Field evaluation of crop yield as affected by nonuniformity of sprinkler-applied water and fertilizers

The determination of target uniformity for sprinkler irrigation system should consider the impacts of nonuniformity of water and fertilizers on crop yield. Field experiments were therefore conducted in north China plains to address the impacts of nonuniformly applied water and fertilizers on winter wheat yield. Irrigation water and fertilizers were applied through a solid set sprinkler system. Three experimental plots were used with seasonal Christiansen uniformity coefficients (arithmetic mean of individual CUs) ranging from 62 to 82%. Each plot was divided into $\text{3}\text{m}\text{×3}\text{m}$ grids. Sprinkler water depth and concentration of fertilizer solution for each grid was measured both below and above the canopy for each individual irrigation event. The spatial distribution of soil moisture for each experimental plot was also measured periodically to determine irrigation times and amounts. On harvest, grain yield and total nitrogen content of plant stems were measured for each grid. The experimental results showed that the uniformity of fertilizer applied increased with sprinkler water uniformity. The distributions of both fertilizers and water applied through sprinkler system can be represented by a normal distribution function. Field experiments also demonstrated that the uniformity of sprinkler-applied water and fertilizers has insignificant effect on winter wheat yield for the studied uniformity range. The current standard for sprinkler uniformity (for example, the target CU is equal to or higher than 75% in China) is high enough for obtaining a reasonable crop yield in dry sub-humid regions.     

Evaluation of a crop water stress index for irrigation scheduling of bermudagrass

This study was conducted to assess crop water stress index (CWSI) of bermudagrass used widely on the recreational sites of the Mediterranean Region and to study the possibilities of utilization of infrared thermometry to schedule irrigation of bermudagrass. Four different irrigation treatments were examined: 100% (I₁), 75% (I₂), 50% (I₃), and 25% (I₄) of the evaporation measured in a Class A pan. In addition, a non-irrigated treatment was set up to determine CWSI values. The status of soil water content and pressure was monitored using a neutron probe and tensiometers. Meanwhile the canopy temperature of bermudagrass was measured with the infrared thermometry. The empirical method was used to compute the CWSI values. In this study, the visual quality of bermudagrass was monitored seasonally using a color scale. The best visual quality was obtained from I₁ and I₂ treatments. Average seasonal CWSI values were determined as 0.086, 0.102, 0.165, and 0.394 for I₁, I₂, I₃, and I₄ irrigation treatments, respectively, and 0.899 for non-irrigated plot. An empirical non-linear equation, Qave=1+⌊6[1+(4.853 CWSIave)^2.27]^−0.559⌋$Q_{\text{ave}}=1+⌊6{[1+{(4.853\text{CWS}{\text{I}}_{\text{ave}})}^{2.27}]}^{-0.559}⌋$, was deduced by fitting to measured data to find a relation between quality and average seasonal CWSI values. It was concluded that the CWSI could be used as a criterion for irrigation timing of bermudagrass. An acceptable color quality could be sustained seasonally if the CWSI value can be kept about 0.10.     

Yield and growth responses of kenaf (Hibiscus cannabinus L.) in a semi-arid tropical environment to irrigation regimes based on leaf water potential

Summary The growth response of kenaf (Hibiscus cannabinus L.) to four irrigation schedules based on leaf water potential _l was evaluated in a semi-arid tropical environment. Total dry matter production was unaffected by regimes in which the mean value of leaf water potential _l (mean of solar noon and dawn value) did not fall below –1.26 MPa. Stem elongation was more sensitive than dry matter accumulation to plant water stress. — The economic yield for paper pulp production (i. e. total plant dry matter production minus that of the foliage and upper 60 cm of stem) increased with the frequency of irrigation. — Growth recovery by kenaf following a period of water stress was examined. Alleviation of water stress 10 weeks after irrigation, when _l was –1.60 MPa, produced stem elongation rates that were greater than those of plants previously receiving irrigation. This ability to withstand water stress and partially compensate in growth following alleviation of the stress indicates that the kenaf crop has stress response features suitable for rainfall only production under semi-arid tropical conditions. — Irrigation schedules based on _l resulted in water applications tailored to crop requirements in that water use increased, and the time interval between irrigation decreased, with increasing canopy development as well as with increasing evaporative demand. However, erratic fluctuations in _l between irrigations make scheduling by this method difficult and the use of daily mean, dawn or noon values of _l for scheduling irrigation of kenaf cannot be recommended in environments of high evaporative demand. The factors contributing to these fluctuations in (_l) are discussed.     

The effect of organic manuring and water quality on water transmission parameters and sodication of a sandy loam soil

Water transmission characteristics under saturated and unsaturated conditions were studied in a sandy loam soil with (F₁) and without (F₀) long-term farmyard manure (FYM) treatments, in relation to sodium adsorption ratios (SAR) and electrolyte concentrations of water. The effect of FYM and ratios of Ca²⁺ : Mg²⁺ in water at a given SAR on sodication of the soil was also studied.Saturated hydraulic conductivity (k) and weighted mean diffusivity ($\text{D}\text{?}$) were slightly higher for F₁ than for F₀, whereas sodication indices like Gapon constant (K_G), Krishnamoorthy-Davis-Overstreet constant (K_KDO) and Vanselow constant (K_V) were slightly smaller. The k and $\text{D}\text{?}$ decreased with an increase of SAR and decrease of electrolyte concentration, the effect of SAR being more pronounced. There was proportionately a sharper decrease in the k and $\text{D}\text{?}$ values at SAR 10 with total electrolyte concentrations of 10–40 meq 1^?1. However, with a total electrolyte concentration of 80 meq 1^?1, there was a smaller drop at SAR 10.A small difference in the build-up of exchangeable sodium percentage (ESP) in F₁ and F₀ treatments at a given SAR suggests that, apart from slightly improving water transmission parameters, the use of FYM also reduces the sodication hazard in a soil irrigated with sodic waters. An increase in the Ca²⁺ : Mg²⁺ ratio from 25:75 to 75:25 slightly decreased the values of K_G, K_KDO and K_V, thus indicating somewhat more preference for Ca²⁺ to Mg²⁺ at a given SAR, which was more so in F₁ soil. This fact could also be expressed in terms of a slight shift of thermodynamic exchange constant (K) and standard free energy change of the exchange reaction (ΔG⁰_r). The presence of some unidentified Na⁺ releasing minerals in the soils studied was observed and correction for exchangeable Na⁺ determination applied.     

Irrigation with coalbed natural gas co-produced water

Water quality is one of the potential concerns associated with the development of coalbed natural gas (CBNG) in the Powder River Basin (PRB) of Wyoming and Montana. Large quantities of water (hereafter referred to as CBNG water) are being co-produced and often discharged in the process of exploring natural gas from coal seams. Use of CBNG water for irrigating croplands may be beneficial if factors associated with soil salinity and sodicity are controlled. This study evaluated effects of five water and three soil treatments applied to a mixed-hay cropland on selected soil chemical properties using a split plot design. Water treatments consisted of Piney Creek water (PC or control), direct irrigation with CBNG water (electrical conductivity or EC of 1.38 dS m⁻¹ and sodium adsorption ratio or SAR of 24.3 mmol^1/2 L^−1/2) with no amendments (NT), CBNG water mixed with solution grade gypsum (G), CBNG water acidified using sulfur burner and mixed with gypsum (GSB) and CBNG water mixed with Piney Creek water (PC/CBNG). Soil treatments consisted of gypsum (G), elemental sulfur (S), combination of these two (GS) and no treatment or the control (NT). Pre (Summer 2003) and post treatment (Fall 2004) soil samples were collected to a depth of 60 cm (top three horizons: A, Bt₁ and Bt₂) to evaluate the effects of treatments on soil pH, EC, SAR, and sulfate (SO₄²⁻) concentrations. Comparisons between pre and post irrigation soil chemistry data indicated CBNG water with no amendments significantly increased (P ≤ 0.05) Na⁺ concentration within the soil profile. Plots treated with a combination of the GSB water treatment and the GS soil amendments were most effective in maintaining the low SAR values at surface soil layer. In all treatment combinations, both EC and SAR increased significantly in the top two sampling depths (A and Bt₁ horizons). Further studies are required to evaluate applications of leaching fractions at the end of each irrigation season for its effectiveness at moving Na⁺ below the rooting zone.     

Daylight crop water stress index for continuous monitoring of water status in apple trees

We studied the suitability of empirical crop water stress index (CWSI) averaged over daylight hours (CWSI_d) for continuous monitoring of water status in apple trees. The relationships between a midday CWSI (CWSI_m) and the CWSI_d and stem water potential (ψ _stem), and soil water deficit (SWD) were investigated. The treatments were: (1) non-stressed where the soil water was close to field capacity and (2) mildly stressed where SWD fluctuated between 0 and a maximum allowable depletion (MAD of 50 %). The linear relationship between canopy and air temperature difference (ΔT) and air vapor pressure deficit (VPD) averaged over daylight hours resulted in a non-water-stressed baseline (NWSBL) with higher correlation (?T = ?0.97 VPD – 0.46, R ² = 0.78, p < 0.001) compared with the conventional midday approach (?T = ?0.59 VPD – 0.67, R ² = 0.51, p < 0.001). Wind speed and solar radiation showed no significant effect on the daylight NWSBL. There was no statistically meaningful relationship between midday ψ _stem and CWSI_m. The CWSI_d agreed well with SWD (R ² = 0.70, p < 0.001), while the correlation between SWD and CWSI_m was substantially weaker (R ² = 0.38, p = 0.033). The CWSI_d exhibited high sensitivity to mild variations in the soil water content, suggesting it as a promising indicator of water availability in the root zone. The CWSI_d is stable under transitional weather conditions as it reflects the daily activity of an apple crop.     

Canopy temperature and water stress of cotton crops with complete and partial ground cover

Summary The use of canopy and air temperature differences to compute a crop water stress index (CWSI) for assessing plant water status was investigated using cotton crop canopies that either fully or partially covered the ground. The complete ground cover canopy condition was studied in a well watered moisture regime in a rainout shelter with measurements made on six Texas cotton race stocks. The partial ground cover canopy situation was investigated in a well watered moisture regime of a commercial cotton variety Paymaster 266 grown in the field. The slope of the nonstressed baseline of the CWSI for a cotton canopy with about 50% ground cover was approximately one-half that reported for full canopies. Values of CWSI calculated with theoretical and empirical procedures agreed more closely under a complete canopy condition than under a partial canopy situation. Values of aerodynamic resistance (r _a ) and canopy resistance for well watered soil moisture conditions (r _ep )were estimated in order to use the theoretical procedure of computing CWSI. Values of r _a ranged from 10 to 15 sm^–1 and r _cp from 50 to 60 sm^–1. Both the theoretical and empirical procedures showed much promise, but more information is needed to develop techniques for evaluating r _a and r _cp under differing canopy and environmental conditions.     

Effect of soil water osmotic potential on growth and water relationships in barley during soil water depletion

Summary Barley plants (Hordeum distichum, L., cv. Zita) grown in a sandy soil in pots were adjusted during a pretreatment period of 5 days to three levels of soil water osmotic potential by percolating 61 of a nutrient solution with additional 0, 22.3 and 44.6 mM KCl. A drying cycle was then started and the plants were harvested when the soil water matric potential had decreased to –1.4 MPa, respectively 6, 7 and 8 days later.No significant differences in dry matter yields, transpiration coefficients and wilting percentages were found between treatments.During the drying cycle leaf water potential ( _l ) decreased concomitantly with decrease in soil water potential ( _s ) with almost constant and similar differences ( _l – _s ) for all treatments despite differences in levels of potentials. The concomitant decrease in leaf osmotic potential () was due partly to dehydration (58%) and partly to increase in leaf solute content (42%) independent of treatment. The part of total osmotic solutes due to K decreased relatively during the drying cycle.Close relationships were found between and _l as functions of relative water content (RWC). Identical curves for the two levels of salt treatment agree with similar concentrations of K, Cl, and ash found for salt treated plants indicating that maximum uptake of macro nutrients may have been reached.During the main part of the drying cycle the turgor potential as function of RWC was higher and decreased less steeply with decreasing RWC in the salt treated than in the non-salt treated plants.In the beginning of the drying cycle additions of KCI lowered the transpiration rates of the salt treated plants resulting in a slower desiccation of the soil and hence an increased growth period. A delay in uptake from a limited soil water supply may be advantageous during intermittent periods of drought.     

Plant indicators of wheat and soybean crop water stress

Summary The onset of water stress within a crop is defined as the time at which the rate of water loss declines below that of a well watered crop in the same locality. The relation to the onset of water stress and soil water status of several readily measured plant parameters was investigated in crops of wheat and soybeans over three years. Evapotranspiration ET was monitored with weighing lysimeters. A noticeable decline in the rate of ET for both wheat and soybeans was detected once 20% to 30% of the total plant available water PAW remained in the 1 m deep lysimeter soil profile. Extension growth of wheat declined when PAW was 33% and 34% in two years of measurement. In soybeans, the decline in the rate of leaf extension coincided with the decline in the rate of ET. Midmorning measurement of exposed leaf water potential _L, covered leaf water potential _CL and covered plant leaf water potential _CP yielded similar results for both wheat and soybeans. Day-to-day variability was least in _CP and most in _L. Values of _CP, _L and _CL decreased rapidly with PAW < 30%. Daily values of leaf diffusive conductance were variable but there was a general decline in conductance with PAW < 30%. It is suggested that _CL may be the easiest and most reliable parameter to monitor as a means of detecting the onset of stress. The results indicated that PAW levels in the root zone of 50% for wheat and 30% for soybean probably do not affect extension growth or plant water status parameters and can thus be used as criteria for irrigation scheduling.Seconded from the Water Research Commission, Pretoria; present address: CSIRO, Division of Irrigation Research, Griffith, N SW 2680, Australia     

Water use by alfalfa,maize, and barley as influenced by available soil water

The availability of soil water is one of the most important determinants of crop production. Field studies were conducted to examine the relationships between relative evapotranspiration ($\text{E}\text{E}{}_{\mathrm{<mtext>max</mtext>}}$) and available water (W) for alfalfa, maize, and barley. Line source sprinkler irrigation systems were used to provide the variations in soil moisture. Actual evapotranspiration (E) was determined using the water balance method. Maximum evapotranspiration (E_max) was the highest E observed among all irrigation levels. Potential evapotranspiration (E₀) was estimated using Penman's equation to characterize the evaporative demand.The results show that the relationships between $\text{E}\text{E}{}_{\mathrm{<mtext>max</mtext>}}$ and W were different for the three crops. For alfalfa, the relationship was dependent on the physical properties of the soil and on E₀. In a clay loam soil, the decline in E from E_max commenced at a higher value of W than in a sandy loam soil. Furthermore, the rate of decline in E from E_max was dependent on E₀ and was greater as E₀ increased. In the sandy loam soil, the relationship between $\text{E}\text{E}{}_{\mathrm{<mtext>max</mtext>}}$ and W was not dependent on E₀. For maize and barley in clay loam soils, $\text{E}\text{E}{}_{\mathrm{<mtext>max</mtext>}}$ as a function of W was linear, and was not dependent on E₀. This study was compared to results reported in the literature, and it was hypothesized that differences were related mainly to the way variation in soil moisture was introduced over the measurement period.     

Photosynthesis,leaf conductance,and water relations of cowpea under saline conditions

Summary Cowpea (Vigna unguiculata L.), grown widely under both irrigated and dryland conditions, is well adapted to drought and high temperature and is moderately salt tolerant. Data on photosynthetic response and regulation of water relations in cowpea under salinity stress is lacking. Therefore, in conjunction with a field plot experiment to establish the leaching requirement of cowpea, measurements were made of carbon dioxide assimilation rates (A) by ¹⁴CO₂ uptake, leaf conductances to H₂O (g₁) by tritum uptake, and to CO₂ (g), and leaf total water potential (_t ¹) and osmotic potential ( ¹).Cowpeas, grown in field plots containing Pachappa fine sandy loam (mixed, thermic, Mollic Haploxeraff), were irrigated daily with saline water (1,350 mg 1^–1 total salt concentration) to achieve leaching fractions of 0.17, 0.13, 0.09, 0.07, and 0.02. Cowpea maintained high leaf water potentials, high rates of CO₂ assimilation and high leaf conductances under moderately saline conditions (high leaching). Values of _t ¹ and ¹ for high leaching were consistently 50 to 200 J kg^–1 higher than for low leaching throughout the day. Calculating ¹ at full leaf turgor eliminated diurnal variation in ¹. As leaching decreased, however, A, g₁, and g, decreased significantly. About 45% of the 1°C assimilated by the leaf was incorporated rapidly into ethanol insoluble compounds. The relationship between A and g₁ for cowpea was similar to that reported for other crops.Contribution from the US Salinity Laboratory, USDA-ARS, 4500 Glenwood Dr., Riverside, CA. 92501, USA