. Impact strength and brittleness decreased when stored outdoors, while, indoor storage did not affect these properties. Batches of drain tubing from two manufacturers, provided each year for 7 consecutive years, were also stored for 5 years. After each year of storage the impact strength and brittleness were determined. The average breakages of impact strength and brittleness of the seven batches of both products increased linearly with storage time. Impact strength and brittleness were affected significantly after an outdoor storage period of 1 year in humid temperate regions. On average, degradation is completed after about 8 years of outdoor storage.     

Effect of sustained saline irrigation on soil salinity and crop yields

Summary Field studies were conducted for a period of ten years (1974 to 1984) on Typic Ustochrept to determine the sustained effects of saline irrigation water electrical conductivity (EC _iw ) 3.2 dS/m, sodium adsorption ratio (SAR) 21 (mmol/1)^1/2 and residual sodium carbonate (RSC) 4me/1, on the build up of salinity in the soil profile and yield of crops grown under fixed rice-wheat and maize/millet-wheat rotations. Saline waters were continuously used with and without the addition of gypsum (at the rate needed to reduce RSC to zero) applied at each irrigation. In maize/millet-wheat rotation, two additional treatments viz. (i) irrigation with 50% extra water over and above the normal 6 cm irrigation, and (ii) irrigation with good water and saline water alternately, were also kept. The results showed that salinity increased rapidly in the profile during the initial years but after five years (1979–1984) the average soluble salt concentration in 0–90 cm soil profile did not appreciably vary and the mean EC _e values under saline water treatment remained almost similar to EC _iw , under both the crop rotations.Saline water irrigation increased pH and Na saturation of the soil, reduced water infiltration rate and decreased yields of maize, rice and wheat. The differences in the build up of salinity and ESP of the soil under the two cropping sequences seemed to be related with the differences in leaching that occurred under rice-wheat and maize/millet-wheat rotations. Application of gypsum increased the removal of Na from the profile, appreciably decreased the pH and Na saturation and improved water infiltration rate and raised crop yields. Application of non-saline and saline waters alternately was found to be a useful practice but irrigation with 50% extra water to meet the leaching requirement did not control salinity and hence lowered crop yields.     

Influence of subsurface drainage on crop production and soil quality in a low-lying acid sulphate soil

Kuttanad, the low-lying tract in Kerala State of south-west India, is a place where drainage problems have caused the agricultural production to remain low. The problem is more severe in the acid sulphate soils of Kuttanad. Besides the problems inherent to acid sulphate soils, the area also experiences problems of flooding, lack of fresh water and intrusion of saline water from the Arabian Sea. A subsurface drainage system consisting of 10 cm diameter clay tiles, each of 60 cm length, was installed at a depth of 1 m with two different spacings of 15 and 30 m for evaluating its influence in improving soil quality and crop production. Many of the critical crop growth parameters in the subsurface drained area, particularly the grain yield and 100 grain weight, were significantly superior to that of the ill-drained areas. Drain spacings up to 30 m was found to significantly improve the productivity of the area. The overall increase in rice yield due to subsurface drainage was 1.36 t/ha. It was also found that subsurface drainage could remove the chemical heterogeneity of soil which is the root cause for patchy crop growth and uneven ripening of rice crop in the area. Acidity in the subsurface drained area was always lower throughout the cropping season. The salinity in the soil could be controlled considerably by subsurface drainage. The iron transformations were not serious enough to cause concern for rice cultivation when subsurface drainage was adopted. Accumulation of sulphates in insoluble form occurred during drainage due to the oxidation of pyrite. Subsurface drainage was also very efficient in leaching sodium, calcium and magnesium. Chloride content in soil decreased drastically during drainage.     

Regional crop modelling in Europe: The impact of climatic conditions and farm characteristics on maize yields

Impacts of climate variability and climate change on regional crop yields are commonly assessed using process-based crop models. These models, however, simulate potential and water limited yields, which do not always relate to observed yields. The latter are largely influenced by crop management, which varies by farm and region. Data on specific management strategies may be obtained at the field level, but at the regional level information about the diversity in management strategies is rarely available and difficult to be considered adequately in process-based crop models. Alternatively, understanding the factors influencing management may provide helpful information to improve simulations at the regional level.In this study, we aim to identify factors at the regional level that explain differences between observed and simulated yields. Observed yield data were provided by the Farm Accountancy Data Network (FADN) and Eurostat. The Crop Growth Monitoring System (CGMS), based on the WOFOST model, was used to simulate potential and water limited maize yields in the EU15 (i.e., the old member states of the European Union). Differences between observed and simulated maize yields were analysed using regression models including: (i) climatic factors (temperature and precipitation), (ii) farm size, (iii) farm intensity, (iv) land use, (v) income and (vi) subsidies. We assumed that the highest yields observed in a region were close to the yield potential as determined by climate and considered the average regional yields as also influenced by management. Model performance was analysed with respect to spatial and temporal yield variability.Results indicate that for potential yield, the model performed unsatisfactory in southern regions, where high temperatures increased observed yields which was in contrast to model simulations. When considering management effects, we find that especially irrigation and the maize area explain much of the differences between observed and simulated yields across regions. Simulations of temporal yield variability also diverted from observed data of which about 80% could be explained by the climatic factors (35%) and farm characteristics (50%) considered in the analysis. However, effects of specific factors differed depending on the regions. Accordingly, we propose different groups of regions with factors related to management which should be considered to improve regional yield simulations with CGMS.     

Simulating water content, crop yield and nitrate-N loss under free and controlled tile drainage with subsurface irrigation using the DSSAT model

In southwestern Ontario, rain-fed crop production frequently fails to achieve its yield potential because of growing-season droughts and/or uneven rainfall distribution. The objective of this study was to determine if the Decision Support System for Agrotechnology Transfer (DSSAT) v4.5 model could adequately simulate corn and soybean yields, near-surface soil water contents, and cumulative nitrate-N losses associated with regular free tile drainage (TD) and controlled tile drainage with optional subsurface irrigation (CDS). The simulations were compared to observations collected between 2000 and 2004 from both TD and CDS field experiments on a Perth clay loam soil at the Essex Region Conservation Authority demonstration farm, Holiday Beach, Ontario, Canada. There was good model-data agreement for crop yields, near-surface (0-30 cm) soil water content and cumulative annual tile nitrate-N loss in both the calibration and validation years. For both TD and CDS, the CENTURY soil C/N model in DSSAT simulated water content and cumulative tile nitrate-N loss with normalized root mean square error (n-RMSE) values ranging from 9.9 to 14.8% and 17.8 to 25.2%, respectively. The CERES-Maize and CROPGRO-Soybean crop system models in the DSSAT simulated corn and soybean yields with n-RMSE values ranging from 4.3 to 14.0%. It was concluded that the DSSAT v4.5 model can be a useful tool for simulating near-surface soil water content, cumulative tile nitrate-N losses, and corn and soybean yields associated with CDS and TD water management systems.     

Effect of brackish water irrigation and straw mulching on soil salinity and crop yields under monsoonal climatic conditions

Fresh water shortages are severally restricting sustainable agriculture development in the North China Plain. The scarcity of fresh water has forced farmers to use brackish water from shallow underground sources, which helps to overcome drought and increase crop yields but also increases the risk of soil salinization. To identify safe and effective ways of using brackish water in this region, field experiments were conducted to evaluate the effect of brackish water irrigation and straw mulching on soil salinity and crop yield in a winter wheat-summer maize double cropping system. The experiment was in a split-plot design. Six rates of straw mulching (0, 4.5, 6.0, 7.5, 15.0 and 30.0 Mg/ha) were assigned to the main plots and two irrigation water qualities (i.e. brackish water with salt content of 3.0-5.0 g/L and fresh water with only 1.27 g salt/L) were applied to subplots. The brackish water irrigation significantly increased the salt content at different soil depths in the upper 1 m soil layer during the two growing seasons. Straw mulching affected the vertical distribution of salt in the brackish water irrigation plots and the average salt content of straw mulch treatments (4.5, 6.0, 7.5, 15.0 and 30.0 Mg/ha) within the 0-20, 20-40 and 0-100 cm soil depths was 10.2, 14.0 and 1.8% lower than that without straw mulch (A₀). No salt accumulation occurred to a depth of 1 m in the brackish water irrigation plots and there was no correlation between the value of SAS (salt accumulated in 1 m of soil) and straw mulch rate. In 2000 and 2001, the salt content within the 0-40 cm soil layer in brackish water irrigation plots increased due to high evaporation rates during April-June, and then decreased up to September as salts were leached by rain. For the fresh water irrigation plots, the salt content remained relatively stable. Straw mulching affected the salt content in the 0-40 cm soil layer in brackish water irrigation plots in different periods of 2000 and 2001, but no correlation between salt content and straw mulch rates was observed except in September of 2000. Unlike for wheat, the yield of maize increased as the straw mulch rate increased according to the equation, y = 0.1589x + 5.3432 (R² = 0.6506). Our results would be helpful in adopting brackish water irrigation and straw mulching in ways that enhance crop yields and reduce the risk of soil salinization. However, long-term effects of brackish water irrigation and straw mulching on soil salinity and crop yield need to be further evaluated for sustainability of the system.     

Effect of frequency of sodic and saline-sodic irrigations and gypsum on the buildup of sodium in soil and crop yields

The effect of three frequencies of irrigation with sodic (high residual alkalinity) and saline-sodic (high residual alkalinity and high NaCl concentration) waters in presence and absence of gypsum application on soil properties and crop yields were investigated under millet (fodder) — wheat — maize (fodder) rotation in a field experiment carried out for 6 years (1986–1992) on a well drained sandy loam Typic Ustochrept soil. Irrespective of the irrigation intervals, sustained use of sodic and salinesodic waters increased pH, electrical conductivity and ESP of the soil and hence significantly decreased crop yields. Application of gypsum decreased ESP and significantly improved crop yields. The beneficial effect of gypsum was lower under saline-sodic irrigation. There were no significant beneficial effects of increasing the frequency of sodic and saline-sodic irrigation, both in presence and absence of applied gypsum, on the yields of wheat and millet (f) crops grown during winter and monsoon seasons, respectively. But decrease in irrigation interval significantly improved yields of maize (f) grown during the hot dry summer period. Frequency of irrigation did not appreciably alter the effectiveness of applied gypsum in wheat and millet (f) but in maize (f), the gypsum treatment was more effective under more frequent irrigation.     

Optimization model of an irrigation reservoir for water allocation and crop planning under various weather conditions

This paper develops a non-linear programming optimization model with an integrated soil water balance, to determine the optimal reservoir release policies, the irrigation allocation to multiple crops and the optimal cropping pattern in irrigated agriculture. Decision variables are the cultivated area and the water allocated to each crop. The objective function of the model maximizes the total farm income, which is based on crop–water production functions, production cost and crop prices. The proposed model is solved using the simulated annealing (SA) global optimization stochastic search algorithm in combination with the stochastic gradient descent algorithm. The rainfall, evapotranspiration and inflow are considered to be stochastic and the model is run for expected values of the above parameters corresponding to different probability of exceedence. By combining various probability levels of rainfall, evapotranspiration and inflow, four weather conditions are distinguished. The model takes into account an irrigation time interval in each growth stage and gives the optimal distribution of area, the water to each crop and the total farm income. The outputs of this model were compared with the results obtained from the model in which the only decision variables are cultivated areas. The model was applied on data from a planned reservoir on the Havrias River in Northern Greece, is sufficiently general and has great potential to be applicable as a decision support tool for cropping patterns of an irrigated area and irrigation scheduling.     

Drainage water salinity of tubewells and pipe drains:: A case study from Pakistan

Drainage water salinity data from 71 public deep tubewells and 79 pipe drainage units near Faisalabad, Pakistan, were studied. Drainage water salinity of the tubewells and the pipe drains remained approximately constant with time. This was attributed to the deep, highly conductive, unconfined aquifer underlying the area, which facilitates lateral groundwater inflow into the drained areas. Tubewells alongside surface drains showed average electrical conductivity, sodium adsorption ratio, and residual sodium carbonate values of 3.2 dS m⁻¹, 17.2 (meq l⁻¹)^0.5, and 6.4 meq l⁻¹, respectively. For pipe drains, which are situated in areas with comparable conditions, the corresponding values were 2.5 dS m⁻¹, 12.2 (meq l⁻¹)^0.5, and 3.7 meq l⁻¹, respectively. Tubewells have an inferior drainage water quality because they attract water from greater depths, where the water is more saline.     

Determination of evapotranspiration from an alfalfa crop irrigated with saline waste water from an electrical power plant

Summary Investigations were carried out in 1989 to determine the evapotranspiration (ET) of alfalfa when irrigated with saline waste water coming from the evaporation of fresh water in the cooling towers of Utah Power and Light Company Electrical Power Plant at Huntington in central Utah, U.S.A. The primary goal is to dispose of the waste water from the power plant by irrigation and to maximize salt deposition in the soil, maximize crop ET, minimize runoff from the soil surface, and minimize leaching to the ground water. Using the Bowen ratio-energy balance method, alfalfa evapotranspiration was measured at an experimental site for each 20-minute period during the 1989 irrigation season. Using a simplified seasonal water balance, the results showed that cumulative irrigation plus rain was less than evapotranspiration for the 1989 irrigation season. This means that for the long term in addition to irrigation and precipitation some water was withdrawn from the soil for alfalfa crop water requirements (ET_a). Short term evaluations showed that because of unforeseen heavy rain (thunder showers) in this mountainous area between irrigations, ET_a was occasionally less than irrigation plus rain. This means the excess water was stored in the soil for later use. The average value for ET_a/ET_p (potential ET) for the 1989 irrigation season was 0.47 but occasionally the ratio was greater than unity. Short-term studies (Hanks et al. 1990 a) indicate that yield and ET_a are likely to decrease only slightly for the coming years if saline irrigation water is applied. This method of investigation can be applied to any industrial processes which produce waste water.     

Numerical evaluation of subsurface trickle irrigation with brackish water

In this study, an assessment for a proposed irrigation system in the El-Salam Canal cultivated land, Egypt, was conducted. A numerical model (HYDRUS-2D/3D) was applied to investigate the effect of irrigation amount, frequency, and emitter depth on the wetted soil volume, soil salinity levels, and deep percolation under subsurface trickle irrigation (SDI) of tomato growing with brackish irrigation water in three different soil types. The simulations indicated that lower irrigation frequency increased the wetted soil volume without significant increase in water percolates below the plant roots. Deep percolation decreased as the amount of irrigation water and emitter depth decreased. With the same amount of irrigation water, the volume of leached soil was larger at lower irrigation frequency. The salinity of irrigation water under SDI with shallow emitter depth did not show any significant effect on increasing the soil salinity above tomato crop salt tolerance. Based on the results, it appears that the use of SDI with brackish irrigation water is an effective method for growing tomato crop in El-Salam Canal cultivated land especially with shallow emitter depth.     

Simulation of soil water dynamics under subsurface drip irrigation from line sources

A mathematical model which describes water flow under subsurface drip lines taking into account root water uptake, evaporation of soil water from the soil surface and hysteresis in the soil water characteristic curve θ(H) is presented. The model performance in simulating soil water dynamics was evaluated by comparing the predicted soil water content values with both those of Hydrus 2D model and those of an analytical solution for a buried single strip source. Soil water distribution patterns for three soils (loamy sand, silt, silty clay loam) and two discharge rates (2 and 4 l m⁻¹ h⁻¹) at four different times are presented. The numerical results showed that the soil wetting pattern mainly depends on soil hydraulic properties; that at a time equal to irrigation duration decreasing the discharge rate of the line sources but maintaining the applied irrigation depth, the vertical and horizontal components of the wetting front were increased; that at a time equal to the total simulation time the discharge rate has no effect on the actual transpiration and actual soil evaporation and a small effect on deep percolation. Also the numerical results showed that when the soil evaporation is neglected the soil water is more easily taken up by the plant roots.     

Effects on crop yields of machinery traffic and soil loosening Part 1. Effects on grass yield of traffic frequency and date of loosening

Two experiments are described which measured the effects on grass yield of field traffic, by a tractor and slurry tanker, and of soil loosening. Traffic in spring was found to produce the greatest reduction in grass yield. Yield reduction following this traffic was mainly limited to first harvest, with the extent of the reduction being dependent on the number of machinery passes within the same wheel tracks and on the degree of damage caused. Soil loosening was found significantly to reduce grass yield at the subsequent harvest.     

A model of lamb production from an autumn catch crop

A model of lambs feeding on an autumn catch crop is described. The program must be supplied with data on the crop of interest, and with utilisation factors for that crop. The program then computes figures on land use and gross margins for management policies chosen by the user.When supplied with available data on catch crops, calculations indicate that under certain circumstances use of such a crop to fatten lambs in the autumn may well be a profitable venture. Thus it is concluded that more data on such crops should be collected.     

Estimating crop coefficients from fraction of ground cover and height

The FAO-56 procedure for estimating the crop coefficient K _c as a function of fraction of ground cover and crop height has been formalized in this study using a density coefficient K _d. The density coefficient is multiplied by a basal K _c representing full cover conditions, K _{cb full}, to produce a basal crop coefficient that represents actual conditions of ET and vegetation coverage when the soil surface is dry. K _{cb full} is estimated primarily as a function of crop height. K _{cb full} can be adjusted for tree crops by multiplying by a reduction factor (F _r) estimated using a mean leaf stomatal resistance term. The estimate for basal crop coefficient, K _cb, is further modified for tree crops if some type of ground-cover exists understory or between trees. The single (mean) crop coefficient is similarly estimated and is adjusted using a K _soil coefficient that represents background evaporation from wet soil. The K _c estimation procedure was applied to the development periods for seven vegetable crops grown in California. The average root mean square error between estimated and measured K _c was 0.13. The K _c estimation procedure was also used to estimate K _c during midseason periods of horticultural crops (trees and vines) reported in the literature. Values for mean leaf stomatal resistance and the F _r reduction factor were derived that explain the literature K _c values and that provide a consistent means to estimate K _c over a broad range of fraction of ground cover.     

稻秸预处理厌氧强化产挥发性脂肪酸研究

利用秸秆厌氧发酵技术进行挥发性脂肪酸(Volatile fatty acids,VFAs)生产,是一种极具前景的秸秆新型高值化利用方向。研究了水热、稀酸、稀碱等预处理方式在中温35℃、高温55℃、超高温70℃等发酵产酸温度条件下的产酸率及过程参数。结果表明,不同预处理条件对挥发性脂肪酸转化率影响从大到小为Ca(OH)_2、HCl、水热预处理,碱性发酵较酸性发酵更有利于厌氧产挥发性脂肪酸进行。Ca(OH)_2能够有效地溶解木质素,35℃时Ca(OH)_2的木质素去除率为63.27%,远高于相同温度的水热处理组的12.45%和HCl处理组的12.40%。中温条件下秸秆挥发性脂肪酸产率最高,虽然35℃时因为温度较低使得秸秆降解率不高,但因该温度下被降解的秸秆转化为挥发性脂肪酸的效率高,从而获得了最高的产酸率,各预处理组单位降解底物产酸率在35℃和70℃分别为0.79~1.20 g/g和0.24~0.51 g/g。最佳的碱处理组35℃的挥发性脂肪酸底物挥发分转化率为0.25 g/g,而70℃时仅为0.09 g/g。研究结果为稻秸厌氧强化产挥发性脂肪酸的预处理和温度等选择提供了一定的理论依据。     

Numerical simulations of steady-state subsurface drainage with vertically decreasing hydraulic conductivity

Most subsurface drainage equations assume either homogeneous, two-layer or three-layer soil conditions. Finite difference simulations were performed to quantify the effect of gradually decreasing hydraulic conductivity on watertable depths for steady-state subsurface drainage. For vertically decreasing hydraulic conductivity, and for cases where drain spacing was based on effective hydraulic conductivity of the 0.5 to 2.0 m layer, mid-spacing watertable depth ranged from 0.282 to 0.900 m. The average value was 0.718 m, which is considerably shallower than the 0.9 m design value used for determining drain spacing. These higher watertables may have detrimental effects on crop yield, especially in arid areas where soil salinity is a problem. The importance of the difference between actual and design watertable depths was mostly related to the type of hydraulic conductivity decrease function, drain depth, and drainage rate. These differences are explained by the position of the drain within the soil profile and the effect of the spacing on the equivalent depth of flow. Using effective hydraulic conductivity of the 0.5 to 3.0 m layer for determining drain spacing reduced the error. For an effective hydraulic conductivity value of 0.3 m/d, the average watertable depth increased from 0.748 m for the 2.0 m auger hole to 0.829 m for the 3.0 m hole. The results presented can be used to estimate the error on watertable depth resulting from ignoring the vertical variations of hydraulic conductivity.