Not all irrigated lands are readily subject to damage from ponded surface waste water. The coarser textured flat lands absorb water rapidly. On the tighter and steeper lands, however, irrigation without surface waste by any method is difficult, and most of the rain that falls may be lost in runoff.

High water tables or accumulation of excess salt have occurred in parts of nearly every irrigated project in the West. Development of drainage systems concurrently with land development of an irrigated project usually prevents trouble.

Another serious problem in the West is how to apply irrigation water to crops without washing away soil. Anything done to improve the structure of the soil usually increases the rate at which the irrigation water is absorbed. An increase in the rate of intake means that larger irrigation streams must be delivered to the fields.

Even though the erodibility of the soil may be reduced somewhat by proper soil management, the effect of the larger streams more than offsets any gain in stability of the soil. Although an improvement in soil structure usually makes necessary a larger irrigation stream, increasing the effective depth of the root zone tends to allow the use of smaller streams.

Water applied to soil immediately after plowing or cultivation causes breakdown of clods and filling of larger pore spaces with soil particles lubricated and made buoyant by water. Water thus tends to settle, or compact, the soil and to increase its density. Gradual consolidation of the cultivated surface continues for several irrigations, after which it becomes more or less stable. With further irrigation, increased intake rates can occur because plant foliage and crop residues usually reduce the velocity of flowing water and increase the wetted areas.

Investigations at Prosser, Wash., have shown that an eightfold variation of furrow intake rates occurred within a given season. This is in addition to variations that occur from season to season, depending on previous crop history, the crop irrigated, moisture content of soil, cultivation, and stream.

Irrigation water applied improperly also causes segregation of the soil particles. The smaller particles are moved downslope and tend to increase the intake rate at the upper end and seal off the lower end of the field. Uniform irrigation throughout the field thus is harder to achieve. Proper irrigation and soil management will help prevent particle segregation.

Uncontrolled irrigation water can cause erosion. It can strip soil from bare sloping fields at an alarming rate: The very water that is so essential to crop growth can also be the means by which the land is ruined.

Changes in absorption rate of soil during the cropping sequence must be kept in mind when the system is designed. Obviously, water need not be in contact with the soil for as long a time when the absorption rate is high as when it is lower to store the same amount of water.

Thus, for the more permeable soils, irrigation streams should be large but should be applied for a short time. Thus the irrigator can get the water across the field rapidly so that no more is absorbed than is desired for good irrigation.

Also (since the rate at which the soil absorbs water declines with time), the shallower the irrigation, the larger the stream needed but for a shorter period of time. Thus, the faster the intake rate of the soil and the less depth applied each irrigation, the larger the stream must be.

When sprinklers are used, the intake rate of the soil governs the size of the stream only as far as the maximum intake rate of the soil is concerned. If water is applied faster than the soil will absorb it, losses of water will occur by surface runoff.

Equally important is the management of irrigation water for good crop production after it has reached the farm. When to irrigate and how much water to apply are questions that continually confront the farmer.

Excessive use of water often can aggravate a drainage problem and reduce yields by allowing deep percolation and leaching of soluble nutrients below root depths. Proper scheduling of irrigations to maintain soil moisture conditions for continuous growth of the plant is important.

To do a good job of irrigating, a farmer needs to know something about how much water is needed to grow a crop; the amount of water that can be stored in the soil-root reservoir; how much of the stored water that can be used before reapplying water; the amount of water withdrawn by the crop at the time he irrigates; and the length of time that water must be in contact with the soil to replace the amount used.

The farmer can eliminate some of the guesswork involved in water management by using the irrigation guides of the Department of Agriculture.

The formulation of irrigation guides requires information about the amount and the rate that crops use water. This phenomenon is referred to as consumptive use or evapotranspiration the water transpired and evaporated from a given area and is an important element in the hydrologic cycle of water movement from the time it falls on the land until it returns to the atmosphere or reaches the ocean. It is the best index of how much water will be needed to produce good crops.

Measurements of consumptive use of water by various crops and natural vegetation have been made under many different physical and climatic conditions. Methods have been developed to estimate consumptive use for a given locality, based on a correlation of climatological data with actual measurements. These methods give the total consumptive requirement of the crop regardless of the source of the water.

In areas where ground water is not a major contributor, the amount of water that must be supplied by irrigation depends on how much precipitation can be utilized to meet the consumptive needs of the crops. The contribution may vary from practically nothing (as at Yuma, Ariz.) to all crop needs. How nearly precipitation meets the needs of the crops depends on the annual amount of precipitation and on the time and way it occurs. It also depends on how much water can be stored as soil moisture in the root zone of the crop at any time. The usable amount varies from place to place and with crop and soil conditions. The more precipitation the root zone of the crop absorbs and stores, the less irrigation water will be required. Thus the consumptive use of the crop minus the effective rainfall gives the net irrigation water requirement.

Values of normal consumptive use of water by each of the major crops and effective rainfall are available for each irrigated area of most States of the West through publications of the State experiment stations or the Soil Conservation Service.

The depth of water required in each irrigation depends on how large the soil moisture reservoir is and how nearly it can be emptied between each irrigation. A medium-textured soil normally stores about 2 inches of usable water per foot of depth of soil. Not all of that water should be removed before irrigating again, however. Usually not more than about 1 inch is removed if the crop is to grow most rapidly. Thus, if the root zone of the crop is 3 feet deep, about 3 inches of water should be stored each irrigation. A farm efficiency of 75 percent would require that 4 inches of depth or 4 acre-inches per acre should be delivered to the farm head-ate for the irrigation. Assuming a medium rate of use of water by the plants of 0.2 inch a day, the 3 inches stored would last 15 days, or the irrigation interval would be every 2 weeks.

The time required to apply an average of 4 acre-inches per acre would depend upon the size of the stream delivered to the farm and the acreage to be irrigated. A stream of 1 cubic foot a second (1 c.f.s.) is approximately equivalent to 1 acre-inch of water an hour. Thus, if the irrigating stream is 4 c.f.s., an average depth of 4 inches could be applied to an acre each hour. At 75 percent efficiency, 3 inches of depth or more would be stored in the crop root zone.

Plants do not withdraw moisture at equal rates from all depths of the root zone. Much of the greatest use is from the top half of the root zone depth. When all available moisture is extracted from any appreciable part of the root zone, growth slows down. Thus irrigation often is considered desirable while 35 percent to 50 percent of the total available moisture is still left in the root zone. An example: A 4-foot root zone that could store 2 inches of usable water per foot of depth would mean that irrigation should start when about 4 inches (or not more than 5 inches) had been used by the crop. The amount of available moisture that a soil can store for plant use influences the frequency, size of streams, size of farm laterals, and other elements of design. In general, inches of available moisture that can be stored in a foot of sand range from 0.25 to 0.75; loamy sands, 0.75 to 1.25; sandy loams, 1.00 to 1.50; fine sandy loams, 1.50 to 2.00; clay loams, 1.75 to 2.25; and clays, 2.00 to 3.00.

Some plants go deeply into the soil. Others have shallow roots. If the roots can extract water from as deep as 6 feet, instead of only 3 feet, it will be more efficient to apply more water at each irrigation and the irrigations will be less frequent. With the greater depths of water applied at each irrigation to the deeper rooted crops, lengths of irrigation run can be greater or the size of irrigation streams may be smaller and still give efficient irrigation.