Irrigation in the East

T. H. Quackenbush and M, D, Thorne.

The irrigated acreage in the 31 States in the humid region in 1954 was 70 percent greater than that in 1949, according to the 1954 Census of Agriculture.

The increase probably occurred as a result of successive years of severe drought and the realization of production increases that can be achieved by irrigation under such conditions.

Many think this "emergency" increase in irrigation is only a natural consequence of the unusual weather and that if a period of years of "normal" rainfall occurs again, the irrigated acreage in the East will be reduced markedly.

Others contend that farming practices have so changed that farmers can no longer afford the risks associated with unfavorable rainfall distribution in any year. They reason that production costs of some crops have increased so much that a further investment for irrigation is necessary to insure production every year.

In order to examine accurately the probable future of irrigation in the humid region, we need to know what the normal situation is with regard to moisture supply for crops and the frequency and extent of the departures from normal that are to be expected. Then we need to know what increases in yield we may expect if we eliminate these periods of moisture deficiency by means of irrigation. We need to know how much water will be required and whether we can obtain this amount of water with legal right to use it for irrigation. Also, we need to know the best methods and the resulting costs of developing the supply, conveying it to the farm site, and applying it to the crop. We need information about the effect of an irrigation program on the other farm operations, such as drainage, fertilization, tillage, and cropping.

Many of the basic principles of irrigation developed through 100 years of experience and 50 years of research in the drier Western States can be applied to the humid areas. In order to take full advantage of this information and adapt it properly to humid conditions, it is important to know the following basic principles that make irrigation in humid climates different from irrigation in the West.

1. Good yields of farm crops can be obtained without irrigation in many years. The margin of profit from irrigation is much smaller while installation costs are comparable. Maximum utilization of equipment and labor therefore is needed to keep irrigation on a sound economic basis.

2. Provisions must be made in humid climates to dispose of surplus rainfall. This requires additional precautions against soil erosion and provisions for adequate drainage.

3. Farm crops generally do not root so deeply as in the and areas. Winter rain and snow can be relied on in most years to replenish the soil moisture that is used from greater depths. This requires lighter and more frequent irrigations and the off-season irrigations (before planting or after the harvest) will not be required.

4. Seasonal requirements of irrigation water are not so great. Short-time, peak-use requirements may be nearly as large, however.

5. It should be kept in mind when considering surface irrigation that a large part of the benefits obtained from land leveling can be charged to the better surface drainage provided.

6. Soils underlain with fine-textured subsoils, which modify root depths, moisture-holding capacities, intake rates, and land leveling operations,are more widespread in the areas being irrigated.

DROUGHT PROBABILITY in the humid region can now be predicted fairly accurately. For example, we can predict that over a great many years an average of 2 years out of 10 will have droughts of a stated severity. We can calculate the probability that a drought of any specified duration will occur during any month of the growing season of any crop. We can also calculate the amount of irrigation water likely to be needed to alleviate this drought. Because assumptions have to be made in this procedure, the results are not completely accurate, but they are probably good enough for most uses for which they are intended.

A drought day, for this purpose, is considered to be any day when the available soil moisture in the root zone has been depleted. Separate computations are then made for various values of available soil moisture--1 inch, 2 inches, 3 inches, and so on.

The rate the crop uses moisture is calculated from weather data, such as sunlight, humidity, and wind, for the desired location. These moisture-use data are then balanced against the actual rainfall recorded at the location in a sort of bookkeeping procedure.

We do this for as many previous years as we have weather records available and establish the patterns, which can be used to predict probability of occurrence of the drought periods in the future.

The equations used to compute moisture use from weather data are being checked continually by actual field measurements under a variety of conditions and probably will be improved further in the coming years to increase the accuracy of prediction. It is not possible, however, to predict when the drought years will fall or when during any one year a drought period will come.

This procedure is adapted particularly to the humid region, where the Soil root zone is at field capacity when the growing season begins. Computations of this sort are not so simple for an and region and probably are not so essential where droughts are to be expected every year and where rainfall may contribute only a minor portion of the water needed for the crop.

A drought survey of this type has been completed for North Carolina, and similar studies have been started for some other Southeastern States. It is planned to extend the studies to cover all the region where the technique is applicable.

The user of this type of information must know the moisture-holding ability of his soil and the depth of rooting of his crop in order to decide which level of available moisture he should use in his situation. Then he can predict with fair accuracy the probability that he will have droughts of varying frequency in the years to come.

Again, it must be emphasized that a study of this type will not tell him which years the drought will come, but only what are the longtime chances of having droughts.

Also, if his chances are 3 out of 10 that at least 20 drought days will occur in July and he has just had 3 successive years with July droughts of this severity, his chances that the coming July will have 20 or more drought days are still 3 out of 10, a fact that often is not realized.

The study in North Carolina shows, for example, that all sections of the State may expect at least 17 drought days in June and July in 2 out of 10 years if the storage capacity for moisture in the soil available to plants is 2 inches. The Coastal Plain can expect a minimum of 34 drought days on the same basis. All parts of the State would require at least 8 inches of irrigation water to overcome the drought that may be expected during the growing season in 9 out of 10 years, with a moisture storage capacity of 2 inches. Except for the mountain regions of the State, the probability that any day in June will be a drought day is at least 30 percent for a moisture storage capacity of 2 inches; however, the probability of a drought day in June is insignificantly low, except for the lower Coastal Plain.

Information of this type for the entire humid region will help put irrigation planning and practice on a much sounder basis.

CROP RESPONSE to irrigation must be known in order to plan an irrigation program if it has been determined that the probability of drought occurrence is such as to indicate appreciable deficiency of water for the crop.

Many determinations of this response have been made, both under experimental conditions and in irrigation practice in the field.

Many of these investigations in the humid region were made to explore the possibilities of irrigation under local conditions. Consequently the data may have only limited application.

At Blacksburg, Va., in 1954, for example, 3 inches of water applied during tasseling and through the milk stage increased corn yields 38 bushels over the nonirrigated yields of 65 bushels an acre. Three additional inches applied before tasseling increased the yield 55 bushels an acre over the unirrigated plots. Full-season irrigation increased the yield 64 bushels an acre over the controls but required 10.5 inches of water. In each instance, water was applied whenever available soil moisture was 50 percent depleted during the specified period.

Information of this sort is valuable to those interested in irrigation in the immediate vicinity. The information given, however, would tell us little about how frequently we could expect this magnitude of response to the same treatments at this location. Nor does it tell us whether we would have measured the same response if the treatments had been applied to corn on another soil type in the vicinity that same year.

Increases in yield from irrigation are more meaningful when we know the frequency of occurrence of the drought condition encountered and when we have a measurement of the moisture conditions of the soil or plants in units that permit us to reproduce them on other soils and in other environments.

Current thinking is that drought surveys will provide the former and that irrigation treatments specified according to moisture stress or moisture tension will provide the latter.

Moisture stress and tension are terms that refer to the energy with which moisture is held in the soil. It is believed they give a good measure of the work that plant roots must do to get the moisture hence of the degree of availability of the moisture.

The percentage of available moisture gives no general indication of the degree of availability. The data from Virginia would thus have much greater applicability when the moisture stress corresponding to the 50-percent available moisture were determined.

Yield results from more than one level of irrigation are required in order to determine the true value of irrigation. Sometimes the conclusion has been drawn that irrigation does not pay in the humid region because only one irrigation treatment was tried and this was an unprofitable one but another rate or frequency of application might have been much better.

Some data in the humid region provide the information in this form.