8. Water conservation and water saving practices and management
8.7. Water conservation in dryland agriculture
8.7.3. Soil management
Soil management practices for water conservation refer to tillage and land-forming practices
that favour rainfall infiltration into the soil, water storage in the soil zone explored by roots, capture of runoff to infiltrate the soil, control of evaporation losses from the soil and weeds, extraction of water by plant roots, and crop emergence and development.
These practices have long been known to have positive impacts on water conservation in dryland farming. However, results of any soil management technology depend upon the soil physical and chemical characteristics, the land-forms and geomorphology, the climate and the kind of implements used. All these factors interact, creating variable responses in terms of crop yields. When a technique is to be introduced in a given environment and it is substantially different from the traditional and well-proved practices adopted by local farmers, it is advisable to perform appropriate testing before it is widely adopted. However, the principles of soil management for water conservation are of general application, regardless of the size of the farm, the traction used, or the farming conditions.
Soil management practices for water conservation, are often common to the practices for soil conservation, i.e. they not only provide for augmenting the soil moisture availability for plant growth but they also contribute to the control of erosion and soil chemical degradation. In many cases they contribute to improving the soil quality. Because these practices produce changes in soil infiltration rates and amounts, soil water storage and runoff volumes, they may also produce relatively important changes in the hydrologic balance at the local field scale and, when widely adopted, they may affect the hydrologic balance at the basin scale.
The soil management practices for water conservation, summarised in Table 8.12, can be grouped as follows:
(a) Runoff control and improved water retention on the soil surface to provide for a higher amount of rainwater that can infiltrate into the soil and a larger time opportunity for the infiltration to occur. These effects are produced by creating a higher roughness of the cropped land where slopes are flat, but do not apply to sloping landscapes, which are discussed later. Practices, many of them traditionally used in dry farming, include:
1. Soil surface tillage, which concern shallow cultivation tillage practices to produce an increased roughness on the soil surface permitting short time storage in small depressions of the rainfall in excess to the infiltration, i.e. limiting overland-flow to give a larger time opportunity for infiltration. This practice is effective for very low sloping land cropped with small grains in soils having good stability of aggregates.
2. Contour tillage, where soil cultivation is made along the land contour and the soil is left with small furrows and ridges that prevent runoff formation and create conditions for the water to be stored until infiltration can be completed. This technique is also effective to control erosion and may be applied to row crops and small grains provided that field slopes are low. When rain could create waterlogging graded furrow tillage across the slope may be more helpful than contour tillage.
3. Using mulches from crop residues or straw applied on the soil surface, which also increases surface roughness which slows the overland-flow and, in addition, improves soil infiltration conditions as mentioned below.
4. Furrow diking to permit the storage of rain water in small basins or pits created along the furrows, which is also particularly useful in sprinkler irrigated furrows to avoid runoff and store the excess applied water until infiltration is completed. This practice is highly effective for sprinklers on moving laterals that often apply water at rates
TABLE 8.12. Soil management techniques for water conservation in agriculture
Soil management techniques Benefits Effectiveness
Water retention on the soil surface and runoff control
Soil surface tillage for increased surface roughness
Ponding of rainfall excess in depressions, larger time opportunity for infiltration
High (flat lands)
Tillage for contour (and graded)
furrows and ridges Runoff and erosion control, storage in furrows,
increased time for infiltration High (low slopes)
Residues and crop mulching Runoff retardation and higher infiltration High (low slopes)
Furrow dikes Rain water storage in furrow basins/pits and
increased infiltration amounts High
Bed surface cultivation Runoff control and increased infiltration Variable Increasing soil infiltration rates
Organic matter for improving aggregation
Improved soil aggregates and increased infiltration
High to very high
Conservation tillage Preserve soil aggregates and infiltration rates High to very high
Mulches, crop residues Soil surface protection, better aggregates and higher infiltration rates
High to very high
Traffic control Less soil compaction and improved water
penetration in the cropped area Variable
Chemicals for aggregates Favours soil aggregates and infiltration rates Medium to high Increasing the soil water storage
capacity
Loosening tillage Increased soil porosity, soil water transmission
and retention High
Subsoiling to open natural or plough made hardpans
Improved soil water transmission and storage, and increasing the soil depth exploitable by roots
High to very high
Deep tillage/profile modification
in clay horizons Increased water penetration and soil depth
exploitable by roots High but costly
Chemical and physical treatments of salt-affected soils
Increased infiltration and available soil water High to very high
Hydrophilic chemicals to
sandy/coarse soils Increase water retention in the soil profile Economic limits
Mixing fine and coarse horizons Increase water transmission and retention Economic limits
Asphalt barriers in sandy soils Decrease deep percolation Limited, costly
Compacting sandy soils Control of deep percolation, higher retention Variable
Control of acidity by liming, and of salinity by gypsum
More intensive and deep rooting, and improvement of aggregation
High Control of soil evaporation
Crop residues and mulching Decrease energy available on soil surface for
evaporation Very high
Shallow tillage Control soil water fluxes to the soil surface High
Chemical surfactants Decrease capillary rise Economic limits Runoff control in sloping areas
Terracing, contour ridges and
strip cropping Reduced runoff, increased infiltration, and
improved soil water storage High to very high Water harvesting (arid lands)
Micro water-harvesting Maximise rainfall infiltration at plant scale High to very high
Micro-watersheds Maximise runoff collection at tree scale High to very high
Runoff farming Maximise runoff collection at field scale High to very high
Water spreading Maximise the use of flood runoff through
diversion for infiltration in cropped fields High to very high
higher than the soil infiltration rate. However the effectiveness of this method depends upon the slope of the furrows and of the land.
5. Bed surface profile, which concerns cultivation on wide beds and is typically used for horticultural row crops. Often beds are permanent and traffic is only practiced in the furrows between beds, which should follow the land contours. The soil aggregation and infiltration are kept undisturbed on the bed and water is captured, stored and infiltrated into the furrows.
(b) Improvement of soil infiltration rates, which refer to a variety of practices that aim at increasing water penetration into the soil and maintaining high rates of infiltration. This limits the rain water that may run off or that evaporates when stored at the soil surface.
These practices refer to:
1. Increasing or maintaining the amount of organic matter in the upper soil layers because this provides for better soil aggregation, which is responsible for the macropores in the soil. Organic matter also preserves soil aggregates, avoiding crusting or sealing at the soil surface, which closes soil pores and significantly decreases the infiltration rates. Maintaining the continuity of macropores through which water penetrates and redistributes within the soil profile is essential.
2. Conservation tillage, including no-tillage and reduced tillage, where residuals of the previous crop are kept on the soil at planting. Mulches protect the soil from direct impact of rain drops, thus controlling crusting and sealing processes resulting from dispersion of aggregates by the rain drop impacts. Conservation tillage also helps to maintain high levels of organic matter in the soil. The soil is also less disturbed by tillage operations which for silty soils, could affect soil aggregates. Therefore, conservation tillage, which is now practised world-wide, is highly effective in improving soil infiltration and controlling erosion in dryland farming.
3. Application of mulches in tree and shrub fruit crops, or use of weed residuals when orchards are not tilled, to improve soil infiltration similar to the processes described above. The effectiveness of this technique is particularly relevant for tropical soils where organic matter is rapidly mineralised and rains may be very intense.
4. Traffic control, i.e. adopting permanent paths for tractors and other equipment, thus limiting soil compaction to these zones, which improves water penetration in the remainder of the cropped area.
5. Application of chemical additives to the soils for strengthening the soil aggregates, thus avoiding soil sealing, and to preserve soil porosity and water pathways in the soil. These may be highly effective for improving infiltration conditions in soils with less stable aggregates.
(c) Increasing the soil water storage capacity by improving the soil water holding characteristics, increasing the depth of the soil root zone, or favouring soil water conditions for water extraction by plant roots. Several practices may be considered:
1. Loosening tillage, which is applied to naturally compacted soils such as heavy silt soils, or to soils compacted by frequent tillage operations and traffic of tractors and equipment. It provides for increased soil porosity, thus enhancing conditions for water transmission and retention in the soil. Conditions for root development are also improved. This technique has variable effectiveness and often has to be repeated quite frequently.
2. Subsoiling for destroying natural or plough made hardpans, which may significantly
improve conditions for water movement downwards and, consequently increase soil water storage. It also provides improved pathways for the roots to develop into deeper layers, thus enlarging the soil depth exploitable by crop roots. Overall, this practice may increase appreciably the amount of water stored in the soil that becomes available for crop use.
3. Deep tillage or soil profile modification when clay horizons overlay more coarse soil horizons. This mixing or even inverting of relative positions of layers, contributes for increasing infiltration, deepening of crop roots, improving the availability of soil water for crop use, and larger water storage volumes.
4. In cold regions, loosening the soil by refilling the soil water storage prior to the soil freezing season. Water changes in volume when freezing and melting occurs and increases soil porosity and as a result, infiltration and water retention in the soil profile.
5. Chemical and physical treatments of salt-affected soils, i.e. treating saline and sodic soils to reduce salt concentration and toxicity to plants. This treatment also provides for reducing the osmotic potential with consequent improvement in the availability of soil water for crops use. It also modifies the physical conditions for water penetration and movement in the soil, thus increasing infiltration, and favours the development of crop roots into deeper layers. Overall, the amount of water stored in the soil usable by crops can be greatly enhanced.
6. Adding fine materials or hydrophilic chemicals to sandy/coarse soils slows the water transmission downwards, increases water retention, controls deep percolation and therefore increases the availability of water in soils with natural low water holding capacity. Similarly, asphalt barriers in sandy soils may be used to decrease deep percolation, but their use is more costly and application is difficult.
7. Compacting coarse textured soils may help reduce infiltration and deep percolation but results are variable and uncertain.
8. Control of acidity by liming, similarly to gypsum application to soils with high pH.
This treatment favours more intensive and deep rooting, better crop development and contributes to improved soil aggregation, thus producing some increase in soil water availability.
(d) Control of soil evaporation, which may be achieved in different ways:
1. Mulching with crop residues, straw, and several other materials including plastic and stones, which is aimed at decreasing the amount of energy available at the soil surface for soil water evaporation. Mulching, which shades the soil, also contributes to control of weeds and therefore of non-beneficial water use
2. Plastic mulching, as used for horticultural field crops to speed up crop emergence and the first stages of crop development, also provides for control of evaporation from the soil. This is particularly true for non transparent plastics that decrease net radiation available at the soil surface.
3. Shallow tillage, also called dust mulching or soil mulching, is a practice consisting of tilling the soil surface, only to a shallow depth, to create a discontinuity between the surface where energy is available and the deeper soil layers where water is retained and from where it would move upwards by capillarity if continuous pathways existed. Shallow tillage is a very common traditional practice in orchards and for the bare fallow period antecedent to crop planting. Besides its effects in directly
controlling soil evaporation, it also provides mechanical control of weeds, and therefore of their transpiration losses. Shallow tillage is highly effective when the high evaporation season is the dry season, i.e. when tillage can be performed before the high evaporation season.
4. Chemical surfactants that limit capillary upwards fluxes are also stated to have potential to limit soil water evaporation. However, there is not enough evidence on the effectiveness of this technique.
(e) Runoff control in sloping areas, which correspond to well known erosion control measures, also provide for water conservation in sloping landscapes. Most of them are associated with modifications of the land forms, reducing both the slope lengths and the slope angles. Generally they consist of:
1. Terracing, which may assume different forms as terraces are designed for both low slope lands cultivated by non-intensive field crops, or for medium to steep slopes, usually adopted for small farms. The first may be designed for infiltration of excess water in the downstream part of the cultivated area, as is common in low rainfall regions, or to drain off that excess water that may be stored in small reservoirs for use by cattle. The second may be designed to naturally drain excess water in the direction of the land slope, or may be designed to infiltrate most of excess water and then have horizontal or near horizontal surfaces. In every case, terraces slow down the overland flow and increase the amount of rainwater that can infiltrate the soil. In low rainfall areas only a small part of the storm rain flows out as runoff. Therefore, the amount of water that infiltrates the soil is significantly increased, mainly when terracing is associated with other soil management practices as described above.
2. Contour ridges, which correspond to a simplification of large terraces on low slope landscapes. Here the natural slope is divided by ridges along the contour lines at spacings which vary inversely with the slope angle. Ridges constitute obstacles to the runoff, that may be drained out by surface drains located immediately upstream of the contour ridges, or that may infiltrate from shallow ditches also located near the ridges. Alternatively, the land slope may be divided by stone walls that decrease the overland flow velocity and favour infiltration. As a result, whatever the solution that is adopted, runoff is controlled and the fraction of rainfall that infiltrates is increased.
3. Strip cropping, which is a technique for retardation of runoff and enhancing infiltration in low sloping areas. The land slope is divided into strips where different crops are cultivated using the contour tillage practice referred to above. Because different crops are used in successive strips they create different conditions for runoff and infiltration (particularly resistance to overland flow), that overall decreases the fraction of rainfall lost to the crops as runoff.
(f) Water harvesting, which are techniques used in arid lands to maximise the fraction of rainfall that is used for crops. Water harvesting, which is already analysed in Chapters 5 and 7, may have different forms that may be grouped as:
1. Micro water-harvesting, referring to a technique where planting is performed on the bottom of wide spaced furrows, so that the large surfaces between the ridges act as rainfall collection areas to maximise infiltration near the plants.
2. Micro-watersheds, where a fraction of the available area is used to collect the rainfall, which flows downslope to infiltrate into the cropped area. The ratio between the cropped area and the collection area generally decreases as the rainfall decreases.
3. Runoff farming, corresponding to techniques that maximise runoff from small areas, which is then stored in small reservoirs for supplemental irrigation or infiltrated in the cropped area.
4. Water spreading, also known as spate irrigation, which consists of diverting flood runoff to the cropped fields where it infiltrates.
The analysis above shows that there are a large variety of traditional and modern soil management practices for water conservation in dryland agriculture. Some of these practices are specific to given environments but others are of more general use, particularly mulching, which may be associated with conservation tillage, and organic matter incorporation. Many of these practices are for dryland farming only but some apply to irrigated agriculture. This is the case for the techniques that enhance soil water storage, that improve infiltration conditions or that help in controlling soil evaporation. A case study on soil management for improved water conservation in the North China Plain is presented in Box 8.2.
The analysis that follows, which refers to irrigated agriculture does not specifically mention most of the soil management practices but their consideration must be included when fully assessing strategies to improve water conservation and saving in water scarce agricultural ecosystems.