Erosion Controls part 1
by Hans G. Jepson, assistant agricultural engineer, Soil Conservation Service
Originally appeared as PREVENTION AND CONTROL OF GULLIES
Farmer’s Bulletin No. 1813, Issued September 1939, Washington, D.C.
U.S. Department of Agriculture
NOTE: Some common practices and specific procedural and specie recommendations back in 1939 have fallen from favor in recent years. For example, in many parts of the southern U.S., Kudzu is seen as an accursed problem. We present this reprint unedited so that you may pick and choose how it might apply to your understanding and problem solving palette. We trust that you will make pertinent and intelligent inquiries in your region with regard to plant varieties and procedures.
It is a common conception that gully control means building check dams, planting trees, plugging gullies with brush, or directly applying to a gully some other individual control measure. This way of thinking focuses attention on devices that stop gullies rather than on ways of farming that prevent gully erosion. It overlooks the limitations of trying to check gullies by some individual control measure and neglects the necessity of properly coordinating means of control. Uncoordinated individual control measures frequently lead to wasted effort, unnecessary expenditures, and disappointment in the performance of the controls used.
A broad, coordinated attack is in general necessary to keep gully erosion under control. A farmer who wishes to keep his fields free from gullies must give first consideration to proper land use and conservation farming on areas that contribute run-off to the gullies. He must have an understanding of the best way to take care of the run-off from those areas. He will make wide use of vegetation in replanting gullies and will therefore need information on the selection of vegetation best suited to local field conditions. He will want to know the particular uses to which plants can best be put and how vegetation can be successfully maintained.
To provide general information useful in such an approach to gully control is the purpose of this bulletin. It describes various types of treatment and states the principles that should be considered in selecting and using these means of saving the land from gullying. It shows how gully-control measures can be used effectively in coordination with recommended soil and water conservation practices.
WHAT GULLIES DO
“Since the achievement of our independence, he is the greatest patriot who stops the most gullies.” If this was true 150 years ago, when Patrick Henry made this statement, it is doubly true today. For gullies are now destroying land in every State. They have eroded many fields so badly that it has been necessary to discontinue the cultivation of areas that were good farm land only a few years ago. Year after year fields are abandoned as the old gullies take more of the land and new ones form.
When lands are gullied, the fertile soil is carried away, and unproductive soil may be deposited on rich bottom lands. Reservoirs and channels are also silted up, which then have to be dredged at great expense. Gullies that cannot be crossed readily by teams and farm machinery divide fields into smaller units and thus increase the cost of cultivation. As gullies tend to drain adjacent soil of its moisture, fields dry out much more rapidly near the gullies. This reduces crop yields on these fields. As these gullies become larger they branch out over the fields; and if they are allowed to develop unchecked, entire fields may have to be abandoned.
Gullies encroach upon public highways; undermine fills, bridges, and culverts; increase maintenance costs; and make travel unsafe. Livestock grazing near the edge of undermined gully banks is endangered. Gullies occasionally extend through a farmstead, undermining the farm buildings and making it necessary to remove them. The unsightly appearance of a gullied farm reduces its market value. These destructive and unsightly ditches have already caused damage that amounts to millions of dollars-much of it a needless loss, had attention been paid to the things that give gullies their start.
HOW GULLIES START
Before we destroyed the native timber, plowed up the virgin sod, and allowed large herds of sheep and cattle to overgraze the range erosion was not a serious problem. Under nature’s cover of vegetation the soil absorbed much of the rainfall and was protected from excessive erosional losses.
In order to make way for the production of crops the land was cleared and plowed. No precautions were taken against loss of soil. On sloping fields the rows were run regardless of the direction of the slope. As a result the amount and velocity of run-off increased until it readily carried away large quantities of soil. At first the soil was removed from the surface in very small rivulets. They gradually became larger, and eventually gullies were formed, which enlarged with each succeeding rain that produced run-off.
Wherever the natural protection of the land is destroyed, the soil is made more vulnerable to erosion. Natural drainageways covered with vegetation once carried the run-off from the land. Stripping these drainageways of their natural cover and cultivating across them (fig. 1, A) or subjecting them to other undesirable farming practices is the beginning of many gullies. Steep slopes cleared for cultivation (fig. 1, C) will soon be badly gullied.
Poorly placed or poorly protected outlets for farm-drainage systems or improperly designed irrigation channels are points where many gullies begin. Improper location and protection of drains for farm roads or highway systems are an invitation to a gully to take the adjacent land. Trails or ruts over sloping fields (fig. 1, B), also contribute to some extent to the formation of gullies. The diversion of run-off into drainageways that are not well enough protected to carry the additional load is one of the surest ways to give gullies a start.
TYPES OF GULLYING
The careless use of land makes it possible for gullies to form. Gullying proceeds by waterfall erosion, channel erosion, erosion by alternate freezing and thawing, or a combination of these three types. Each type of gullying has a characteristic form (fig. 2), which may be modified to a considerable extent by local soil characteristics. If the underlying soil materials are soft and easily incised, deep, straight-walled gullies are formed; if the subsoil consists of plastic, resistant clays, the gullies are relatively shallow with sloping banks.
Water falling over the edge of a gully or the bank of a ditch forms deep and very rapidly extending gullies. Their characteristic form is a U-shaped cross section (fig. 2, A).
A small vertical overfall usually develops in the lower reaches of a drainageway, and water falling over it undermines the edge of the bank, which caves in, and the waterfall moves upstream. As the overfall advances up the slope its vertical height increases, since it usually leaves a relatively flat slope below. This undermining goes on rapidly, particularly if the surface soil is underlain by sand or easily eroded subsoil.
In this manner gullies often start in the bank of natural watercourses that have been eroded to a great depth. They extend back into the drainage area and grow deeper up the slope, often attaining depths of 50 to 60 feet or more. As they extend backward and cross lateral drainageways or natural depressions, waterfalls are in turn formed in the sides of these depressions, and branch gullies develop. This branching may continue until a network of gullies covers the entire drainage area. Their growth is dependent mainly on soil characteristics, depth of overfall, and the size of the contributing drainage area rather than on the slope of the land. They may extend very rapidly even through almost level land. They frequently grow at a rate of 30 to 50 feet a year, depending on the amount of run-off and the character of the soil. Some of them have been known to advance several hundred feet during a single heavy rain.
In the Pacific Southwest some of the larger gullies formed in this way are known as barrancas; in the Northwest they are called coulees; in the Colorado Basin they are commonly referred to as arroyos or washes.
Channel, or ditch, erosion is essentially a scouring away of the soil by concentrated run-off as it flows over unprotected depressions. Gullies formed by channel erosion usually have sloping heads and sides (fig. 2, B). In fact, these gullies are often referred to as V-gullies. As the scouring continues the gully becomes longer, deeper, and wider. V-gullies often attain lengths of 1 mile or more and depths and widths of 20 to 40 feet. The extension in length is usually much faster than the widening of the gully because a greater volume of run-off passes over the gully head than over the sides. Usually the gully does not advance beyond the divide of the drainage area, but it may continue to widen and deepen for years. The rate at which the gully deepens is very rapid on the upper part of the area, where the slopes are comparatively steep, and generally it decreases in the lower reaches as the slope decreases. Silting, rather than erosion, may occur if the channel or ditch emerges into a wide, flatbottom drainageway. The increased volume of water, however, may offset the effect of moderate changes in slope along the lower reaches.
Channel erosion and waterfall erosion are commonly found in the same gully. The extension of the vertical head is usually by waterfall erosion; and the scouring of the sloping bottom and sides by channel erosion extends the depth and width. Gullies frequently start by channel erosion, and as an overfall develops at the head of the gully, the gully continues to develop by waterfall erosion. Gullies formed by channel erosion alone are often a series of closely spaced, parallel, V-shaped gullies in the upper reaches of drainage areas, where slopes are fairly steep and the contributing area small. Channel erosion is usually present in gullies caused by waterfall erosion, except those newly formed.
EROSION BY ALTERNATE FREEZING AND THAWING
Erosion by alternate freezing and thawing is prevalent in parts of the South, where alternate freezing and thawing temperatures are common in the winter and precipitation is generally in the form of rain. Alternate freezing and thawing loosens the soil, which sloughs off and is then carried away by heavy rains. This occurs on all slopes of a gully bank (fig. 2, C), and particularly on southern exposures. Gullies formed in this way may extend in all directions, as the direction of growth is not determined by the slopes of a field. Gully erosion by alternate freezing and thawing usually supplements waterfall and channel erosion, particularly in the Southern States. It may continue for years as the only form of erosion in gullies that are near a drainage divide and have little or no contributing drainage area.
In certain localities, during or immediately following prolonged rains, and especially following periods of alternate freezing and thawing, mass movements of soil in the form of slides, earth flows, and slumps take place on steep slopes. A considerable number of gullies have their beginning in these disturbed or galled areas, particularly where the displacement exposes highly erodible subsurface material. These mass movements usually develop conditions that are conducive to gullying.
PREVENTION OF GULLIES
This bulletin deals with the control of gullies that begin as a result of man’s abuse of the land. We can prevent these gullies. To prevent the formation of a gully is much better and easier than to control it once it has formed. We can never prevent erosion entirely because natural deterioration will continue as long as there is action by wind, water, or frost. But this natural geologic erosion is generally so gradual and moves at so slow a pace that it does no appreciable harm.
It is the erosion that arises from improper land use and methods of tillage that those who work the land can control. If a farm is free of gullies (fig. 3) it will usually be found that the operator, under a well-developed land use program, uses good farming methods and is constantly on the alert against gullying. Wherever and whenever he finds a danger spot he immediately takes steps to prevent the formation of a gully. It is easy to stop a gully when it has just begun to form, but if the gully is neglected for some time it can usually be checked only with considerable work and at no little expense. It should not be assumed, however, that just because no gullies are visible on a field no erosion is taking place. There may be considerable sheet erosion, which is a forerunner of gullying unless it is checked.
Constant vigilance is necessary to forestall gullying that starts from some practice that may seem of no consequence. The thoughtless driving of a wagon up or down a slope in a field made soft by rain leaves a deep rut that may develop into a large gully unless it is in some way filled in immediately. Filling the rut with straw or manure, or even spading it full, will usually prevent further damage.
Many farms have deep gullies because the feed lots were not properly located or because the stock trails cut deep into sloping pasture lanes (fig. 1, B). Such gullies can usually be prevented by shifting fences as need arises and by rotating feed lots if gullies should begin to form. Care should be used in the location and protection of drainageways, of watering places for livestock, and of roads or trails.
Unless caused by such minor things as drain outlets or stock trails, gullying is usually preceded by sheet erosion. A close examination of sheet erosion shows that the soil is not removed in strictly uniform sheets or layers, as is so often supposed, but that numerous small rivulets are formed, which might be classified as miniature gullies. They are so small and close together that this process of soil removal is usually spoken of as sheet erosion. When water from several of these rivulets collects, larger depressions are formed, and they may finally become gullies. It thus is evident that where heavy sheet erosion has been under way for a period of time, gullying is probably imminent. Little sheet erosion occurs on a farm if its steep slopes are covered with trees and shrubs, its flat land and moderate slopes farmed according to approved cropping and tillage practices, and intervening areas reserved for permanent grasses (fig. 4).
The first step in preventing gullies is to plan or replan the farm so as to get the best possible use of the land. This will include the retirement to permanent cover of such areas as are definitely too steep to farm; the utilization of the better agricultural land for cultivated crops; the placing of moderately sloping and eroded areas in meadow or pasture, if such areas cannot be economically tilled. Good land use may require that the general field pattern be considerably changed and only the most suitable areas used for crops. In many instances fences will have to be reset and field roads rerouted to get the best arrangements.
The best known methods of controlling erosion on those slopes that must be tilled are crop rotations, cover crops, strip cropping, and contour cultivation, alone or in combination with terracing where it is required. These practices are discussed in Farmers’ Bulletins 1776, Strip Cropping for Soil Conservation; 1758, Cover Crops for Soil Conservation; and 1789, Terracing for Soil and Water Conservation. Insofar as the methods of farming described in these bulletins control erosion they aid directly in preventing the formation of gullies. The use of fertilizers and the conservative grazing of pasture or range also protect the land from erosion.
One of the most important considerations in land use that prevents gullies is the proper disposal of excess run-off water from the fields. Every farm has its own natural drainage patterns, which, in general, it is difficult to change. This pattern includes all water-conveying depressions or channels of either continuous or intermittent flow. Except for minor variations it should be followed in planning fields and, especially, in locating outlets for terraces or diversion ditches. Natural drainageways should be used, wherever possible, and they should be left in sod in order that they may continue to carry water without gullying. The extra ground that may be cropped by farming these drainageways can in no way compensate for the damage that will occur if severe gullying is begun by cultivating the drainageways with the rest of the field.
If the general drainage pattern is not considered or followed in planning for land use, either a very expensive artificial system to dispose of surplus run-off will be required or the water must be concentrated in undesirable locations that will eventually gully. If a natural drainage unit is not complete on one farm, but covers two or more farms, a better water-disposal plan can be worked out if the farms are considered collectively.
If the results of careless cultural practices could have been foreseen or if just a little time had been spent in checking the beginning of a small gully, many of the present large gullies would never have formed. Once the gullies have formed and prevention is too late, it is still possible to stop serious erosion in the gullies and cover most of them with vegetation.
PLANNING GULLY CONTROL LAND USE AND CONTROL COSTS
The size of both the gully and its drainage area are of great importance in planning the control of gullies. In order to avoid confusion as to what will be considered large or small gullies, it seems desirable to classify them. The differences of opinion as to size groupings makes such classification difficult. A large gully in the northeastern part of the United States may be considered a medium-sized gully in the Southeast. The following classification of gullies according to size is suggested for general use. The range of these groupings is necessarily wide in order to accommodate the wide variations in field conditions.
- Small gully: Less than 3 feet deep.
- Medium-sized gully: Three to 15 feet deep.
- Large gully: Over 15 feet deep.
It is recommended that gullies be further classified according to the size of their drainage area. A small drainage area is 5 acres or less; a medium-sized drainage area, 5 to 50 acres; and a large drainage area, over 50 acres. According to this classification a small gully with a medium-sized drainage area is a gully less than 3 feet in depth with a watershed of 5 to 50 acres.
If a gully directly menaces a building or a highway structure it is a rather simple matter to determine how much money may be spent to protect this property as its value is usually known or can be readily estimated. Estimating the damage of gullying in a field or the justifiable expenditure for checking it is not so easy, as it is much more difficult to determine the true value of the land and the exact extent of present and ultimate damage.
“How much am I justified in spending?” This is the first question every farmer asks himself when he plans to stop gullying on his land. If a gully has already advanced within a short distance of its drainage divide it may be unwise to spend very much in controlling the gully because it can do only a little more damage. After a gully has eaten its way to the head of the watershed it usually ceases to be active, and if it is protected from livestock, natural revegetation will reclaim it over a period of time. If a gully has advanced only a short distance into the watershed there is more justification for establishing immediate control (fig. 5).
The actual acreage of the land surface destroyed by a gully may be relatively small, but this gully may so dissect a field or threaten adjacent areas as to hamper or endanger farming operations on the entire watershed, either immediately or in the future. And just one badly gullied field on a farm creates an unsightly appearance that reduces the value of the entire farm. The value of a farm on which gullying is active may therefore decrease much more rapidly than the present state of gullying would indicate.
Although severely gullied land has little immediate value some control measures are usually warranted on all such areas if only to protect adjacent lands. But it is well to determine what is the most economical and suitable protection for each gullied area. The cost of controlling a gully and the type of protection should always be considered in relation to the use that can be made of the gullied land as well as the protection to adjacent areas that such control will afford.
For example, a badly gullied field that could not be restored to usefulness as cropland except at too great expense may be retired to woodland, for which purpose it may have considerable value. If this is done, little expenditure for gully control need be made because a protected plantation over the larger part of the drainage area will ordinarily prevent further gullying.
Less seriously damaged areas may be used for permanent pasture or meadow, provided a suitable cover can be established and the cost of controlling the gullies can be kept commensurate with the returns to be expected from the land. Land that is to go into permanent pasture or meadow often requires considerable treatment to provide a satisfactory grass cover and to facilitate future farming operations.
More expensive gully treatment is usually required on gullied land to be retained for crops, but the fertility or value of the land may justify it. In general, however, it may be said that complete, immediate reclamation of gullies is too costly unless the gullies are still so shallow that they can be crossed with machinery or terraces. The expense of installing the control measures necessary to check gullies on highly erodible areas to be kept in cultivation is frequently considerable. On these areas it is usually more economical to check active erosion within the larger gullies and then reclaim the gullies over a period of years through the use of vegetation.
There is a special type of treatment in gully control that is peculiar to the arid and semiarid parts of the United States. It has been used on large barrancas and arroyos in locations where land values are high or where it has been found necessary to protect reservoirs and irrigation systems from excessive silting. For example, where a large gully is biting into valuable orchard land it may be justifiable to fill the gully completely through the use of one or more silt-retention dams. These dams can also be used if silt deposits are being carried into irrigation systems by an actively cutting gully, even though the gully head may be under control. The silt load can be materially reduced by placing large multiple-lift, overpour dams in the alluvial-fill valleys. These structures are located at sites where the grade of both the gully and valley floor is mild and where the valley floor is sufficiently broad to permit the spreading of water. When properly located, these dams serve to stabilize active lateral cutting and they will gradually rebuild the gully by collecting silt. They often serve the twofold purpose of holding back silt and providing water for supplemental irrigation.
TREATMENT OF GULLIED DRAINAGE AREAS
Complete gully control includes proper treatment of the drainage area as well as of the gully itself. If the control treatment is applied only to the gully, it is likely that all control efforts are being directed at the results and the cause neglected. Neglecting the drainage area usually makes it much more difficult, if not impractical, to control a gully satisfactorily and often leads to the formation of new gullies faster than the old ones can be checked. For example, if an eroding field is terraced and nothing further is done to control erosion on adjacent fields it is likely that gullies will gradually encroach upon the terraced field and damage it.
If a drainage area is gullied, it will require more complete treatment than if the gullies were not present. For example, regular cropping and tillage practices that conserve the soil may permit the safe production of farm crops on an ungullied drainage area, but if gullies are present it may be necessary to put a permanent cover of vegetation on the area. Or a relatively flat, ungullied drainage area may be cropped without excessive soil and water losses if only crop rotations, contour tillage, and strip cropping are practiced, whereas the presence of a few gullies on that area would make it necessary to use terraces in addition to these other conservation practices if the same crops were to be produced without excessive soil loss and further gullying checked.
The plan of treatment for gully control, in order to be complete, should thus include treatment of the drainage area based on good land use and soiland water-conserving practices such as strip cropping, contour tillage, crop rotations, and cover crops. These should be combined with terracing or contour furrowing where applicable. Only in this way is it possible to achieve a measure of success in the control of gullies or any assurance that such control will be more than short-lived.
A soil can absorb rainfall up to a certain rate. When this rate is exceeded some of the water begins to run off. That is why heavy and rapid downpours are much more likely to cause harmful erosion than equivalent amounts of rain that fall over longer periods. This fact is so generally understood that special terms have come into use to express this relation. For example, a slow, steady rain is ordinarily called a “ground soaker,” and a dashing downpour is commonly referred to as a “gully washer.” The more rapid the rate of rainfall, other conditions being the same, the greater the proportion of rain lost as surface run-off.
It is the run-off that loosens the soil and carries it away. Without run-off there would be no gully erosion. Furthermore, the degree of gullying is directly associated with the rate of run-off. A high rate is usually more destructive than a low rate. The rate of run-off is generally a more accurate indication of the probable damage that run-off will cause than the amount of run-off. The rate of run-off is dependent on the size, shape, and slope of the drainage area; the extent, nature, and condition of the soil and cover; the intensity and duration of the rainfall; and the slope and condition of the drainage channels. High rainfall intensities of long duration, poor plant cover, saturated, frozen, or impervious soils, and steep slopes all contribute to high run-off rates.
One must estimate the probable rate and amount of run-off from a particular drainage area before it is possible to design structures to control the run-off or even to know what structures can best be used. Run-off rates are generally of more significance than the total amounts of run-off discharged in computing required discharge capacities for control structures. A possible exception would be where control is obtained through run-off retention or absorption, in which case the amount of run-off may become the deciding factor rather than the maximum rates. Terraces, diversion ditches, drainageways, and spillways must have sufficient capacity to carry the run-off resulting from the maximum rainfall intensities likely to occur during the probable life of the control structure. This period is generally from 5 to 10 years for small check dams, terraces, and diversion ditches and from 20 to 50 years for the more expensive soil-saving dams. The maximum rainfall intensity that is likely to occur during a period of 5 years is usually spoken of as having a frequency of 5 years, and similarly for any other number of years. For example, a dam designed with sufficient spillway capacity to handle the run-off resulting from a rainfall intensity that the records show will probably occur only once in 10 years is said to be designed on a 10-year rainfall intensity-frequency.
It is evident that considerable experience is required to estimate run-off from a drainage area. An inexperienced man is apt to be considerably in error in his estimates. Table 1 is included for the convenience and use of those inexperienced in making run-off determinations. Unless an individual is entirely familiar with the procedure of estimating run-off rates it is suggested that he use this table in arriving at run-off values for gully-control structures. Its use will insure reasonably safe design values for even the more adverse conditions that may be encountered in the field. The run-off rates given in table 1 should not be used for the design of large or expensive permanent dams, which are usually designed for a rainfall intensity-frequency exceeding 10 years.
Suppose it is desired to know the probable run-off from a rolling cultivated field comprising about 35 acres, located in southern Iowa. In table 1 find “rolling cultivated.” Southern Iowa is in group 2, so follow the line from the “2” opposite “rolling cultivated” across the page to the column headed 35 acres. The figure here is 132. It indicates that the probable rate of run-off on a 10-year frequency from this 35-acre drainage area will be about 132 cubic feet per second. It is important that the drainage areas be properly classified according to location in group 1, 2, or 3, as it can be seen that there is considerable variation in the run-off rates in each of these groups.
It frequently occurs that a drainage area is a composite of pasture, woods, and cultivated land. In order to arrive at the run-off from such an area, the run-off from the several units should be estimated in this way. Suppose a drainage area (located in group 2) consists of 60 acres of land with slopes of between 10 and 30 percent, which is classified as “hilly,” 30 acres, or one-half of this drainage area, is in timber, and the remainder is in pasture. From table 1 the run-off from 60 acres of hilly timber is found to be 75 cubic feet per second, one-half of which would be 37 ½ cubic feet per second. The run-off from 60 acres of hilly pasture would be 148 cubic feet per second, one-half of which would be 74 cubic feet per second. Adding these, the total probable run-off is found to be 111 ½ cubic feet per second. It is important to note that the proper figure is computed in this manner and not by adding the runoff from a 30-acre watershed of timber and a 30-acre watershed of pasture.
Applicable methods of handling the run-off to facilitate gully control differ for various types and sizes of gullies and drainage areas. Other important factors that in part determine the best method to be used are soil types and climatic conditions. From the standpoint of economy and practicability, the methods should be considered in the following order:
- Retention of run-off on the drainage area.
- Diversion of run-off above the gullied area.
- Conveyance of the run-off through the gully.
In field practice, local conditions often prohibit the use of certain of these methods, but they should always be given consideration in the order named. No method should be discarded as impractical until thorough investigation has proved it to be so. Frequently the most practical solution will be the use of a combination of the three methods. It should be kept in mind that, wherever practical, vegetation should be reestablished in gullied areas. The methods of controlling run-off are to aid in such establishment and are not to be considered as substitutes that will make unnecessary the use of vegetation on either the drainage area or in the gully.
Whatever the method of run-off control used, it should be carefully planned and executed. Haphazard work will lead to disappointment and may actually aggravate rather than alleviate the conditions being treated. Any work that is done should further the general plan for control of the gullied area.
RETENTION OF RUN-OFF
Keeping the soil and moisture on a gullied field by means of proper land use and approved cropping and tillage practices lessens considerably the amount of run-off to be carried away through the gully and thereby reduces the amount of treatment needed in the gully itself. If these practices still permit so much run-off to enter the gully that vegetation cannot be established there, the use of subsoiling, contour furrows or ridges, listing, level terraces with closed ends, or earth fills to impound water may also be advisable. These erosion controls are not difficult to construct, and they retain considerable moisture for crops. If they can be applied over the entire drainage area of small gullies or even of medium-sized gullies that have small to medium- sized drainage areas on which the soils are absorptive, the slopes moderate, and rainfall low, little or no control may be necessary in the gully itself.
On areas having absorptive soils, subsoiling small watersheds above gullies to a depth of 1 to 2 feet has proved effective in reducing the run-off sufficiently to permit satisfactory establishment of a vegetative cover in the gully. Subsoiling requires the use of a special subsoiling machine. Machines of various widths may be used, depending on the power available. The chisel points are usually spaced on 1- to 2-foot centers, depending on the depth of penetration desired. Subsoiling should be done on the contour and may be either in strips or continuous on the field, depending on the amount of absorption required. It is frequently used in conjunction with contour ridges or absorptive-type terraces. A small subsoiled strip immediately above contour ridges or terraces will open up the soil so that much more run-off can be absorbed.
CONTOUR FURROWS AND LISTING
Contour furrows or ridges are primarily small ditches or ridges constructed across the slope with a plow, lister, or terracing machine (fig. 6). Their storage capacity is dependent on their water cross-sectional area and their spacing. Various storage capacities can be obtained by regulating the size and spacing of the furrows.
The storage capacity for contour furrows or ridges with various spacings and water cross-sectional areas is given in table 2. If, for example, it is desired to store 2.5 inches of run-off in contour furrows having a water cross section of 1.25 square feet, the furrows should be spaced about 6 feet apart. The storage capacity of the furrows given in table 2 does not include the water that seeps into the soil. The ends of the furrows or ridges are usually turned uphill and the channels blocked at occasional intervals so that, in the event of a break, all the water cannot drain from the furrow and thus encourage gullying. Lister furrows (fig. 7) have been used extensively on cultivated land in the Great Plains to keep rainfall on the fields. On gullied watersheds they can be used to keep water out of the gullies.
Absorptive-type terraces are constructed larger than contour ridges, and they provide more storage capacity, are spaced father apart, and can be farmed over if necessary. Their ends can be left partly or completely open, depending on the necessity for some drainage as a factor of safety against overtopping. Where these terraces are to be depended upon to retain all of the excess rainfall, they should be used on only moderate land slopes (generally less than 2 percent) and pervious soil types. It will usually be necessary to provide storage for at least 2 to 4 inches of run-off in areas of low rainfall, and a capacity of as much as 6 to 7 inches may be necessary in areas with high rainfall. The expense and difficulty of providing a storage capacity of 6 to 7 inches usually makes the use of adequate retention measures impractical in areas where rainfall is high. Furthermore, if crops are to be grown on a field containing these terraces, the retention of an excessive amount of water may damage the crops before the water can be absorbed by the soil.
On cultivated areas with absorptive soils, small- or medium-sized gullies with small watersheds can sometimes be reclaimed by placing a series of earth fills across the gullies. Their spacing is dependent on the slope of the gullies. The use of this method is limited to areas where sufficient storage capacity can be provided above the earth fills to retain the major portion of the run-off that is commonly discharged from the drainage area. The earth fills are extended above the ground level, and short diversion ditches or spur terraces are sometimes used to lead overflow away from the end of the fills in order to prevent damage by erosion. Where a series of earth fills are used and the overflow is diverted from alternate sides, somewhat of a sirup-pan retention is effected. The diverted water is finally either impounded in the gully or absorbed by the soil. The use of this or similar methods requires considerable attention. As the storage capacity of the small reservoirs is gradually reduced by silt deposition, the amount of overflow will increase and may start erosion unless sufficient plant cover has become established to afford protection.
On fields where the measures discussed in this section do not provide sufficient storage capacity to retain the run-off and keep it out of the gullies, drainage-type terraces or diversion ditches may also have to be used to divert part of the run-off.