This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
In addition to the water lost by natural evaporation and by shallow cultivation, a vast loss is sustained owing to the moisture that is given off from the leaves of the crops. Stephen Hales (b. 1677, d. 1761) was the first to discover that leaves gave off moisture, and an account will be found in his Vegetable Staticks, or Experiments on the Sap of Vegetables, published in 1727. The quantity of water taken out of the soil by various crops is stated by H. W. Wiley, in his Agricultural Analysis, to be as follows per acre: -
Equal to Inches of Rain per Acre.
Grape vines ...
The accuracy of these figures may be doubted. If an acre of Sunflowers, for instance, required 5619 tons of water (equivalent to 56 in. of rain) to mature, it is obvious that they could not be grown in many parts of eastern Great Britain, where the total annual rainfall only averages 24 or 25 in., or 2400 to 2500 tons per annum. As the Sunflower crop would require only about 150 days at the most to mature, from start to finish, something like 38 tons of water (or .38 in. of rain) would have to fall on an acre of ground each day. Assuming that 10,000 Sunflowers were grown to the acre, this would mean that each plant would absorb and transpire about 8 1/2 lb. of water per day.
Professor Bentley, in his Manual of Botany, states that "a common Sunflower, 3 1/2 ft. high and weighing 3 lb., gives off on an average 20 oz. of water; and a Cabbage plant about 19 oz. of fluid in a single day".
It may be remarked that a Sunflower 3 1/2 ft. high and weighing 3 lb. is a poor specimen. Taking an average specimen, 6 ft. high and about 6 lb. in weight, it bears about 30 leaves, each with a superficial area of about 45 sq. in. The total transpiration leaf surface for one Sunflower is therefore about 1350 sq. in. Assuming that a plant of this size will transpire only 24 oz. of water per day for 150 days, each plant will transpire in the season 225 lb. of water from its leaves, or over 1000 tons for a crop of 10,000 plants to the acre. Assuming also that each plant, when fully grown, contains 4 lb. of water, that would give 40,000 lb., or about 18 tons, more moisture taken from the soil. It would therefore appear that an acre of Sunflowers would require about 1020 tons of water (equal to 10 in. of rain) in the course of the year.
The figures on p. 120 give an idea as to the approximate quantity of water taken out of the soil during the growing season by various crops.
It will thus be seen that such crops as Sunflowers, Jerusalem Artichokes, and Cabbage crops require at least 10 in. of rain in 150 days to enable them to flourish, while Beetroot and Lettuces require over 23 in. of rain, and Runner Beans require about 26 in. in the course of about 100 days. If all the rain that falls is not absorbed by the soil, it is evident that the crops must suffer, unless moisture can be kept round the roots in some way.
Mr. A. D. Hall, in his book on The Soil, calculates that 300 lb. of water transpired is equivalent to 1 lb. of dry matter. It is obvious that the amount of water given off will depend largely upon the season, whether wet or dry, hot or cold, and also upon the way the crops are cultivated, and whether they are planted at proper distances apart, and are in a free-growing healthy condition, free from insect pests and fungoid diseases. The nature of the crop itself must also be taken into account. Some plants containing very little dry material (e.g. Lettuces, Turnips) would give off more moisture than others of a more woody nature. And again, some very fleshy plants, like most of the Cactaceae, many of the Euphorbiaceae, and of the Asclepidese (like Stapelias, Haworthias), and such plants as Stonecrops, are specially adapted to conserve their moisture even in the hottest weather, owing to the very few stomata on their surfaces. Many of these are protected from the glaring sun by hairs, spines, bristles, or by a waxy "bloom", consequently the amount of moisture given off from their surfaces is very small.