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Chapter VII. Irrigated forest plantations

1. Introduction
2. Irrigation with permanent water supply
3. Irrigation with an intermittent water supply: Rainwater harvesting
4. Irrigation with waste water

1. Introduction

Irrigated forest plantations can be established for the commercial production of fuelwood, posts, construction lumber, and fodder. The use of irrigation practices also allows the use of more exacting fast-growing tree and shrub species. In many instances, the availability of wood from irrigated plantations will lessen the destruction of the natural vegetation.

In arid zones, irrigated forest plantations can be achieved using:

- a dependable and permanent water supply;

- an intermittent water supply;

- waste water.

2. Irrigation with permanent water supply

Depending upon the amount of water which can be made available from a dependable water supply (well, dam, river...), permanent irrigation systems can be established. Different designs of permanent irrigation systems can be chosen, depending on the prevailing conditions. Three types of such systems are reviewed in the following sections: gravity systems, sprinkler systems and localized systems.

2.1 Gravity systems

Gravity irrigation systems are characterized by the manner in which the irrigation stream is controlled by the soil surface. Four types can be distinguished: surface flooding, border check, basin and furrow irrigation.

Surface flooding - This system resembles the inundation that sometimes takes place on flat lands along rivers and it is the simplest form of permanent irrigation. On gently sloping land that requires little preparation, surface flooding is easy to implement. In essence, water is released from main ditches and allowed to spread over the surface. However simple, this method generally has been inadequate for tree and shrub crops, as it is difficult to obtain uniform distribution of the water. Also, there is a risk that the root system of the plants can become deprived of oxygen because of waterlogging.

Border check - In this method, parallel earth ridges guide the flowing water as it moves down the slope over the strips which vary from 3 to 30 meters in width and can be more than 100 meters in length. A relatively large flow of water is needed. The land should have a uniform moderate slope parallel to the checks. Careful land preparation is necessary to ensure stability of the ground. The method is suited to medium-textured, deep, permeable soils. On sandy soils, infiltration would be excessive unless the strips are short. Slow percolation renders it unsuitable for heavy soils. A drainage ditch should be sited at the end of the strip to carry away any excess water. Design involves achieving an optimum balance between soil type; slope, width, and length of strips; and water flow so that the desired depth of water will be applied uniformly to the compartment without excessive percolation at the input end. In agroforestry applications, it could be ideal because the trees could be grown along the check, the width of which could be sufficient for one or two rows of trees.

Basin irrigation - This is a system in which the field or compartment is divided into small units each of which has a level surface. The basins are filled with water which is all allowed to infiltrate, any excess being drained off. When leaching salts from the soil, the depth of water can be maintained for considerable periods by allowing continuous flow into the basins. The method entails relatively high labour inputs.

Furrow irrigation - This is a common method to distribute irrigation water. Furrows are built from the main feeder channel in parallel lines spaced at regular intervals to permit the wetting of the tree rooting zone. The width of the furrows and their spacing depend, in large part, upon the permeability of the soil. The heavier the soil, the larger and the wider apart the furrows must be; the opposite applies in more porous soils. The method requires relatively high labour inputs and a high degree of skill and experience in directing water from supply channels into the furrows and controlling its flow. A disadvantage of the system is that tree roots tend to develop linearly along the furrow; trees tend to lean across the furrows and windthrow sometimes occurs later in the rotation. Unevenness of furrow levels and distribution of water can develop when soil conditions are not uniform. Regular maintenance of the furrows is important.
2.2 Sprinkler systems

Sprinkler systems are most applicable in areas of irregular topography where land grading is not feasible, in irregularly sloped areas, or when rapid application of relatively small quantities of water is desired. Applications to forestry are somewhat limited by crop height and costs, but such systems are applicable in the early establishment phase of forest plantation crops.

Figure 7.1 Drip system of irrigation

Figure 7.2 Field application of the drip system using a hand water pump, a drum and tubing.

All sprinkler systems have in common a source of water under pressure, a system of pipelines to deliver water to the point of delivery, and nozzles through which the water is distributed.

The advantages of sprinkler systems are that they are applicable to areas of irregular topography and shape without levelling; they can be used in areas of high watertable or where there is a hard pan near the surface without increasing soil salinity; the amount and rate of water application are easily controlled so that runoff and deep percolation can be avoided, this gives them advantages in areas of high permeability. They can utilize a small, continuous supply of water better than gravity methods; water is distributed evenly if it is not too greatly affected by wind; they do not require land for water channels, ditches, and borders, thus saving land and the maintenance costs and inconveniences of open distribution systems.

2.3 Localized systems

Localized irrigation systems, an umbrella term for trickle, drip, drop, or sip irrigation methods, are among those that cause wetting of only part of the soil, i.e. that at the base of and surrounding the root system of the plant. They are characterized by slow and low rates of application of water to the plant rooting zone through distribution pipes and orifices or nozzles organized either under or above the soil surface.

The basic components include a pressurized supply of water, a control head, a main line with laterals, and distributors (Figure 7.1). To create the appropriate pressure in the water supply usually requires a pump and storage tanks or reservoir. The control head is usually sited at the highest point in the field and connected to the water supply. The advantages of localized irrigation systems include the facts that, within limits, they can accommodate undulating ground, are relatively easy to manage, have relatively low labour costs, and are simple to operate. The principal problem of localized irrigation systems is the susceptibility of the smaller pipes and the distributors to clogging by sand, silt, organic matter, algae, bacterial slimes, and precipitation of nutrients, colloidal materials, or lime. Root system size or spread and depth being a function of the volume of water applied at each irrigation, root development can be restricted through inadequate watering. Also, trees can die very rapidly if water is withheld for even a short period: thus the water supply must be reliable (Figure 7.2).

3. Irrigation with an intermittent water supply: Rainwater harvesting

Rainwater harvesting as a means of providing water seasonally, over longer periods, has been used in arid areas for thousands of years to grow agricultural crops and trees for fruit, amenity, and other purposes. Rainwater harvesting essentially involves two components: (1) a catchment or collector area, usually prepared in some manner to improve runoff efficiency, and (2) a smaller water storage area in which crop or tree plants are grown or where water is stored in small tanks or other structures for future use.

In the case of tree planting, rainwater is used directly without storage requirement. Four techniques are particularly relevant:

- runoff farming.

- desert strip-farming,

- contour terrace farming, and

- flood water spreading.

Runoff farming - Watersheds are divided into several micro-watersheds depending upon the area needed for each tree. The collection area for each micro-watershed may range from 20 to 1000 square meters, depending upon the area precipitation and tree water requirements. Micro-catchment procedures are used in complex terrain where other water harvesting techniques may be difficult to apply (Figure 7.3).

In a typical situation, a series of well-designed micro-watersheds with appropriate dimensions are prepared. At the lowest point, a basin is dug about 40 centimeters deep and a tree is planted. The depression collects and stores the runoff from the rest of the micro-watershed that feeds the plant. At the root zone, soil should be at least 1.5 meters deep. Diagonal distances between the lowest corner to the farthest contributing corner should be between 5 and 30 meters.

This technique is particularly successful in years of normal precipitation. In dry years, most annual crops will fail. It is therefore advisable to select drought-resistant plants for use with this system.

Desert strip-farming - Although only a very small percentage of the rainfall reaches major stream channels in arid and semi-arid regions, considerable runoff occurs on many of the gently sloping watershed areas. Desert strip-farming makes use of this water by employing a series of terraces that shed water onto a neighboring strip of productive soil (Figure 7.4).

Depending upon the topography, soil characteristics, and climatic conditions of the site, two types of micro-watersheds can be used: (a) one-sided micro-watershed for moderately permeable soils and natural land slope greater than 6 per cent, and (b) two-sided micro-watershed for highly permeable soils with natural land slope less than 4 per cent (Figure 7.5).

Figure 7.3 Runoff farming concept

Figure 7.4 Desert strip farming concept.

Figure 7.5 Two sided micro-watershed with different treatment applied to the collector sources.

Contour terrace - The purpose of terracing is to retard and collect all runoff between the terraces. If the runoff is properly managed, enough water can be added to the soil of the terrace to improve tree growth significantly. Terraces are essential on steep slopes where all woody vegetation has been destroyed and is not likely to be reestablished before severe erosion occurs. Terraces should be large enough to hold or carry the heaviest ten-year rain.

Contour planting consists of placing long, low barriers perpendicular to the gradients, along contour lines which intercept and retain runoff and silt. The barriers can be of stone, logs, earth or hedge.

Floodwater spreading

In arid regions, rainfall usually falls during short, intense storms. The water swiftly drains away into washes and gullies and is lost to the region. Sometimes floods occur, often in areas untouched by the storm. Waterspreading is a practice of deliberately diverting the floodwaters from their natural courses and spreading them over adjacent floodplains or detaining them on valley floors (Figure 7.6). The wet floodplains or valley floors are then used to grow tree or forage crops.

Site selection is the key to success in floodwater farming. Three principal types of sites are preferred: (1) slopes below escarpments, (2) alluvial deltas, or (3) floodplains.

While potential sites are found in many arid and semi-arid regions, waterspreading systems require careful design and engineering to withstand flood waters. They must be selected so as to optimize topography, soil type, and vegetation.

4. Irrigation with waste water

The use of waste water for agricultural crops and tree production constitutes an interesting approach for many arid countries. It can be justified by the following facts:

- Creation of a resource from waste water. Fertilizer equivalent contained in waste water from an urban center averaging 10,000 inhabitants without polluting industry amounts to some US$30,000.

- The technique can be considered an economical method for waste water treatment.

- It is a water saving device so essential for dry regions where water resources are scarce.

Figure 7.6 Sketch of a water spreading system.

Several trials have shown that irrigation with waste water improve tree growth. However, a number of facts remain to be documented, such as the optimum size of equipment, the most adequate techniques and the resolution of health problems associated with some techniques in practice.

Depending upon the available water resource and the type of exploitation, three situations can be distinguished:

- untreated waste water,

- partially treated waste water, and

- completely treated waste water.

4.1 Untreated waste water

The use of untreated waste water is usually impracticable on account of the odor. Untreated waste water is either transported by tankers from houses and factories and poured into prepared trenches, or spread directly in the area. Odor could be rendered less obnoxious by mixing untreated waste water with urea or ammonium sulphate and muriate of potash. There are two problems associated with this situation. The first is the organization of waste water transport, and the second, the agreement between the producer and the user of waste water as to when the water is needed by the user.

4.2 Partially treated waste water

This technique aims at reducing the cost of water treatment by replacing the water softening plant with a simple primary water softening device. Soil will complement the full treatment (Figure 7.7). This technique is also used to recuperate fertilizer contained in the semi-treated water.

4.3 Completely treated waste water

This situation is very common in many arid countries. Urban centers and nearby plants treat their water. Agricultural farms nearby can use this water by connecting water pipes to the main treated water canal. This treated water can be totally used for growing agricultural crops and forest trees, partly reused by the plant or poured into a nearby canal (Figure 7.8).

4.4 Technical considerations for irrigated plantations using waste water

The use of waste water for irrigated forest plantations requires a periodical chemical water analysis because the quality of waste water varies considerably. In addition to water pH, consideration should be given to the variation in the amount of chloride, sulfate, free ammonia, water fluorine, phosphate, zinc, boron and silica. Soils for plantations should have a good porosity without a high infiltration rate. Soil absorption capacity should be monitored to follow its evolution.

Figure 7.7 Partially treated waste water

Figure 7.8 Completely treated waster

4.5 Health aspects

There are a number of health hazards associated with the use of waste water. Contamination of soil and vegetation, effluent runoff polluting water canals, waste water infiltration to groundwater, germ dispersal through biotic and wind factor. This necessitates limiting the use of waste water when untreated or partially treated to sites with low slope, and with moderately permeable soil. Sites should also be distant from human settlements. Irrigation techniques by gravity limit the direct contact of waste water with the plant but have the disadvantage of contaminating underground water if waste water is applied in great quantity. Sprinkler irrigation techniques limit the risk of underground water pollution but lead to direct contact of waste water with plants. Localized irrigation systems can prove the most apt techniques provided the problems of clogging of hoses are solved.

Total security can be ensured only through a set of preventive measures adapted to each site:

- selection of suitable tree crop (forest tree species for wood and fuelwood are the least affected);

- selection of appropriate site and irrigation techniques;

- addition of appropriate decontamination products to waste water.

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