Response of tall fescue to irrigation water salinity,leaching fraction,and irrigation frequency

A long-term study in the rhizotron at the U.S. Salinity Laboratory established the yield and evapotranspiration of tall fescue as a function of irrigation water salinity, leaching fraction, and irrigation frequency. As the salt concentration of the irrigation water increased or leaching fraction decreased, dry matter production was reduced significantly. Differences in production because of irrigation frequency, however, were insignificant. With low stress (high leaching, L = 0.27, and low salinity water, S = 1 dS/m) annual dry matter yields were 2.0 kg/m², compared to annual yields of 1.4 kg/m² with high stress (low leaching, L = 0.09, and high salinity water, S = 4 dS/m).Annual evapotranspiration dropped from 1860 mm for low stress treatments to 1170 mm for high stress. Soil evaporation was negligible for the mature grass stand. In concurrence with several models, relative dry matter production was proportional to relative water use.The salt tolerance of treatments dominated by osmotic potential was in agreement with that published for tall fescue. As matric potential decreased among treatments yields fell significantly below that predicted by the salt tolerance model.     

Prediction of surface irrigation advance using soil intake properties

A simple method for predicting surface irrigation advance trajectories using infiltration parameters and inflow rate as inputs was developed. The difference between the inflow rate and the sum of infiltration rates over the wetted portion of the field equals the flow rate available for advance. An average (characteristic) infiltration rate ahead of the wet portion is computed using a fixed time step. An advance step (for a fixed time step) is calculated from the ratio of the flow rate available for advance and characteristic infiltration rate. Predictions of advance by the proposed method were compared with field observations, with the kinematic wave model, and with analytical solutions of Philip and Farrell (1964). In all cases, the method provided predictions that were in good agreement with field observations, and performed similarly to the kinematic wave model. The method offers a simple and efficient tool for prediction and evaluation of surface irrigation systems under various soil types and variable inflow rates. The method is particularly useful for predictions in fields with spatially and temporally variable intake properties.     

Determining leaching fraction from field measurements of soil electrical conductivity

Theory for determining leaching fraction from measurements of soil electrical conductivity is presented with experimental data to substantiate its validity.     

Predicting soil salinity in response to different irrigation practices, soil types and rainfall scenarios

A model was developed to predict rootzone salinity under different irrigation practices on different soil types, with similar rainfall but different monthly distributions. A rootzone daily water and salt balance was performed using eight scenarios: two soil types (coarse textured vs. fine textured), two multi-year series of actual rainfall data and two irrigation practices (surface with fixed number of irrigations and ET-based sprinkler irrigation). All factors influenced the mean electrical conductivity (EC) of the rootzone in the growing season (EC_eS): (i) Surface irrigation led to lower EC_eS than sprinkler irrigation; (ii) Winter-concentrated rainfall caused lower EC_eS than rainfall distributed uniformly throughout the year; and (iii) Coarser-textured soil usually resulted in lower EC_eS than the finer textured. The EC_eS was related to the total precipitation of the hydrologic year and to the annual leaching fraction (LF) but surprisingly not to the seasonal LF. In most cases, the model predicted lower EC_eS than the FAO steady-state approach. Therefore, considering these site-specific features could lead to lower leaching requirements and the safe use of higher salinity water.     

Estimations of permissible irrigation rates from soil properties in high frequency irrigation

The effect of irrigation dynamics and soil physical properties on the permissible rate of application was analyzed using the analytic solution of Richard’s equation for periodic flux type boundary conditions typical to high frequency irrigation. Dynamics of irrigation regime was defined by its instantaneous application rate, R ₀, its frequency, and the duty cycle which is the ratio of irrigation duration and irrigation interval (period). Soil properties were saturation, hydraulic conductivity, diffusivity, and Gardner’s soil type coefficient. Fourier Transform was used to derive a closed form analytical expression for the maximal permissible value of R ₀, which would not result in water logging and saturation at soil surface. The analytical expression ties the three irrigation parameters with the above three soil parameters together. Prevention of aeration stress by restricting the moisture content at the soil surface, to become less than the minimal air pore volume (drainable porosity), was also used as an upper constraint of moisture at the surface soil. The effect of irrigation frequency and duty cycle on the permissible R ₀ values was analyzed and computed regarding three soil types: coarse sand, sand, and sandy loam. Under short duration periods of water application resulting from either small values of duty cycle or short irrigation periods (high frequency), or both, the soil surface would not become saturated even for very high R ₀ values. The maximal application depth V _max, depends on both the duty cycle and the frequency. For a given soil, V _max remains essentially constant per irrigation periods of 1 h or less, typical to pulsed irrigation and independent of the duty cycle. For periods longer than 1 day for coarse soils, V _max increases with the duty cycle while for sandy loam soils, the increase in V _max becomes insignificant for duty cycles larger than 0.3. The computed values of maximal R ₀ and V _max based on the water logging concept are much higher than the ones used in irrigation design and should be considered as upper permissible limits only. The permissible values of R ₀ and V _max are much lower if the aeration stress concept is adopted and found in the range used in practice. This analysis may be useful for the design of precision irrigation for high frequency microdrip, high frequency trickle irrigation, as well as for trunk diameter measurement (TDM) irrigation methods.     

灌溉水质和灌水方式对红壤斥水性及其理化性质的影响

以蒸馏水为对照,选取再生水和稀释2,4,6倍再生水等4种低质水,采用连续灌溉和再生水-蒸馏水交替灌溉等2种灌溉方式对红壤进行1年的处理,以此探明灌溉水质和灌水方式对酸性土壤斥水性、pH、盐分及有机质质量比的影响.结果表明:2种灌溉方式下红壤均产生亚临界斥水性,连续灌溉下红壤接触角比交替灌溉平均增加了23.1°;2种灌溉方式均使红壤酸性增强,盐分和有机质质量比增加,其中连续灌溉下的增长率大于交替灌溉下增长率;连续灌溉下红壤斥水性、pH、盐分和有机质质量比随灌溉水质的变化均可用二次函数来描述,决定系数均在0.9以上;交替灌溉下,红壤斥水性和盐分与灌溉水质的关系可用二次函数来表示,决定系数均在0.9以上,而土壤pH和有机质质量比与灌溉水质相关性不明显;2种灌溉方式下,红壤斥水性与pH呈负相关,与盐分质量比呈正相关,与土壤有机质质量比在连续灌溉下表现为正相关,而交替灌溉下则相反.采用再生水-蒸馏水交替灌溉方式能抑制土壤斥水性的产生.     

Nitrogen leaching during sprinkler irrigation of a Dutch clay soil

Short-circuiting, which is vertical movement of free water through large continuous pores in an unsaturated soil matrix, was measured in the field in large columns from a cracked Dutch clay soil. The columns had been fertilized with chemical nitrogen fertilizer at a rate of 80 kg N ha^?1. Sprinkler irrigation (with an average intensity of 18 mm h^?1 and applied quantities varying from 8 mm to 22 mm) resulted in strong nitrogen leaching from the columns. Losses, which averaged 30%, resulted from mass-flow due to short-circuiting. Redox measurements suggested that no denitrification occurred. Application of only 6 mm of water slightly reduced nitrogen losses to approximately 15%. Losses could be reduced to 8% by applying the fertilizer to a wet soil surface which had just been sprinkled, and by sprinkling again the next day. In that way, the fertilizer grains dissolve and nitrogen diffuses into the surface soil, allowing less nitrogen movement along the soil surface towards the vertical cracks during the next sprinkling.     

Induction of soil water repellency following irrigation with treated wastewater: effects of irrigation water quality and soil texture

We hypothesized that organic matter (OM) content originating from treated wastewater (TWW) irrigation and soil texture dominate the intensity of soil water repellency. The relationship between soil texture, wastewater treatment level, and water repellency was examined in a 3-year lysimeter experiment (2008–2010). Soil type–water quality combinations, consisting of three soils with different specific surface area (SSA) and four levels of water quality differing in OM content, were tested. In each year, water repellency developed in all TWW quality treatments, but not in freshwater-irrigated controls. At the end of each year (except 2009), the highest degree of repellency was exhibited by sandy soil treated with the lowest quality TWW (highest OM content). The lowest degree of water repellency was consistently exhibited by the soil with the highest SSA irrigated with the highest quality TWW (lowest OM content). Water quality, rather than SSA, was the dominant factor in determining degree of repellency induced by TWW irrigation.     

Correcting soil water balance calculations for dew, fog, and light rainfall

In Mediterranean climates, adoption and use of the ET-based scheduling method is limited to regions characterized by considerable contributions to evapotranspiration from fog interception, dew, and light rainfall. While the crop evapotranspiration is often accurately estimated, the water balance is frequently in error because a considerable portion of the energy expended is used to vaporize water from the plant surfaces rather from inside the leaves (i.e., transpiration). Growers in regions with considerable fog, dew, and light rainfall are hesitant to use ET-based scheduling because the cumulative crop evapotranspiration between irrigations is often considerably higher than the soil water depletion. A correction for these surface contributions is clearly needed to improve the water balance calculations and to enhance adoption of the ET-based scheduling method. In this paper, we present a simple, practical method to estimate the contribution of fog interception, dew, and light rainfall to daily crop evapotranspiration, and we show how to use the method to improve water balance calculations.     

石羊河流域咸水灌溉对土壤物理性质的影响

为揭示咸水灌溉对土壤水力特性的影响机制,探求西北内陆干旱地区的合理灌溉模式,对石羊河流域中游地区开展制种玉米咸水灌溉田间试验,通过设置4种灌溉水矿化度水平(0.71,3.00,6.00,9.00 g/L),研究土壤盐分对土壤容重、孔隙度、质地和饱和导水率的影响.研究结果表明:对土壤进行咸水灌溉会引起土壤容重、孔隙度的改变,从而改变土壤饱和导水率.随着灌水次数的增多,各个处理土壤容重都越来越大,孔隙度越来越小,从而使得饱和导水率越来越小.此外咸水灌溉还会引起土壤质地的改变,特别是黏粒含量的增大,会使土壤饱和导水率减小.研究成果对咸水资源的高效利用及农业的可持续发展,具有重要的理论意义和实用价值.     

Simulation of bromide and nitrate leaching under heavy rainfall and high-intensity irrigation rates in North China Plain

Heavy rainfall and irrigations during the summer months in the North China Plain may cause losses of nitrogen because of nitrate leaching. The objectives of this study were to characterize the leaching of accumulated N in soil profiles, and to determine the usefulness of Br⁻ as a tracer of surface-applied N fertilizer under heavy rainfall and high irrigation rates. A field experiment with bare plots was conducted near Beijing from 5 July to 6 September 2006. The experiment included three treatments: no irrigation (rainfall only, I0), farmers’ practice irrigation (rainfall plus 100 mm irrigation, I100) and high-intensity irrigation (rainfall plus 500 mm irrigation, I500), with three replicates. Transport of surface-applied Br⁻ and NO₃⁻ (assuming no initial NO₃⁻ in the soil profile) and accumulated NO₃⁻ in soil profiles were all simulated with the HYDRUS-1D model. The model simulation results showed that Br⁻ leached through the soil profile faster than NO₃⁻. When Br⁻ was used as a tracer for surface-applied N fertilizer to estimate nitrate leaching losses, the amount of N leaching may be overestimated by about 10%. Water drainage and nitrate leaching were dramatically increased as the irrigation rate was increased. The amounts of N leaching out of the 2.1-m soil profile under I0, I100 and I500 treatments were 195 ± 84, 392 ± 136 and 612 ± 211 kg N ha⁻¹, equivalent to about 20 ± 5%, 40 ± 6% and 62 ± 7% of the accumulative N in the soil profile, respectively. N was leached more deeply as the irrigation rate increased. The larger amount of initial accumulated N was in soil profile, the higher percentage of N leaching was. N leaching was also simulated in summer under different weather conditions from 1986 to 2006. The results indicated that nitrate leaching in rainy years were significantly higher than those in dry and normal years. Increasing the irrigation times and decreasing the single irrigation rate after fertilizer application should be recommended.     

Effects of wastewater irrigation on soil sodicity and nutrient leaching in calcareous soils

Soil column studies were conducted with two soils to assess the effects of irrigation with wastewater on soil and groundwater quality. Upon the application of wastewater, exchange occurred between solution sodium (Na⁺) and exchangeable cations (Ca²⁺, Mg²⁺, K⁺), whereby these cations were released into solution. The average exchangeable sodium percentage (ESP) of the soils increased during leaching from 9 to 21 and 28.8 to 29.7 after applying 5.0 and 3.5 l (about 7 and 6 pore volumes) of wastewater to the soils columns, respectively. Adverse effect of high Na⁺ concentration in the wastewater on raising ESP was less pronounced in the soil having initial high ESP than in the soil with low initial ESP. Salinity of the soils was also increased with the application of wastewater and Mg²⁺ and K⁺ were leached from the soils. These losses would be more severe on soils having a low cation exchange capacity and if, uncorrected could lead eventually to their deficiencies for plant growth. When the soil columns were leached with distilled water the flow rate of one soil decreased to zero after 2.2 pore volume indicating damage to soil structure. Irrigation with wastewater, which is generally more sodic and saline than regional groundwater, increases the rate of soil sodification of shallow groundwater. A relatively simple chromatographic model was used to estimate final ESP profiles in the soils assuming the condition of local equilibrium. This approach had a limited success for one of the soil. Since the final leached concentrations are in good agreement with those of wastewater, we attribute these differences to non-uniform flow through the column. In terms of practical soil and water management, our study reveals that relatively simple means can be useful to predict the water quality in soils, their discharge to ground water, and the hazard of soil structure deterioration.     

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

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

滴灌水温对土壤入渗和土壤温度的影响

为了解水温对滴灌土壤入渗特征和土壤温度的影响,研制一套恒温试验装置,可使水温变化控制在±0.5 ℃范围内,选择5,20,35 ℃作为试验水温,进行不同水温室内滴灌入渗试验,分析各水温下土壤水分入渗和土壤温度变化特征.结果表明:在相同时段内,随滴灌水温升高,水平和垂直湿润锋运移距离增大,垂直湿润锋运移速率增大.分别建立水平、垂直湿润锋运移距离与入渗时间和滴灌水温的关系模型,决定系数R²均大于0.99.湿润土体平均含水量与入渗时间关系不大,但随入渗水温的升高而减小.土壤水分扩散率与水温成正比;水温升高,饱和导水率随之增大,二者呈指数函数关系;土壤吸持水分的能力随温度的升高而降低.不同灌溉水温改变了土体中的温度分布,随着距滴头距离的增加,由水温引起的土壤温度的变化量逐渐减小.结论可为指导大田和温室滴灌技术提供理论依据.     

Effect of irrigation methods,management and salinity of irrigation water on tomato yield,soil moisture and salinity distribution

The increasing demand for irrigation water to secure food for growing populations with limited water supply suggests re-thinking the use of non-conventional water resources. The latter includes saline drainage water, brackish groundwater and treated waste water. The effects of using saline drainage water (electrical conductivity of 4.2–4.8 dS m⁻¹) to irrigate field-grown tomato (Lycopersicon esculentum Mill cv Floradade) using drip and furrow irrigation systems were evaluated, together with the distribution of soil moisture and salt. The saline water was either diluted to different salinity levels using fresh water (blended) or used cyclically with fresh water. The results of two seasons of study (2001 and 2002) showed that increasing salinity resulted in decreased leaf area index, plant dry weight, fruit total yield and individual fruit weight. In all cases, the growth parameters and yield as well as the water use efficiency were greater for drip irrigated tomato plants than furrow-irrigated plants. However, furrow irrigation produced higher individual fruit weight. The electrical conductivity of the soil solution (extracted 48 h after irrigation) showed greater fluctuations when cyclic water management was used compared to those plots irrigated with blended water. In both drip and furrow irrigation, measurements of soil moisture one day after irrigation, showed that soil moisture was higher at the top 20 cm layer and at the location of the irrigation water source; soil moisture was at a minimum in the root zone (20–40 cm layer), but showed a gradual increase at 40–60 and 60–90 cm and was stable at 90–120 cm depth. Soil water content decreased gradually as the distance from the irrigation water source increased. In addition, a few days after irrigation, the soil moisture content decreased, but the deficit was most pronounced in the surface layer. Soil salinity at the irrigation source was lower at a depth of 15 cm (surface layer) than that at 30 and 60 cm, and was minimal in deeper layers (i.e. 90 cm). Salinity increased as the distance from the irrigation source increased particularly in the surface layer. The results indicated that the salinity followed the water front. We concluded that the careful and efficient management of irrigation with saline water can leave the groundwater salinity levels unaffected and recommended the use of drip irrigation as the fruit yield per unit of water used was on average one-third higher than when using furrow irrigation.     

Influence of irrigation water quality on soil infiltration

The quality of municipal waste water is affected by its chemical composion and by the process used to treat it. If suspended solids limit infiltration, their removal by the municipal treatment facility is a factor to consider in the planning and management of water resources. The effect of water quality and sprinkler water application rate on bare soil infiltration was investigated. Treated wastewater and clean water were applied to a bare soil at two different application rates, 25 mm/hr and 100 mm/hr. The change in each soil infiltration rate after eight runs for each water quality was determined. The results showed that the reduction in infiltration rate was greatest for the soil under the treated wastewater application due to the effect of accumulation of suspended solids at the soil surface.     

Implications of precipitation patterns and antecedent soil water content for leaching of pesticides from arable land

An improved understanding of how precipitation patterns control pesticide leaching from structured soils prone to macropore flow could lead to practical mitigation strategies that would help farmers minimize losses by optimizing application timings. A sensitivity analysis of the macropore flow model MACRO was therefore carried out to examine the influence of antecedent soil water content and precipitation patterns on pesticide leaching to drainage systems and groundwater. One thousand model runs were executed (20 four-year weather data series, 50 application dates per season) for both autumn and spring applications of a hypothetical moderately sorbed and quickly degraded herbicide for one of three national scenarios for pesticide risk assessment in Sweden (Näsbygård, a loamy moraine soil in Scania, southern Sweden). Rapid and direct transport of pesticides in macropores to drainage systems and shallow groundwater was predicted to occur rather infrequently in spring (in 4 of the 20 years) and even more rarely in autumn. For autumn applications, the soil water deficit at application (SWD_tot) and medium-term precipitation (30–90 days after application) were the two most sensitive variables controlling pesticide leaching. For spring applications, total leaching was most closely linked to rainfall the following winter, while short-term precipitation (5 days after application) and the antecedent soil water deficit were the two most important predictors for maximum pesticide concentrations in drainflow. The potential for reducing leaching by restricting applications to periods of low risk was investigated. The results showed that avoiding applications on wet soil in autumn could potentially reduce total pesticide losses by a factor of two to three. Similarly, the risk of acute toxicological effects in surface waters following pesticide applications in spring could be reduced by a factor of 2–3 by avoiding application when 5-day weather forecasts predict precipitation >10 mm.     

Determination of soil hydrologic properties under simulated rainfall condition

Two important soil hydrologic properties viz. soil water retention (h-θ relationship) and unsaturated hydraulic conductivity (K-θ relationship), were determined under simulated rainfall conditions. The h-θ relationship was determined using a rainfall simulator infiltrometer (RSI). The resulting h-θ relation was then used as input to the Van Genuchten's model (VGM) for determining the K-θ relationship. In order to validate the results obtained through RSI-VGM combination, the commonly adopted instantaneous profile method (IPM) was also applied to develop the K-θ relationship independently. Functional sensitivity analysis, conducted to simulate the soil water storage using the model Soil Water Actual Transpiration Rate (SWATR), showed that the simulated results obtained through RSI-VGM combination were in agreement with those from IPM.     

Nitrate leaching and soil nitrate content as affected by irrigation uniformity in a carrot field

High value crops such as carrot planted in coarse soils of the Southern San Joaquin Valley in California are prime candidates for nitrate leaching through irrigation nonuniformity. A 2-year study was carried out to explore the impact of irrigation uniformity on nitrate leaching. Irrigation uniformity was measured using catchcans. Soil nitrate (NO₃-N) and ammonium (NH₄-N) contents were measured from soil sampled at different depths and times during two growing seasons. Nitrate leaching was determined using ion-exchange resin bags at 1-m depth sampled three times during each season. Although, soil NO₃-N as well as seasonal irrigation was significantly higher along the lateral irrigation pipe than between the sprinklers, nitrate leaching was not significantly higher. As expected, soil nitrate content decreased as percolation increased for both years. Nitrate leaching, as estimated by anion-exchange resin bags, was positively correlated to soil NO₃-N content but was not correlated to irrigation depth, irrigation uniformity, or deep percolation. Field variation in saturated hydraulic conductivity (K_s), soil organic matter (OM), and soil water retention at field capacity had limited effect on NO₃-N and NH₄-N distributions in the profile and on nitrate leaching. The results of this experiment suggest that irrigation nonuniformity has less impact on nitrate movement than suggested by earlier studies.     

降水预报准确率对变量灌溉水分管理的影响

为充分利用降水,提高变量灌溉水分管理精度,将降水预报信息与变量灌溉制度结合生成变量灌溉处方图,在评估2016年和2017年不同降水等级预报准确率的基础上,研究了未来3 d降水预报信息对冬小麦和夏玉米变量灌溉制度的影响.结果表明,两年冬小麦和夏玉米生育期内,预见期1,2,3 d的降水预报信息准确率差异不大,无雨预报准确率最高,两年平均为83.3%,大雨和暴雨次之,为51.7%,中雨预报准确率最差,为23.0%.结合降水预报信息制定变量灌溉制度,2016年冬小麦和2017年夏玉米生育期内分别减少灌水量8 mm和16 mm.根据目前预报信息准确率,建议无雨和小雨预报时,直接按设计灌水定额灌溉;中雨预报时,可采用灌水定额的80%实施灌溉;大雨和暴雨预报时,可适当推迟灌溉.