C.19 Land clearing
C.20 Flood protection
C.21 Drainage
C.22 Land grading
C.23 Physical, chemical and organic aids and amendments
C.24 Reclamation leaching
C.25 Duration of the reclamation period
C.26 Irrigation engineering requirements
Land Development Requirements and Limitations
Land Conditions affecting Development Costs
Area-specific investment costs may be incurred to develop land for irrigated agriculture. These are discussed in this section under eight headings: land clearing of vegetation and rocks; flood protection; drainage; land grading or levelling; physical, chemical and organic aids and amendments; reclamation leaching; duration of the reclamation period; and irrigation engineering. The suitability of land in terms of the measures required to develop it include both physical and economic evaluation as discussed below and in Chapter 7.
C.19.1 Forested areas
C.19.2 Areas of persistent weeds
C.19.3 Removal of rocks and stones
In both forested and rocky areas the factors to take into account in assessing land units for clearing are:
i. the cost of clearing;
ii. the value of timber or other products;
iii. damage to the land as a result of the clearing operation, and subsequent effects on the land use.
The destruction of the topsoil is most serious if it is very thin and contains most of the organic matter, and overlies nearly sterile subsoil. This topsoil contains most of the nutrients and should be protected; if mechanical clearing methods are used, the topsoil may be lost with the rootball leaving an unfavourable, infertile and often acid material for cultivation. In fertile river basins, the subsoils may be intractable clays. The mechanical clearing may compact the clay severely. The topsoil with its more favourable physical characteristics may be removed or mixed in.
The land classifier should indicate areas of land that are particularly susceptible to damage and recommend where it may be possible to remove the topsoil for later replacement. This is seldom practicable, and he should point out the areas that might be cleared by hand and those that are less vulnerable and more suited to mechanical clearing. He may also recommend areas where careful supervision of windrowing is especially important, in order to ensure that it is on the contour and to avoid the common practice of blocking natural drainage lines by pushing the material into depressions.
Wherever practical, he will advocate hand methods of clearing even though these are slower than mechanical methods. The rate of clearing in regions of the world where shifting cultivation is practised can often keep pace with the rate that an area can be physically settled, where vast areas are hastily cleared by heavy equipment ahead of settlement, secondary jungle often results and a repeat clearing operation is necessary.
Removal costs depend on size and type of vegetation, local labour costs, equipment available and the area involved. Costs rise steeply as the size of individual bushes and trees and density of stand increases. Using modern equipment and in comparison with clearing costs for light brush (sage), a thick stand of pine 30-45 cm in trunk diameter could cost 40 times as much and dense jungle 120 times as much. For large tracts of land (over 2 000 ha) very heavy machinery can halve the cost of jungle clearing but the resulting damage must also be taken into account. Wherever possible, shearing blades should be used to avoid large holes and the extraction of rootballs (Clarke 1980).
Sandy soils tend to cost less to clear than fine textured soils. Clearing large trees with bulldozers tends to leave large holes where the tree stood, and soil clinging to the roots is carried to the windrows in preparation for burning. Land grading is therefore usually necessary regardless of whether sprinkler or surface irrigation is to be employed.
Hand methods of clearing can be greatly accelerated by the use of chainsaws. The cost of hand methods (plus the use of chainsaws) can be estimated by timing the following operations:
i. Underbrushing - this is to cut, as close as is possible to ground level, all grasses, vines and small diameter trees (less than 10-15 cm), facilitate access for the chainsaw crews and also, once the dead material dries off, to provide a good dry base for the subsequent burning.ii. Felling - following underbrushing chainsaw gangs (normally one operator and two assistants) cut everything as close as possible to ground level; height of stump will vary depending upon buttress heights. If possible, all trees should be felled in the same direction to facilitate later operations, and should be felled clear of natural waterways.
iii. Burning - this is to remove all the leaves and as many of the branches as possible. The cut and felled vegetation should be left to dry before burning. This normally takes six to eight weeks depending on sunshine and humidity. Burning should not be delayed more than three months after felling because of re-growth when the green leaves will hinder burning. To increase the chances of drying and a good burn these operations are best carried out in the dry season. It is essential to have a good burn because re-lighting and subsequent operations are much more difficult in half-burnt vegetation.
iv. Stacking - after burning all the remaining wood should be cut into pieces that can be handled by manpower. These pieces should be stacked on stumps and retired. In this way a large proportion of the stem itself is removed without leaving a big hole. Extremely large diameter pieces should be cut so that they may be rolled away to the boundaries of the plot where they are left to rot. The process of stacking and reburning may need to be carried out several times before a satisfactory result is achieved. |
If there is commercially acceptable timber in the area to be cleared, a slightly modified procedure will have to be adopted so that this timber can be removed before general cutting and burning. Stacking may also have to be modified to take account of the levelling, grading or contouring of the land for irrigation, and to account for future firewood or charcoal needs. Land can sometimes be cleared in return for the wood on it.
It is important for the land evaluator to understand the implications of the methods and dangers of clearing activities as he is usually in a unique position to advise on these matters, should different areas need different treatment. He may also have to estimate the costs of land clearing for evaluation purposes. Table 51 (A and B) gives two estimates of labour requirements based on conditions in Indonesia according to a survey by Gajah Mada university, and by M. Ross of the Transmigration Area Development.
Table 51 LABOUR REQUIREMENTS FOR CLEARING VEGETATION IN SUMATRA
A) GAJAH MADA UNIVERSITY SURVEY (Rimbobujang in Sumatra)
|
Land clearing requirements: |
Man-days/ha | |
|
Underbrushing all vegetation less |
than 10 cm diameter 10 | |
|
Cutting trees (chainsaw) |
2 | |
|
Cutting-off crown and branches |
6-8 | |
|
First burning |
1 | |
|
Cutting remaining branches |
3 | |
|
Piling |
60 | |
|
Second burning |
2 | |
|
Cutting and piling |
100 | |
|
Third burning |
2 | |
|
|
Total: |
186 |
B) TRANSMIGRATION AREA DEVELOPMENT (Ross) (Muara Marah)
|
Land clearing requirements: |
Man-days/ha | |
|
Underbrushing |
18 | |
|
Felling: Chainsaw operators |
3 | |
|
|
Assistants |
3 |
|
Burning |
2 | |
|
First restacking |
18 | |
|
Reburning |
3 | |
|
Final clearing of rows: Chainsaw operators |
2 | |
|
Assistants |
2 | |
|
|
Total: |
51 |
These figures are for the following vegetation conditions at Muara Marah:
|
Stem diameter range in cm |
No. of stems (range) |
|
0-15 |
not counted |
|
5-29 |
76 - 107 |
|
30 - 39 |
16 - 45 |
|
40 - 49 |
14 - 34 |
|
50 - 59 |
16 - 19 |
|
greater than 60 |
22 - 27 |
|
Totals per ha |
149 - 217 1/ |
1/ These totals do not correspond with the minimum and maximum number of stems given in this table.Source: Clarke 1980
The three main methods of destroying persistent weeds used in land reclamation are as follows and they may be used separately! or in combination:
i. mechanical cultivation;
ii. flooding;
iii. chemical control.
Many million of hectares in Asia with irrigation potential are covered with stands of alang-alang or ladang (Imperata cylindrica) and this must be destroyed as completely as possible before settlement. The land characteristics at particular locations may indicate which of the above three methods of control or their combinations are preferable. This particular weed, and other persistent weeds often have very deep (30-40 cm) underground rhizomes. The areas are usually cultivated with heavy disc harrows but within a month or two after harrowing the cut rhizomes send up new shoots. The problem therefore tends to be multiplied, and in settlement areas, if the settlers do not arrive shortly after the fallowing, the infestation becomes worse. Where it is possible, flooding of the land in readiness for cultivation can help the farmer to keep the weed under control. Repeated cultivation is necessary to cut up the rhizomes so that they die eventually, but this is costly. Five or six well-timed cultivations may be efficacious. However, even if cutting is carried out successfully, some doubt will remain that sufficient depth has been achieved for complete control. Hence, chemical control methods may be necessary using either a systemic weedkiller that destroys the whole plant or repeated defoliation. The land classifier may have to advise on the terrain conditions for applying the weedkiller by mechanical methods, if large areas are to be treated. At some sites, water availability for spraying may be a problem and ultra low volume spraying at 2 litres/ha may be tried.
The land evaluator may be called on to estimate the cost of removal of rocks and stones and it may be a factor in determining whether land is suitable or not suitable in a 'provisionally-irrigable' classification. The methods of removal vary from hand, to mechanical removal, crushing, or blasting. The location of rocks or boulders may be an important aspect in the alignment of irrigation canals or pipelines in classifying 'irrigable land'. Field size and shape may be affected and result in a downgrading of the land suitability class.
Stones (20-40 cm in diameter) and cobbles (7-20 cm in diameter) are usually removed from the tillage zone although some crops, e.g. pasture and orchard, suffer little loss of production from them. Removal costs should be a consideration in assigning land suitability classes.
A method of estimating the cost of stone removal used by the US Bureau of Reclamation is to remove and pile all stones or cobbles from the surface and upper 20 cm depth from a 21 x 21 ft area (0.01 ac) and then to measure or estimate the volume of the stone heap. Thus each 10 inch diameter stone from this area is equivalent to 1 yd3/ac in the area as a whole. A metric equivalent of this method might use an area of 10 m x 10 m (0.01 ha) for excavation. Each 26.7 cm atone found within this area would then be approximately equivalent to 1.0 m3 of stones per hectare.
About 2.3 man-hours per cubic metre are required for manual picking of stones. The cost of transporting the stone must also be taken into consideration.
Overflow hazards from rivers or drainage ways often influence the use, management and development costs of affected portions of an irrigation project. Any lands located in areas susceptible to such damage should be evaluated in terms of the benefits and costs from flood protection measures. These might be simple measures such as the extra surface drainage construction of small earth embankments, or gabion boxes (wire boxes filled with stones), or more sophisticated structures. Often flood damage will be eliminated by upstream constructions which are part of the measures to make more water available for irrigation. Reduction or elimination of flooding is frequently a benefit of large-scale projects.
Before classification, the land evaluator should liaise with the project hydrologist and engineer on the effect of proposed project works on future flooding.
Lands subject to severe and frequent damaging floods are generally excluded from an irrigation project. Deep water rice and floating rice are possibilities if the extent and timing of the water rise and fall is predictable (see heading A.13).
Sound evaluation of flood hazards and the associated land development costs are difficult because no two situations are exactly alike.
Run-off from adjacent hillsides is a common problem on lands lying at the base of hills. The problem is particularly serious in erosive areas subject to torrential and damaging rainfall during parts of the year. Under such conditions, soil, stones and vegetative debris from the hillside may overflow the crop land to be evaluated. Stones and cobbles on the surface of the soil and observable severe erosion on the hillside will be indications of existing or potential flood problems. Land subject to such damage is less suitable for irrigation development than land similar in other respects. If the condition is very severe, land subject to this type of run-off should be excluded from the 'provisionally-irrigable' area.
The need to remove excess water and salts from an irrigated river basin (Figure 18) necessitates a network of surface or subsurface drains. Drainage is discussed in two FAO Irrigation and Drainage Papers particularly. No. 28 and No- 38, Drainage Design Factors (1980), also in the USBR Drainage Manual and in Luthin et al. (1957).
Drainage costs are an important criteria in the classification of land, especially in arid and semi-arid areas where salinity and sodicity must be controlled. Initially, the evaluation is often carried out in advance of the detailed drainage studies and frequently the classifier does not have the necessary information on which to base the assessment at the time of the survey. Therefore, it is most important that the classification is modified at later stages on the basis of the drainage studies. Many problems have been caused by the land classifier determining the costs of drainage when, in fact, this should be done by a qualified drainage engineer.
Drainage investigations are directed toward determining the prevailing depths, slopes and fluctuations in level of the groundwater surface; the presence or absence of confined water tables (i.e. water under pressure below a slowly permeable strata); and the thickness and permeability of soil and substrata layers which may retard water transmission.
Figure 18 Flow diagram for water and salt circulation in an irrigated river basin
Source: Westcot 1979 (in FAO 1979a)
Soundly conducted drainage investigations require a network of cased observation wells of known elevation, or existing domestic wells; piezometer installations to detect water tables; numerous deep borings to determine the variability of substrata materials; and field tests for permeability. Three methods for obtaining in-place horizontal permeability data are commonly used. These are the auger-hole (or shallow well pump-out) test, the piezometer test, and the shallow well pump-in test. The 'permeameter' test is used to determine the vertical permeability of a narrow zone. In most drainage studies, knowledge of the horizontal permeability obtained by one of these tests is considered to be sufficient: it being assumed that vertical permeability will be adequate for water to reach the saturated zone from which it will be drained horizontally. If there is cause to suspect the presence of slowly permeable layers above the saturated zone, the 'ring-permeameter' test (described by Winger 1965 and in FAO 1984, Soils Bulletin No. 52) provides a method of determining the vertical permeability of these layers which, although complex and rather slow, gives uniformly dependable results at a reasonable cost. An important necessity for the proper conduct of this test is the installation of pairs of tensiometers and of piezometers to confirm the fulfillment of the requirements of Darcy's Law for the movement of liquids through saturated material, on which the subsequent calculation of permeability is based.
Several formulae have been developed to estimate required drain spacings from data on permeability and depth to barrier. The method used by the USBR (Dumm 1968) takes into account crop water requirements, irrigation efficiency, leaching requirements, desired water table depth, rainfall characteristics and specific yield. Because of the importance of drainage to the success of an irrigation project, shortened methods for estimation of drainage requirements should be avoided unless their validity in the particular area has been proven.
Drainage within the field is of little benefit if the drainage network as a whole is neglected or the outlets are liable to blockage. The land evaluator in liaison with the drainage engineer will ensure that all existing drainage ways and areas which will require outlet surface drains are made a part of the development plan.
The assessment of drainage requirements can be facilitated by the table of permeabilities in Table 52 for comparing and classifying permeability for different soils and substratum materials.
For further guidance on drainage design see FAO Irrigation and Drainage Paper No. 38 and other publications. Also see Section C.25.
Land grading and levelling requirements are based on an appraisal of topography and the need for modifications to suit the choice of irrigation technique specified in the land utilization type. There are four aspects of topography that have special bearing on land levelling and grading for surface irrigation: 1) slope, 2) microrelief, 3) macrorelief, and 4) cover. The land classifier must achieve competence in distinguishing and evaluating those topographic features that are significant. Considerable experience is required to achieve acceptable accuracy in estimating the costs of levelling from field observations. Topographic maps do not always give sufficient information for accurate assessments. Guidance and training may be provided by an experienced agricultural engineer engaged in detailed layout studies. Detailed farm layouts of representative areas showing the costs of land grading can provide the best guidelines. If done properly, evaluation of the topography based on experience and field layout studies is adequate for most planning studies.
Table 52 COMPARISON AND CLASSIFICATION OF PERMEABILITY FOR DIFFERENT SOIL AND SUBSTRATUM MATERIALS
|
Textural grades and/or substratum materials (1) 1/ |
Metres per day |
Soil Survey Index No. (5) |
Classification of Normal Rates (Indices and descriptive classes) 2/ |
||||
|
Maximum (2) |
Minimum (3) |
Normal (4) |
Key Class (6) |
Drainability Index No. (7) |
Survey Key Class (8) |
||
|
Heavy clays |
0.5 |
<0.001 |
0.01 |
1 |
Very slow |
1 |
Very poor |
|
Medium clays |
0.6 |
0.002 |
0.02 |
1 |
Very slow |
1 |
Very poor |
|
Silty clay |
0.6 |
0.002 |
0.04 |
2 |
Slow |
2 |
Poor |
|
Sandy clay |
0.6 |
0.007 |
0.05 |
2 |
Slow |
2 |
Poor |
|
Silty clay loam |
0.7 |
0.005 |
0.12 |
2 |
Slow |
3 |
Fair |
|
Clay loam |
1.2 |
0.02 |
0.15 |
2 |
Slow |
3 |
Fair |
|
Silts |
0.6 |
0.005 |
0.1 |
2 |
Slow |
3 |
Fair |
|
Silt loam |
3 |
0.01 |
0.3 |
3 |
Mod. slow |
3 |
Fair |
|
Sandy clay loam |
3 |
0.02 |
0.5 |
3 |
Mod. slow |
3 |
Fair |
|
Loam |
3.5 |
0.05 |
0.6 |
3 |
Mod. slow |
4 |
Good |
|
Fine sandy loam |
3.5 |
0.1 |
1 |
4 |
Mod. rapid |
4 |
Good |
|
Sandy loam |
4 |
0.1 |
1 |
4 |
Mod. rapid |
4 |
Good |
|
Coarse sandy loam |
5 |
0.3 |
2 |
5 |
Rapid |
5 |
Very good |
|
Loamy fine sand |
4 |
0.3 |
2 |
5 |
Rapid |
5 |
Very good |
|
Loamy sand |
5 |
0.4 |
2.5 |
5 |
Rapid |
5 |
Very good |
|
Loamy coarse sand |
6 |
2 |
3 |
5 |
Rapid |
5 |
Very good |
|
Fine sand and very fine sand |
12 |
0.1 |
2 |
5 |
Rapid |
5 |
Very good |
|
Medium sand |
60 |
2 |
4 |
6 |
Very rapid |
5 |
Very good |
|
Coarse sand |
120 |
6 |
12 |
7 |
Excessive |
5 |
Very good |
|
Gravelly clays to gravelly clay loams |
1 |
<0.001 |
1 |
2 |
Slow |
3 |
Fair |
|
Gravelly silts to loams |
4 |
0.005 |
1 |
4 |
Mod. rapid |
4 |
Good |
|
Gravelly fine sandy loams to fine sands |
60 |
2 |
6 |
6 |
Very rapid |
5 |
Very good |
|
Very gravelly clays to very gravelly sandy loams |
12 |
1 |
3 |
5 |
Rapid |
5 |
Very good |
|
Very gravelly silts to loams |
60 |
3 |
6 |
6 |
Very rapid |
5 |
Very good |
|
Very gravelly fine sandy loams to fine sands |
120 |
6 |
12 |
7 |
Excessive |
5 |
Very good |
|
Mixed pea gravels and sands |
60 |
1.5 |
12 |
7 |
Excessive |
5 |
Very good |
|
Pea gravels clean |
240 |
24 |
48 |
7 |
Excessive |
5 |
Very good |
|
Gravel, cobble and sands (mixed) |
120 |
2 |
36 |
7 |
Excessive |
5 |
Very good |
|
Clean gravels 1 |
200 |
36 |
77 |
7 |
Excessive |
5 |
Very good |
|
Cobble and gravel 1 |
800 |
72 |
120 |
7 |
Excessive |
5 |
Very good |
|
Cobble 2 |
400 |
120 |
240 |
7 |
Excessive |
5 |
Very good |
|
"S" loose gravelly, |
|
|
|
|
|
|
|
|
"Gypsy" etc. 3/ |
24 |
1 |
12 |
7 |
Excessive |
5 |
Very good |
|
"S" marly or limey, soft to semi-hard |
4 |
0.05 |
0.5 |
4 |
Moderate |
3 |
Pair |
|
"S" marly or limey, semi-hard to hard |
0.05 |
<0.005 |
0.03 |
2 |
Slow |
2 |
Poor |
|
"S" clayey to limey, compact to very hard |
0.05 |
<0.005 |
0.01 |
1 |
Very slow |
1 |
Very poor |
|
Lightly cemented gravels |
77 |
0.1 |
12 |
7 |
Excessive |
5 |
Very good |
|
Any creviced or fractured rock |
77 |
0.005 |
6 |
6 |
Very rapid |
5 |
Very good |
|
Porous rocks including semi-hard & hard caliche |
77 |
0.005 |
6 |
6 |
Very rapid |
5 |
Very good |
|
Uniform bedrock few or no fractures or crevices |
0.005 |
<0.0001 |
>0.0005 |
1 |
Very slow |
1 |
Very poor |
|
Gypsum beds |
|
|
|
1 |
Excessive |
5 |
Very poor |
1/ Textural grades are classified on the basis of normal structures and do not include highly dispersed - soils containing excess exchangeable sodium ions. Data are from all known sources.2/ These indices compare rates of water transmittal only. Drainability of an area is influenced also by depth to impervious layers, stratification, thickness and position of aquifers, slope and the rate of water intake and storage capacity of soils.
3/ Includes a wide variety of commonly unconsolidated substratum and subsoil materials (sand, silts, clays and gravels) with various degrees of weathering, illuviation and cementation.
Note: Table prepared by Ralph M. Parsons Co
The degree of slope acceptable for irrigation development and therefore the cost of land levelling depends on i) the anticipated method of irrigation, ii) intensity and amount of rainfall, iii) susceptibility of the soil to erosion, and iv) planned cropping system. Slopes of 50% or more are commonly surface irrigated in traditional Asian terraced systems; however, such land would generally not be considered- suitable for development today. In the USA, gravity irrigation on slopes greater than about 12% is seldom practised. With sprinkler or drip systems, limitations on slope due to an erosion hazard or the operation of farm machinery are important. Slopes of 20% are currently considered the maximum acceptable in the USA for cultivated crops irrigated by sprinklers. In areas that experience severe thunderstorms, the maximum usable slope may be less. Land devoted to dense cover crop or grass may permit irrigation of steeper slopes than for row or field crops.
Although excessive slope is the most frequent problem, lack of slope may also be a limitation. Excessive flatness may result in higher grading costs to increase the slope and achieve the smooth uniform surface necessary for uniform distribution of irrigation water. Extremely gentle gradients may make irrigation of slowly permeable soils difficult because standing water induces scaling and waterlogging. Very permeable soils and extremely flat topography may prevent uniform irrigation without excessive deep percolation and water use. On the other hand, very flat land provides an opportunity to use really efficient surface irrigation methods such as basin and border strip, where soils are suitable.
In estimating the cost of grading, the field boundaries and the type of surface irrigation (small basins, large basins, furrow and border strip lengths, etc.) must be determined. There is an interrelationship between the irrigation field size and the amount of land grading required. Where grading will cause damage by exposing subsurface horizons and hardpans, alternative development options should be considered.
An estimate of the land grading requirements is an essential part of a land classification study if surface irrigation is to be used. If possible, the land classifier should know from discussions with economists the maximum allowable cost for land development before the field study begins. There are no specific methods or approaches that must be used. The intuition for estimating the required moving of earth is gained primarily through experience. Topographic maps and detailed farm layouts of representative areas are valuable for correlating estimates on similar areas. Average cut and fill needed within a field and its conversion to the estimated volume of material that must be moved is one method. The estimate of cut and fill can be made by evaluating the difference between the microrelief s highs and lows and averaging them for the field. This approach implies an average cut over half of an area with fill in the remaining portion. Tables can be developed to show the volume represented by the various differences- If topographic maps with the elevations recorded for each reading are available, they can provide a good guide to highs and lows. Depth of topsoil, subsoil quality, the presence of gypsiferous or other substratum must all be appraised.
How smooth the surface should be for efficient irrigation may vary with gradient, the accepted gravity irrigation method, water quality, anticipated depth to the water table, and cropping. Less precise grading is usually needed as the gradient increases, and with less efficient irrigation methods such as used by small farmers with small basins and short furrows. There is a trade-off between the cost of land grading and the benefits of efficient water use. If water use efficiency will inevitably be low, there will be no point in achieving more than a smoothing of the land in the direction of the slope, except in rice basin systems where depth of standing water is critical.
The volume of earth to be moved for construction of farm laterals, drains and farm structures should sometimes be included in estimates of the total land grading cost. Although land grading costs are based primarily on total volume of earth to be moved, other factors may influence the total cost. Unit costs for grading vary with the depth of cuts, length of haul, how smooth the surface must be, soil texture (which affects plasticity and the range of moisture conditions under which they can be worked), and field size (where it is more difficult to manoeuvre large equipment).
In mechanized systems of agriculture the grading costs are interrelated with the choice of field size and shape to minimize the costs of operating farm machinery once the land is developed. Field size and shape in this context are determined primarily by the land's macrorelief. Other factors are the limit of irrigation runs on soils with excessive infiltration rates, and the importance of length of slope in the control of erosion. In complex topography where slopes change frequently in both lateral and transverse directions surface irrigation for mechanized agriculture may be impracticable. As the field becomes smaller and irrigation runs shorter, labour requirements increase, a more complex farm irrigation system is needed, machinery operating costs increase, the proportion of unproductive land increases, and the irrigation efficiencies decrease. The minimum economic field size and length of run established in the specifications are based primarily on these factors. In these circumstances estimating the field size must precede the estimation of land grading costs. Field boundaries usually lie on the more prominent topographic features and the less prominent relief within may be graded to permit gravity flow of water. Other features, such as ownership boundaries, bodies of unarable land, boundaries of land in a use precluding irrigation, that might interrupt irrigation flow may also define field boundaries. Features that determine field size must be defined by observation. This requires considerable experience and judgement. In the more general land classification studies it is not practical to define each field. In such situations, an estimate of the field size is achieved by comparing the landform with similar areas where detailed farm layouts have been completed or with irrigated areas with similar topography. Appropriate field sizes can be associated with different land units.
Table 53, as an example, shows an evaluation of field size and shape in relation to suitability for mechanized farming in the USA. Table 54 shows the amount of earth to be moved at various depths of cut and fill which, together with local unit costs, can be used to calculate grading costs.
Table 53 EVALUATION OF IRRIGATED FIELD SIZE FOR MECHANIZED FARMING
|
|
Critical Limits |
|||
|
s1 |
s2 |
s3 |
n |
|
|
Field size, minimum (ha) |
8.0 |
3.6 |
2 |
1 |
|
Length of run, minimum (m) 1/ |
390 |
120 |
100 |
50 |
|
Dimensions (m) |
390 x 200 |
120 x 300 |
100 x 200 |
50 x 200 |
1/Consideration must be given to water intake rates when assessing the length appropriate for a given soil.
Table 54 GRADING ESTIMATES IN TERMS OF CUT AND PILL
|
Type of Grading |
Light |
Medium |
Heavy |
|
Average cut and fill (cm) |
7.5 |
15 |
30 |
|
Earth moving (m3/ha) |
375 |
750 |
1 500 |
|
(yd3/ac) |
200 |
400 |
800 |
Note: 100 yd3/ac equivalent to 189 m/ha.
Finally, the effect of earthmoving on the physical productivity of the land must be evaluated. This may depend on depth of topsoil, the quality of the subsoil, presence of gypsiferous layers and other characteristics.
C.23.1 Physical aids to reclamation such aids include:
C.23.2 Chemical and organic amendments
The development of land may include the need for physical, chemical and organic amelioration treatments. Apart from leaching, which is described in the next section, the special land improvements that may be required can be divided under two headings.
i. deep ploughing, especially on stratified soils with permeable and impermeable layers, or on soils with gypsum layers within reach of the plough;ii. subsoiling, especially to break an indurated B-horizon or lime layer:
iii. profile inversion, where the upper subsoil has undesirable properties (lower and upper subsoils are inverted and then the top soil is replaced);
iv. sanding, involving the spreading and mixing of sand into the upper horizons of fine texture soils (not effective on heavy clay soils).
Chemical amendments are very often necessary in the reclamation of saline-sodic and sodic soils to neutralize free sodium and to supply a cation that will replace sodium in the exchange complex. Gypsum is by far the most commonly used amendment. Phosphor gypsum, which is a by-product of superphosphate and is available relatively cheaply in countries with superphosphate manufacturing plants, can be effective even at low rates of application due to the small particle size of the material. Shainberg (personal communication) and others have shown that it produces very significant effects on the electrolytic properties of water repellant soils and produces rapid improvements in the physical condition of the silt/clay fraction. Other amendments that may be used are calcium chloride, calcium carbonate and waste lime from sugar mills (a mixture of alkaline calcium compounds). Acidifying materials such as sulphuric acid, sulphur and iron sulphate serve to reclaim sodic soils by neutralizing soda and reacting with lime in calcareous soils to produce gypsum which furnishes the desired soluble calcium. An alternative effective way of solubilizing CaCO3 in the soil itself is to build up the organic matter level by growing green manure crops or by adding organic manures. This lowers the pH by increasing the carbon dioxide concentration in the soil. The growing of a reclamation crop can often be the most effective way to improve saline-sodic soils following leaching. Mulching such soils with organic materials can also have spectacular effects (Eavis and Cumberbatch 1977).
The land evaluator, when assessing the need for amendments, if these are chemical and related to the amount of sodium to be removed, can initially calculate the theoretical gypsum requirement:
'Initial ESP' is the measured value before reclamation. 'Final ESP' is the desired value which is often taken as 10, a level of exchangeable sodium at which no noticeable peptization results. For example, if initial ESP = 30, final ESP = 10 and CEC = 24:
Since 1 me of gypsum/100 g of soil is equivalent to 860 ppm of gypsum and since one hectare of soil to a depth of 20 cm may be taken to weigh 3.1 million kg, the amount of gypsum theoretically required to treat this depth of soil will be:
Gypsum requirement/ha/20 cm = 860 x 106 x 3.106 x 4.8 = 12 400 kg.
In practice, the gypsum is likely to be impure and a correction factor for percentage purity must be used. Furthermore, the efficiency of replacement of sodium by calcium is not 100%, partly because of the presence of free sodium in the soil. Therefore, it is recommended that the amount of gypsum to be applied be increased in accordance with the equivalents of free sodium carbonate and bicarbonate (FAO/Unesco 1973). USBR studies in Idaho (unpublished) have shown that, in general, gypsum is only 60-75% efficient in replacing exchangeable sodium; a finding which can be used to adjust the calculated requirement. Table 55 shows the amount of other amendments that would be as effective as one tonne of pure gypsum, if they were locally more economic. The possibility of the improvement being achieved without the use of amendments by increasing the electrolyte content by use of water of moderate salinity levels should also be considered where the water available does not have a high SAR value. The importance of the SAR value will depend on the clay minerals present in the soil, with SAR generally less than 10 for 2:1 type minerals and somewhat higher for 1;1 type minerals. Saline water (5-8 dS/m) with additions of gypsum or calcium chloride to lower the SAR below the appropriate limiting value would usually be appropriate for the initial leaching of saline-sodic or sodic soils.
Gypsiferous soils have special reclamation requirements as discussed by Mousli (1979). The soil may i) contain gypsiferous material throughout, ii) be a calcareous gypsic soil, iii) be a soil containing a layer of solid gypsum at a depth of less or more than 150 cm, iv) be a sandy gypsiferous soil, or v) be a stony gypsiferous soil. The high solubility of gypsum causes a high osmotic pressure that reduces water extraction by plants, though at higher EC values than for saline soils. The soil solution is saturated with calcium which results in the fixation of the trace elements (Fe, Mn, Cu and Zn) in less available forms. A hard pan or impervious layer prevents root and water penetration. The solution of gypsum and its leaching out from the soil during irrigation causes an increase in plasticity and a decrease in cohesion and structure of some soils. It may also cause collapse of imperfectly lined canals. Gypsiferous soils tend to be susceptible to erosion due to lack of cohesion and structure. To improve the soil profile of gypsiferous soils, the incorporation of organic materials, deep ploughing and the careful management of irrigation water are important.
Table 55 AMOUNTS OF CHEMICAL AMENDMENTS EQUIVALENT TO ONE TONNE OF GYPSUM
|
Amendment |
Tons |
|
Gypsum (CaSO4, 2H2O) |
1.00 |
|
Calcium chloride (CaCl2.2H2O) |
0.85 |
|
Limestone (CaCO3) |
0.58 |
|
Sulphur |
0.19 |
|
Sulphuric acid |
0.57 |
|
Iron sulphate (FeSO4.7H2O) |
1.62 |
|
Aluminium sulphate (Al2 (SO4)3. 18H2O) |
129 |
|
Calcium polysulphide (CaSO4) 24% sulphur |
0.77 |
Source: FAO/Unesco 1973
In evaluating the costs and benefits of physical, chemical and organic aids and amendments, the land evaluator should note the guidelines in Chapter 7.
Some soils have such high concentrations of salts prior to irrigation that an initial leaching is required before agricultural production can begin. The amount of water that must be applied to reclaim a saline root zone by leaching depends primarily on the initial soil salinity levels and the technique of applying water. Typically, about 70% of the soluble salts initially present in a saline soil profile will be removed by leaching with a depth of water equivalent to the depth of soil to be reclaimed, if water is ponded continuously on the soil surface and drainage is adequate (Hoffman 1980).
The relationship between the fraction of salt remaining in the profile, C/Co (where Co is the initial salt concentration and C is the salt concentration during reclamation), and the amount of water leaching through the soil profile by continuous ponding per unit depth of soil, d/d, (US Salinity Laboratory Staff, in preparation) can be approximated by:"
(C/Co).(dw/ds) = 0.3 when dw/ds is greater than 0.3.
The data for this relationship, illustrated in Figure 19, include soil types ranging from peat to sandy loam to clay. The |equation can be refined by taking the salt concentration of the applied water (Ci) into account. This is done by substituting (C - Ci)/(Co - Ci) for C/Co. Such refinement improves the assessment of dw as Ci increases or as complete reclamation is approached (i.e. as C approaches Ci).
Figure 19 Depth of water per unit depth of soil required to leach a saline soil by continuous or intermittent ponding or to leach a soil inherently high in boron (US Salinity Laboratory Staff, in preparation)
The amount of water required for leaching soluble salts can be reduced by intermittent applications of ponded water or by sprinkling. The differences in leaching efficiency among the leaching methods are caused primarily by differences in the effect of diffusion of salts to primary flow channels, or by the larger percentage of water flowing through the fine pores of the soil mass in the unsaturated case. The relationship between C/Co and dw/ds for intermittent ponding (US Salinity Laboratory staff, in preparation) illustrated in Figure 19, can be approximated by:
C/Co. (dw/ds) = 0.1 when dw/ds exceeds 0.1.
The relationship for intermittent ponding was derived from four field trials where the depth of water applied each cycle ranged from 50 to 150 mm with corresponding ponding intervals ranging from weekly to monthly. To remove about 70% of the soluble salts initially present by intermittent ponding, a depth of water equal to about one-third of the depth of soil to be reclaimed is required. This is only one-third of the amount required where continuous ponding was used. However, these are the results of trials under controlled experimental conditions and in field practice the required uniformity of application might not be achievable to make intermittent leaching a favourable practical proposition.
Leaching efficiency by sprinkling is similar to that for intermittent ponding. In some cases, efficiency may be improved further, particularly where low application rates are maintained or where sprinkling is intermittent. Sprinkling has the added advantage over ponding that precise land levelling is not required. A disadvantage of intermittent ponding and sprinkling is that a longer period is required and on low intake soils evaporation losses may approach or exceed infiltration. Great care is necessary to ensure uniformity of application of the water. If a salt tolerant crop is the first crop to be planted on the land, it may be possible to complete the primary reclamation leaching during the lifetime of that crop.
Excess boron is generally more difficult to leach than soluble salts because it may be tightly sorbed to soil particles. The origin of the boron may determine the amount of water required for reclamation. Soils inherently high in boron seem to hold boron with more tenacity than soils where boron has been added in the irrigation water. The former require more leaching for initial reclamation and often require additional leaching periodically to remove boron released from the soil subsequently. As with soluble salts, the relationships between C/Co and dw/ds in leaching soils inherently high in boron (US Salinity Laboratory Staffs, in preparation), illustrated in Figure 19, can be approximated by
(C/Co). (dw/ds) = 0.6 when dw/ds exceeds 0.6.
Thus for soils inherently high in boron, the amount of water required to remove a given fraction of boron is about twice that required to remove soluble salts by continuous ponding. Boron leaching efficiency does not appear to be significantly influenced by the method of water application.
It may be necessary for the land evaluator to cost the use of water to reclaim different areas. The characteristic appropriate in first approximations is the volume or depth of water required. A limiting salt concentration (ECe) for the initial soil condition may be established to divide land that it is worthwhile leaching and reclaiming, from that which is not.
Lands that must be reclaimed by grading and leaching may not be immediately suitable for the desired final cropping and land use. In some cases it will be several years before the crop yields are optimal. The length of the reclamation period can greatly affect the feasibility of a project, and in general, the shorter the reclamation period the better. The uniformity of crop growth in the early years may be poor and it may be desirable to grow crops of lower value that add to the organic and nutrient contents of the soil. There may be subsidence problems due to the dissolution of gypsum and to insufficient compaction on land that has been filled in 'cut and fill' operations.
One major factor of economic importance is the stage at which drainage is installed in the fields. In terms of Net Present Value it is very much more costly to install drainage early in the life of the project than later. Consequently, the temptation has been to delay the installation of field drains or, in many cases, to ignore drainage altogether. This has had disastrous consequences in irrigation projects in arid or semi-arid areas. Nevertheless, it may be bad economics to install drainage early in a project, if the water table is very deep. During the years after starting to irrigate, the water table often rises to the point where drainage is essential. For different areas of land, the classifier may have to decide in which year following project year 1, the drainage must be installed. This is assessed on the basis of the depth of the water table, and the expected rate in its rise. In poorly permeable soils where there is likely to be a perched water table, the drainage may have to be installed from the start of irrigation. Sometimes there is a need for drainage while salts are being leached during reclamation, but normally it is not worth catering for the extra drainage in drainage design but rather to apply the water over a longer period of time.
On relatively permeable rice land, seepage and percolation losses are often excessive in the first years after initial development. Usually a period of about seven years is required for the percolation rates to reduce as a result of the accumulation of fine-grained material in the floor of the paddy fields, acting as a seal.
Determination of the period of time over which the land improves to full productivity, the length of time to the installation of field drains, and accompanying effects on production and costs may be used as critical limits of the reclamation period.
The assessment of land suitability or limitations with regard to irrigation engineering may concern i) the development of new lands for irrigation; and ii) the rehabilitation of irrigation schemes.
i. New lands
A preliminary assessment of land suitability for the irrigation and drainage engineering works frequently has to be carried out by the land classifier in the early stages of a project as part of the general survey. Later the engineers may need to survey the possible routes of the irrigation and drainage networks in great detail, and make further topographic maps.
The important considerations in the early surveys are:
a. the topographic features of the land that influence the flow of water by gravity or the elevation and distance to which water must be pumped (see also B.14. Location);b. the depths of barriers that can act as obstructions to the constructing of canals, drains and other structures or affect grading and land levelling operations (i.e. aspects that have not been assessed under other headings);
c. the presence of unstable subsurface materials that may lead to subsidence problems;
d. the permeabilities of soils on which canals and drains will be constructed and the associated losses of water for unlined or lined channels;
e. the substratum condition as it affects the installation of permanent structures such as diversion weirs, storage reservoirs, etc.;
f. soil conditions for installing field and main drainage (i.e. depth to barrier, nature of barrier, etc.):
g. conditions for access to sites for project construction;
h. the location of dugwells or tubewells in respect not only to water, but also to the land that will be irrigated, to obtain the best advantages in terms of energy-saving and topography;
i. the size and shape of potential management units or fields (see also heading B.14);
j. the positioning of bunds or levees according to topography and changes in soil texture or other land characteristics, thus improving the efficiency of water use and productivity;
k. the assessment of basin sizes, furrow lengths etc. (Table 48) in relation to the earthmoving costs, and the acceptable slopes and microrelief after grading (see also heading B.15);
l. the matching of water supply and demand and the scheduling of water in terms of frequency, rate and duration of application. The design of the canal or pipe networks to the field and the engineering costs depend on any one or all of these factors.
Some of the above may be class-determining independently of the assessments already made under earlier heads.
ii. Rehabilitation of irrigation schemes
In rehabilitation schemes, quite different assessments may be required depending on, for example, whether the scheme is in an Asian rice area, or in an arid or semi-arid area subject to waterlogging and salinity problems. Other categories also occur in the intermediate rainfall zones.
In the Asian rice land situation, rehabilitation often involves upgrading the primary, secondary and tertiary water supply networks or the installation of improved water control structures (diversion weirs, measuring devices, storage structures, etc.). The land evaluator may be called upon to evaluate land suitabilities relating to the improvement of these engineering works.
In the rehabilitation of saline, sodic and waterlogged land in arid and semi-arid areas, surveys are generally required for the engineering works, especially topographic surveys and groundwater level surveys for the proper location of irrigation and drainage channels. If very high construction costs are implicated, the land suitability class of the associated land may be downgraded accordingly.
The most important evaluations under this heading are those that exclude land from development because of excessive costs to develop it for irrigation or to drain it.
