D.27 Long-term prevention of salinity and sodicity hazards
D.28 Control of groundwater and surface water
D.29 Long-term erosion hazard
D.30 Other environmental hazards
The long-term conservation and environmental risks of irrigated agriculture should never be neglected in view of the historical associations between irrigation, land degradation, human diseases and other adverse side effects. The standard of conservation and environmental protection achieved will depend on the cropping, management and irrigation alternatives initially selected for the project, and on the management of resources after project implementation. The latter depends on the social and economic context, and particularly on the determination of funding agencies and governments to provide finance and satisfactory control measures. Historically, land degradation due to soil erosion, salinity, sodicity and waterlogging has resulted in the decline and fall of civilizations from the time that they ceased to undertake the long-term conservation measures that were needed. Health hazards are also a feature of irrigation schemes as already discussed in Section 5.8.
Four headings for evaluation are given in this section, namely, long-term salinity and sodicity hazard; control of groundwater; soil erosion- and the environment.
The measures required to prevent land degradation due to salinization and sodification should be recognized by the land evaluator who may make certain recommendations for prevention. The assessment under this heading is concerned with whether or not the necessary preventative measures will be implemented. The measures may be too sophisticated or too difficult or costly for farmers and existing government organizations to carry through, in which case it may be irresponsible to recommend the development of such land which should be classified as Not Suitable.
Water supplies and quality can be degraded by various factors including inadequate drainage, overpumping of aquifers and saline intrusion, or through deforestation and degradation of watersheds. The same criteria as discussed in the previous section apply to these risks. If the government agency or farmer is unlikely to implement conservation measures, it may be necessary to classify the land as Not Suitable.
Problems of water table control can arise both in areas threatened by salinization and in areas that are not threatened. In the latter case, though there may be a favourable salt balance, the presence of excess water, even if only periodic, can sometimes lead to the abandonment of land by farmers. Even if only for short periods this can result in weed growth and other problems that make further farming unviable, and the land reverts to natural vegetation or extensive agriculture (e.g. livestock) without irrigation.
Saline intrusion due to overpumping of aquifers frequently occurs in irrigation projects relying on groundwater. Wells must be monitored to ensure that the safe yield is not exceeded. The organizing authorities may need power to curb the use of water and to prevent the over-exploitation of the water resources. Without this power, the land may receive progressively more saline water until it has to be abandoned.
Degradation of catchments or drainage basins may affect the quantity of water for irrigation. Deforestation and erosion can lead to changes in seasonal stream flow patterns, and sedimentation of storage reservoirs may occur. The conservation of vegetation and soil in the catchment is of great long-term importance in the viability of many irrigation schemes.
Soil erosion in arid and semi-arid areas may occur from wind or water; in humid areas erosion due to water is usually the major hazard.
Erosion due to water: Alternative methods for assessing erosion due to water are:
i. the Universal Soil Loss Equation (USLA);
ii. the FAO Soil Degradation Assessment (FAOSDA) method;
iii. the Soil Loss Estimator for Southern Africa (SLEMSA);
iv. local methods based mainly on slope;
v. observed present situation in similarly irrigated areas.
All the above methods must be adjusted to take account of the additional hazard from gully erosion.
Whichever method is used for calculating or estimating soil loss, the critical limits and factor ratings can be applied in a similar way, by means of the following steps:
i. decide upon maximum acceptable limits of soil loss corresponding to each rating. These values form the critical limits for all crops or LUTs;
ii. for each land unit, calculate the soil loss on the basis of climate, soil and topographic factors only, i.e. omitting the land use factor;
iii. for each crop or LOT in turn, multiply the soil loss calculated in step ii. by a land use factor;
iv. repeating steps ii. and iii. gives estimates of soil loss for each land unit - LOT combination. These estimates are compared with the values in step i. to give a factor rating with respect to erosion hazard.
Wind erosion and dune formation: Strong winds, especially in arid and semi-arid areas, may necessitate the use of windbreaks. It may be essential to plant these several years ahead of the start of irrigated cropping. The erosion hazard must generally be rated on the basis of existing local experience, or from experience in similar situations. There have been cases of irrigation projects being completely debilitated by wind, the effects not only being soil loss but also dune formation and the filling of channels for water supply and drainage with soil.
The possibility of adverse effects from irrigation projects on human health and wildlife has already been discussed in Sections 5.8 and 5.7 respectively.