The driest season is in September and October.
The peninsula of Arabia, with Syria, its continuation to the northwest, has some of the characteristics of the hottest and driest parts Arabia.
In the north, the driest and best months are October, November and December; in the south, December, January, February and March.
In the western and southwestern parts the summer months are the wettest and the spring months are the driest; thus, at El Paso the rainfall amounts to 2.2 in.
Summer is the rainy season, and May, June and July the driest months of the year.
More than half the rainfall occurs from July to November, the wettest month being September, with an average of 2.95 in.; the driest month is April, with an average of 1 14 in.
That in the two driest consecutive years will be about 75 °A of the mean for the 50 years.
With further experience it has become obvious that very few reservoirs are capable of equalizing the full flow of the three consecutive driest years, and each engineer, in estimating the yield of such reservoirs, has deducted from the quantity ascertained on the assumption that they do so, a certain quantity representing, according to his judgment, the overflow which in one or more of such years might be lost from the reservoir.
The actual size of the reservoir which would certainly yield the assumed supply throughout the driest periods has therefore been largely a matter of judgment.
3, where the total flow for the driest year measures too on the vertical percentage scale; the horizontal time scale being divided into calendar months.
Throughout the dry period under consideration, we find from the diagram the following quantities (in gallons per acre of drainage area) and corresponding ratios: - On comparing columns 3 and 6 or 4 and 7 it appears that so great is the increase required in the size of a reservoir in relation to its increased yield, that only in the most favourable places for reservoir construction, or under the most pressing need, can it be worth while to go beyond the capacity necessary to render uniform the flow of the two or three driest consecutive years.
It must also be remembered that the total capacity of a reservoir must be greater than its net available capacity, in order that in the driest seasons fish life may be maintained and no foul water may be drawn off.
With the exception of the alkali flats, no portion of the desert is devoid of vegetation, even in the driest seasons.
Besides, his style is that of the driest annalist.
At the time when it is hottest and driest on the coast it is raining heavily in the Andes, and the rivers are full.
Along the coast the autumn months are the wettest and the spring months are the driest; for example, at Galveston the rainfall amounts to 5.7 in.
It will find sustenance equally on the driest of soils as on the fattest pastures; upland and fen, arable and moorland, are alike to it, provided only the ground be open enough.
The western part of the province is driest, as the rain clouds often pass over the lower levels but are caught by the eastern hills.
Falling yearly; the amount decreases as one proceeds northward, and Leipzig, with an average annual rainfall of 17 in., enjoys the driest climate.
The winters are long and marked by exceedingly low temperatures, but as they are the driest season of the year, the extremes are not so disagreeable as they would be in a more humid region.
Soc. Arts, 1801, xix., 75) stated that all kinds of willows invariably throve best on the driest spots of some wet land planted by him.
Nearly all the moisture that is precipitated during six or seven months is stored up in the form of snow, and is gradually diffused in the course of the succeeding summer; even in the hottest and driest seasons the reserves accumulated during a long preceding period of years in the form of glaciers are available to maintain the regular flow of the greater streams. Nor is this all; the lakes that fill several of the main valleys on the southern side of the Alps are somewhat above the level of the plains of Lombardy and Venetia, and afford an inexhaustible supply of water, which, from a remote period, has been used for that system of irrigation to which they owe their proverbial fertility.
The narrow valleys between the hills are very fertile, having a rich soil and an abundant water-supply even in the driest seasons.
The north-east winds acquire their greatest frequency from March to June and in November, which are accordingly the driest portions of the year.
It thus happens that the driest climates in the east are those which have to south-westwards the broadest extent of mountainous ground, and that the wettest eastern climates are those which are least protected by high lands on the west.
The driest climates of the east are in Tweeddale about Kelso and Jedburgh, the low grounds of East Lothian, and those on the Moray Firth from Elgin round to Dornoch.
Cluny in the Grand Port (south-eastern) district has a mean annual rainfall of 145 in.; Albion on the west coast is the driest station, with a mean annual rainfall of 31 in.
The south-east counties are the driest portions of the United States.
The rods are reputed to be most durable when from the driest ground, and to be especially good where the bottom is chalky.
Fishing has always been important, the numerous good harbours giving security to fishing-boats; and the fact that this coast is the mildest and almost the sunniest, though by no means the driest, part of Great Britain has led to the establishment of many health >>
In the Western Division and along the south coast the driest month is usually April or May, while in the Eastern Division it is February or March.
It is equally satisfactory to know that there is a nearly constant ratio on any given area (exceeding perhaps 1000 acres) between the true mean annual rainfall, the rainfall of the driest year, the two driest consecutive years and any other groups of driest consecutive years.
Thus in any period of 50 years the driest year (not at an individual gauge but upon such an area) will be about 63% of the mean for the 50 years.
That in the three driest consecutive years will be about 80% of the mean for the 50 years.
That in the four driest consecutive years will be about 83% of themean for the 50 years.
That in the five driest consecutive years will be about 85% of the mean for the 50 years.
That in the six driest consecutive years will be about 862% of the mean for the 50 years.
Thus the benefit to the fisheries and to the riparian owners generally is beyond all question; but the cost to the water authority of conferring that benefit is also very great - commonly (according to the proportion of the natural flow intended to be rendered uniform) 20 to 35% of ' The volume of compensation water is usually fixed as a given fraction of the so-called " available supply " (which by a convention that has served its purpose well, is understood to be the average flow of the stream during the three consecutive driest years).
In conformity with the above-mentioned convention (by which compensation water is determined as a certain fraction of the average flow during the three driest consecutive years) the available supply or flow from a given area is still understood to be the average annual rainfall during those years, less the corresponding evaporation and absorption by vegetation.
If the reservoir were larger it might equalize the flow of the four or more driest consecutive years, which would be somewhat greater than that of the three; if smaller, we might only be able to count upon the average of the flow of the two driest consecutive years, and there are many reservoirs which will not yield continuously the average flow of the stream even in the single driest year.
This capacity is represented by the height of the line a'a' (drawn parallel to a a from the point of maximum surplus f) vertically above the point of greatest deficiency c, and equal, on the vertical scale, to the difference between the height c = 48% and g= 78% or 30% of the stream-flow during the driest year.
This is true, but it is only so because we have made our reservoir large enough to contain in addition to its stock of 19%, at the beginning of the year, all the surplus water that passes during the earlier months in this driest year with its least favourable time-distribution of flow.
In assuming a demand at the beginning of the year below the mean, resulting in an overflow equal in this case to b e at the end of February and increasing our reservoir to meet it, we assume also that some additional supply to that reservoir beyond the 11% of the streamflow from the driest year can be obtained from the previous year.
In relation to this supply from the previous year the most trying assumption is that the rainfall of that year, together with that of the driest year, will be the rainfall of the two driest consecutive years.
We have already seen that while the rainfall of the driest of 50 years is about 63% of the mean, that of the driest two consecutive years is about 15% of the mean.
It follows, therefore, that the year immediately preceding the driest cannot have a rainfall less than about 87% of the mean.
As the loss by evaporation is a deduction lying between a constant figure and a direct proportional to the rainfall, we should err on the safe side in assuming the flow in the second driest year to be increased proportionally to the rainfall, or by the difference between 63 and 87 equal to 24% of the mean of 50 years.
This 24% of the 50 years' mean flow is 38% of the driest year's flow in fig.
3, and is therefore much more than sufficient to ensure the reservoir beginning the driest year with a stock equal to the greatest deficiency-19% - of the cumulative flow of that year beyond the cumulative demand.
Having determined this evaporation for the second driest consecutive year and deducted it from the rainfall - which, as above stated, cannot be less than 87% of the mean of 50 years - we may, as shown on fig.
The whole diagram shows, by the greater gradient of the unbroken straight lines, the greater demand which can be satisfied by the enlargement of the reservoir to the extent necessary to equalize the flow of the two driest consecutive years.
In the illustration the c' h is a little greater, measuring 471% of the flow of the driest year.
In the same way we may group in a single diagram any number of consecutive driest years, and either ascertain the reservoir capacity necessary for a given uniform yield (represented cumulatively by a straight line corresponding with a'a', but drawn over all the years instead of one), or conversely, having set up a vertical from the most trying point in the line of cumulative flow (c or c in fig.
3 - representing, in percentage of the total annual flow of the driest year, the capacity of reservoir which it may be convenient to provide) we may draw a straight line a"' a" of uniform yield from the head of that vertical to the previous point of maximum excess of cumulative flow.
The line a" a" drawn from zero parallel to the first line, produced to the boundaries of the diagram, will cut the vertical at the end of the first year at the percentage of the driest year's flow which may be safely drawn continuously from the reservoir throughout the two years.
We find on the left-hand scale of yield that the height of the ordinate drawn to the 50-inch mean rainfall curve from 200,000 on the capacity scale, is 1457 gallons per day per acre; and the straight radial line, which cuts the point of intersection of the curved line and the co-ordinates, tells us that this reservoir will equalize the flow of the two driest consecutive years.
Similarly, if we wish to equalize the flow of the three driest consecutive years we change the co-ordinates to the radial line figured 3, and thus find that the available capacity of the reservoir must be 276,000 gallons per acre, and that in consideration of the additional expense of such a reservoir we shall increase the daily yield to 1612 gallons per acre.
In this case the left-hand radial line passes through the point at which the coordinates meet, showing that the reservoir will just equalize the flow of the driest year.
Similarly, the yield from any given reservoir, or the capacity required for any yield, corresponding with any mean rainfall from 30 to 100 in., and with the flow over any period, from the driest year to the six or more consecutive driest years, may be determined from the diagram.
M., and emptying into Upstart Bay; it receives numerous tributaries in its course, and carries a large body of fresh water even in the driest seasons.
The official figures in inches for the station at Port Blair, which is situated in by far the driest part of the settlement, were: - A tidal observatory has also been maintained at Port Blair since 1880.
In the middle, eastern and north-eastern parts of Texas the spring months are the wettest and the winter months are the driest; for example, at Waco the rainfall amounts to 4.5 in.
Mountain areas of io,000 acres and upwards, largely covered with moorland, upon nearly imper meable rocks with few water-bearing fissures, yield in temperate climates, towards the end of the driest seasons, and therefore solely from underground, between a fifth and .a quarter of a cubic foot per second per 1000 acres.
Empirical rules have grown up assigning to each district, according to its average rainfall, a particular number of days' supply, independently of any inflow, as the contents of the reservoir necessary to secure a given yield throughout the driest seasons.