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.     

An improved Lewis-Milne equation for the advance phase of border irrigation

Summary The Lewis-Milne (LM) equation has been widely applied for design of border irrigation systems. This equation is based on the concept of mass conservation while the momentum balance is replaced by the assumption of a constant surface water depth. Although this constant water depth depends on the inflow rate, slope and roughness of the infiltrating surface, no explicit relation has been derived for its estimation. Assuming negligible border slope, the present study theoretically treats the constant depth in the LM equation by utilizing the simple dam-break wave solution along with boundary layer theory. The wave front is analyzed separately from the rest of the advancing water by considering both friction and infiltration effects on the momentum balance. The resulting equations in their general form are too complicated for closed-form solutions. Solutions are therefore given for specialized cases and the mean depth of flow is presented as a function of the initial water depth at the inlet, the surface roughness and the rate of infiltration. The solution is calibrated and tested using experimental data.Abbreviations a (t) advance length - c mean depth in LM equation - c _f friction factor - c _h Chezy's friction coefficient - g acceleration due to gravity - h(x, t) water depth - h ₀ water depth at the upstream end - i() rate of infiltration - f(x, t) discharge - q₀ constant inflow discharge - S _f energy loss gradient or frictional slope - S₀ bed slope - t time - u(x, t) mean velocity along the water depth - x distance - Y() cumulative infiltration - (t) distance separating two flow regions - infiltration opportunity time     

Optimization of groundwater abstraction system and distribution pipe in pressurized irrigation systems for minimum cost

A tool named DOPIR (Dimensioning Of Pressurized IRrigation) was developed to optimize the process of water abstraction from an aquifer for pressurized irrigation systems. This tool integrates the main factors throughout the irrigation process, from the water source to the emitter. The objective is to minimize the total cost of water abstraction and application (C _T) (investment (C _a) + operation (C _op) per unit of irrigated area according to the type of aquifer, crop water requirement and electricity rate periods. To highlight the usefulness of this tool, DOPIR has been applied to a corn crop in Spain with a permanent sprinkler irrigation system, considering two types of aquifer: confined and unconfined. The effects of parameters such as the static water table in the aquifer (SWT), irrigated area (S), number of subunits in the plot (NS), sprinkler and lateral pipe spacing, and average application rate (ARa) on C _T have been analyzed. Results show that energy cost (C _e) is the most important component of C _T (50–72 % in the case studies). Thus, it is very important to adapt the design and management of the irrigation and pumping system throughout the irrigation season to the energy rate periods.     

Procedure for predicting and designing moving sprinkler application patterns

Summary Water application pattern, WAP, is one of the most important factors that determine the instantaneous and the cumulative application rates of moving irrigation machines. The mathematical background of a procedure to predict and design the WAP of moving irrigation machines is introduced. It includes a mathematical analysis of the effect of pressure head, height and spacing between emitters on the WAP, and a nomograph that presents this analysis graphically and illustrates the design procedure of the application pattern of irrigation machines.Abbreviations P()a water application rate at a normalized radial distance from the emitter [m/s] - ka number of linear segments needed to represent the pattern - s/Ra normalized radial distance from the emitter - Ra wetted radius [m] - sa radial distance from the emitter [m] - n _j n _i/ha normalized water application rate at point - j, ha maximum water application rate [m/s] n _j water application rate at point j [m/s] - _j =m _j/Ra normalized radial distance of point j from emitter - m _ja radial distance of point - ja from emitter [m], CWAP - (x)a Cumulative Water Application Pattern: amount of water per unit area applied at a distance - xa from the travel path of the emitter [m³/m²] - xa distance from the travel path of the emitter [m] - T _xa time of application at a distance - xa from the travel path of the emitter [s] - va velocity of propagation of the machine [m/s] - k ₁a the outmost linear segment that its radial distance from the emitter - m _k1a is smaller than the distance of the travel path from the emitter - x, T _ja time at which the - j ^tha linear segment (ring) stops influencing the point located at a distance - xa from the emitter - ₁, ₂, ₃a dimensionless numbers derived by dimensional analysis - ua water jet velocity [m/s] - ga gravity acceleration [m/s²] - da nozzle diameter [m], v kinematic viscosity [m²/s] - Ha emitters height [m] - , a regression analysis coefficients - Paa Pattern fit coefficient for water application - F(r)a normalized desired water application pattern [1/m] - f(r)a normalized actual water application pattern [1/m] - La common distance on which - F(r) and f(r)a are defined [m], SP spacing interval between emitters [m] - DSa dimensionless spacing interval between emitters - DSa variation of dimensionless spacing interval - Paa variation of Pa coefficient - Pa pressure head [kPa]     

Wind effects on solid set sprinkler irrigation depth and yield of maize (<Emphasis Type="Italic">Zea mays</Emphasis>)

A field experiment was performed to study the effect of the space and time variability of water application on maize (Zea mays) yield when irrigated by a solid set sprinkler system. A solid set sprinkler irrigation layout, typical of the new irrigation developments in the Ebro basin of Spain, was considered. Analyses were performed (1) to study the variability of the water application depth in each irrigation event and in the seasonal irrigation and (2) to relate the spatial variability in crop yield to the variability of the applied irrigation and to the soil physical properties. The results of this research showed that a significant part of the variability in the Christiansen coefficient of uniformity (CU), and wind drift and evaporation losses were explained by the wind speed alone. Seasonal irrigation uniformity (CU of 88%) was higher than the average uniformity of the individual irrigation events (CU of 80%). The uniformity of soil water recharge was lower than the irrigation uniformity, and the relationship between both variables was statistically significant. Results indicated that grain yield variability was partly dictated by the water deficit resulting from the non-uniformity of water distribution during the crop season. The spatial variability of irrigation water depth when the wind speed was higher than 2 m s^–1 was correlated with the spatial variability of grain yield, indicating that a proper selection of the wind conditions is required in order to attain high yield in sprinkler-irrigated maize.     

Assessing whole-field uniformity of stationary sprinkler irrigation systems

The procedure established in the literature for the evaluation of stationary sprinkler irrigation systems is limited in space and time since it is based on a sample of precipitation taken around one sprinkler during a given period of the whole irrigation event. This procedure also ignores what happens in the soil after water infiltrates. A model of the drop trajectory and of the water distribution pattern is formulated here for simulating precipitation from single sprinklers. The operating pressure determines sprinkler flow and maximum throw. Wind and evaporation distort the distribution patterns. The water distribution of individual sprinklers is overlapped to generate precipitation over the whole field and to calculate a coefficient of uniformity. Field effective uniformity is then calculated by averaging precipitation over the extension of plant roots or water redistribution within the soil profile. Application of the model has shown the impact of system management and design, field topography and wind on irrigation uniformity. Management factors such as lateral operation time or riser inclination may account for a large part of the field precipitation variations. A rough topography may also reduce uniformity significantly. Wind speed is important when it exceeds 1.8–2 m s^–1. The allowable maximum pressure loss of 20% fixed as a design criterion seems an overly strict limit when other factors may overcome pressure loss as sources of non-uniformity. The sources of non-uniformity have different scales of variation. Large-scale sources, such as lateral operation time or pressure loss, are not dampened by the crop or soil. Sources of smaller-scale variation, such as wind or inclination of the sprinkler riser, are better compensated by the crop and soil. The application of this kind of model to the design and management of sprinkler irrigation systems is discussed. Received: 9 May 1997     

The effects of pressure, nozzle diameter and meteorological conditions on the performance of agricultural impact sprinklers

This study evaluates agricultural impact sprinklers under different combinations of pressure (p), nozzle diameter (D) and meteorological conditions. The radial curve (Rad) of an isolated sprinkler, i.e., the water distribution along the wetted radius, was evaluated through 25 tests. Christiansen's uniformity coefficient (CUC) and the wind drift and evaporation losses (WDEL) were evaluated for a solid-set system using 52 tests.The Rad constitutes the footprint of a sprinkler. The CUC is intimately connected to the Rad. The Rad must be characterized under calm conditions. Very low winds, especially prevailing winds, significantly distort the water distribution. The vector average of the wind velocity (V’) is recommended as a better explanatory variable than the more popular arithmetic average (V). We recommend characterizing the Rad under indoor conditions or under conditions that meet V’ < 0.6 m s⁻¹ in open-air conditions.The Rad was mostly affected by the sprinkler model. V’ was the main explanatory variable for the CUC; p was significant as well. V was the main variable explaining the WDEL; the air temperature (T) was significant, too.Sprinkler irrigation simulators simplify the selection of a solid-set system for farmers, designers and advisors. However, the quality of the simulations greatly depends on the characterization of the Rad. This work provides useful recommendations in this area.     

Effects of applied water and sprinkler irrigation uniformity on alfalfa growth and hay yield

This study was conducted to investigate the effects of applied water and sprinkler irrigation uniformity on alfalfa (Medicago sativa L.) growth and hay yield in a semi-arid region. Field experiments were carried out in 2006 in Varamin, Iran, on three plots of 25 m × 30 m. Each plot was subdivided into 25 subplots of 5 m × 6 m. Different irrigation depths and sprinkler water uniformities were obtained by various scenarios of sprinkler nozzle pressure. In each plot, applied water was measured at 250 points (125 points above and 125 points below canopy) and the soil water content of 40 cm deep below soil surface was monitored at 25 points, each in the center of a subplot, throughout the irrigation season. The results showed that sprinkler water and soil water content uniformity varied between 66-78 and 88-91%, respectively. The findings revealed that soil water content uniformity was around 20% higher than sprinkler water uniformity. The irrigation uniformity below the canopy was estimated to be 2.5% greater than above the canopy, and canopy-intercepted water could account for 11-15% of the total seasonal applied water. Evaluation showed that alfalfa leaf area index relies more heavily on farm water application uniformity than hay yield and crop height. The experimental results illustrated that water distribution in sprinkler irrigation systems has a direct effect on alfalfa growth, hay yield and water productivity such that the applied water reduction and the increased sprinkler water uniformity led to an increased alfalfa water productivity of 2.41 kg m⁻³.     

Kinetic energy in sprinkler irrigation: different sources of drop diameter and velocity

Sprinkler kinetic energy has been linked to a number of problems in irrigated fields. This work presents the characterization of sprinkler drop kinetic energy and specific power from low-speed photographic drop data using a commercial impact sprinkler and three operating pressures. The spatial variability of specific power (W m⁻²) was assessed for different sprinkler spacings, showing different patterns in rectangular and triangular spacings. The specific power uniformity coefficient ranged from 38–77%, depending on sprinkler spacing and operating pressure. An attempt was made to characterize specific power from estimated (measured diameter and estimated velocity) and simulated data (using a ballistic model). While estimated data produced adequate results, simulated data resulted in a large overestimation. Discrepancies in kinetic variables between measured and simulated drop data permit to conclude that it is important to continue experimental drop characterization efforts as well as sprinkler simulation model development.     

Sprinkler evaporation losses in alfalfa during solid-set sprinkler irrigation in semiarid areas

Gross sprinkler evaporation losses (SEL_g) can be large and decrease irrigation application efficiency. However, it is not universally established how much of the SEL_g contributes to decrease the crop evapotranspiration during the sprinkler irrigation and how much are the net sprinkler losses (SEL_n). The components of SEL were the wind drift and evaporation losses (WDEL) and the water intercepted by the crop (IL). The gross WDEL (WDEL_g) and evapotranspiration (ET) were measured simultaneously in two alfalfa (Medicago sativa L.) plots, one being irrigated (moist, MT) and the other one not being irrigated (dry, DT). Catch can measurements, mass gains, and losses in the lysimeters and micrometeorological measurements were performed to establish net WDEL (WDEL_n) during the irrigation and net IL (IL_n) after the irrigation as the difference between ET_MT and ET_DT. Also, equations to estimate IL_n and net sprinkler evaporation losses (SEL_n) were developed. IL_n was strongly related to vapor pressure deficit (VPD). SEL_n were 8.3 % of the total applied water. During daytime irrigations, SEL_n was 9.8 % of the irrigation water and slightly less than WDEL_g (10.9 %). During nighttime irrigations, SEL_n were slightly greater than WDEL_g (5.4 and 3.7 %, respectively). SEL_n was mainly a function of wind speed.     

Effective irrigation uniformity as related to root zone depth

Summary In models used for relating the yield to irrigation uniformity it has been assumed that the spatial distribution of irrigation water, as measured at the soil surface, is indeed the water distribution at any depth throughout the root zone. In the present paper the distribution of infiltrated water within the soil bulk, as determined by an analytic solution of the two-dimensional unsaturated flow equation, did not conform to this assumption. A new alternative definition of irrigation uniformity is proposed under the assumption that water uptake by roots does not affect the flux distribution within the soil profile. In this analysis the spatial distribution of irrigation water flux at the soil surface, which is the upper boundary condition of the flow equation, is assumed to be a sine function. The solution to this problem indicates that there is a damping effect, which increases with soil depth, on the surface flux fluctuations. Furthermore, the actual irrigation uniformity at a given depth below the soil surface depends upon the initial uniformity at the surface and the distance between adjacent water sources. The closer the water sources are to each other, the shallower is the depth needed to damp the oscillations down to a certain level. This may explain why the actual uniformity of drip irrigation is high while the detailed distribution is very nonuniform and on the other hand, why the actual uniformity of sprinkler guns is low while the detailed actual distribution is close to uniform. Two uniformity coefficients are derived in this study: 1. A depth dependent coefficient which is made up of the damping factor that multiplies the flux fluctuations at the soil surface; 2. An effective uniformity coefficient, which is an average of the depth dependent coefficient over a part or the entire root zone. Different degrees of uniformity are expected when water is applied by different irrigation systems having similar uniformity coefficients at the soil surface, but dissimilar distances between the emitters. Assuming that crop yield depends to some extent on the uniformity of water depth actually available to the roots, the yields associated with such irrigation systems will probably also vary.     

The transport of chloride and non-diffusible solutes through soil

Summary The mean velocity at which water flowed through large undisturbed cores of soil was determined from the breakthrough of surface-applied Cl^–, using a transfer function based on the normal distribution of the logarithm of cumulative drainage. For soils ranging in texture from sandy loam to silty clay loam, mean pore water velocities varied from 7 to 30 cm h^–1 for an input rate of 2 cm h^–1. Antibiotic-resistant Escherichia coli applied to the soil surface appeared to be transported through large pores only (> 10–15 m diameter), and the relative concentration in the effluent (C/C₀) did not change significantly with effluent volume. Mean C/C₀ values for E. coli in these soils, which ranged from 0.003 to 0.94, could be predicted from the mean pore water velocity derived from Cl^– transport.     

Assessing whole-field sprinkler irrigation application uniformity

In order to assess whole-field sprinkler irrigation uniformity, an experiment was conducted to obtain water distribution profiles at 23 different pressures for each of five different sprinklers: Nelson R33, Nelson R33LP, Nelson R33 with road guard, Nelson R33LP with road guard, and Rainbird Mini Paw/LG-3. A mathematical model was developed to account for pressure variation throughout a fixed sprinkler system on a 10-ha field and to evaluate sprinkler irrigation uniformity for the whole field using interpolated water distribution profiles from the experimental data. The relationships between irrigation application uniformity and sprinkler pressure, sprinkler spacing, pressure variation, sprinkler type, and field topography were studied using the model. The results show that the coefficient of uniformity, CU, decreases rapidly when the pressure is below the low end of the manufacturer-recommended range; however, CU changes very little with pressure within the manufacturer-recommended range. The system application uniformity, CU_sys, is usually less when pressure variations at different locations in a field are considered, and a simple previously published equation to predict CU_sys is shown to closely approximate the CU from a more stringent calculation method. It was found that the impact of pressure variation (within the tested ranges) on application uniformity is less than that of the sprinkler spacing. Also, the effect of field topography on sprinkler application uniformity is relatively small for the cases tested herein.     

Evaluation of the Penman-Monteith model for estimating soybean evapotranspiration

The Penman-Monteith model with a variable surface canopy resistance (r_cv) was evaluated to estimate hourly and daily crop evapotranspiration (ET_c) over a soybean canopy for different soil water status and atmospheric conditions. The hourly values of r_cv were computed as a function of environmental variables (air temperature, vapor pressure deficit, net radiation) and a normalized soil water factor (F), which varies between 0 (wilting point, _WP) and 1 (field capacity, _FC). The performance of the Penman-Monteith model (ET_PM) was evaluated using hourly and daily values of ET_c obtained from the combined aerodynamic method (ET_R). On an hourly basis, the overall standard error of estimate (SEE) and the absolute relative error (ARE) were 0.06 mm h^–1 (41 W m^–2) and 4.2%, respectively. On a daily basis, the SEE was 0.47 mm day^–1 and the ARE was 2.5%. The largest disagreements between ET_PM and ET_R were observed, on the hourly scale, under the combined influence of windy and dry atmospheric conditions. However, this did not affect daily estimates, since nighttime underestimations cancelled out daytime overestimations. Thus, daily performances of the Penman-Monteith model were good under soil water contents ranging from 0.31 to 0.2 (_FC and _WP being 0.33 and 0.17, respectively) and LAI ranging from 0.3 to 4.0. For this validation period, calculated values of r_cv and F ranged between 44 s m^–1 and 551 s m^–1 and between 0.19 and 0.88, respectively.Communicated by R. Evans     

Response of alfalfa (<Emphasis Type="Italic">Medicago sativa</Emphasis> L.) to diurnal and nocturnal saline sprinkler irrigations. I: total dry matter and hay quality

Little information is available on the quantitative effects on crops of saline sprinkler irrigations and the presumable beneficial effects of nocturnal versus diurnal irrigations. We measured crude protein content, carbon isotope discrimination and total dry matter (TDM) of alfalfa (Medicago sativa L.) subject to diurnal and nocturnal saline sprinkler irrigations. The work was carried out in Zaragoza (Spain) during the 2004–2006 growing seasons with a triple line source sprinkler system using synthetic saline waters dominated by NaCl with an irrigation water EC ranging from 0.5 to 5.6 dS m⁻¹. The quality of alfalfa hay assessed through its crude protein concentration was not significantly affected by salinity. Carbon isotope discrimination, an indicator of the effect of osmotic stress on plant water status, tended to decrease with increases in salinity. Based on a piecewise linear response model, alfalfa grown under saline sprinkler irrigation was shown to be more tolerant (threshold soil salinity, EC_e = 3.5 dS m⁻¹) than in previous experiments under surface irrigation (threshold EC_e = 2.0 dS m⁻¹) at relatively low salinity values, but became more sensitive at higher salinity values as shown by the higher absolute slope (13.4%) for sprinkler as compared to surface irrigation (7.3%). No significant differences in TDM were found between diurnal and nocturnal saline sprinkler irrigations. The recommended practice of irrigating at night for sprinkler irrigation using saline water is therefore not supported by our results in alfalfa grown under semiarid conditions.     

Spatial and temporal distributions of nitrogen and crop yield as affected by nonuniformity of sprinkler fertigation

Deep percolation and nitrate leaching are important considerations in the design of sprinkler systems. Field experiments were therefore conducted to investigate the influence of nonuniformity of sprinkler irrigation on deep percolation and spatial distributions of nitrogen and crop yield during the growing season of winter wheat at an experiment station in Beijing, China. Three experimental plots of a sandy clay loam soil in the 0–40 cm depth interval and a loamy clay soil below 40 cm were irrigated with a sprinkler irrigation system that had a seasonal averaged Christiansen irrigation uniformity coefficient (CU) varying from 72 to 84%. Except for the fertilizer applied before planting, fertilizer was applied with the sprinkler irrigation system. The corresponding seasonal averaged CU for fertigation varied from 71 to 85%. Daily observation of matrix water potentials in the root zone showed that little deep percolation occurred. Consequently, the effect of sprinkler uniformity on deep percolation was minor during the irrigation season for the soil tested. Intensive gravimetric soil core samplings were conducted several times during the irrigation season in a grid of 5 m × 5 m for each plot to determine the spatial and temporal variation of NH₄-N and NO₃-N contents. Soil NH₄-N and NO₃-N exhibited high spatial variability in depth and time during the irrigation season with CU values ranging from 23 to 97% and the coefficient of variation ranging from 0.04 to 1.06. A higher uniformity of sprinkler fertigation produced a more uniform distribution of NH₄-N, but the distribution of NO₃-N was not related to fertigation. Rather it was related to the spatial variability of NO₃-N before fertigation began. At harvest, the distribution of dry matter above ground, nitrogen uptake, and yield were measured and the results indicated that sprinkler fertigation uniformity had insignificant effects on the parameters mentioned above. Field experimental results obtained from this study suggest that sprinkler irrigation if properly managed can be used as an efficient and environment-friendly method of applying water and fertilizers.     

The effect of soil surface temperature on the crop water stress index

Summary A coupled soil-vegetation energy balance model which treats the canopy foliage as one layer and the soil surface as another layer was validated againt a set of field data and compared with a single-layer model of a vegetation canopy. The two-layer model was used to predict the effect of increases in soil surface temperature (T _s ) due to the drying of the soil surface, on the vegetation temperature (T _v ). In the absence of any change in stomatal resistance the impact of soil surface drying on the Crop Water Stress Index (CSWI) calculated from T _v was predicted. Field data came from a wheat crop growing on a frequently irrigated plot (W) and a plot left un watered (D) until the soil water depletion reached 100 mm. Vegetation and soil surface temperatures were measured by infrared thermometers from tillering to physiological maturity, with meteorological variables recorded simultaneously. Stomatal resistances were measured with a diffusion porometer intensively over five days when the leaf area index was between 5 and 8. The T _v predicted by the single-layer and the two-layer models accounted for 87% and 88% of the variance of measured values respectively, and both regression lines were close to the 11 relationship. Study of the effect of T _s on the CWSI with the two-layer model indicated that the CWSI was sensitive to changes in T _s . The overestimation of crop water stress calculated from the CWSI was predicted to be greater at low leaf area indices and high levels of stomatal resistance. The implications for this bias when using the CWSI for irrigation scheduling are discussed.List of Symbols C Sensible heat flux from the soil-vegetation system (W m^–2) - c _{l shade} Mean stomatal conductance of the shaded leaf area (m s^–1) - c _{l sun} Mean stomatal conductance of the sunlit leaf area (m s^–1) - c _max Maximum stomatal conductance (m s^–1) - c ₀ Minimum stomatal conductance (m s^–1) - C _p Specific heat at constant pressure (J kg^–1 °C^–1) - C _s Sensible heat flux from the soil (W m^–2) - C _v Sensible heat flux from the vegetation (W m^–2) - c _v Bulk stomatal conductance of the vegetation (m s^–1) - CWSI Crop Water Stress Index (dimensionless) - e _a Vapor pressure at the reference height (kPa) - e _b Vapor pressure at the virtual source/sink height of heat exchange (kPa) - e ₀ ^* Saturated vapor pressure at T ₀ (kPa) - e _s Vapor pressure at the soil surface (kPa) - e _v ^* Saturated vapor pressure at T _v (kPa) - G Soil heat flux (Wm^–2) - GLAI Green leaf area index (dimensionless) - GLAI_shade Green shaded leaf area index (dimensionless) - GLAI_sun Green sunlit leaf area index (dimensionless) - k Extinction coefficient for photosynthetically active radiation (dimensionless) - k ₁ Damping exponent for Eq. A 5 (m² W^–1) - LAI Leaf area index (dimensionless) - LE Latent heat flux from the soil-vegetation system (W m^–2) - LE _s Latent heat flux from the soil (W m^–2) - LE _v Latent heat flux from the vegetation (W m^–2) - p _a Density of air (kg m^–3) - PAR_a Photosynthetically active radiation above the canopy (W m^–2) - PAR_u Photosynthetically active radiation under the canopy (W m^–2) - r _a Aerodynamic resistance (s m^–1) - r _b Heat exchange resistance between the vegetation and the adjacent air boundary layer (s m^–1) - r _c Bulk stomatal resistance of the vegetation (s m^–1) - R _n Net radiation above the canopy (W m^–2) - R _s Net radiation flux at the soil surface (W m^–2) - r _st Mean stomatal resistance of leaves in the canopy (s m^–1) - R _v Net radiation absorbed by the vegetation (W m^–2) - r _w Heat exchange resistance between the soil surface and the boundary layer (s m^–1) - S Photosynthetically active radiation on the shaded leaves (W m^–2) - S _d Diffuse photosynthetically active radiation (W m ^–2) - S ₀ Photosynthetically active radiation on a surface perpendicular to the beams (W m^–2) - T _a Air temperature at the reference height (°C) - T _b Temperature at the virtual source/sink height of heat exchange (°C) - T ₀ Aerodynamic temperature (°C) - T _s Soil surface temperature (°C) - T _v Vegetation temperature (°C) - w ₀ Single scattering albedo (dimensionless) - Psychrometric constant (kPa °C) - ₀ Cosine of solar zenith angle (dimensionless)     

Determining water–yield relationship,water use efficiency,crop and pan coefficients for silage maize in a semiarid region

A field study was conducted to determine effects of seasonal deficit irrigation on plant cob, leaf, stem and total fresh yield, plant height and water use efficiency (WUE) of silage maize for a 2-year period in the semiarid region. In addition, the crop and pan coefficients k _c and k _p of silage maize were determined in full irrigation conditions. Irrigations were applied when approximately 50% of the usable soil moisture was consumed in the effective rooting depth at the full irrigation treatment. In deficit irrigation treatments, irrigations were applied at the rates of 80, 60, 40, 20 and 0% of full irrigation treatment on the same day. Irrigation water was applied by hose-drawn traveler with a line of sprinklers. Increasing water deficits resulted in a relatively lower cob, leaf, stem and total fresh yields. The linear relationship between evapotranspiration and total fresh yield were obtained. Similarly, WUE was the highest in full irrigation conditions and the lowest in continuous stress conditions. According to the averaged values of 2 years, yield response factor (k _y) was 1.51 for silage maize. When combined values of 2 years, seasonal pan coefficient (k _p) and seasonal crop coefficient (k _c) were determined as 0.84 and as 1.01 for silage maize, respectively.     

基于弹道理论有风条件下折射式喷头喷灌均匀度研究

为计算有风条件下折射式喷头水量分布及喷灌均匀度,以弹道轨迹理论为基础,依据风速分布模型,建立有风条件下折射式单喷头水量分布计算方法,采用该方法模拟出有风条件下Nelson D3000型喷头倒挂安装方式下水量分布特性,通过与实测资料进行对比,验证了模拟具有较高的准确度,可应用于有风条件下折射式喷头水量分布计算。在此基础上,选用4.76 mm(24号)喷嘴直径,模拟出不工况下单喷头水量分布,计算出组合情况下喷灌均匀度,分析了风速、风向、喷头间距、工作压力和安装高度5种因素对喷灌均匀度的影响,并对蒸发漂移损失进行了分析。结果表明:95%的置信区间下,喷头布置间距对喷灌均匀度的影响最显著,其次是安装高度和喷头工作压力,风速和风向对喷灌均匀度影响不显著。风速、喷头工作压力和安装高度都会对蒸发漂移损失产生影响,其中工作压力影响最大。当选用Nelson D3000型喷头在风速小于6 m/s的环境下喷灌时,应将喷头安装间距固定在2.13~3.04 m范围内。另外,该安装间距范围内,喷头安装高度和喷灌压力增大后,喷灌均匀度增大的效果不明显,因此应采用低压喷灌以降低喷灌系统运行成本;考虑到较高的喷头安装高度会产生较大的蒸发漂移损失,喷灌时还应适当降低喷头安装高度,以提高喷灌水分利用率。     

Evapotranspiration and crop coefficients of irrigated biomass sorghum for energy production

New cultivars of sorghum for biomass energy production are currently available. This crop has a positive energy balance being irrigation water the largest energy consumer during the growing cycle. Thence, it is important to know the biomass sorghum water requirements, in order to minimize irrigation losses, thus saving water and energy. The objective of this study was to quantify the water use and crop coefficients of irrigated biomass sorghum without soil water limitations during two growing seasons. A weighing lysimeter located in Albacete (Central Spain) was used to measure the daily biomass sorghum evapotranspiration (ET_c) throughout the growing season under sprinkler irrigation. Seasonal lysimeter ET_c was 721 mm in 2007 and 691 mm in 2010. The 4 % higher ET_c value in 2007 was due to an 8 % higher evaporative demand in that year. Maximum average K _c values of 1.17 in 2007 and 1.21 in 2010 were reached during the mid-season stage. The average K _c values for the 2 years of study were K _c-ini: 0.64 and K _c-mid: 1.19. The seasonal evaporation component was estimated to be about 18 % of ET_c. The average basal K _c (K _cb) values for the two study years were K _cb-ini: 0.11 and K _cb-mid: 1.17. The good linear relationship found between K _cb values and the fraction of ground cover (f _c) and the excellent agreement found between Normalized Difference Vegetation Index and different biophysical parameters, such as K _cb and f _c, will allow monitoring and estimating the spatially distributed water requirements of biomass sorghum at field and regional scales.