Management of declining groundwater in the Trans Indo-Gangetic Plain (India): Some options

The average productivity of rice–wheat sequence is quite impressive in the Trans Indo-Gangetic Plain (India) but these gains are over-shadowed due to declining groundwater, particularly in the areas, where groundwater quality is either good or marginal. The groundwater decline can be reversed through artificial groundwater recharge and by adopting suitable land and water management practices. Groundwater recharge is found technically feasible through vertical shafts conducting water from the ground surface directly to aquifers, after it has been passed through a sand-gravel filter. The recharge rate through this system is almost equal to a shallow cavity/filter well yield (about 11 l/s) and its cost is estimated at about INR 10/100 m³ (1 US$ = 45 INR). Further study in the Kaithal and Karnal districts of Haryana for stabilizing watertable within 6–7 m, which permits continuous use of shallow tubewell technology, indicated that the rice area could be supported at 60% of cultivable command area (CCA) and wheat between 65 and 80% of CCA with the existing management practices. The cultivation of wheat crop is sustainable in larger area, mainly due to its medium water requirement, salt resistance characteristics and consistent market demand resulting in assured returns. There is a possibility of supporting rice at a higher level, if part of the area (up to 10%) is left fallow and used for rainwater conservation and recharge. The fallow area may be subsequently put under early rabi (winter) crops like mustard, gram and other pulses. The effect of varying irrigation and fallowing would increase 23% equivalent wheat yield by changing land and water management practices. The analysis further indicated that the adoption of proposed irrigation management practices might stabilize watertable at desired level of 6–7 m in 10–15 years in high (3–4 m), 5 years in medium (5–10 m) and 40 years in deep (>10 m) watertable areas.     

Estimating spatial mean soil water contents of sloping jujube orchards using temporal stability

Estimating spatial mean soil water contents from point-scale measurements is important to improve soil water management in sloping land of semiarid areas. Temporal stability analysis, as a statistical technique to estimate soil water content, is an effective tool in terms of facilitating the upscaling estimation of mean values. The objective of this study was to examine temporal stability of soil water profiles (0–20, 20–40, 40–60 and 0–60 cm) in sloping jujube (Zizyphus jujuba) orchards and to estimate field mean root-zone soil water based on temporal stability analysis in the Yuanzegou catchment of the Chinese Loess Plateau, using soil water observations under both dry and wet soil conditions. The results showed that different time-stable locations were identified for different depths and the temporal stability of soil water content in 20–40 cm was significantly (P < 0.05) weaker than that in other depths. Moreover, these time-stable locations had relatively high clay contents, relatively mild slopes and relatively planar surfaces compared to the corresponding field means. Statistical analysis revealed that the temporal stability of root zone soil water (0–60 cm) was higher in either dry or wet season than that including both, and soil water exhibited very low temporal stability during the transition period from dry to wet. Based on the temporal stability analysis, field mean soil water contents were estimated reasonably (R² from 0.9560 to 0.9873) from the point measurements of these time-stable locations. Since the terrains in this study are typical in the hilly regions of the Loess Plateau, the results presented here should improve soil water management in sloping orchards in the Loess Plateau.     

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.     

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.     

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 quality assessment of different reservoir types in relation to nutrient solution use in hydroponics

Hydroponics requires good quality water. For this purpose, water quality is based on concentrations of specific ions and phytotoxic substances as well as the presence of organisms and substances that can clog irrigation systems. Here, four irrigation reservoirs, i.e. two rainwater ponds, a peat ditch, and a natural lake, were analyzed to determine whether or not they conform to water quality guidelines. Based on our data, the four reservoirs could be divided into two categories in respect to their water quality. The two rainwater ponds belong to the category characterized by low input of ionic strength (480 μmol m⁻¹), low concentration of unwanted ions, such as SO₄²⁻ (63 μmol l⁻¹) and Zn²⁺ (3.9 μmol l⁻¹), a moderate bacterial population (lg 4.9 CFU m⁻¹), and moderate algae density (lg 6.0 cells ml⁻¹). The rainwater ponds were found to contain a good diversity in bacteria (45 species from 25 genera), and a poor diversity of algae (15 species from 4 groups). The other category, to which the peat ditch and natural lake belong, is characterized by a high ionic strength (12,200 μmol l⁻¹), high concentrations of alkali ions (Mg²⁺: 890 μmol l⁻¹; Ca²⁺: 3.260 μmol l⁻¹; K⁺: 470 μmol l⁻¹), a moderate bacterial (lg 4.7 CFU ml⁻¹), but low algae density (lg 5.0 cells ml⁻¹). In comparison to the first category, the diversity of the bacteria was poor (seven species from three genera). However, in sharp contrast was the rich algal community detected in the peat ditch, for which 32 species from six groups were found, whereas in the natural lake, only one group with seven species was identified. In all reservoirs, species of the genera Paenibacillus and Bacillus were detected, and small green algae, e.g. Scenedesmus spp., also dominated in each case. Overall, the bacterial and algal densities showed wide fluctuations between water sources, and neither caused filter clogging as observed in investigations of others. The quality of the rainwater investigated was assessed to be well suited for use in hydroponics due to appropriate nutrient concentration (except Zn²⁺ in one pond), and lack of potential bacterial and algal development. However, we recommend water from the natural lake and the peat ditch to be used with care because of the high nutrient concentration.     

Effect of different water supply regimes on growth and size hierarchy in spring wheat populations under mulched with clear plastic film

This study was conducted to determine the effect of water supply regimes and plastic film mulching on the harvest index (HI), reproductive allocation (RA) and size hierarchy in spring wheat (Triticum aestivum L.) populations, and to explore their mechanism in relation to size hierarchy and life-history strategies.The grain yield, biological yield (aboveground biomass), HI and RA of spring wheat decreased significantly (p < 0.001) along the water control gradient (irrigative amount decreased 132 → 66 → 0 mm) either mulching or non-mulching, size hierarchy (as measured by the Gini coefficient (G) of aboveground biomass per plant) always increased. HI and RA in mulching treatment were significantly lower than non-mulching (p < 0.05). Meanwhile, the number and weight of barren shoots and the ratio of barren shoot biomass to total shoot biomass were significantly greater (p < 0.05) in mulched populations than non-mulched controls both at booting, flowering and ripening stages. RA and HI were both negatively correlated to average G of the populations. These results suggested that size hierarchies in spring wheat populations are closely correlated with the water regime in the field, and that under greater drought stress there are relatively smaller plants with lower HI (size-dependent reproductive allocation). Size hierarchy is an index of competitive status in plant populations under stress environments. Agriculturally, greater size hierarchy may result in growth redundancy, which is detrimental to reproductive allocation and consequently, grain yield. The results support the view that stand uniformity in field crops is an important mechanism for increasing grain yield.From tilling to ripening stages, the tendency of Gini coefficient (G) shows obvious differences between mulched populations and non-mulched controls. At booting and flowering, the G was significantly higher in mulched populations than non-mulched controls, and it was just contrary at ripening.Appreciable growth redundancy occurred in spring wheat populations mulched with plastic film, which may result from the exacerbated interplant competition and self-thinning. Thus, spring wheat cultivation with plastic film mulching does not always mean high-efficiency, although there is a remarkable increase in grain yields.     

Assessment of the effect of tie-ridging on smallholder maize yields in Malawi

Tie-ridging is being promoted in Malawi as an on-field rainwater harvesting technique to ensure a maize (Zea mais L.) crop during a dry or drought year. Resource-poor smallholder farmers are likely to take up tie-ridging if it increases and not decreases maize yield in most years. A numerical study was conducted to calculate the expected maize yield gain due to tie-ridging taking into account the probability of occurrence of drought, dry, normal and wet years (climatic uncertainty). Mean yields due to tie-ridging in drought, dry, normal and wet years at different N levels were derived from observed smallholder maize yield data using a linear nitrogen response model and field-observed retained rainwater amounts in tie-ridged fields. Simulation results indicate that tie-ridging will result in hybrid maize yield gain in a drought year (1050 kg ha⁻¹) and dry year (560 kg ha⁻¹). There will be a hybrid maize yield loss in a normal year (350 kg ha⁻¹) and wet year (700 kg ha⁻¹). For local maize, there will be a yield gain in a drought year (500 kg ha⁻¹), dry year (220 kg ha⁻¹) and normal year (120 kg ha⁻¹). There will be a slight yield loss in a wet year (60 kg ha⁻¹). Considering observed probability of the occurrence of drought, dry, normal and wet years in Malawi, the study reveals that there will be no hybrid maize yield gain in any coming year with tie-ridging. For local maize, the expected yield gain in any coming year was positive (133.3 kg ha⁻¹) but this gain is less than the minimum gain required considering the opportunity cost of labour (142.5 kg ha⁻¹). Thus under the smallholder conditions and climate of Malawi, the expected yield gain in any coming year due to tie-ridging is likely to be minimal and uneconomic.     

Subsurface drain flow and crop yield predictions for different drain spacings using DRAINMOD

DRAINMOD was run for 15 years to predict and compare drain flow for three drain spacings and crop yield for four drain spacings at the Southeastern Purdue Agricultural Center (SEPAC). Data from two continuous years of daily drain flow from one spacing were used to calibrate the eight most uncertain parameters using a multi-objective calibration function and an automatic calibration method. The model was tested using the remaining field data for the 5, 10, and 20 m drain spacings for drain flow and the additional 40 m spacing for yield predictions. Nash–Sutcliffe efficiency (EF) for daily drain flow simulations for the calibration years and drain spacing ranged from 0.62 to 0.79. The daily EF for model testing ranged from −0.66 to 0.81, with the average deviations of 0.01 to 0.07 cm/day and standard errors of 0.03–0.17 cm/day. On a monthly basis, 91% of plot years had EF values over 0.5 and 76% over 0.6 for years with on-site rainfall data. The total yearly drain flow was predicted within ±25% in 71% of plot years, and within ±50% in 93% of plot years with on-site rainfall data. Statistical tests of daily drain flow EF values for three spacings and percent errors of crop relative yield for four spacings indicated that the reliability of the model is not significantly different among different spacings, supporting the use of DRAINMOD to study the efficiencies of different drain spacings and to guide the drain spacing design for specific soils. In general, the model correctly predicted the pattern of yearly relative yield change. The relative corn (Zea mays L.) and soybean (Glycine max L.) yields were well predicted on average, with percent errors ranging from 1.3 to 9.7% for corn and from −3.3 to 10.3% for soybean.     

Drainage effects on spatial variability of soil electrical conductivity in a vertisol

Spatial variability of soil electrical conductivity (EC) is characterized in a 33 ha plot before and 2 years after drainage initiation. Measurements of EC were made in a square grid at 50 m spacing and at 0–20, 20–40, 40–60 and 0–60 cm depths. Both mean EC values and coefficients of variation (CV) are reduced after drainage. The frequency histograms show that EC fits to a lognormal distribution before drainage, whereas it seems to be normally distributed after drainage initiation. The bimodality found in histograms before drainage was not observed after it. Spatial structure of soil EC is strongest at 0–20 cm before drainage and it is weaker at greatest depths. Nevertheless, the semi-variogram at 40–60 cm after drainage shows a more remarkable spatial structure. EC spatial variability shows anisotropy before drainage, which was related to topography. However, directional semi-variograms after drainage did not show such anisotropy. In conclusion, drainage not only reduces EC values, but also notably changes EC spatial variability.     

Soil salinity reduction and prediction of salt dynamics in the coastal ricelands of Bangladesh

Field experiments were conducted in moderately saline and saline soils during the 1996 dry and wet seasons and the 1997 dry season to document salt dynamics and establish their relationship with local hydrology. Topsoil (0–15 cm) salinity in the dry season varied from 4.0 to 9.0 dS m⁻¹ in moderately saline soils at Mirzapur and from 5.0 to 12.0 dS m⁻¹ in saline soils at Barodanga. In wet season, the corresponding figures were from 1.5 to 2.5 dS m⁻¹ and from 2.0 to 3.0 dS m⁻¹, respectively. Dry season cropping significantly reduced topsoil salinity at both the research sites. Overall peak salinity in non-plowed cropped lands was 25–38% lower than that of fallow lands, and in plowed cropped lands it was about 30–40% less than the non-plowed cropped lands.Multiple linear and non-linear regression models were developed to predict topsoil salinity of the fallow land for both moderately saline and saline soils by using daily rainfall and evaporation as independent variables. The prediction level was not significantly improved when a non-linear model was employed in place of linear model. Therefore, a linear model may be used to predict topsoil salinity of the coastal ricelands of Bangladesh.     

Effects of irrigation method on chile pepper yield and Phytophthora root rot incidence

A field experiment was conducted in 1995 and 1996 to examine the effects of different irrigation methods on yields and Phytophthora root rot disease of chile plants (Capsicum annum New Mexico `6–4'). Three irrigation methods, daily drip, 3-day drip, and alternate row furrow irrigation, were applied to plots infested with P. capsici and uninfested plots. For both years, the drip irrigation (either daily or 3-day) created higher marketable green chile yields than the alternate row furrow irrigation (p < 0.05), and the yields between the daily and 3-day drip irrigation were statistically similar. The effect of irrigation on marketable combined yields was similar to that on green chile yields. In 1995, root rot disease incidence in the infested plots was significantly higher under alternate row furrow irrigation than for daily and 3-day drip irrigation. There was no disease development in the uninfested plots regardless of the irrigation method. The disease decreased green chile yield by 55% (p < 0.1), and combined yield (green + red chile) by 36% (p < 0.1) in 1995 compared to that in uninfested plots in alternate row furrow irrigation. In 1996, however, no disease occurred in any treatment. The results suggested that drip irrigation increases chile yield through providing either favorable soil moisture conditions or unfavorable conditions for Phytophthora propagation.     

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.     

Cotton yield and applied water relationships under drip irrigation

Different irrigation scheduling methods and amounts of water ranging from deficit to excessive amounts were used in cotton (Gossypium hirsutum L.) irrigation studies from 1988 to 1999, at Lubbock, TX. Irrigation scheduling treatments based on canopy temperature (T_c) were emphasized in each year. Surface drip irrigation and recommended production practices for the area were used. The objective was to use the 12-year database to estimate the effect of irrigation and growing season temperature on cotton yield. Yields in the irrigation studies were then compared with those for the northwest Texas production region. An irrigation input of 58 cm or total water application of 74 cm was estimated to produce maximum lint yield. Sources of the total water supply for the maximum yielding treatments for each year averaged 74% from irrigation and 26% from rain. Lint yield response to irrigation up to the point of maximum yield was approximated as 11.4 kg ha⁻¹ cm⁻¹ of irrigation between the limits of 5 and 54 cm with lint yields ranging from 855 to 1630 kg ha⁻¹. The intra-year maximum lint yield treatments were not limited by water input, and their inter-year range of 300 kg ha⁻¹ was not correlated with the quantity of irrigation. The maximum lint yields were linearly related to monthly and seasonal heat units (HU) with significant regressions for July (P=0.15), August (P=0.07), and from May to September (P=0.01). The fluctuation of maximum yearly lint yields and the response to HU in the irrigation studies were similar to the average yields in the surrounding production region. The rate of lint yield increase with HU was slightly higher in the irrigation studies than in the surrounding production area and was attributed to minimal water stress. Managing irrigation based on real-time measurements of T_c produced maximum cotton yields without applying excessive irrigation.     

Irrigation management and hydrosalinity balance in a semi-arid area of the middle Ebro river basin (Spain)

The analysis of irrigation and drainage management and their effects on the loading of salts is important for the control of on-site and off-site salinity effects of irrigated agriculture in semi-arid areas. We evaluated the irrigation management and performed the hydrosalinity balance in the D-XI hydrological basin of the Monegros II system (Aragón, Spain) by measuring or estimating the volume, salt concentration and salt mass in the water inputs (irrigation, precipitation and Canal seepage) and outputs (evapotranspiration and drainage) during the period June 1997–September 1998. This area is irrigated by solid-set sprinklers and center pivots, and corn and alfalfa account for 90% of the 470 ha irrigated land. The soils are low in salts (only 10% of the irrigated land is salt-affected), but shallow (<2 m) and impervious lutites high in salts (average EC_e=10.8 dS m⁻¹) and sodium (average SAR_e=20 (meq l⁻¹)^0.5) are present in about 30% of the study area.The global irrigation efficiency was high (Seasonal Irrigation Performance Index=92%), although the precipitation events were not sufficiently incorporated in the scheduling of irrigation and the low irrigation efficiencies (60%) obtained at the beginning of the irrigated season could be improved by minimising the large post-planting irrigation depths given to corn to promote its emergence. The salinity of the irrigation water was low (EC=0.36 dS m⁻¹), but the drainage waters were saline (EC=7.5 dS m⁻¹) and sodic (SAR=10.3 (meq l⁻¹)^0.5) (average values for the 1998 hydrological year) due to the dissolution and transport of the salts present in the lutites. The discharge salt loading was linearly correlated (P<0.001) with the volume of drainage. The slope of the daily mass of salts in the drainage waters versus the daily volume of drainage increased at a rate 25% higher in 1997 (7.6 kg m⁻³) than in 1998 (6.1 kg m⁻³) due to the higher precipitation in 1997 and the subsequent rising of the saline watertables in equilibrium with the saline lutites. Drainage volumes depended (P<0.001) on irrigation volumes and were very low (194 mm for the 1998 hydrological year), whereas the salt loading was moderate (13.5 Mg ha⁻¹ for the 1998 hydrological year) taking into account the vast amount of salts stored within the lutites. We concluded that the efficient irrigation and the low salinity of the irrigation water in the study area allowed for a reasonable control of the salt loading conveyed by the irrigation return flows without compromising the salinization of the soil’s root-zone.     

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.     

Spatial variability of solutes in a pecan orchard surface-irrigated with untreated effluents in the upper Rio Grande River basin

Untreated effluents are blended with water from the Rio Grande River and used for irrigation in the Juarez Valley of northern Mexico. Effluents are a source of nutrients, but may also be a source of heavy metal contamination. This study was conducted to characterize deposition patterns of selected metals, salts, and total nitrogen in a 6 ha pecan (Carya illinoenisis K.) orchard which had healthy-to-stunted trees with dieback. Orchard soil was collected along multiple transects to depths of 1.2 m, with spacing every 20 m. All solutes showed a magnitude variability in particular ions. Chromium, Ni, Pb, and Cd concentrations averaged <14 mg kg⁻¹. Soil Na, Ca, K, Mg, SO₄, Cl and NO₃–N averaged <100 mg kg⁻¹. Total N was <0.21%. Most solutes accumulated at the soil surface with the exception of Na and SO₄. Linear semi-variograms best described spatial metal deposition and surface clay content with a range of influence >189 m. Spherical semi-variograms best described spatial distribution of salts and total N, but accounted <50% of the variability. The solubility of solutes in moderately alkaline irrigation water and their specific behavior in calcareous soils likely affected deposition patterns. Estimated metal loads from irrigation over a 15-year period were <3 kg ha⁻¹, but about 187 Mg ha⁻¹ for total dissolved solids (salts). Pecan leaf tissue showed no signs of heavy metal accumulation. Suboptimum pecan growth was associated with salt accumulation in a clayey area with low permeability. Salts, in particular Na, rather than metals may be the most important inorganic contaminants for irrigated agriculture in this region. Salt loads in irrigation waters are expected to increase as agriculture increasingly relies on urban effluents too expensive to convert to potable water.     

Grain yield and water use in a long-term fertilization trial in Northwest China

The wheat- (Triticum aestivum L.) and corn- (Zea mays L.) rotation system is important for food security in Northwest China. Grain yield and water-use efficiency [WUE: grain yield/estimated evapotranspiration (ET)] were recorded during a 24-year fertilization trial in Pingliang (Gansu, China). Mean yields of wheat for the 16 years, starting in 1981, ranged from 1.29 Mg ha⁻¹ for unfertilized plots (CK) to 4.71 Mg ha⁻¹ for plots that received manure (M) annually with nitrogen (N) and phosphorus (P) fertilizers (MNP). Corn yields for the 6 years, starting in 1979, averaged 2.29 and 5.61 Mg ha⁻¹ for the same respective treatments. Whether the years were dry, normal or wet, average grain yields and WUEs for both crops were consistently highest in the MNP and lowest in the CK treatment, and were always lower in the N than in the M treatment and in all others treatments that received N along with P fertilizers. More importantly, WUEs for MNP and for straw along with N annually and P every second year (SNP) were always higher than the other fertilized treatments in dry years. Compared to yield data, coefficients of variance (CV) for WUEs were consistently low for all treatments, suggesting that WUEs were relatively stable from year to year. Yields and WUEs declined over time, except in the CK and MNP treatments for wheat. Declined yields of wheat for the N and M treatments were comparable, and the decline for the NP treatment was similar to that for the SNP treatment. Likewise, corn yields and WUEs declined for all treatments. Grain yields were significantly correlated with ET, with slopes ranging from 0.5 to 1.27 kg m⁻³ for wheat and from 1.15 to 2.03 kg m⁻³ for corn. Balanced fertilization and long-term addition of organic material to soil should be encouraged in this region to maximize the use of stored soil water, arrest grain yields decline, and ensure sustainable productivity using this intensive cereal cropping system.     

Water–yield relations and optimal irrigation scheduling of wheat in the Mediterranean region

Crop yield is primarily water-limited in areas of West Asia and North Africa with a Mediterranean climate. Ten years of supplemental irrigation (SI) experiments in northern Syria were conducted to evaluate water–yield relations for bread wheat (Triticum aestivum L.) and durum wheat (Triticum turgidum L.), and optimal irrigation scheduling was proposed for various rainfall conditions. The sensitive growth stages of wheat to water stress were from stem elongation to booting, followed by anthesis, and grain-filling. Water stress to which crop subjected depends on rainfall and its distribution during the growing season; the stress started from early March (stem-elongation stage) or even in seedling stage in a dry year, and from mid-April (anthesis) in an average or wet year. Crop yield linearly increased with increase in evapotranspiration (ET), with an increase of 160 kg for bread wheat and of 116 kg for durum wheat per 10 mm increase of ET above the threshold of 200 mm. Water-use efficiency (WUE) with a yield ≥3 t ha⁻¹ was ca. 60% higher than that with yield <3 t ha⁻¹; this emphasises the importance of that to achieve effective use of water, optimal water supply and relatively high yields need to be ensured. Quadratic crop production functions with the total applied water were developed and used to estimate the levels of irrigation water for maximizing yield, net profit and levels to which the crops could be under-irrigated without reducing income below that which would be earned for full SI under limited water resources. The analysis suggested that irrigation scenarios for maximizing crop yield and/or the net profit under limited land resource conditions should not be recommended. The SI scenarios for maximizing the profit under limited water resource conditions or for a targeted yield of 4–5 t ha⁻¹ were recommended for sustainable utilization of water resources and higher WUE. The time of irrigation was also suggested on the basis of crop sensitivity index to water stress taking rainfall probability and available soil water into account.     

Irrigation with saline water in the reclaimed marsh soils of south-west Spain: impact on soil properties and cotton and sugar beet crops

The drained and irrigated marshes in south-west Spain are formed on soils of alluvial origin from the ancient Guadalquivir river estuary. The most important characteristics of these soils are the high clay content (about 70%), high salinity, and a shallow, extremely saline, water table. The reclaimed area near Lebrija, called Sector B-XII (about 15,000 ha), has been under cultivation since 1978. Some years, however, water supply for irrigation is limited due to drought periods. The objective of this work was to evaluate the effects of irrigation with high and moderately saline waters on soil properties and growth and yield of cotton and sugar beet crops. The experiments were carried out during 1997 and 1998 in a farm plot of 12.5 ha (250 m×500 m) in which a drainage system had been installed, consisting of cylindrical ceramic sections (0.3 m long) forming pipes 250 m long, buried at a depth of 1 m and spaced at intervals of 10 m. These drains discharge into a collecting channel perpendicular to the drains. Two subplots of 0.5 ha (20 m×250 m) each were selected. In 1997 cotton was growing in both subplots, and irrigation was applied by furrows. One subplot (A) was irrigated with fresh water (0.9 dS m⁻¹) during the whole season, while in the other subplot (B) one of the irrigations (at flowering stage) was with water of high salinity (22.7 dS m⁻¹). During 1998 both subplots were cropped with sugar beet. Subplot A was irrigated with fresh water (1.7 dS m⁻¹) during the whole season, while in subplot B two of the irrigations were with moderately saline water (5.9–7.0 dS m⁻¹). Several measurement sites were established in each subplot. Water content profile, tensiometric profile, water table level, drainage water flow, soil salinity, and crop development and yield were monitored. The results showed that after the irrigation with high saline water (subplot B) in 1997 (cotton), the soil salinity increased. This increase was more noticeable in the top layer (0–0.3 m depth). In contrast, for the same dates, the soil of subplot A showed no changes. After five irrigations with fresh water, the salinity of the soil in the subplot B reached values similar to those before the application of saline water. In 1998 (sugar beet) the application of moderately saline water in subplot B also increased soil salinity, but this increase was lower than in 1997. The irrigation with high saline water affected crop development. Cotton growth was reduced in comparison with that in the subplot irrigated only with fresh water. Despite this negative effect on crop development, the crop yield was the same as in the subplot A. Sugar beet development did not show differences between subplots, but yield was higher in subplot B than in subplot A.