Direct evaporation from soil under a row crop canopy

A simplified measurement of the soil evaporation (E) component of evapotranspiration (ET) is needed to obtain independent measurements of transpiration (T) and to evaluate the effects of E and T on ET. Our objective in this study was to evaluate the use of small lysimeters placed under a crop canopy to measure the E component. Lysimeters were constructed of rigid PVC pipe sections, 20.3 cm in diameter and 20, 10, or 5 cm long. Water loss from the lysimeters was recorded daily. The water content of the soil surrounding the lysimeters was measured gravimetrically from composite 1-cm-increment cores sampled daily. The results reported are for two drying cycles of 16 and 13 days in July 1975 and 1976. In order for the lysimeters to behave as the surrounding soil, the water content of the lysimeters must be higher than the soil outside to compensate for changes under the natural conditions due to plant uptake, drainage and upward flow. Since the lysimeters depend on a set of compensating factors to directly measure E, estimates of E from them should be used with caution. A better use of the lysimeters would be to establish a relationship between lysimeter E and the surface soil water content and then use surface water content measurements to infer E.     

作物和土壤旱情的温差模型

本文根据邯郸地区的观测资料,建立了小麦、玉米和棉花地的土壤相对含水量和14时冠气温差(Tc-Ta)的作物旱情温差模型RW=a+b(Tc-Ta)_(14)和裸地土壤相对含水量和14时土气温差(Ts—Ta)的土壤旱情温差模型RW=a′+b′(Ts-Ta)_(14)。经检验,平均相对误差小于20％,适于灌溉管理和大面积监测旱情之用,具有快速、简便的特点。本文还提出了冠层温度的覆盖度校正方法和进行旱涝等级划分的温差指标。     

A three-resistance model of crop and forest evaporation

Summary The evaporation of deep crops such as forests is usually considered in terms of the two-resistance Penman-Monteith model, though this conflates two of the three resistances actually involved, i.e. the canopy resistancer _c between the transpiring leaves and the top of the canopy, and the resistancer _s due to the stomates of the leaves. A review of the literature on these and the aerodynamic resistancer _a (between the crop and the atmosphere) shows how distinctly different they are, and therefore how inappropriate it is to lump any two together.Once the soil has dried substantially,r _s depends approximately onM ^–2, whereM is the fractional available soil moisture.As regards grassed surfaces,r _a is 300/u s m^–1, whereu is the wind speed at 2 m.With 2 Figures     

Quantification of crop yields under rainfed conditions using a simple soil water balance model

Summary The concept of water requirement satisfaction index was used to quantify the crop yield under rainfed conditions, choosing pearl millet as an example. The relationship between the water requirement satisfaction index and yield was observed to beY = exp (— 18.023+4.173 lnx), indicating the exponential behaviour of the yield as affected by the water availability to the crop. The correlation between the observed and expected yields was found to be 0.94. Yields were simulated for the years 1901–85 using the mathematical relationship. Simulation results showed that the years with commencement of rainy season from 25th to 28th week are most suited for millet production in the study area. The maximum crop failures were observed with 7 years per decade during the years 1901–40. The results also showed that the concept ofWRSI can be used in assessing the effects of soil moisture stress on crop yields at field level.

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Zusammenfassung Das Konzept des Water Requirement Satisfaction Index (Wasser-Bedarf-Sättigungs-Index,WRSI) wurde verwendet, um Ernteprognosen ohne Bewässerung am Beispiel von Perlenhirse zu erstellen. Das Verhältnis zwischenWRSI und Ernteertrag konnte dabei alsy = exp (— 18.023+4.173 lnx) beschrieben werden, was auf eine Beeinflussung der exponentiellen Entwicklung des Ernteertrags durch die Menge verfügbaren Wassers hinweist. Die Übereinstimmung zwischen erwarteten und beobachteten Erträgen lag bei 0.94. Die Ernteerträge für die Jahre 1901–1985 wurden damit simuliert. Die Ergebnisse dieser Simulation zeigten, daß Jahre, in denen der Beginn der Regenzeit zwischen die 25. und 28. Woche fällt, am besten zur Hirseproduktion in der untersuchten Gegend geeignet scheinen. Maximale Ernteausfälle wurden mit sieben Jahren pro Dekade zwischen 1901 und 1940 registriert. Die Ergebnisse zeigen, daß das Konzept desWRSI auch zur Einschätzung von Auswirkungen der Bodenfeuchtigkeitsbelastung auf Ernteerträge auf dem Feld verwendet werden kann.

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With 1 Figure     

Application of the Priestley-Taylor evaporation model to assess the influence of soil moisture on the evaporation from a large weighing lysimeter and class a pan

The influence of soil moisture on evaporation from a 6-m grass-covered lysimeter and from Class A pans was assessed for one summer using the -parameter of the Priestley-Taylor evaporation model appropriate for the individual surfaces computed on a daily basis. Net radiation over the pan was estimated from above-grass measurements using a correlation established between the two, using measurements made in the previous two summers. Changes in heat storage of the water were considered in the derivation of for the pan. A unique relationship for the particular conditions of the site was determined between the for the lysimeter and soil moisture, approaching 1.29 at soil moisture near field capacity, but decreasing to as low as 0.5 for dry soil. The corresponding relationship for the pan showed more scatter, but this was improved by using 5-day running means of evaporation and stratifying the data in terms of wind speed to yield a family of curves. Values for at wet soil conditions varied from 1.07 for 100 km day^–1 wind run to 1.17 for 250 km day^–1 wind run. For each curve, values of increased by about 20%; as the soil dried. The relationships may be used to reduce observed Class A pan evaporation to equivalent values for wet-soil conditions and to estimate near-surface soil moisture and actual evapotranspiration for this particular site. Extension of the technique to other areas requires derivation of similar relationships appropriate for those other locations     

Effects of different long-term crop straw management practices on ammonia volatilization from subtropical calcareous agricultural soil

长期秸秆还田或秸秆焚烧会显著影响土壤肥力及土壤氮素循环,但该措施对土壤氨挥发的影响仍尚不明确。本研究利用秸秆还田长期定位试验小区,研究了无秸秆配施(CK),配施100%或50%秸秆(SI1, SI2)和配施50%秸秆焚烧(SI2B)对土壤氨挥发的影响。结果表明:氨挥发在小麦季持续38天,而玉米季持续7–10天。秸秆还田显著影响混施基肥期的土壤氨挥发而非表施追肥期。与CK相比, SI1和SI2分别降低了35.1%和16.1%的年累积氨排放,可能因为秸秆的高C/N比及较高的微生物活性促进了无机氮的固定降低土壤NH4+的浓度。SI2B比SI2增加了29.9%的氨排放。因此,长期合理的秸秆还田可为石灰性旱地土壤氨挥发减排提供选择和依据。     

Comparison of different methods for estimating soil surface evaporation in a bare field

In this paper, three methods for estimating soil evaporation in a bare field were evaluated: evaporation ratio method (k ratio), complementary relationship and bulk equation. Micro-lysimeters were used to measure the actual evaporation for validation of the three methods. For the k ratio method, pan evaporation was used as the reference evaporation instead of the value obtained from the Penman–Monteith equation. This result is important for areas where meteorological data are unavailable. The results showed that, for daytime evaporation, the k ratio and bulk equation produced a good fit with the observation data, while the complementary relationship generated a larger deviation from the measured data. We recommend that the k ratio method and bulk equation could be used to calculate daytime soil evaporation with high accuracy when soil water content and pan evaporation data or meteorological data are available, while the complementary relationship could be used for a rough estimation when pan evaporation is available. All the methods could be applied to calculate cumulative evaporation.     

Dealing with drought: The challenge of using water system technologies to break dryland poverty traps

We explore strategies among farmers in semi-arid Tanzania to cope with drought, and investigate if access to a local supplemental irrigation system (the Ndiva system) can improve coping capacity. Results show high dependency on local ecosystem services when harvests fail, and indicate that farmers commonly exhaust asset holdings during droughts. Ndiva access did not have any direct effects on coping capacity, but seemed to have some indirect effects. Drawing on our findings we discuss the complexity of escaping persistent dryland poverty, and outline the circumstances under which small-scale water system technologies, such as Ndiva irrigation, may help.     

Study of crop coefficient and the ratio of soil evaporation to evapotranspiration in an irrigated maize field in an arid area of Yellow River Basin in China

A field experiment was conducted in a maize field in 2006 in an arid area of the Yellow River Basin in China. The daytime evapotranspiration (ET_c) and soil evaporation beneath the maize canopy (E _g) were measured by Bowen ratio energy balance method and micro-lysimeters, respectively. The results showed that the total ET_c during maize growth season was 696 mm, and the maximum values occurred at about 90–140 days after sowing. The crop coefficient (K _c), which was calculated from the ratio of ET_c to reference evapotranspiration (ET₀), was quite different from the values reported by other researchers in similar climate areas, with average values of 0.34, 0.47, 1.0 and 0.9 for initial, development, mid-season and late-season stages, respectively. High correlations between leaf area index (LAI) and average K _c for every 4 days were obtained. The total E _g was 201.4 mm with average values ranged from 0.92 to 2.05 for four growth stages of maize; and accounted for around 28.9 % of ET_c. The ratio E _g/ET_c showed high negative relationship with LAI. These results were very important in precise management of irrigation for maize in Yellow River Basin areas.     

Effects of turfgrass evaporation on external temperatures of buildings

Summary Evaporative cooling of air by urban vegetation has been considered to be primarily a mesoscale process. Recent studies, however, indicate that localized evaporation may be equally as effective at lowering air temperatures and may substantially reduce summer cooling loads in buildings. An experimental study was conducted in a moderately humid environment to explore how changes in evaporation and the surface energy balance of turfgrass may affect external temperatures of buildings. Surface temperatures of a building shell and the energy balance of adjacent bermudagrass were measured as evaporation from the grass decreased during two drying cycles. In spite of large decreases in latent heat flux and increases in sensible heat flux from the turfgrass, no systematic changes in wall temperature were observed. If evaporative cooling occurred, it was obscured by other environmental factors. Wall temperatures did vary in response to changes in irradiance and wind speed. Changes in wind speed had a considerable effect on wall temperatures during periods of predominantly free convection. When forced convection was dominant, changes in wind speed had a relatively small effect on wall temperatures. Results support the view that evaporative cooling of air is mainly a mesoscale process, and that changes in evaporation within an individual landscapes are unlikely to have a major effect on building energy balance in many climatic regimes.With 12 Figures     

Modeling plant density and ponding water effects on flooded rice evapotranspiration and crop coefficients: critical discussion about the concepts used in current methods

Theoretical and Applied Climatology - The aim of the study is to propose new modeling approaches for daily estimations of crop coefficient K c for flooded rice (Oryza sativa L., ssp. indica) under...     

Effects of ozone air pollution on crop pollinators and pollination

Human driven environmental changes increase the concentrations of polluting reactive compounds in the troposphere, such as ozone and nitrogen oxides. These changes lead to biodiversity losses and alter plant physiology and plant-pollinator interactions, essential for pollination services, with potential consequences for agricultural production. Here we used 133 unique sampling events from NW Europe to investigate how air pollution (ozone and nitrogen oxides) and other sources of nitrogen is related to pollinator visitation rate and their contribution to agricultural production, also considering possible interactive effects with landscape quality and pesticide input. We showed that ozone modulates the effect of pesticide exposure and temperature on crop pollinators, increasing the probability of negative impacts on crop pollination. Indeed, when ozone levels are highest, the strength of the effect of pesticide on pollinators is more than double then when ozone levels are intermediate. This indicates that air pollution should be considered in management plans and policies aiming to safeguard biodiversity and promote more sustainable food production practices.     

红壤旱地棉田间作小气候效应初步研究

为研究棉田间作对田间小气候的影响,共设置4个处理:棉花单作、棉花间作花生、棉花间作甘薯、棉花间作大豆,研究不同间作棉田群体对不同层次的光强、土壤温度以及棉花农艺性状及产量的影响。结果表明,与棉花单作系统相比,间作系统可通过改变受光结构、土壤温度、土壤含水量等促进棉花生长发育,利于棉花种植获得高产;棉花间作大豆模式是较适宜在红壤旱地推广的间作模式。     

气象因子对棉花纤维品质的影响

以邯棉333和鲁棉研27号为材料,进行了气象因子与棉纤维品质相关性研究。结果表明：纤维指标受气象因子影响大小顺序：比强度大于伸长率大于麦克隆值大于2.5%跨长。从棉纤维整个发育期分析,影响2.5%跨长、比强度和伸长率的主要因子分别为日平均最低温度、平均相对湿度和日较差。影响麦克隆值的主要因子为日平均气温、大于15 ℃有效积温和平均日照时数;从棉纤维发育各阶段分析,整齐度主要受发育初期日平均最低气温影响,麦克隆值和伸长率主要受伸长期气象因子影响;各品质指标与次生壁增厚期气象因子均具有显著相关性,为影响棉纤维品质的关键期;麦克隆值还受成熟期平均相对湿度的影响。研究结果可为棉纤维品质生态区划和品质生态模型定量化提供依据。     

水分胁迫对华北平原冬小麦地上部分及产量的影响

以“济麦-22”为供试品种,利用中国气象局固城生态环境与农业气象试验站大型根系观测系统,研究冬小麦在重度干旱胁迫（≤40.0%）、轻中度干旱胁迫（40.1%-55.0%）和适宜（55.1%-80.0%）3种水分胁迫条件下地上部分对水分胁迫的响应,以探索水分胁迫对华北平原冬小麦产量的影响,分析不同水分胁迫对冬小麦产量的影响程度。结果表明：华北平原冬小麦在轻中度干旱胁迫和重度干旱胁迫下,小麦全生育期的天数缩短,株高、叶面积及灌浆速率均呈不同程度的减少。3种水分胁迫的株高增长量为适宜>轻中度胁迫>重度胁迫,灌浆速率为适宜>轻中度胁迫>重度胁迫。土壤水分胁迫引起冬小麦物质分配更多地向支持生长的茎秆转移,在生长发育过程中受到水分胁迫,小麦产量将降低,重度胁迫条件下小麦产量为适宜水分条件的69%。     

The conductance of a maize crop and the underlying soil to ozone under various environmental conditions

Flux measurements of ozone and water vapour employing the eddy correlation technique were used to determine the surface conductance and canopy conductance to ozone. In the surface conductance to ozone, all surfaces at which ozone is destroyed and the transport process to these surfaces are included. The canopy conductance to ozone represents the ozone uptake of transpiring plant parts. The surface conductance to ozone of the maize crop and the underlying soil was generally larger than the canopy conductance to ozone. This means that beside the uptake by stomata, there was another important ozone sink. Under wet soil surface conditions, the surface conductance and the canopy conductance to ozone coincided. This indicates that the resistance of wet soil and the remaining plant parts (cuticle) to ozone was much larger than the stomatal or soil resistance. On the other hand, under dry soil conditions the conductances differ, largely caused by a variation in the transport process to the soil. The transport of ozone to soil increased with increasing friction velocity (u _*) and decreased with increasing atmospheric stability, leaf area index (LAI) or crop height (h). These effects for midday (unstable) conditions were parameterized with an in-crop aerodynamic resistance,r _inc in a very straightforward way;r _inc=13.9 LAIh/u _*+67 (cc.=0.77). If the ozone flux in air pollution models is described with a simple resistance model (Big Leaf model), the extra destruction at the soil should be modelled using an in-crop aerodynamic resistance. For these measurements the ozone flux to the soil was 0–65% of the total ozone flux measured above the crop. Under wet soil conditions, this was less than 20%; under dry soil conditions, this was 30–65%.     

Measurement of total and condensed water mixing ratios in warm-based cumulus clouds by a jet engine evaporation technique

Summary A technique has been developed for measuring total water mixing ratio and condensed water mixing ratio in warm-based convective clouds. The jet engine compressor on a Learjet 24 is employed as an evaporator to convert all cloud condensate to vapour and the vapour density in the compressed (bleed) air is measured with a Lyman-alpha humidiometer.Sources of error in the measurement are i) a contaminant in the air lines which must be evaluated regularly by baselining the measurement against a dewpoint measurement in clear air; ii) nonisokinetic sampling at the jet inlet, which has been evaluated by monitoring the volume flow rate of air at the inlet.The Lyman-alpha humidiometer has been found to be robust and serviceable, with very slow drift properties. The drift is monitored by introducing dry nitrogen into the measurement cell.

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Zusammenfassung Eine Technik wurde entwickelt, um die Gesamt- und Kondenswassermischungsverhältnisse in konvektiven Wolken mit warmer Basis zu messen. Ein Düsentriebwerkskompressor auf einer Learjet 24 dient als Verdunster, um alles kondensierte Wasser in Dampf umzuwandeln, dessen Dichte in der ausströmenden komprimierten Luft dann mittels eines Lyman-alpha-Feuchtemeßgeräts bestimmt wird.Fehlerquellen sind zum einen die Verunreinigung der Luftstraßen, die regelmäßig ausgewertet werden muß, indem der Hintergrundswert durch Taupunktmessungen in klarer Luft bestimmt wird; zum anderen die nicht-isokinetische Probenentnahme am Triebwerkseinlaß, die aus der Volumsflußrate der Luft am Einlaß bestimmt wird.Das Lyman-alpha-Feuchtemeßgerät erwies sich als robust und wartungsextensiv, u. a. aufgrund seiner geringen Drift. Die Drift wird mittels in die Meßkammer eingeführten trokkenen Stickstoffes aufgezeichnet.

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With 10 Figures