Irrigation with saline water: benefits and environmental impact

doi:10.1016/S0378-3774(98)00120-6

Agricultural Water Management

Volume 40, Issues 2–3, May 1999, Pages 183-194

https://doi.org/10.1016/S0378-3774(98)00120-6 Get rights and content

Abstract

The shortage of water resources of good quality is becoming an important issue in the arid and semi-arid zones. For this reason the availability of water resources of marginal quality such as drainage water, saline groundwater and treated wastewater has become an important consideration. Nevertheless, the use of these waters in irrigated lands requires the control of soil salinity by means of leaching and drainage of excess water and salt. However, the leaching of salts, soil microelements and agro-chemicals can lower the quality of the drainage water in the irrigation scheme. The irrigation return flows with water or poor quality are a source of pollution of the surface water bodies situated downstream of the drainage outlet. Deep percolation could also contaminate the groundwater. Therefore, irrigation with saline water requires a comprehensive analysis even beyond the area where water is applied. The problem should be treated beyond the scope of the irrigation scheme, taking into consideration the groundwater and downstream surface water resources of the river basin. Consequently, the sustainable use of saline water in irrigated agriculture requires the control of soil salinity at the field level, a decrease in the amount of drainage water, and the disposal of the irrigation return flows in such a way that minimizes the side effects on the quality of downstream water resources. This paper describes the guidelines for a preliminary evaluation of the suitability of water for irrigation and the key factors for salinity control in lands irrigated with saline water. Options to improve the quality of the drainage water, strategies for the reuse of this water and alternatives for disposal of the outflow are also analysed. The final goal is to obtain sustainable agriculture and maintain the quality of the water resources in the river basin.

Introduction

In the arid and semi-arid zones water resources of good quality are becoming more and more scarce and are allocated with priority to urban water supply. For this reason there is an increasing need to irrigate with water of certain salt content, like groundwater, drainage water and treated wastewater.

Table 1 shows a classification of waters of different origins which allow a comparison among the salinity of drainage water, groundwater and surface water that is still used in zones with sufficient resources.

When a water resource is available to irrigate potentially a farm or an irrigation scheme the first step is to evaluate its suitability for this purpose. If the results of such an evaluation show the hazard of soil salinization, a careful analysis of the problem is needed, since it is possible to use saline waters for irrigation under proper land use and water management conditions.

To avoid the process of secondary salinization of the soil due to irrigation with water of high salt content, the salts added have to be washed out at least from the upper root zone, which is most active in the absorption of water and nutrients. The salts dissolved in the percolation water in part can be accumulated in the deeper subsoil, and in part discharged by natural drainage or through subsurface drainage systems in waterlogged lands. These systems have to be installed to control the water table level in order to permit leaching and disposal of the salts and to avoid the capillary rise of saline groundwater from the water table. In irrigated lands subsurface drainage is not only essential to provide good aeration and adequate water content in the root zone but also to maintain a favourable balance of salts.

However, the deterioration of the quality of the drainage water and the irrigation return flow and its side effects on water resources at the river basin level, necessitates a broader management programme with a wider context than salinity control at the field and farm levels. Reduction of the amount of drainage water, and disposal of the irrigation return flows in such a way that prevent the pollution of the water resources situated downstream, are the key issues to be taken into consideration for a strategy of irrigation with saline water.

Section snippets

Water and salt balances in an irrigated soil

The key factors to avoid the process of secondary soil salinization can be easily identified if water and salt balances in the root zone of an irrigated soil and the saturated zone below the water table are considered. The components of these balances can be identified in the scheme of the water cycle in an irrigated soil (Fig. 1).

The water balance in the root zone of an irrigated soil in a given time interval is the following: $I+P+G= ET_{c} +R+ Δ W$ where

I	effective amount of irrigation water infiltrated

Evaluation of the suitability of water for irrigation

Knowledge of the chemical composition of the water is necessary but not sufficient to evaluate its suitability for irrigation. Other factors such as climate, soil characteristics, drainage conditions and the irrigation method should be considered in order to define the appropriate land use and water management.

In this evaluation the aridity of the zone defined by the precipitation deficit, namely, the effective rainfall less the actual evapotranspiration, has to be considered. The soil

Leaching the salt added with the irrigation water

If the EC_i is lower than 0.7 dS/m, losses of water by deep percolation, that are generally higher than 15 percent of the amount of water applied, are sufficient to achieve an acceptable salt content in the soil solution (FAO, 1980). When waters of higher salinity – with values of EC_i between 0.7 and 3.0 dS/m and higher – are applied, more leaching is required to maintain a long-term adequate salinity content. In this case, percolation water can be insufficient in the whole irrigated field or in

Influence of leaching on irrigation and drainage requirements

The salt equilibrium equation can be used to calculate the leaching requirement during the irrigation season and to compare the result obtained with the average amount of percolation expected under the current irrigation practice. If the amount of percolation water (R) covers the leaching requirements (R^*) the salinization risk is controlled. In this case the application efficiency could be improved in order to save water. A security margin (R > 1.3R^*) is advisable (FAO, 1980) because the

The problem of the drainage water quality

The drainage water does not only contain dissolved salts but also soil microelements, such a boron, selenium, molybdenum, arsenic, etc. Boron, which is frequent in coastal areas of volcanic origin, can be toxic for plants in small concentrations. Selenium has been detected in drainage waters of irrigation schemes of the San Joaquin Valley, California, in concentrations that become toxic for the fauna that lives in the lakes into which the drainage systems discharge (Hoffman, 1990). Another

Disposal of secondary drainage water

The volume of irrigation return flows can be reduced drastically if the leaching fraction is diminished and by reusing the primary drainage water. However, at a certain point a certain flow of secondary drainage water remains, which can have a very high salt concentration (Table 1).

In coastal areas, where drainage systems discharge into tidal rivers saline contamination is not so important, because the water of the river is already saline. However, a strict chemical control is required,

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