The Sahara moisture percent contribution is the lowest among the proportions of the three sources assumed to contribute to the AVMs in Cairo, Fig. 1., with a maximum value of 8% in Winter and 12% in Summertime. The Sahara vapor contribution is systematically higher in Summertime than in Winter. The negative relationship between the Saharan percent contribution and S reflects the role of the Saharan vapor source in the local drought, even in Winter. The high S values of the AVMs in Summertime represent a "lost chance" as the estival moisture cannot lead to precipitation due to the prevailing high temperatures. The hibernal intrusion of the Tropospheric (Saharan) downdrift also stands behind the abortion of precipitation in Winter. Rainfall events over Cairo and the Nile Delta are exceptional but may occur by a rare sudden breakage in the Hadley Cell regime governing the regional drought conditions in the southeastern Mediterranean basin. Rare precipitation events would occur, however, a few times per year, by the sudden advent of exceptional confrontation of external humid hot-stream with external cold air-masses. Infrequent significant precipitation may also take place, a few times per decade, via the inception of a regional deep and vast depression cell that leads to sharp adiabatic cooling, as the one called "Dragon" shower of 12-14 March 2020. Compared to the contributions of the Marine vapor and ET concerning S, shown in Figs. 2 and 3, the relationship of the Sahara vapor contribution with S, Fig. 1, shows a salient low dispersion in the two seasons. This behavior reflects the consistency of the Tropospheric (Sahara) downdrift that produces adiabatic heating the year-round.
Fig. 2 is showing that the Marine moisture contribution dominates the scene in Winter and Summer in Cairo AVMs, as expected. In both seasons, the percent contributions and S of this northern moisture source show a wide range of values and faint positive trend of x and y variables in Summer vs. a weak negative trend in Winter. The relationship between the two variables is not significant since dispersion is considerable compared to that of the Sahara vapor contribution behavior shown in Fig. 1. Such considerable scattering reflects the oscillation of the Marine source input in a prominent daily cycle with the wind gusts passing southward, especially in Summertime, Fig 23. Few Summer data-points, however, show lower Marine contribution than most of Winter data points. These few points correspond to some condensates collected in the Spring of the year 2019, not in Summertime (S values are known to be sometimes lower in Spring than in Winter). Recently, research workers (Bonne et al., 2019) have recommended that relative humidity and sea surface temperature (not wind speed) only are the two variables that control deuterium excess of the Marine vapor within the PBL.
The attitude of the ET moisture input vs. S in Cairo AVMs, Fig. 3, works as a vertical mirror image of the behavior of the Marine vapor input vs. S, shown in Fig. 2. The high dispersion of the relationship between ET contribution and S in Winter and Summer reflects the variation in the biological activity of the Nile Delta vegetation in response to the current S values in the two seasons. However, a bulky scatter appears at a higher S level in the hot season and shows a weak negative trend, compared to a significant positive regression with less dispersion for the cold season. The opposite trends are expressions of different ET fluxes and as byproducts of the Marine source dominance. The diurnal temperature increase, in the two seasons, however, induces higher ET signals vs. the nocturnal ET fading that starts by evening. On the contrary, the ET contribution signal is showing low dispersion in Fig 9 for its relationship with d18O of the AVMs, in the two seasons, with a steeper regression in Winter. Such a steep regression is related to the Winter crops that produce low ET fluxes, isotopically enriched, in Winter.
The relationships of the Sahara vapor WP with S Fig. 4 show the same behavior of the relationships of the Sahara percent contributions with S, Fig. 1, except the linear regressions with a slightly larger negative slope, and lower locus, in Winter than in Summertime. The higher WP values in Summer reflect stronger downdrift of Tropospheric (Sahara) air layers in the hot season. Albeit the remarkably low WP values of the Sahara source in the local AVMs in the two seasons, their isotopic impact on the isotopic ratio of the AVMs is sharp due to the profound isotopic depletion in the Sahara vapor.
Fig. 5 is showing the relationships of the Marine moisture WP with S. The behaviors of such relationships sharply differ from the relationships of the percent contributions with S, Fig. 2, both for Winter and Summertime. Here, the two seasons show positive linear regressions and much less dispersion. The WP-S regression has more significant R2 value in Summer than in Winter; 0.7702 and 0.4398, respectively. The higher WP levels in Summer agree with the visible dominance of the northern windblown to Cairo City in the hot season, Fig 23. Also, the highest S value in Summer is almost double the highest S value in Winter, due to significant Marine vapor southward flux in Summer. The remarkable estival WP values for the Marine vapor source lead to an accentuated isotopic depletion in Cairo AVMs in Summertime.
The ET WP relationship with S, Fig. 6, is like the ET percent contribution relationship with S, Fig. 3, except for the signs of the two regressions. The Summertime data-points, however, show much dispersion in Figs 3 and 5. The WP ranges of ET for the hibernal and estival seasons are comparable and reflect the permanent biological activities in the cultivated Nile Delta lands. These activities produce about the same absolute ET moisture despite the different ET fluxes in the two seasons. This piece of information is precious since it reveals the moisture-distribution that the percent contribution may hide. The wide WP range of the vegetative source in the two seasons is a direct reflection of the weather conditions and the cropping pattern in the Nile Delta since ET is absent by night, something that accentuates the diurnal signal of ET WP in the two seasons. However, the hibernal ET WP data-points appear at a lower S position and show much more significant regression than the estival ET WP data-points.
The Sahara vapor percent contribution in relationship with d18O of the AVMs, Fig. 7, is higher in Summer than in Winter, but both show high scattering. Compared to the behavior of the relationships of both the percent contributions, of the Marine moisture and ET, with d18O, shown in Figs 8 and 9, the Sahara vapor source percent contribution concerning d18O shows a notably high dispersion. This chaos reflects not only a daily cycle but, more importantly, the permanent presence of such drought source in Cairo AVMs the year-round. However, Figs. 1 and 4 show that the percent contributions and WP values for the Saharan vapor concerning S have extremely low dispersions and visible negative regressions in both Winter and Summer. However, Fig. 7 also illustrates that the Sahara vapor source percent contributions are showing up at all the observed d18O range of the local AVMs in both Winter and Summer. Nonetheless, such a vapor source has a slightly higher contribution at the moderate and low 18O contents of the local AVMs in Summer vs. its low contributions at the full d18O range in Winter. The last statement could, however, be an artifact produced by the mixing process of the regional vapor sources. Otherwise, there is a seasonal change in the d18O or S value of the Sahara pole; a change that nobody can verify for the time being.
The percent contribution of the Marine vapor concerning d18O of the AVMs, Fig. 8, has a wide range of values that are almost comparable for Winter and Summer (few data-points to the left-hand side correspond to Spring, not to Summertime). Compared to the behavior of the relationship of the percent contribution of the Sahara vapor with d18O shown in Fig. 7, the corresponding relationships for the Marine vapor in Fig. 8 are showing low scattering and negative linear regressions with steeper slope (-0.1504) in Winter than in Summer (-0.0872). This trend reflects the Marine vapor dominance the year-round, and that it has extensive daily cycles that are unequally partitioned between the daytime and nighttime. The high Marine contributions in the two seasons, however, are associated with the isotopically depleted AVMs. The Marine vapor source negative regressions with d18O for the two seasons are opposite to the ET positive regressions shown in Fig. 9 as if the opposition is a vertical mirror image.
Fig. 9 is showing that the ET percent contribution with d18O values of the AVMs is slightly higher in Winter than in Summertime (except for the few Spring data-points shown to the right-hand side below the regression line). Both relationships, however, have a significant positive regression. Compared to the behavior of the percent contributions of the Sahara and Marine vapor, Figs 7 and 8, respectively, the ET data-points have very low dispersion. Such positive trends reflect the permanent presence of the ET vapor source the year-round and thanks to its diurnal pulse and nocturnal fading. Higher ET percent contributions are associated with isotopically enriched AVMs in Winter (keeping the few Spring data-points excluded). However, the last statement could be an artifact of the mixing process of the three vapor sources if the isotopic composition of the ET pole was not further enriched in Winter but more depleted in Summertime. The steeper positive ET regression with d18O in Winter shown in Fig 9 agrees with the ET high flux released out from the Nile Delta crops in the cold season, and such a high hibernal ET flux is more than expected for the cold season. However, the last statement could be an artifact of the mixing process of the three vapor sources. Otherwise, the isotopic composition of the ET vapor pole would have a remarkable enrichment in Winter and more depletion in the Summertime. Such an isotopic shift is to verify in the future. Nonetheless, the isotopic depletion of some data-points of the AVMs in Summertime may result from the isotopically depleted ET flux of a Summer crop (e.g., Corn). However, the situation is so beautifully complex to be justified by a single crop criterion since other data-points show visible isotopic enrichment in Summertime, as shown in Fig. 14 bottom corner diagram. The interactive diagram in Fig. 15 shows that the decrease of ET contribution in Summer is responsible for the significant isotopic depletion in the estival AVMs (as the higher contribution of the Marine vapor source is the real reason for such isotopic depletion in the hot season). The decrease in ET contributions in Summertime is a byproduct of the estival sharp increase in the Marine vapor source contributions. Thus, the Marine source is actively transferring its depleted isotopic fingerprint to the AVMs, preferably in Summertime. As the available data is showing ET contributions in the two seasons, we may look at the point at 30% in Summer in Fig 14. The ET flux from the Nile Delta (via the ratio of the ET flux to the applied irrigation water) is about 35 BCM in Summer. The total AVMs in the Nile Delta in the hot season would amount to 100 BCM, with 65 BCM mass directly flowing out from the northeastern Mediterranean basin, and the rest is from the ET. Such 100 BCM moisture corresponds to the approximate value calculated using the regional specific humidity calculations reported elsewhere (Specific Humidity Calculator_7_SUMMARY_OF_MEAN_VALUES_and_Project Operation Zone_4, in the Folder Water Generation Project_Specific Humidity in the Folder Hydro.)
The Sahara WP values in relationship with d18O of the local AVMs, Fig. 10, is systematically higher in Summer than in Winter. However, the Sahara WP values are showing higher dispersion in Summer than in Winter. The distribution of the data-points for the two seasons over the full d18O range is related to the omnipresence of Tropospheric downdrift the year-round. Such drought effect occurs e without correlation with the d18O content of the AVMs. However, in Summer, there is a data-point cluster of this moisture source concentrated at the moderate and depleted isotopic signatures of the local AVMs.
The Marine WP values Fig. 11, is much higher in Summer than in Winter. The data-points for the two seasons, however, show high scattering. Compared to the behavior of the WP values of the ET source, Fig 12, the Marine WP values are showing extensive dispersion, especially in Summer, with non-significant regression in the two seasons. The reason for the high dispersion is that the Marine vapor fluxes experience periodic diurnal and nocturnal change between convection and advection, respectively. However, the Marine vapor data-points visibly show some clusters at the moderate and depleted isotopic compositions of the AVMs in Summertime.
Fig. 12 is showing that the relationship of WP values of the ET source with d18O of the local AVMs has significant positive regressions in the two seasons, with a slightly steeper slope, 0.9617, in Winter than in Summer, 0.9539. Compared to the behavior of the corresponding contributions of the Sahara vapor and the Marine vapor, Figs. 10 and 11, respectively, the relationships of ET WP with d18O is showing very low dispersion and significant positive repressions for the two seasons. The two configurations reflect the perpetual release of diurnal ET flux the year-round, but with more impact on the AVMs isotopic enrichment in Winter.
To this extent, we show what happens to the contributions of three vapor sources when we used one fixed value for the isotopic composition of each vapor source and two values for S, one for Winter and the other for Summer. For more practical purposes, one refers to these two values as exhalation in Winter, and inhalation in Summer to consider what happens when dealing with the "inhalation" of the S values in the Summertime vs. the "exhalation" that occurs in Winter. By the term "inhalation," we mean the significant increase in the S value for each vapor origin in Summertime, in contrast to the term "exhalation," which means a remarkable decrease of S value for each vapor origin in Winter, Fig 21. Such changes in the S values correspond to the acute change in the temperature and RH in the two seasons. To keep things simple, we assumed, however, that no change in the isotopic composition of the three vapor sources takes place. The reduction of the specific humidity in Winter will result in the shrinkage of the curved wedge shown by the CLAW model, Fig 21, and consequently, the shift in the percent contributions, and the WP values, calculated by the TIMAM model, for the three vapor sources contributing to the AVM. Other users may, however, appraise for making some change in S and d18O values for the three vapor sources. Such changes are to introduce by the user as required for his experimentation.
Fig. 13. The increase of the Marine vapor source percent contribution (top-left diagram) leads to isotopic depletion in the local AVMs. In contrast, progressive AVM isotopic enrichment goes with ET source percent contribution increase (middle-left diagram). In Summertime, the Marine vapor source contribution (top-right diagram) increases at high S values while the ET source contribution (middle-right diagram) increases at low S values. Under the hot Summertime weather conditions, the cultivated crops in the Nile Delta react by releasing higher ET flux, especially when the S values are low. The high ET contributions at the low S values in Summertime is due to high ET fluxes from the Nile Delta vegetation under the apogee temperatures prevailing in the hot season. In contrast, the lack of trends in Winter, for both the Marine and ET contributions with S, reflects apparent moisture stagnation in the cold season due to the steadiness of the northern wind replaced by the southern wind that partially resists the ET signal transmission to Cairo city in the cold season, Fig 23. The relationship between the percent contribution of the Sahara vapor source with S values (bottom-right diagram) shows a curvilinear increase with S decrease in the two seasons, with higher estival contributions of the Sahara moisture at higher S values, as in Fig 1. This observation reflects the active role of the Sahara vapor source in the regional drought the year-round. On the contrary, the Sahara moisture source has no definite relationship with d18O of the AVMs (bottom-left diagram) the year-round, as in Fig 10.
Fig 14. The negative relationship between ET percent contribution and S (top left diagram) is showing lower ET contribution in Summer than in Winter despite the high ET flux in the hot season. This odd trend is an artifact (byproduct) of the massive impact of the Marine vapor contribution increase in Summertime (top right diagram). In contrast, the bottom diagrams show the inverse trends for the relationship between the ET and Marine contribution, on the y-axis, and d18O ratio, on the x-axis. The left bottom diagram is showing a positive relationship between ET contribution and d18O in Winter and Summer. The tight bottom diagram shows a negative relationship between the Marine contribution and d18O in the two seasons. The higher Marine vapor contribution in that diagram shows a data-point cluster at the isotopically depleted atmospheric vapor mixtures in Summer. Despite the observations mentioned above, the isotopic compositions of the AVMs almost cover the full d18O range in the two seasons. ET contribution increase associated with dryness and isotopic enrichment (in Winter and at the low S value in Summer). In contrast, the increase of the Marine contribution associated with wetness and isotopic depletion of the AVMs (in Summertime).
Fig. 15. Interactive diagrams based on the left bottom diagram shown in Fig 14. These four diagrams are to run in Excel to show the impact of the different isotopic compositions and percent contributions of the used three vapor sources on the isotopic composition of the local AVMs. These water vapor mixtures visibly become more isotopically depleted in Summertime at the low percent contribution of the ET vapor source when the Marine vapor contribution is at maximum. Besides, the Sahara vapor endmember has a higher contribution to the Summertime than Winter AVM. Also, the estival increase in the Sahara vapor proportion, to 10-12% in Summer, partially leads to the isotopic depletion in the AVM in the hot season (black square in the left bottom diagram). Albeit the shallow S values for the Sahara vapor origin, its extreme isotopic depletion stands behind its perceivable impact on the isotopic composition of the local AVMs, especially in Summertime. The visible hibernal isotopic enrichment of the AVM is the direct result of the high ET vapor source contribution in Winter (the black square in the left bottom diagram shifts upward).
The water liquid phase that would condensate from an AVM, with-10 per mil for d18O, will have the d18O value of about 0 per mil for the obtained precipitation. This example is of primary importance for the interpretation of the isotopic ratios of the Nile water at the river middle African head reaches. Assuming a dominant ET recycling origin, for the significant precipitation events that make most of the runoff that goes to the River Nile, at its up reaches, and assuming -10 per mil for d18O of the AVMs at the river head reaches, and 0 per mil for d18O for mid-African precipitation, the runoff water that goes to the river course would show d18O of about +1 per mil, or so, in the African rainforest heights upstream. Then, the river water gets more isotopically enriched by evaporation as it flows across the long northward pathway that finally reaches the flat downstream terrains of Egypt. In contrast, if, by quite an argument, the condensation of a dominant Ocean water vapor source was, instead, to mostly form the primary precipitation events at the Nile upstream territories, the middle-African runoff that goes to the river would be marked by accentuated isotopic depletion, which is not the case. The isotopic enrichment of the Nile water, at its head reaches, visibly reveals the dominance of ET recycling, at the upstream territories (Ref), and is to attribute to massive ET flux of the dense tropical rainforests. Any widescale clearance of the tropical forests in the middle African territories will catastrophically diminish the Nile water discharge towards the downstream countries. The interactive diagrams, shown in Fig 15, supply an excellent tool for the interpretation, not only of the isotopic composition of the AVMs downtown Cairo city but also for understanding the enriched Nile water isotopic composition in the upstream mountainous regions, as we have just explained in the last few sentences.
Moreover, we believe that the accelerated deforestation in tropical Africa (Neef, 2020) will lead to diminishing the ET proportion in the African AVMs that induce precipitation at the Nile upstream countries. High ET recycling in the Nile's head reaches stands behind the enriched isotopic composition of the upstream river water, Fig. 15. As such, a significant depletion in the isotopic signature of the Nile water will visibly be due to the substitution of a fraction of the ET origin by the Ocean vapor in the rainforest tropical African territories due to unfortunate massive deforestation. Hundred years ago, the rainforests territories were covering about 14% of the total land worldwide, but today it is only about 6%, and the human activity has removed about half of the tropical rainforests.
Fig 16. The relationships of the ratio of the contribution of ET vapor source to that of the Marine vapor with the isotopic composition (left-hand side diagram) and the S values (right-hand side diagram) for the AVMs show two inverse regressions and seasonal fingerprints, 1) ET to Marine ratio is increasing with the enrichment in 18O (left diagram). 2) ET to Marine ratio is decreasing with the increase of the S values (right diagram). The horizontal line shown at the y-axis value of unity, in both diagrams, is to use to follow the deviations from the equity of both vapor sources contribution.
Fig.17. The four Korean-style diagrams shown in this figure illustrate the differences between the ternary and binary mixing models and what they have in common. The ternary mixing charts (top charts) are more successful than the binary blending (bottom charts). The top sketches are showing the relationship between the d18O and S values of the AVMs s in the Nile Delta apex in Winter (left charts) and Summer and Winter together (right charts). S no account can ignore the ET contribution in the atmospheric moisture over such a vast delta, the ET source participation was to include in the ternary mixing model. The ET vapor source has the effect of dragging the data-point towards the top right corner in each of the top diagrams. The position of the literature data-points, taken from Gat et al., 1995, for the atmospheric moisture over the eastern Mediterranean Sea, January 1995, is astonishing. Gat data-points are lying nearby the top left corner, in the four diagrams, i.e., to the left side of our Winter data-points, for Cairo city. The surprising Gat data-points positions reflect not only the extreme drought under the impact of the shallow moisture content of the Sahara vapor but also show the presence of an ET component in the vapor samples collected using a standard cryogenic procedure in Winter of the year 1995. What Gat and co-workers have sampled and measured was not a pure Marine moisture, but mixtures of Marine atmospheric vapor affected by the Tropospheric downdrift over the Sahara, plus ET component from the European continent. Drought appears, on the x-axis, on four diagrams, via the low S values (top charts) and the mixing ratio, w, values (bottom graphs). Comparatively, Gat's data-points configuration indicates that even in Winter, the air masses over the Nile Delta apex at Cairo city have higher humidity contents than that of the air-masses over the open eastern Mediterranean water surface. Unfortunately, Gat and co-workers have not reported any isotopic or humidity data for the east Mediterranean basin in Summertime. However, we may assume that the isotopic composition of the atmospheric moisture over the eastern Mediterranean will stay constant the year-round (and show the same range of values as that of Gat's Winter isotopic contents). Only the S and w values will significantly increase in Summertime in this Marine basin. Gat data-points will, virtually, move (on an oblique line with the same trend as that of our data-points for Cairo city) to the right-hand side bottom corner of the four diagrams if Gat and co-workers have also sampled the atmospheric mixtures over the eastern Mediterranean in the Summertime of the year 1995, as they did in Winter of the same year. Nevertheless, Gat's virtual data-points in the new position, for such an imaginary Summertime campaign, will appear in the two diagrams above our Cairo data-points and the oblique binary mixing line of the Marine and the Sahara vapor. The presence of our data-points below the Gat imaginary estival campaign might indicate that our isotopic compositions for Cairo city have isotopic depletion compared to the eastern Mediterranean basin vapor. What, then, would be the meaning of such isotopic depletion in our data-points? Would the assumed depletion in our data-points indicate the impact of climate change (namely, via air temperature increase) over during the 22-24 years between 1995 (Gat's sampling date) and2017/2019 (dates of our work)? The answer is no. The reason for such a net negative response is that any significant increase in air temperature will lead to a decrease in the value of the equilibrium fractionation factor, and this leads to a more enriched isotopic composition for the AVM corresponding to our collected liquid-phase water samples. In contrast, we are looking for a plausible reason for the thought depletion in our 18O values (of the local AVMs assumed in equilibrium with the liquid phases that we have collected in Winter and Summertime) compared to the eastern Mediterranean vapor isotopic composition. The sole reason for negating the depletion discrepancy in our data-points is the mixing dynamics of three vapor sources, including the ET component as an important isotopic enrichment controller in the regional AVMs. Such a biological controller governs the positions of our data-points shift towards the top right corner in the four diagrams, even if the contribution of the ET source is not significant. We conclude, then, that Gat's vapor samples had a substantial ET component diffused from the European continent, and he has not reported such a potential component over the open seawater surface of the eastern Mediterranean basin. If Gat's vapor samples had not any ET component, his data-points would precisely show up on the binary mixing line of the Marine and the Sara sources in the four diagrams, not above such a line. This argument says that Gat data-points are more enriched than expected from the simple binary mixing between the Sahara and the Marine vapor sources. Such enrichment is certainly due to the impact of the continental ET component in Gat's vapor samples. Noteworthy also is to mention that the limits of our experimental data-points for the local AVMs have governed the isotopic composition and specific humidity for the Marine vapor source. However, the compositions of our three vapor sources are tentative and may be subject to seasonal shifts but never to the extent to force our Marine pole to appear in the position and place of Gat's Marine vapor data-points. When the Cavity Ring-Down Spectroscopy, CRDS, devices provide full isotopic data by the continuous and direct measurement for the local AVMs, such controversy may get to an end. However, we are not the only research workers who mention critics of Gat data. A preprint, unpublished discussion paper (Cox et al., 2012) has already contended issues in the d-excess values for the eastern Mediterranean surface water reported by Gat in 1996 for his 1988/89 data and attributed such a discrepancy to faulty isotopic measurement by Gat teamwork.
Fig. 18. The ternary Piper diagram is showing the contributions of the three vapor sources composing the local AVM data-points. Our data-points are mostly aligned close to the right-hand side of the Piper triangle. The right-side is a binary (Marine and ET) mixing line. The Sahara vapor contribution increase drags the data-points to the left-hand side, towards the Piper inner space. Such a triangle supplies complementary information to add to the data-points orientation pointing towards the top corner of the Korean-style space to the right-hand side of the Sahara-Marine binary mixing line (top diagrams in Fig 17). The Korean-style chart shows the binary mixture of the Marine and Sahara vapor, where ET contribution increase triggers the data-points to the top right corner. The Korean style space is superior to the Piper triangle as we do not need to know the percent contribution of the three vapor sources in each AVM. However, the Piper triangle is superior as it shows the percent input of each vapor source. Piper triangle, however, would fail if the contribution of one of the three vapor sources was not available. Nonetheless, we have plotted Gat's data-points of January 1995 (Gat et al., 1995) on the Piper diagram, using the binary Marine and Sahara vapor mixing, just for illustration purposes, via ignoring the ET component data in TIMAM model and Macro. The ET component was to ignore in Gat's data-points processing for this purpose. The reason is that no calculation is possible via our TIMAM model for the Gat-data-points using his isotopic ratios and S values as we do not know the d18O and S values to use for the ET source contributing to Gat's data-points. Still, we may assume it, in other work, as being a continental (European) ET component for which we may find the corresponding isotopic and humidity data in the literature. However, the most critical problem in Gat isotopic data is that each of his collected vapor samples was to obtain in about one day, and thus the provided temperature and RH data were also mean values over time and space. This obstacle is challenging due to the bulk nature of Gat's isotopic and meteorological data in terms of the long time needed to collect each vapor sample via the standard cryogenic vapor procedure. The long time required for collecting each vapor sample means that the isotopic ratio for each vapor sample was an average value to report overtime and on the ship navigation trajectory. This situation leads to unavoidable irregularities as the isotopic ratio of the atmospheric vapor, and the corresponding meteorological data will change every hour. In contrast, our samples have an excellent temporal resolution of one hour at one location. However, we may assume that the visual projection of Gat data-points would show a virtual ternary mixing configuration that indicates about 15% ET contribution, 10% Saharan vapor component, and 75% for the proper Mediterranean vapor source, as a first approximation. Albeit its strange shortages, Gat data reflect that each of his vapor samples is to primarily represent a mixture of at least two vapor sources, namely the Marine vapor and the Tropospheric moisture (Sahara vapor). This observation also means that there is no isotope and humidity data possible for any pure Marine vapor pole over the eastern Mediterranean Sea. The isotopic and humidity data for a pure Marine vapor source would never exist over such a semi-closed sea. The Mediterranean, contrary to the open oceans, unavoidably gets the impacts of the continental moisture fluxes, out from the European, Asian, and African continents, via the wind activity. Accordingly, we are right in assuming the isotopic composition and specific humidity reported in this work for the theoretical Marine vapor source as constrained by fitting Cairo city data-points by the Macro of TIMAM model. If the late statement is not the last word in the quantification of the isotopic composition of the atmospheric vapor of the region, it offers an outline direction on the long road. No atmospheric vapor source is a "pure" vapor source No single statement, however, can settle every conflict in that issue without using an hourly resolution for the continuous and direct measurement of the isotopic composition of the local AVMs using the adequate CRDS devices. Moreover, the satellite Isoscape isotopic model could not fill such a gap.
Fig. 19. The top diagram is showing the opposite trends of the two dominant vapor sources, the Marine vapor (circles) and ET (triangles) for the relationship of the d18O values of the atmospheric vapor mixtures (on the y-axis) with percent contribution of each source in the AVM (on the x-axis) in Winter (blue symbols) and Summertime (yellow symbols). The contribution of the Sahara vapor, shown by small rectangles at the left-hand side of the top diagram, has minimum values that rarely exceed 10%. The low ET contributions are associated with the isotopically depleted AVMs in both seasons, the moderate ET contributions associate with the moderate to high isotopic enrichment of the AVMs in Winter, and the high ET contributions associate with the isotopically enriched AVMs. The high Marine contributions associate with the AVMs isotopic depletion in Summer. The association of the extremely high Marine vapor contributions with the extreme depletion of the AVMs puts a useful constraint on the isotopic composition to assign to the theoretical "pure" Marine vapor source of the eastern Mediterranean Sea. The bottom diagram is showing a set of quasi-linear relationships between the S values (on the y-axis) and the percent contributions on the x-axis. The S value shown on the y-axis is not the S value for the AVM but the S value for each of the three vapor sources in the mixture, as calculated by TIMAM Model. The ET moisture shows low to moderate contributions at low to moderate specific ET_S values in Winter, while both variables slightly increase in Summertime. The Marine moisture source shows moderate to high contributions at moderate to high S Marine values in Winter while showing moderate to very high contributions associated with moderate to extremely high Marine_S values in the Summertime.
Fig. 20. The diagram shows that the high ET WP values, on the x-axis, associate with highly enriched isotopic signals of the AVMs (on the y-axis) in the two seasons, with steeper regression in Winter. In contrast, the chaos of the WP relationship with the d18O values of the AVMs is to observe for the Marine vapor source, especially in Summer (Risi et al., 2020). The low WP values for the Sahara vapor source cover the full range of the d18O range of the AVMs, with a slight increase in Summer. The considerable variation of the WP distribution for the three vapor sources with the isotopic signature of the AVM is a direct expression of the aerodynamic processes at the Nile Delta apex.
Figure 21. Two curved wedge frameworks produced by the CLAW model (three blue curves for Winter and three red curves for Summertime) including our data-points for Winter (blue and black squares) and Summertime (red circles and red spots). The Mediterranean data-points (Gat Jan 1995) appear as purple void squares, to the left-hand side, inside the blue "Winter exhalation" framework but outside the red" Summer inhalation" framework. The difference between the data-points distributions in Winter and Summer is visible inside the two frameworks. However, the diagram setup is for the relationship between d18O and S values for the AVMs, while Fig 20 shows the three vapor sources composing the local AVM using the TIMAM model for the relationship between d18O of the AVMs and the individual WP values for each vapor origin. It is astonishing to remark that S values in Cairo AVMs in Summertime exceed S values in the AVMs at the Southern Amazon basin.
Figure 22. The daily evaporation rates, downtown Cairo city, for the years 2018, 2019, and 2020. The Winter season is showing a low evaporation rate plateau, with an average of 4 mm per day, while the average of the estival plateau is at 12 mm per day. The Spring season shows an ascending evaporation rate trend (primarily due to temperature increase under low RH values), with sharp oscillations between 6 and 16 mm per day, that may reach 22 mm per day under the impact of hot and dry air-mass surges. Autumn is showing a visible descendent trend, via the rapid decrease in temperature while the RH values are still high.
Figure 23. Guided by the shown wind rose, we assume a mean northern wind speed of 5 km/hr. This speed will impose a 36-hr delay on the diurnal and nocturnal Marine isotopic and humidity signals to arrive at Cairo across 180 km (the distance between the Mediterranean coast and Cairo city). The ET vapor source has a 24-hr delay for its diurnal isotopic and humidity signals to cross a 120 km distance between a point in the middle of the northern sector of the Nile Delta (i.e., 60 km to the south of the Mediterranean coast) and Cairo city.
Figure 24. The Time-controlled dynamics, for the change in the percent contributions of the three vapor sources making up the local AVM, at different geographic keystones to the north of Cairo, namely, 1, at the Mediterranean coast, i.e., +180 km, 2. at +120 km, 3. at +60 km, and 4- downtown Cairo. The top diagram is showing elusive horizontal mirror-image for the Marine, and ET, percent contributions downtown Cairo (large blue and green squares, respectively) as the Marine %input increase by nighttime in Cairo is visibly counter-intuitive. Such nocturnal Marine contribution increase in Cairo is an artifact imposed by the time-lag caused by wind direction, trajectory, and speed since the Marine windblown mostly starts at the Mediterranean coast. Compared to the Marine signal, the ET signal trajectory is shorter (considered here at +120 km, i.e., at the north of the central cultivated lands of the Nile Delta). The diurnal increase in the ET source percent contribution is normal behavior as transpiration fades out by nighttime. The behavior of the Sahara vapor percent contribution (small void red squares, bottom diagram) is like that of the ET source percent contribution as the Sahara signal also has a short trajectory (considered here at +120 km), and the active diurnal wind helps in its transmission to the capital mostly by daytime. Assuming diurnal and nocturnal northern wind speeds of 8.88 and 3.75 km per hour, respectively, the northern diurnal wind needs 1.69 days to reach Cairo while the nightly northern wind needs four days. Alternatively, for simplifying, we consider a mean speed of 5 km per hour for the northern wind the year-round, and we change the distances of the initiation geographical keystones from Cairo for both the Marine and ET percent contribution signals. The top left corner labels marked as +180, and +60 km (top diagram) show two initiation keystones for the Marine vapor source, one at +180 km to the north of Cairo, i.e., at the Mediterranean coast, and the other at +60 km, i.e., inland in the Nile Delta nearby the capital. The percent contribution of the coastal Marine vapor source (at +180 km) will be subject to acute modifications as it travels southward to Cairo. It will reach the capital after 36 hours, i.e., with a 12-hour time-lag, while the ET source signal, initiated at the +120 km keystone to the north of Cairo, will have no time-lag, as it takes 24 hours to reach the capital when transported by the same northern wind speed of 5 km per hour. The notable time-lag for the Marine percent contribution will lead to the significant dispersion observed in Figs. 11 and 20 for the relationship between d18O and WP of the Marine vapor source. Such a high scattering (Risi et al., 2020) is due to the active blend mingling of the Marine vapor controlled by the dominant northern wind activity. The Marine vapor is isotopically enriched, and has higher WP values, by daytime; still, the inverse is right for nighttime. These differently marked isotopic-humidity signatures of the Marine vapor source by daytime and nighttime will actively mix along with the signal trajectory southwards, and such blending will cause the observed high dispersion for the relationship between d18O and WP of the Marine vapor in Cairo downtown, plot Figs. 11 and 20. There is no such scattering for the relationship between d18O and WP for the ET source since the ET signal goes only by the daytime. At the coast, there is no ET component, i.e., the local AVM is just a binary mixture of the Marine and Tropospheric vapor sources. It is noteworthy to observe the increase in the percent contributions of ET and Sahara vapor sources by daytime vs. the decrease in their nocturnal contributions. In contrast, the change in the Marine vapor source contribution depends on its signal downtown Cairo city and its lag-time controlled by the wind speed and distance of the concerned location from the Mediterranean coast. The aerodynamics prevailing between Cairo and the Mediterranean coast will always result in the shown different shifts in the signal arrival of the three vapor sources to the capital, according to the corresponding time-lag for each (12 hours for the Marine contribution, but zero hours for the ET and Sahara contributions). The vertical grey stripes in the top and bottom diagrams show the nighttime hours each day.
Fig. 7 has two vertical axes. The reader must keep an eye on plot #11 in Figs 6 to get an idea about the relationship between the Marine vapor source mass contribution, WP, and the isotopic ratio of the AVM. The primary vertical axis in Fig. 7 reports the relationship between d18O of the AVM and time (large grey squares) while the secondary vertical axis is in use to communicate the relationship between the percent contributions of the three vapor sources and time on the x-axis. Accordingly, please do not refer to the Marine data-points in Fig. 7 and wrongly look for elusive projections for them on the primary vertical axis in this diagram. Otherwise, the wrong projection will lead to the false impression that the Marine vapor source data-points show up enriched isotopic compositions. Visibly, this alarm-statement also applies to the ET vapor source data-points.
Figures 1 to 12. The twelve diagrams are showing comparisons of three vapor source contribution in the local AVMs, at Cairo city, namely the Tropospheric (Sahara) vapor, Marine vapor, and ET. The first and second rows have the specific humidity, S, values on the x-axis, while the third and fourth rows show 18O. The first and third rows have the percent input on the y-axis, while the 2nd and fourth rows show the weight parameter, WP, with WP value = S * input % / 100.