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.     

Effects of different ridge:furrow ratios and supplemental irrigation on crop production in ridge and furrow rainfall harvesting system with mulches

The ridge and furrow rainfall harvesting (RFRH) system with mulches is being promoted to increase water availability for crops for higher and stable agricultural production in many areas of the Loess Plateau in northwest China. In the system, plastic-covered ridges serve as rainfall-harvesting zones and stone-, straw- or film-mulched furrows serve as planting zones. To adopt this system more effectively, a field study (using corn as an indicator crop) was conducted to determine the effects of different ridge:furrow ratios and supplemental irrigation on crop yield and water use efficiency (WUE) in the RFRH system with mulches during the growing seasons of 1998 and 1999.The results indicated that the ridge:furrow ratios had a significant effect on crop yield and yield components. The 120:60 cm ridge and furrow (120 cm wide ridge and 60 cm wide furrow) system increased yield by 27.9%, seed weight per head by 14.8%, seed number per head by 7.4% and 1000-seed weight by 4.7%, compared with the 60:60 cm ridge and furrow (60 cm wide ridge and 60 cm wide furrow) system. No differences in WUE were found between the two ratio systems. For corn and winter wheat, the optimum ridge:furrow ratio seems to be 1:1 in the 300-mm rainfall area, 1:2 in the 400-mm rainfall area and 1:4 in the 500-mm rainfall area. The optimum ridge:furrow ratio seems to be 1:3 for millet in the 300-mm rainfall area, although it is unnecessary to adopt RFRH practice in regions with more than 400 mm rainfall. The most effective ridge size for crop production seems 60 cm in the Loess Plateau. Implementing supplemental irrigation in the RFRH system is also a useful way to deal with the temporal problem of moisture deficits. In the case of corn, supplemental irrigation at its critical growth stage can increase both grain yield and WUE by 20%. The combination of in situ RFRH system with supplemental irrigation practice will make the RFRH system more attractive.     

Effect of drip irrigation on squash (Cucurbita pepo) yield and water-use efficiency in sandy calcareous soils amended with clay deposits

Water research studies in Saudi Arabia clearly showed sever depletion of groundwater. Therefore, the scientifically applied research program related to water saving and conservation in agriculture is essential, where agricultural activities account for more than 85% of the total water consumed. This study aims to investigate the effect of four irrigation levels, two irrigation methods and three clay deposits on water-use efficiency (WUE) of squash and the distributions of salts and roots in sandy calcareous soils. A field experiment was conducted at the college experimental station in 2002 and 2003. It consists of three clay deposits, three rates (CO = 0, C2 = 1.0 and C3 = 2.0%), four irrigation levels (T₁ = 60, T₂ = 80, T₃ = 100 and T₄ = 120% of Eto) using surface (IM1) and subsurface (IM2) drip irrigation.Results indicated that squash fruit yield was significantly increased with the increase in irrigation water level for each season. Generally, WUE values were increased as linearly with applied irrigation water and decreased at the highest irrigation level. Types of clay deposits significantly affected fruit yields compared with the control. The yield increase was 12.8, 8.35 and 6.4% for Khulays, Dhruma and Rawdat clay deposits, respectively. The differences between surface and subsurface drip on fruit yields and WUE were also significant. Results indicated that moisture content of subsurface-treated layer increased dramatically, while salts were accumulated at the surface and away from the emitters in subsurface drip irrigation. Intensive root proliferation is observed in the clay-amended subsurface layer compared with non-amended soil. The advantages of subsurface drip irrigation were related to the relative decrease in salt accumulation in the root zone area where the plant roots were active and water content was relatively higher.     

Response of two varieties of lentil to soil salanity

Two varieties of lentil were grown in tanks filled with clay, and were irrigated with waters containing three different levels of salinity. Salinity affected the germination and survival of the seedlings; the pre-dawn leaf-water potential and maximum osmotic adjustment; the development of leaf area, dry matter and number of flowers, and, finally, the yield.Lentil has a high water-use efficiency, about 2 kg m⁻³ under non-saline conditions, much higher than legumes such as broadbean and soybean. The crop, however, is much more salt sensitive and can only be grown on non-saline soils. At an EC_e of 2 dS/m, the limit between non-saline and slightly saline soils, the yield reduction is about 20% and at an EC_e of 3 dS/m it is 90–100%.The salt tolerance classification, made after a greenhouse experiment with nutritive solutions, was not confirmed by the experiments reported here.     

Impact of treated wastewater irrigation on quality attributes and contamination of tomato fruit

A field experiment was conducted in 1999 and 2000 to investigate the effect of different treatments of potable and treated wastewater on the quality of tomato fruit (Lycopersicon esculentum L. Mill) in Jordan. Tomato seedlings (cvs. GS₁₂ and RS₅₈₉₉₅₆) were furrow irrigated with different mixtures of potable and wastewater (1:0, 1:1, 1:3, and 0:1). The BOD, and SS of the treated effluent used were 34 and 35 mg/l, respectively. Irrigation with treated wastewater did not affect fruit pH, increased their size up to 2 cm in diameter, and weight up to 78.7 g. Additionally, a decrease of 1.5% in the SSC, 0.59 kg in firmness, and 5.1% in weight loss of tomato fruit were recorded. The 0:1 application of treated wastewater resulted in an increased microbial contamination (TC 1.56×10⁴ and 4.7×10² CFU/100 g; FC 3×10² and 130 CFU/100 g; TBC 188×10² and 205×10² CFU/100 g) on the surface of the fruit (skin) for GS₁₂ and RS₅₉₉₉₅₆ varieties, respectively. There was a negligible contamination on fruit scar, and nil in fruit flesh. Contamination increased exponentially with increasing the proportions of treated wastewater application. Since treated wastewater was highly contaminated with total coliform (up to 42.0 CFU/100 ml) and total bacterial count (up to 7.820 CFU/100 ml), hence, contamination was aggravated with increasing the percentage of treated wastewater. It is suggested that the treated wastewater can be used as an alternative for irrigation of tomatoes eaten after cooking, but not for those taken as raw provided that the effluent quality is continuously monitored to avoid contamination.     

Nitrogen loss through subsurface drainage effluent in coastal rice field from India

Experiments were conducted to estimate nitrogen loss through drainage effluent in subsurface drained farmers’ field at a coastal site near Machilipatnam, Andhra Pradesh, India. The concentration of three forms of nitrogen, namely, NH₄–N, NO₂–N and NO₃–N in the subsurface drainage effluent from 15, 35 and 55 m drain spacing areas were measured in 1999 and 2000. The area with 15 m spacing was already reclaimed during 1986–1998 by the subsurface drainage system. The soil salinity of the root zone was brought down from an initial high of 35 to 4 dS m⁻¹. The subsurface drainage system with 35 and 55 m drain spacing was laid in the adjoining area and commissioned in 1998. Earlier raising of any crop in the area with 35 and 55 m spacings was not possible due to very high salinity, sodicity and poor drainage conditions. The nitrate-nitrogen loss dominated in reclaimed land with 15 m spacing whereas ammonium-nitrogen loss dominated in the land that was highly saline and in the initial stage of reclamation by the subsurface drainage technology with 35 and 55 m drain spacing. The total nitrogen loss of 3.75 kg per ha per year in 15 m drain spacing area was minimum and 23.53 kg per ha per year in 35 m drain spacing area was maximum. The nitrate-nitrogen loss contributed the maximum of 82% and ammonium- and nitrite-nitrogen contributed 11 and 7%, respectively, in 15 m drain spacing area whereas the ammonium losses contributed 93 and 82% in 35 and 55 m drain spacing areas, respectively. The losses in the form of nitrite and nitrate remained negligible in 35 m drain spacing area, but the losses to the tune of 8 and 15% in the form of nitrite and nitrate, respectively, occurred in 55 m drain spacing area.     

Trickle and sprinkler irrigation of potato (Solanum tuberosum L.) in the Middle Anatolian Region in Turkey

The potato (Solanum tuberosum L.) is widely planted in the Middle Anatolian Region, especially in the Nigde-Nevsehir district where 25% of the total potato growing area is located and produces 44% of the total yield. In recent years, the farmers in the Nigde-Nevsehir district have been applying high amounts of nitrogen (N) fertilizers (sometimes more than 900 kg N ha⁻¹) and frequent irrigation at high rates in order to get a much higher yield. This situation results in increased irrigation and fertilization costs as well as polluted ground water resources and soil. Thus, it is critical to know the water and nitrogen requirements of the crop, as well as how to improve irrigation efficiency. Field experiments were conducted in the Nigde-Nevsehir (arid) region on a Fluvents (Entisols) soil to determine water and nitrogen requirements of potato crops under sprinkler and trickle irrigation methods. Irrigation treatments were based on Class A pan evaporation and nitrogen levels were formed with different nitrogen concentrations.The highest yield, averaging 47,505 kg ha⁻¹, was measured in sprinkler-irrigated plots at the 60 g m⁻³ nitrogen concentration level in the irrigation treatment with limited irrigation (480 mm). Statistically higher tuber yields were obtained at the 45 and 60 g m⁻³ nitrogen concentration levels in irrigation treatments with full and limited irrigation. Maximum yields were obtained with about 17% less water in the sprinkler method as compared to the trickle method (not statistically significant). On the loam and sandy loam soils, tuber yields were reduced by deficit irrigation corresponding to 70% and 74% of evapotranspiration in sprinkler and trickle irrigations, respectively. Water use of the potato crop ranged from 490 to 760 mm for sprinkler-irrigated plots and 565–830 mm for trickle-irrigated treatments. The highest water use efficiency (WUE) levels of 7.37 and 4.79 kg m⁻³ were obtained in sprinkle and trickle irrigated plots, respectively. There were inverse effects of irrigation and nitrogen levels on the WUE of the potato crops. Significant linear relationships were found between tuber yield and water use for both irrigation methods. Yield response factors were calculated at 1.05 for sprinkler methods and 0.68 for trickle methods. There were statistically significant linear and polynomial relationships between tuber yield and nitrogen amounts used in trickle and sprinkler-irrigated treatments, respectively. In sprinkler-irrigated treatments, the maximum tuber yield was obtained with 199 kg N ha⁻¹. The tuber cumulative nitrogen use efficiency (NUE_cu) and incremental nitrogen use efficiency (NUE_in) were affected quite differently by water, nitrogen levels and years. NUE_cu varied from 16 to 472 g kg⁻¹ and NUE_in varied from 75 to 1035 g kg⁻¹ depending on the irrigation method. In both years, the NH₄-N concentrations were lower than NO₃-N, and thus the removed nitrogen and nitrogen losses were found to be 19–87 kg ha⁻¹ for sprinkler methods and 25–89 kg ha⁻¹ for trickle methods. Nitrogen losses in sprinkler methods reached 76%, which were higher than losses in trickle methods.     

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.     

Optimal applied water and nitrogen for corn

Water and fertilizer applications should be limited due to scarce resources and environmental protection aspects. An analysis of crop yield production and profit maximization was conducted to determine the optimal water and nitrogen allocation. In this analysis based on crop production and cost functions, a series of equations for determination of applied water and nitrogen for three conditions of maximum yield (w_m and N_m, respectively), maximum profit under limited land (w_l and N_l, respectively) and maximum profit under limited water (w_w and N_w, respectively) were derived. The associated crop production function was determined from the results of a corn experiment with four levels of nitrogen fertilization and varying amounts of applied water via a line source of sprinkler irrigation. The previously derived equations were also applied to the experimental field data and finally the optimum amounts of applied water and nitrogen were determined at different conditions (w_m, w_l, and w_w for water and N_m, N_l, and N_w for nitrogen, respectively). At present market value (15.55 Rls/m³ for water and 652 Rls/kg for nitrogen) the amounts of w_m, w_l, and w_w were 1.0, 0.99, and 0.74 m, respectively, and the amounts of N_m, N_l, and N_w were 212, 212, and 206 kg N/ha, respectively. Because of the low price of nitrogen, the optimum amounts of nitrogen at three mentioned conditions were similar. But if the price of nitrogen and water are increased (i.e. 50000 Rls/kg N and 100 Rls/m³ water), then the amounts of applied nitrogen and water at the mentioned three conditions would be 212, 67, and 61 kg N/ha, and 1.00, 0.93, 0.84 m, respectively. When water is limiting, the optimum amount of applied water would not be different by changing the water price, however, it may be increased by a little amount when the nitrogen price is increased.     

Responses of five almond cultivars to irrigation: Photosynthesis and leaf water potential

In the Trás-os-Montes region, almond orchards are usually planted in the dry soils on the upper valley of the Douro river and are typically cultivated under non-irrigated conditions, leading to low yields. This study aimed to compare the physiological responses of five almond varieties (Francoli, Ferragnès, Glorieta, Lauranne and Masbovera) growing under non-irrigated and irrigated conditions. In irrigated conditions, all cultivars had higher photosynthetic rates, with maximum rates in a range of 10–12 μmol CO₂ m⁻² s⁻¹. Study of daily photosynthesis (June–August) indicates that, irrigated plants showed maximal values at 11 h (32 °C), while in water stressed ones highest values were found at 9 h (28 °C). The irrigation induced an increase in photosynthesis of around 173% in Lauranne, 187% in Francoli, 204% in Glorieta, 266% in Masbovera and 331% in Ferragnès. In relation to values of water potential that allow half-rate of photosynthesis (ψ_w50), they were calculated as −2.95, −2.50, −3.10, −3.20 and −3.30 MPa for Ferragnès, Glorieta, Masbovera, Francoli and Lauranne, respectively.     

Wheat water productivity and yield in a cool highland environment: Effect of early sowing with supplemental irrigation

The Central Anatolian Plateau of Turkey is a typical cool highland rainfed wheat area with an annual rainfall of 300–500 mm. Due to suboptimal seasonal rainfall amounts and distribution, wheat yields in the region are low and fluctuate substantially over seasons. Delayed sowing due to late rainfall affects early crop establishment before winter frost and causes substantial reduction in yield. A 4-year field study (1998/1999 to 2001/2002) was carried out at Ankara Research Institute of Rural Services to assess the impact of early sowing with supplemental irrigation (SI) and management options during other dry spells on the productivity of a bread wheat cultivar, “Bezostia”. Treatments included early sowing with 50 mm irrigation and normal sowing with no irrigation as main plots. Four spring (SI) levels occupied the sub-plots. These are rainfed (no-irrigation), full irrigation to sature crop water requirements and two deficit irrigation levels of 1/3 and 2/3 at the full irrigation treatments.Results showed that early establishment of the crop, using 50 mm of irrigation water at sowing, increased grain yield by over 65% and adding about 2.0 t/ha to the average rainfed yield of 3.2 t/ha. Early sowing with SI allowed early crop emergence and development of good stand before being subjected to the winter frost. As a result, the crop used rainwater more efficiently. Additional supplemental irrigation in the spring also increased yield significantly. Grain yields of 5120, 5170 and 5350 kg/ha were obtained by applying 1/3, 2/3 and full SI, respectively. The mean productivity of irrigation water given at sowing was 3.70 kg/m³ with maximum value of 4.5 kg/m³. Water productivity of 1/3, 2/3 and full SI were 2.39, 1.46 and 1.27 kg/m³, respectively, compared to rainwater productivity of 0.96 kg/m³.     

The effects of different irrigation regimes on cucumber (Cucumbis sativus L.) yield and yield characteristics under open field conditions

A study was conducted to determine the effects of different drip irrigation regimes on yield and yield components of cucumber (Cucumbis sativus L.) and to determine a threshold value for crop water stress index (CWSI) based on irrigation programming. Four different irrigation treatments as 50 (T-50), 75 (T-75), 100 (T-100) and 125% (T-125) of irrigation water applied/cumulative pan evaporation (IW/CPE) ratio with 3-day-period were studied.Seasonal crop evapotranspiration (ET_c) values were 633, 740, 815 and 903 mm in the 1st year and were 679, 777, 875 and 990 mm in the 2nd year for T-50, T-75, T-100 and T-125, respectively. Seasonal irrigation water amounts were 542, 677, 813 and 949 mm in 2002 and 576, 725, 875 and 1025 mm in 2003, respectively. Maximum marketable fruit yield was from T-100 treatment with 76.65 t ha⁻¹ in 2002 and 68.13 t ha⁻¹ in 2003. Fruit yield was reduced significantly, as irrigation rate was decreased. The water use efficiency (WUE) ranged from 7.37 to 9.40 kg m⁻³ and 6.32 to 7.79 kg m⁻³ in 2002 and 2003, respectively, while irrigation water use efficiencies (IWUE) were between 7.02 and 9.93 kg m⁻³ in 2002 and between 6.11 and 8.82 kg m⁻³ in 2003.When the irrigation rate was decreased, crop transpiration rate decreased as well resulting in increased crop canopy temperatures and CWSI values and resulted in reduced yield. The results indicated that a seasonal mean CWSI value of 0.20 would result in decreased yield. Therefore, a CWSI = 0.20 could be taken as a threshold value to start irrigation for cucumber grown in open field under semi-arid conditions.Results of this study demonstrate that 1.00 IW/CPE water applications by a drip system in a 3-day irrigation frequency would be optimal for growth in semiarid regions.     

Water requirement of drip irrigated tomatoes grown in greenhouse in tropical environment

Four different levels of drip fertigated irrigation equivalent to 100, 75, 50 and 25% of crop evapotranspiration (ET_c), based on Penman–Monteith (PM) method, were tested for their effect on crop growth, crop yield, and water productivity. Tomato (Lycopersicon esculentum, Troy 489 variety) plants were grown in a poly-net greenhouse. Results were compared with the open cultivation system as a control. Two modes of irrigation application namely continuous and intermittent were used. The distribution uniformity, emitter flow rate and pressure head were used to evaluate the performance of drip irrigation system with emitters of 2, 4, 6, and 8 l/h discharge. The results revealed that the optimum water requirement for the Troy 489 variety of tomato is around 75% of the ET_c. Based on this, the actual irrigation water for tomato crop in tropical greenhouse could be recommended between 4.1 and 5.6 mm day⁻¹ or equivalent to 0.3–0.4 l plant⁻¹ day⁻¹. Statistically, the effect of depth of water application on the crop growth, yield and irrigation water productivity was significant, while the irrigation mode did not show any effect on the crop performance. Drip irrigation at 75% of ET_c provided the maximum crop yields and irrigation water productivity. Based on the observed climatic data inside the greenhouse, the calculated ET_c matched the 75–80% of the ET_c computed with the climatic parameters observed in the open environment. The distribution uniformity dropped from 93.4 to 90.6%. The emitter flow rate was also dropped by about 5–10% over the experimental period. This is due to clogging caused by minerals of fertilizer and algae in the emitters. It was recommended that the cleaning of irrigation equipments (pipe and emitter) should be done at least once during the entire cultivation period.     

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.     

Development and evaluation of integrated water and nitrogen model for maize

Maize (Zea mays L.) is an important food crop for irrigated regions in the world. Its growth and production may be estimated by different crop models in which various relationships between growth and environmental parameters are used. For simulation of maize growth and grain yield, a simulation model was developed (Maize Simulation Model, MSM). Dynamic flow of water, nitrogen (N) movement, and heat flow through the soil were simulated in unsteady state conditions by numerical analysis in soil depth of 0–1.8 m. Hourly potential evapotranspiration [ET_p(t)] for maize field was estimated directly by Penman–Monteith method. Hourly potential evaporation [E_p(t)] was estimated based on ET_p(t) and canopy shadow projection. Actual evaporation of soil surface was estimated based on its potential value, relative humidity of air, water pressure head and temperature at soil surface layer. Actual transpiration (T_a(t)) was estimated based on soil water content and root distribution at each soil layer. Hourly N uptake by plant was simulated by N mass flow and diffusion processes. Hourly top dry matter production (HDMA^j + 1, where j is number of hours after planting) was estimated by hourly corrected intercepted radiation (RSLT^j + 1) by plant leaves [determined from leaf area index (LAI^j + 1)] with air temperature, the maximum and minimum plant top N concentration and the amounts of nitrogen uptake. The value of LAI^j + 1 at each hour was estimated by the accumulated top dry matter production at previous hour using an empirical equation. Maize grain yield was estimated by a relationship between harvest index and seasonal plant top dry matter production. The model was calibrated using data obtained under field conditions by a line source sprinkler irrigation. When the values of water and nitrogen application were optimum, grain yield (moisture content of 15.5%) was 16.2 Mg ha⁻¹. Model was validated using two independent experimental data obtained from other experiments in the Badjgah (Fars province). The experimental results validated the proposed simulation model fairly well.     

Phenology based irrigation scheduling and determination of crop coefficient of winter maize in rice fallow of eastern India

Vast rainfed rice area (12 million ha) of eastern India remains fallow after rainy season rice due to lack of appropriate water and crop management strategies inspite of having favourable natural resources, human labourers and good market prospects. In this study, a short duration crop, maize, was tried as test crop with different levels of irrigation during winter season after rainy season rice to increase productivity and cropping intensity of rainfed rice area of the region. Maize hybrid of 120 days duration was grown with phenology based irrigation scheduling viz., one irrigation at early vegetative stage, one irrigation at tassel initiation, two irrigation at tassel initiation + grain filling, three irrigation at early vegetative + tassel initiation + grain filling and four irrigation at early vegetative + tassel initiation + silking + grain-filling stages. Study revealed that one irrigation at tassel initiation stage was more beneficial than that of at early vegetative stage. Upto three irrigation, water use efficiency (WUE) was increased linearly with increased number of irrigation. With four irrigations, the yield was higher, but WUE was lower than that of three irrigations, which might be due to increased water application resulted in increase crop water use without a corresponding increase of yield for the crop with four irrigations. The crop coefficients (K_c) at different stages of the crop were derived after computing actual water use using field water balance approach. The crop coefficients of 0.42–0.47, 0.90–0.97, 1.25–1.33, and 0.58–0.61 were derived at initial, development, mid and late season, respectively with three to four irrigation. Study showed that leaf area index (LAI) was significantly correlated with K_c values with the R² values of 0.93. When LAI exceeded 3.0, the K_c value was 1. Study revealed that the K_c values for the development and mid season stage were slightly higher to that obtained by the procedure proposed by FAO, which might be due to local advection.     

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.     

Sediment phosphorus dynamics for three tile fed drainage ditches in Northeast Indiana

Phosphorus (P) losses from agricultural lands degrade surface waters due to anthropogenic eutrophication. Previous studies focused on plot-to-field scale P loss and reductions from best management practices (BMP's), little information in intense agricultural catchments has been gathered on the dynamics influencing P beyond the edge of the field. This study was conducted to examine the phosphorus equilibrium between the water column and sediments in three tile fed drainage ditches in Northeast Indiana. Surface water and sediment samples were collected and analyzed for organic carbon (C), particle size and P from sites along three ditches with similar soils and land use at sites within each watershed draining approximately 300 and 1500 ha on each ditch. Organic C, silt and clay fractions of the bottom sediments decreased with increasing drainage area. Soluble P concentrations were low in Ditch A, but increased with increasing drainage area (0.02–0.05 mg P L⁻¹). Overall, the P concentrations were higher in the Ditches B and C (0.06–0.09 mg P L⁻¹). Exchangeable P, P partitioning index and equilibrium P concentrations (EPC_o) decreased with increasing drainage area by as much as 95, 93 and 100%, respectively, except in one catchment area with a confined animal feeding operation between sampling points, where ExP and EPC_o increased by 4 and 116%, respectively. Aluminum sulfate and calcium carbonate treatment of ditch sediments reduced exchangeable P and sediment EPC_o in this study. Results from this study indicated some watershed characteristics, as well as sediment physiochemical properties, affect ditch sediment and water P equilibrium and buffering capacity. Furthermore, this study demonstrated that managers could potentially use chemical treatment of the ditches to increase the temporary retention of P in ditches and maybe reducing sediment P availability.     

Review of measured crop water productivity values for irrigated wheat,rice, cotton and maize

The great challenge of the agricultural sector is to produce more food from less water, which can be achieved by increasing Crop Water Productivity (CWP). Based on a review of 84 literature sources with results of experiments not older than 25 years, it was found that the ranges of CWP of wheat, rice, cotton and maize exceed in all cases those reported by FAO earlier. Globally measured average CWP values per unit water depletion are 1.09, 1.09, 0.65, 0.23 and 1.80 kg m⁻³ for wheat, rice, cotton_seed, cotton_lint and maize, respectively. The range of CWP is very large (wheat, 0.6–1.7 kg m⁻³; rice, 0.6–1.6 kg m⁻³; cotton_seed, 0.41–0.95 kg m⁻³; cotton_lint, 0.14–0.33 kg m⁻³ and maize, 1.1–2.7 kg m⁻³) and thus offers tremendous opportunities for maintaining or increasing agricultural production with 20–40% less water resources. The variability of CWP can be ascribed to: (i) climate; (ii) irrigation water management and (iii) soil (nutrient) management, among others. The vapour pressure deficit is inversely related to CWP. Vapour pressure deficit decreases with latitude, and thus favourable areas for water wise irrigated agriculture are located at the higher latitudes. The most outstanding conclusion is that CWP can be increased significantly if irrigation is reduced and crop water deficit is intendently induced.     

Fate of gypsum surface — applied to an Appalachian hill pasture watershed

Low pH soils leached with gypsum solutions have been shown to sorb gypsum thereby increasing calcium and improving root growth of some crops. However, in some situations, exchangeable aluminum is leached which could enter waterways and potentially cause adverse off-site impacts. Due to relatively high solubility, gypsum itself has potential to impact stream water chemistry. In this study, 8250 kg ha⁻¹ gypsum was applied to plots in a steep 4.3 ha-pasture watershed in central Appalachia. Changes in the 0–40 cm soil profile and changes in water chemistry at a flume below a seep which drains the watershed were measured in response to this application. After the end of the second growing season, and a total rainfall of 1.7 m, 46% of the applied gypsum calcium was still in the top 40 cm of soil. The rainfall had a 11.4% efficiency rate of transporting gypsum out of the top 40 cm compared to movement of a saturated solution through an inert medium. Of the applied gypsum sulfate, 3.4% exited the watershed through stream flow. Maximum gypsum concentrations in stream flow, which occurred during a storm event, were 34 ppm or <1.5% of saturation levels. No detectable aluminum was measured in stream flow at the flume. These findings indicate that as long as a minor part of watersheds such as this one has gypsum applied at rates less than 10,000 kg ha⁻¹, off-site environment impacts should be minimal.