Precipitation trends,evapotranspiration and the ecological zones of Nigeria

Summary An attempt is made to use the ratio of precipitation to potential evapotranspiration to qualify earlier climatic classifications of Nigeria for proper ecological zonation.Results show that in the pure forest belt located south of latitude 7° N the value is greater than 0.75, while in the middle belt (7–10° N) belonging to the wooded savanna and in areas further north gradually approaching steppe-type vegetation of pure Sahel, the values of below 0.40.Modulations of values (hence the eco-zones) appear to be responses to variable precipitation, especially in drought years. It is suggested that this notwithstanding, irreversible trends in land-surface degradation are mainly due to uncontrolled human interference in relation to large-scale agriculture in areas where is less than 0.40 in the Sudan-Shael belt of Nigeria. This needs to be re-appraised, if this desertification trend is to be checked

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Zusammenfassung In vorliegender Arbeit wird der Versuch unternommen, mittels des Quotienten von Niederschlag und potentieller Evapotranspiration frühere klimatische Klassifikationen der ökologischen Zonen von Nigeria genauer zu bestimmen.Die Ergebnisse zeigen, daß im reinen Waldgürtel südlich des 7. Breitengrades-Werte über 0.75 auftreten, während im mittleren (7–10° N), zur Waldsavanne gehörigen Gürtel, und in weiter nördlich gelegenen Gebieten, die zunehmend die steppenartige Vegetation der reinen Sahelzone aufweisen,-Werte unter 0.40 auftreten.Die Schwankungen der-Werte (forthin Ökozonen) scheinen die Reaktion auf unterschiedlichen Niederschlag besonders in Dürrejahren zu sein. Ungeachtet dessen ist die Annahme naheliegend, daß die irreversiblen Trends der Verschlechterung der Landoberflächen vor allem unkontrollierten menschlichen Eingriffen in Zusammenhang mit großflächigem Landbau in Gebieten des Sudan-Sahel-Gürtel Nigerias zuzuschreiben sind, wo weniger als 0.40 beträgt. Diesem Aspekt muß eine neuerliche Untersuchung gewidmet werden, um diesen Trend zur Desertifikation zu überprüfen.

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Sensitivity analysis of monthly reference crop evapotranspiration trends in Iran: a qualitative approach

Theoretical and Applied Climatology - The main objective of this study was to analyze the sensitivity of the monthly reference crop evapotranspiration (ETo) trends to key climatic factors (minimum...     

A survey of temporal and spatial reference crop evapotranspiration trends in Iran from 1960 to 2005

Reference crop evapotranspiration (ET₀) is one of the most important climatic parameters which plays a key role in estimating crop water demand and scheduling irrigation. Under global warming and climate change conditions, it is needed to survey the trend of ET₀ in Iran. In this study, ET₀ values were determined based on FAO-56 Penman-Monteith equation over 32 synoptic meteorological stations during 1960–2005; and analyzed spatially and temporally in monthly, seasonal and annual time scales. After removing the significant lag-1 serial correlation effect by pre-whitening, non-parametric statistical Mann–Kendall (MK) test was used to detect the trends. The slope of the changes was determined by Sen’s slope estimator. In order to facilitate in trend analysis, the 10 moving average low pass filter were also applied on the normalized annual ET₀ time series. Annual ET₀ time series and filtered ones were then classified by hierarchical clustering in three clusters and then mapped in order to show the patterns of different clusters. Results showed that the significant decreasing trends were more considerable than increasing ones. Among surveyed stations, and on an annual time scale, the highest and lowest annual values of Sen’s slope estimator were observed in Tabas with (+) 72.14 mm per decade and Shahrud with (?) 62.22 mm per decade, respectively. Results also indicated that the clustered map based on normalized and filtered annual ET₀ time series is in accordance with another map which showed spatial distribution of increasing, decreasing and non-significant trends of ET₀ on annually time scale. Exploratory and visual analysis of smoothed time series showed increasing trend in recent years especially after 1980 and 1995. In brief, the upward trend of ET₀ in recent years is a crucial issue with regard to the high cost of dam construction for agricultural aims in arid and semi-arid regions e.g. Iran.     

WRF/ARPEGE-CLIMAT simulated climate trends over West Africa

The Weather Regional Forecast (WRF) model is used in this study to downscale low-resolution data over West Africa. First, the performance of the regional model is estimated through contemporary period experiments (1981?C1990) forced by ARPEGE-CLIMAT GCM output (ARPEGE) and ERA-40 re-analyses. Key features of the West African monsoon circulation are reasonably well represented. WRF atmospheric dynamics and summer rainfall compare better to observations than ARPEGE forcing data. WRF simulated moisture transport over West Africa is also consistent in both structure and variability with re-analyses, emphasizing the substantial role played by the West African Monsoon (WAM) and African Easterly Jet (AEJ) flows. The statistical significance of potential climate changes for the A2 scenario between 2032 and 2041 is enhanced in the downscaling from ARPEGE by the regional experiments, with substantial rainfall increases over the Guinea Gulf and eastern Sahel. Future scenario WRF simulations are characterized by higher temperatures over the eastern Tropical Atlantic suggesting more evaporation available locally. This leads to increased moisture advection towards eastern regions of the Guinea Gulf where rainfall is enhanced through a strengthened WAM flow, supporting surface moisture convergence over West Africa. Warmer conditions over both the Mediterranean region and northeastern Sahel could also participate in enhancing moisture transport within the AEJ. The strengthening of the thermal gradient between the Sahara and Guinean regions, particularly pronounced north of 10°N, would support an intensification of the AEJ northwards, given the dependance of the jet to the position/intensity of the meridional gradient. In turn, mid-tropospheric moisture divergence tends to be favored within the AEJ region supporting southwards deflection of moist air and contributing to deep moist convection over the Sahel where late summer rainfall regimes are sustained in the context of the A2 scenario regional projections. In conclusion, WRF proved to be a valuable and efficient tool to help downscaling GCM projections over West Africa, and thus assessing issues such as water resources vulnerability locally.     

Spatiotemporal reference evapotranspiration changes in humid and semi-arid regions of Iran: past trends and future projections

Theoretical and Applied Climatology - This study was undertaken to investigate the reference evapotranspiration (ET0) changes in semi-arid and humid regions of Iran during the past...     

Improved predictability of droughts over southern Africa using the standardized precipitation evapotranspiration index and ENSO

The provision of timely and reliable climate information on which to base management decisions remains a critical component in drought planning for southern Africa. In this observational study, we have not only proposed a forecasting scheme which caters for timeliness and reliability but improved relevance of the climate information by using a novel drought index called the standardised precipitation evapotranspiration index (SPEI), instead of the traditional precipitation only based index, the standardised precipitation index (SPI). The SPEI which includes temperature and other climatic factors in its construction has a more robust connection to ENSO than the SPI. Consequently, the developed ENSO-SPEI prediction scheme can provide quantitative information about the spatial extent and severity of predicted drought conditions in a way that reflects more closely the level of risk in the global warming context of the sub region. However, it is established that the ENSO significant regional impact is restricted only to the period December–March, implying a revisit to the traditional ENSO-based forecast scheme which essentially divides the rainfall season into the two periods, October to December and January to March. Although the prediction of ENSO events has increased with the refinement of numerical models, this work has demonstrated that the prediction of drought impacts related to ENSO is also a reality based only on observations. A large temporal lag is observed between the development of ENSO phenomena (typically in May of the previous year) and the identification of regional SPEI defined drought conditions. It has been shown that using the Southern Africa Regional Climate Outlook Forum’s (SARCOF) traditional 3-month averaged Nino 3.4 SST index (June to August) as a predictor does not have an added advantage over using only the May SST index values. In this regard, the extended lead time and improved skill demonstrated in this study could immensely benefit regional decision makers.     

Drought in the Eastern Cape region of South Africa and trends in rainfall characteristics

Much of the Eastern Cape province in South Africa has been experiencing a severe drought since 2015. This drought has had major socio-economic effects particularly on the large impoverished rural population as well as on some urban areas where supplied water services have broken down in several cases. The region is influenced by both midlatitude and tropical systems leading to a complex regional meteorology that hitherto has not been much studied compared to other parts of South Africa. Here, the ongoing drought is examined in the context of long-term trends and the interannual rainfall variability of the region. Although the region has experienced drought in all seasons since 2015, focus here is placed on the spring (September–November) which shows the most consistent and robust signal. On average, this season contributes between about 25–35% of the annual rainfall total. Based on CHIRPS data, it is found that this season shows a significant decreasing trend in both rainfall totals as well as the number of rainfall days (but not heavy rainfall days) for spring over most of the province since 1981. On interannual time scales, the results indicate that dry (wet) springs over the Eastern Cape are associated with a cyclonic (anticyclonic) anomaly southeast of South Africa as part of a shift in the zonal wavenumber 3 pattern in the midlatitudes. Over the landmass, a stronger (weaker) Botswana High is also apparent with increased (decreased) subsidence over and near the Eastern Cape which is less (more) favourable for cloud band development and hence reduced (enhanced) rainfall during dry (wet) springs. Analysis of mid-century (2040–2060) CMIP5 rainfall projections suggests that there may be a flattening of the annual cycle over the Eastern Cape with the winter becoming wetter and the summer drier. For the spring season of interest here, the multi-model projections also indicate drying but less pronounced than that projected for the summer.

     

珠江流域实际蒸散发与潜在蒸散发的关系研究

采用水量平衡模型和Penman公式分别计算了珠江流域七个子流域1961—2000年实际蒸散发(I_(ETa))和潜在蒸散发(I_(ETp)),并对供水条件变化下I_(ETa)与I_(ETp)的关系进行了定量化分析,对各子流域I_(ETa)和I_(ETp)关系的理论从属性进行判定,主要结论如下:1)珠江流域年实际蒸散发量远低于潜在蒸散发量,多数子流域I_(ETa)值不到I_(ETp)值的1/2。7个流域面积加权平均I_(ETa)为681.4 mm/a,I_(ETp)为1 560.8 mm/a。从蒸散发的变异性来看,则实际蒸散发I_(ETa)的变异性明显要高于潜在蒸散发I_(ETp)。2)东江、西江、北江、柳江和盘江等5个流域实际蒸散发I_(ETa)都与降水量呈现正相关关系,韩江、郁江两个流域I_(ETa)随降水变化的变化趋势不明显。各子流域的潜在蒸散发I_(ETp)与降水量呈现显著负相关关系。7个子流域平均情况下,随着降水量的增加,I_(ETa)呈现明显的增加趋势,而I_(ETp)呈现明显的下降趋势。3)通过对降水量P与实际蒸散发I_(ETa)及潜在蒸散发I_(ETp)的联合回归方程P-IET回归系数的T检验,判定韩江、柳江和盘江等三个子流域以及七流域面积加权平均I_(ETa)与P和I_(ETp)与P的关系满足理论意义上的严格互补相关;东江、西江、北江等三个流域I_(ETa)与P和I_(ETp)与P的关系满足"非对称"互补相关。4)基于极端干旱和极端湿润的边界条件,推导出非对称条件下的实际蒸散发互补相关理论模型。     

New alternatives for reference evapotranspiration estimation in West Africa using limited weather data and ancillary data supply strategies.

Accurate estimation of reference evapotranspiration (ET ₀ ) is essential for the computation of crop water requirements, irrigation scheduling, and water resources management. In this context, having a battery of alternative local calibrated ET ₀ estimation methods is of great interest for any irrigation advisory service. The development of irrigation advisory services will be a major breakthrough for West African agriculture. In the case of many West African countries, the high number of meteorological inputs required by the Penman-Monteith equation has been indicated as constraining. The present paper investigates for the first time in Ghana, the estimation ability of artificial intelligence-based models (Artificial Neural Networks (ANNs) and Gene Expression Programing (GEPs)), and ancillary/external approaches for modeling reference evapotranspiration (ET ₀ ) using limited weather data. According to the results of this study, GEPs have emerged as a very interesting alternative for ET ₀ estimation at all the locations of Ghana which have been evaluated in this study under different scenarios of meteorological data availability. The adoption of ancillary/external approaches has been also successful, moreover in the southern locations. The interesting results obtained in this study using GEPs and some ancillary approaches could be a reference for future studies about ET ₀ estimation in West Africa.     

Interrelationship of rainfall,temperature and reference evapotranspiration trends and their net response to the climate change in Central India

The monthly rainfall data from 1901 to 2011 and maximum and minimum temperature data from 1901 to 2005 are used along with the reference evapotranspiration (ET₀) to analyze the climate trend of 45 stations of Madhya Pradesh. ET₀ is calculated by the Hargreaves method from 1901 to 2005 and the computed data is then used for trend analysis. The temporal variation and the spatial distribution of trend are studied for seasonal and annual series with the Mann-Kendall (MK) test and Sen’s estimator of slope. The percentage of change is used to find the rate of change in 111 years (rainfall) and 105 years (temperatures and ET₀). Interrelationships among these variables are analyzed to see the dependency of one variable on the other. The results indicate a decreasing rainfall and increasing temperatures and ET₀ trend. A similar pattern is noticeable in all seasons except for monsoon season in temperature and ET₀ trend analysis. The highest increase of temperature is noticed during post-monsoon and winter. Rainfall shows a notable decrease in the monsoon season. The entire state of Madhya Pradesh is considered as a single unit, and the calculation of overall net change in the amount of the rainfall, temperatures (maximum and minimum) and ET₀ is done to estimate the total loss or gain in monthly, seasonal and annual series. The results show net loss or deficit in the amount of rainfall and the net gain or excess in the temperature and ET₀ amount.     

Adaptation to climate change and variability: farmer responses to intra-seasonal precipitation trends in South Africa

We describe the nature of recent (50 year) rainfall variability in the summer rainfall zone, South Africa, and how variability is recognised and responded to on the ground by farmers. Using daily rainfall data and self-organising mapping (SOM) we identify 12 internally homogeneous rainfall regions displaying differing parameters of precipitation change. Three regions, characterised by changing onset and timing of rains, rainfall frequencies and intensities, in Limpopo, North West and KwaZulu Natal provinces, were selected to investigate farmer perceptions of, and responses to, rainfall parameter changes. Village and household level analyses demonstrate that the trends and variabilities in precipitation parameters differentiated by the SOM analysis were clearly recognised by people living in the areas in which they occurred. A range of specific coping and adaptation strategies are employed by farmers to respond to climate shifts, some generic across regions and some facilitated by specific local factors. The study has begun to understand the complexity of coping and adaptation, and the factors that influence the decisions that are taken.     

The role of evapotranspiration in climate

Summary A knowledge of the moisture balance at the earth's surface is essential to an understanding of climate. Precipitation and its areal distribution have been investigated in much detail. Evapotranspiration, which is the reverse of precipitation and represents the combined evaporation from the soil surface and transpiration from plants, is little understood and seldom measured. Actual evapotranspiration depends on climatic factors but is limited by the amount of available moisture in the soil. On the other hand, potential evapotranspiration which may be defined as the amount of water which would be lost from a surface completely covered with vegetation if there is sufficient water in the soil at all times for the use of the vegetation depends on climate alone.In order to evaluate the moisture factor in climate, the moisture supply or the precipitation must be compared with the water need or the potential evapotranspiration. The distribution of precipitation through the year never coincides with, and seldom parallels, the distribution of potential evapotranspiration. When the precipitation is in excess of need, the surplus goes to recharge ground water and produce runoff. When the precipitation does not equal the need, there is a deficiency which results in drought.There are various methods of measuring evapotranspiration, all of which are subject to many limitations, and only two of which give promise of yielding acceptable results. The first of these, the so called vapor transfer method, is of especial interest because it is the only one in which measurements may be made of a natural surface without disturbing it in any way. It is not yet a practical method because it requires greater precision in instrumentation than is feasible at present. The second, utilizing a soil tank or evapotranspirometer, is more artificial but it can provide reasonable results if used with care. The evapotranspirometer consists of a sunken open topped tank filled to ground level with soil on which vegetation is planted. A permanent water table is maintained in the soil at a given depth. A large area surrounding the tank must have a vegetation cover the same as in the tank. In addition, soil moisture both inside and outside the tanks must be maintained at the same level. The evapotranspirometer has proven to be a useful and inexpensive instrument for measuring potential evapotranspiration. Soil tank evapotranspirometers have been installed in thirteen localities during the last few years.Pending further theoretical work on the problem of evapotranspiration and the accumulation of observations an empirical formula was devised from data of irrigation projects and watersheds to give the potential evapotranspiration from climatological data alone. The formula has been applied with considerable success by investigators in various parts of the world.The relation between potential evapotranspiration and precipitation at nine European stations is discussed (see fig. 6). Potential evapotranspiration follows a uniform pattern through the year in most of Europe. It is negligible in the winter months as far south as central France, northern Italy, and southern Russia and reaches only 3 cm a month in southern Spain. It rises to a maximum in July that ranges from 10 cm in northern Scandinavia to 17 cm in southern Europe. Precipitation, on the other hand, is highly variable from one region to another. In southeastern Europe it is low, resulting in a large summer water deficiency (56.9 cm at Athens, Greece). This deficiency exists in the summertime over most of the rest of Europe, with the exception of certain restricted regions such as around Zurich, Switzerland. Over western Europe where the precipitation is greater and more uniform through the year the summer water deficiency is smaller. It is 10.6 cm at Birmingham, England and only 2.1 cm at Vard, Norway. In times of excess rainfall water is stored in the soil. The part of this water that is within reach of roots is used before the plants begin to suffer. From studies in western United States it was found that under ordinary circumstances the amount of water stored in the root zone that is available to plants is equivalent to 10 cm of rainfall.The uses as well as the limitations of potential evapotranspiration are only partially known. Even with its present limited use in climatology, hydrology, agriculture, and soil tractionability, it has shown itself to be a powerful tool. With more information concerning the nature of potential evapotranspiration and knowledge of its areal distribution from the tropics to the arctic, its usefulness should increase manifold.

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Zusammenfassung Die Kenntnis der Feuchtigkeitsbilanz an der Erdoberfläche ist für das Verständnis des Klimas unentbehrlich. Der Niederschlag und seine räumliche Verteilung sind in allen Einzelheiten untersucht. Evapotranspiration dagegen, die das Zusammenwirken der Verdunstung von der Erdoberfläche und der Transpiration von den Pflanzen und somit gleichzeitig das Gegenstück zum Niederschlag darstellt, ist wenig bekannt und nur selten untersucht. Die tatsächliche Evapotranspiration hängt von Klimafaktoren ab, ist jedoch durch die Menge der verfügbaren Bodenfeuchtigkeit begrenzt. Anderseits hängt die potentielle Evapotranspiration nur vom Klima allein ab, da sie durch die Wassermenge definiert ist, die von einer vollständig mit Vegetation bedeckten Fläche abgegeben wird, falls für den Bedarf der Vegetation ständig genügend Wasser zur Verfügung steht.Um den Feuchtigkeitsfaktor im Klima zu untersuchen, muß die Feuchtigkeitszufuhr durch Niederschlag mit dem Wasserbedarf oder der potentiellen Evapotranspiration verglichen werden. Die Niederschlagsverteilung über das Jahr deckt sich kaum je mit dem Verlauf der potentiellen Evapotranspiration. Wenn der Niederschlag den Bedarf übersteigt, wird der Überschuß zur Speicherung der Grundwasserreserven und zur Steigerung des Abflusses verwendet; bleibt jedoch der Niederschlag hinter dem Bedarf zurück, so entsteht ein Defizit, das zu Dürre führt.Zur Messung der Evapotranspiration bestehen verschiedene Methoden; diese unterliegen durchwegs gewissen Einschränkungen und nur zwei davon lassen annehmbare Resultate erwarten. Die eine der beiden, die sogenannte Dampfaustauschmethode bietet besonderes Interesse, da sie allein auf Messungen an einer natürlichen Fläche ohne Störungsursachen beruht; doch ist sie noch nicht in der Praxis verwendbar, da sie größere instrumentelle Präzision erfordert, als zur Zeit erreichbar ist. Die zweite Methode, bei der ein Gefäß im Boden (Evapotranspirometer) verwendet wird, arbeitet stärker mit künstlichen Bedingungen, kann jedoch bei sorgfältiger Anwendung vernünftige Resultate liefern. Das Evapotranspirometer besteht aus einem versenkten, oben offenen Gefäß, das bis zum Erdbodenniveau mit Erde gefüllt ist, auf der Vegetation gepflanzt wird; ein konstanter Wasserspiegel wird im Boden in bestimmter Höhe gehalten. Die weitere Umgebung des Gefäßes muß dieselbe Vegetationsdecke haben wie das Gefäß selbst. Zudem muß die Bodenfeuchtigkeit innerhalb und außerhalb des Evapotranspirometers auf gleicher Höhe gehalten werden. Das Gerät hat sich als nützliches und billiges Instrument zur Messung der potentiellen Evapotranspiration erwiesen. In den letzten Jahren wurden solche Bodengefäßevapotranspirometer (insgesamt 13) an verschiedenen Orten installiertVorgängig weiterer theoretischer Untersuchungen über das Problem der Evapotranspiration und der Sammlung von Beobachtungsmaterial wurde auf Grund von Erfahrungen an Bewässerungsanlagen eine empirische Formel zur Bestimmung der Evapotranspiration aus klimatologischen Daten aufgestellt; diese Formel wurde in verschiedenen Teilen der Erde mit gutem Erfolg angewandt.Die potentielle Evapotranspiration an neun europäischen Stationen besitzt im allgemeinen einen einheitlichen Jahresverlauf mit sehr kleinen Werten im Winter und einem Maximum im Juli. Anderseits zeigt der Niederschlag große Unterschiede zwischen den einzelnen Gegenden. Während er in Südosteuropa bei gro\em Sommerdefizit gering ist, erreicht er in Westeuropa größere Mengen. In Zeiten von Überschuß wird Regenwasser im Boden gespeichert; soweit sich dieses Wasser in der Reichweite der Wurzeln befindet, wird es ausgenützt, bevor die Pflanzen zu welken beginnen. Bei Untersuchungen in den westlichen Vereinigten Staaten wurde festgestellt, daß unter gewöhnlichen Bedingungen die im Wurzelbereich gespeicherte Wassermenge, die den Pflanzen zur Verfügung steht, einer Niederschlagsmenge von 10 cm entspricht.Die Verwendungsmöglichkeiten wie auch die Grenzen der potentiellen Evapotranspiration sind erst teilweise bekannt; aber auch mit ihrer gegenwärtigen beschränkten Anwendung in Klimatologie, Hydrologie, Landwirtschaft und Bodenbearbeitung hat sie sich schon als wertvolles Hilfsmittel erwiesen. Mit zunehmenden Kenntnissen über die Eigenschaften der potentiellen Evapotranspiration und über ihre geographische Verteilung von den Tropen bis zu den Polargebieten dürfte ihre Bedeutung stark anwachsen.

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Résumé Pour comprendre le climat il est indispensable de connaître le bilan de l'humidité de la surface de la terre. Si les précipitations et leur distribution géographique ont été étudiées dans tous leurs détails, l'évapotranspiration par contre, qui représente l'effet combiné de l'évaporation de la surface du sol et de la transpiration végétale et forme ainsi la contre-partie des précipitations, est peu connue et n'a fait que rarement l'objet d'études. L'évapotranspiration effective dépend de facteurs climatiques mais est limitée par la quantité disponible d'humidité du sol. D'autre part l'évapotranspiration potentielle ne dépend que du climat puis-qu'elle est définie par la quantité d'eau fournie par une surface entièrement recouverte de végétation au cas où il y a assez d'eau pour subvenir aux besoins de celle-ci.Pour étudier le facteur humidité d'un climat, il faut comparer l'apport humide des précipitations au besoin en eau ou à l'évapotranspiration potentielle. Il est rare que la distribution annuelle des pluies corresponde à la courbe d'évapotranspiration potentielle. Des précipitations excédant le besoins alimentent les réserves d'eau souterraine et accroissent l'écoulement; une pluie inférieure aux besoins crée un déficit conduisant à la sécheresse.Il existe différentes méthodes de mesure de l'évapotranspiration; elles sont toutes insuffisantes et deux d'entre elles seulement permettent d'espérer des résultats acceptables. L'une, appeléemethode d'èchange de vapeur, est particulièrement intéressante parcequ'elle repose sur des mesures sur le terrain sans modifier les conditions naturelles; elle n'est cependant pas applicable en pratique car elle exige une précision instrumentale supérieure à celle que l'on peut atteindre actuellement. L'ature méthode qui se sert d'un récipient, ditevapotranspiromètre, introduit davantage de conditions artificielles; bien appliquée elle fournit cependant des résultats raisonnables. L'évapotranspiromètre est un récipient enterré, ouvert vers le haut et rempli de terre jusqu'au niveau du sol et dans lequel on sème la même végétation qu'aux alentours; on maintient à l'intérieur un niveau d'eau constant. En outre l'humidité du sol doit être la même à l'extérieur et à l'intérieur de l'évapotranspiromètre qui s'est révélécomme un instrument utile et peu coûteux. On a installé ces dernières années treize de ces appareils en différents endroits.Sans attendre de nouvelles recherches théoriques ni la collation de matériel d'observations, on a établi sur la base d'expériences faites dans des installations d'irrigation une formule empirique permettant de déterminer l'évapotranspiration à partir de données climatologiques; cette formule a été utilisée avec succès en différents points du globe.L'évapotranspiration potentielle de neuf stations européennes présente en général une marche annuelle uniforme avec très petites valeurs en hiver et un maximum en juillet. D'autre part les précipitations accusent de grandes différences suivant les régions; tandis que dans le Sud-Est européen elles sont faibles et déficitaires en été, elles sont plus importantes en Europe occidentale. Aux époques d'excédent, l'eau de pluie s'accumule dans le sol; tant que cette eau est à portée des racines, elle est utilisée par les plantes avant que celles-ci ne commencent à se faner. Lors de recherches dans l'Ouest des Etats-Unis, on a constaté que dans les conditions normales l'eau de réserve qui peut atteindre les racines et qui est à disposition des plantes correspond à une pluie de 10 cm.On ne connaît que partiellement aujourd'hui les possibilités d'application et les insuffisances de la notion d'évapotranspiration potentielle; celle-ci s'est cependant révélée utile en climatologie, en hydrologie et en agriculture. Lorsque les propriétés de l'évapotranspiration potentielle et sa distribution géographique des tropiques aux régions polaires seront mieux connues, cette notion verra son importance s'accroître fortement.

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With 6 Figures.

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The material in this paper is adapted in part from previously published papers of the authors. Grateful acknowledgement is made to the Editors of the Geographical Review and the Annals of the Association of American Geographers for permission to reprint brief selections without change.     

Methodological differences in projected potential evapotranspiration

There is growing concern that the higher temperatures expected with climate change will exacerbate drought extent, duration and severity by enhancing evaporative demand. Temperature-based estimates of potential evapotranspiration (PET) are popular for many eminently practical reasons and have served well in many research and management settings. However, a number of recent publications have questioned whether it is appropriate to use temperature-based PET estimates for long-term evaporative demand and drought projections, demonstrating that PET does not always track temperature. Where precipitation changes are modest, methodologically driven differences in the magnitude or direction of PET trends could lead to contrasting drought projections. Here I calculate PET by three methods (Hamon, Priestley-Taylor and Penman) and evaluate whether different techniques introduce disparities in the sign of PET change, the degree of model agreement, or the magnitude of those changes. Changes in temperature-based Hamon PET were more significantly and consistently positive than trends in PET estimated by other methods, and where methods agreed that summer PET would increase, trends in temperature-based PET were often larger in magnitude. The discrepancies in PET trends appear to derive from regional changes in incoming shortwave radiation, wind speed and humidity -- phenomena simpler equations cannot capture. Because multiple variables can influence trends in PET, it may be more justifiable to use data-intensive methods, where the source(s) of uncertainty can be identified, rather than using simpler methods that could mask important trends.     

Simulating evapotranspiration in a semi-arid environment

Summary In this work, simulations with the mesoscale meteorological model FOOT3DK for a semi-arid research site in southern morocco are presented. The main aim of this study is to introduce two different ways to improve the soil moisture distribution towards a more realistic pattern. One of them resembles the availability of groundwater resources below the lower boundary of the soil part of the model, the other one resembles irrigation practices in the region. Additionally, we introduce a newly derived land use/land cover data set obtained from analysis of LANDSAT data and compare the simulation results to those obtained with the USGS GLCC data. To evaluate the results with the refinements in soil moisture and land use/land cover, we focus on evapotranspiration, as the quantity which is most tentative to the changes in soil moisture and is an important part of the local hydrological cycle. To evaluate the importance of sub-grid scale surface heterogeneity in soil moisture and land use/land cover, we present simulations with enhanced surface resolution. Simulation results are compared to point measurements at different sites in the research area for validation.The results show, that a deep groundwater table and irrigation of parts of the research area can be represented by the methods we used. Simulated transpiration is overestimated compared to measured values, but this is due to the maximum approach used in this work. Finer tuning of the artificial enhancement of soil moisture with the two methods presented here are expected to lead to realistic distributions of evapotranspiration and related quantities, therewith drastically enhancing simulation accuracy for this site. As uncertainties of soil moisture distribution and restricted representation of soil moisture dynamics in meteorological models is a common problem especially for arid and semi-arid sites, we expect our results to be useful for meteorological simulations in other arid or semi-arid areas as well.     

Linking climate change and health outcomes: Examining the relationship between temperature,precipitation and birth weight in Africa

This paper examined the relationship between birth weight, precipitation, and temperature in 19 African countries. We matched recorded birth weights from Demographic and Health Surveys covering 1986 through 2010 with gridded monthly precipitation and temperature data derived from satellite and ground-based weather stations. Observed weather patterns during various stages of pregnancy were also used to examine the effect of temperature and precipitation on birth weight outcomes. In our empirical model we allowed the effect of weather factors to vary by the dominant food production strategy (livelihood zone) in a given region as well as by household wealth, mother's education and birth season. This allowed us to determine if certain populations are more or less vulnerable to unexpected weather changes after adjusting for known covariates. Finally we measured effect size by observing differences in birth weight outcomes in women who have one low birth weight experience and at least one healthy birth weight baby. The results indicated that climate does indeed impact birth weight and at a level comparable, in some cases, to the impact of increasing women's education or household electricity status.     

Variability and trends of local/regional scale surface climate in northern Africa during the twentieth century

Four regions are detected in northern Africa (20° W–40° E, 0–30° N) by applying the cluster analysis method on the annual rainfall anomalies of the period 1901–2000. The first region (R1), an arid land, covers essentially the north of 17.75° N from west to east of the study zone. The second region (R2), a semiarid land with a Sahelian climate, less warm than the dry climate of R1, is centred on Chad, with almost regular extension to the west towards Mauritania, and to the east, including the north of the Central African Republic and the Sudan. The region 3 (R3), a wet land, is centred on the Ivory Coast and covers totally Liberia, the south part of Ghana, Togo, Benin and the southwest of Nigeria. The fourth region (R4), corresponding to the wet equatorial forest, covers a part of Senegal, the Central Africa, the south of Sudan and a part of Ethiopia. An analysis of observed temperature and precipitation variability and trends throughout the twentieth century over these regions is presented. Summer, winter and annual data are examined using a range of variability measures. Statistically, significant warming trends are found over the majority of regions. The trends have a magnitude of up to 1.5 K per century. Only a few precipitation trends are statistically significant. Regional temperature and precipitation show pronounced variability at scales from interannual to multi-decadal. The interannual variability shows significant variations and trends throughout the century, the latter being mostly negative for precipitation and both positive and negative for temperature. Temperature and precipitation anomalies show a chaotic-type behaviour in which the regional conditions oscillate around the long-term mean trend and occasionally fall into long-lasting (up to 10 years or more) anomaly regimes. A generally modest temporal correlation is found between anomalies of different regions and between temperature and precipitation anomalies for the same region. This correlation is mostly positive for temperature in cases of adjacent regions. Several cases of negative interregional precipitation anomaly correlation are found. The El Niño Southern Oscillation significantly affects the anomaly variability patterns over a number of regions, mainly regions 3 (R3) and 4 (R4), while the North Atlantic Oscillation significantly affects the variability over arid and semiarid regions, R1 and R2.     

An analysis of yearly trends in growing degree days and the relationship between growing degree day values and reference evapotranspiration in Turpan area, China

The growth and development of crops is commonly regarded as a function of time alone. However, this approach can be inadequate due to temperatures which vary from year to year caused by global climate change. This prompted the development of the growing degree day concept, which incorporates information on both the passage of time and the temperature experienced by the crop plant during that time. Crop water requirements, which are estimated by multiplying reference evapotranspiration values by a crop-specific coefficient, play a crucial role in the management of hydrologic cycles on arable land. Consequently, it would be useful to identify the relationships between cumulative growing degree days and reference evapotranspiration, in order to develop new methods for predicting crop growth and development periods and calculating reference evapotranspiration. This paper describes annual trends in cumulative growing degree days values and their impact on grape growth. Three different methods for calculating cumulative growing degree days values were evaluated as well. Several key findings were achieved. First, for the period between 1952 and 1995, the cumulative growing degree days values for specific days of the year were normally distributed. Second, the relationship between the relative cumulative growing degree days value and the passage of time can be accurately described by using a cubic polynomial function. Third, the day-to-day change in the average relative cumulative reference evapotranspiration can be described using an exponential function of time, which can be used to calculate the relative cumulative reference evapotranspiration value for any given day of the year. Fourth, there was a significant correlation between the relative cumulative growing degree days and cumulative reference evapotranspiration values during the period between grape budding and maturity, which can be described using a cubic polynomial function. Finally, a new method for determining the ET0 value for any given day of the year was developed; this method requires only a knowledge of the CGDD-at-year-end and no sophisticated meteorological data.