Chapter 1. Introduction

Need for drainage of irrigated lands

Large-scale development of irrigation has taken place in many arid and semi-arid areas since the late nineteenth century. Although irrigation has greatly increased the agricultural production potential, recharge brought about by seepage losses from the irrigation network and deep percolation from farm irrigation has accumulated into the underlying groundwater. A rise in water table results when irrigation-induced recharge is greater than the natural discharge. In many irrigated areas around the world, rising water tables have subsequently led to waterlogging and associated salinity problems. This has happened where drainage development has not kept pace with irrigation development or where maintenance of drainage facilities has largely been neglected. As an example of the historical rise in the groundwater table after the introduction of large-scale irrigation, Figure 1 shows the elevations of the ground surface and the variations of the phreatic level in an irrigated area in Punjab, Pakistan.

Figure 1. Rise of the groundwater table in Punjab, Pakistan

Source: Bhutta and Wolters, 2000.

Salinization affects about 20-30 million ha of the world's 260 million ha of irrigated land FAO (2000). To maintain favourable moisture conditions for optimal crop growth and to control soil salinity, drainage development is indispensable especially in saline groundwater zones. Smedema et al. (2000) estimate that current drainage improvement programmes cover less than 0.5 million ha per year, insufficient in their view to balance the current growth of affected drainage areas. They estimate that: 10-20 percent of the irrigated land is already equipped with drainage; 20-40 percent of the irrigated area is not in need of any artificial drainage; while 40-60 percent is in need of drainage but remains without drainage facilities. Table 1 shows examples from countries in Central Asia and the Near East to illustrate their observations.

Table 1. Salinized and drained areas compared with total irrigated area, Central Asia and the Near East

In Central Asia, the present drainage infrastructure is insufficient to control irrigation-induced waterlogging and salinity with a comparatively small percentage of subsurface drained land. In addition, the poor state of drainage networks (due to lack of maintenance) has exacerbated waterlogging and salinity (FAO, 1997a).

In the Near East, which is a region subject to salinity problems due to the prevailing climate conditions, an average of about 29 percent of the irrigated areas have salinity problems. Table 1 shows that for 12 countries in the Near East on average about 34 percent of the irrigated area has been provided with drainage facilities. For most countries no figures are available on the area under surface versus subsurface drainage (FAO, 1997b).

In Pakistan, 13 percent of the irrigated area is reportedly suffering from severe salinity problems in spite of the efforts made to provide drainage in irrigated areas. Salinity problems persist because of deficiencies in water policies and the low priority attached to the allocation of resources for the operation and maintenance (O&M) of drainage facilities in favour of initiating new projects (Martínez Beltrán and Kielen, 2000).

On the other hand, under the influence of the growing world population and the increasing demand for food, there is a trend of irrigation intensification. To supplement scarce surface water resources, groundwater is exploited through tubewell development, mainly in fresh groundwater zones, all over the world. In many of these areas, the water table is declining due to overexploitation of groundwater resources. Therefore, problems of waterlogging and related salinization in irrigated agriculture are confined principally to saline groundwater zones. However, the salinization and sodification of agricultural lands resulting from irrigation with marginal and poor-quality water (mainly groundwater) is increasing rapidly. Although firm figures are not currently available, many cases have been reported and documented for major irrigated areas in the world including South Asia, Southeast Asia, Central Asia, North Africa, the Near East, Australia, and the United States of America.

Need for water conservation and reuse

In response to the increasing world population and economic growth, water withdrawals for human consumption will increase, so increasing the competition for water between municipal, industrial, agricultural, environmental and recreational needs. If present trends continue with water withdrawal under present practices and policies, it is estimated that by 2025 water stress will increase in more than 60 percent of the world (Cosgrove and Rijsberman, 2000).

In this respect, providing food for the growing population is a major challenge as agriculture is already by far the largest water consumer in most regions in the world, except North America and Europe. On a global basis, agriculture accounts for 69 percent of all water withdrawals (FAO, 2000). Although the water resources for agriculture are often overused and misused, the general belief is that irrigated agriculture has to expand by 20-30 percent in area by 2025 in order to produce sufficient food for the growing world population. In order to avoid exacerbating the water crisis and to prevent considerable food shortages, the productivity of water use needs to increase. In other words, the amount of food produced with the same amount of water needs to increase. This is possible through the conservation and reuse of the available water resources in the agriculture sector, including usable drainage waters.

The overuse and misuse of water in irrigated agriculture has not only resulted in large-scale waterlogging and salinity and overexploitation of groundwater resources, but also in the depriving of downstream users of sufficient water and in the pollution of fresh water resources with contaminated irrigation return flow and deep percolation losses. Water pollution adds to the competition for scarce water resources as it makes them less suitable for other potential beneficial downstream uses. Furthermore, it might cause severe environmental pollution and threaten public health (Box 1).

Towards drainage water management

Until ten years ago drainage water management received little attention. Drainage research tended to focus on design issues, while evaluation dealt largely with the performance of the installed system in relation to the design criteria (Snellen, 1997). After the 1992 Earth Summit, the international irrigation and drainage community focused its full attention on drainage water management. Agenda 21 not only stresses the need for drainage as a necessary complement to irrigation development in arid and semi-arid areas, but at the same time it urges the conservation and recycling of freshwater resources in a context of integrated resource management (UNCED, 1992). In many countries these concerns and especially the concern for water quality degradation have resulted in drainage water disposal regulations to maintain the water quality standards of freshwater bodies for other uses, i.e. agricultural, municipal, industrial, environmental and recreational uses.

Scope of this publication

This publication focuses on the management of drainage water from existing drainage systems located in irrigated areas in arid and semi-arid regions. It does not address design considerations for new drainage systems in detail as these will be the subject of the forthcoming FAO Irrigation and Drainage Paper Planning and design of land drainage systems.

For existing drainage facilities, planners, decision-makers and engineers have a number of drainage water management options available to attain their development goals, e.g. reducing the waterlogged and salinized area in a certain drainage basin whilst maintaining water quality for downstream users. The options can be divided into four broad groups of measures: (i) water conservation, (ii) drainage water reuse, (iii) drainage water disposal, and (iv) drainage water treatment. Each of these options has certain potential impacts on the hydrology and water quality in an area, and where more than one option is applied at a site, interactions and trade-offs occur. Therefore, planners, decision-makers and engineers need to have a framework for selecting from among the various possibilities and for evaluating the impact and the contribution towards the development goals. Furthermore, technical expertise and guidelines on each of the options are required to enable enhanced assessment of the impact of the differing options.

The objective of this publication is twofold:

1. To present a framework that will enable planners, decision-makers and engineers to select from among the differing drainage water management options and to evaluate their impact and contribution towards the development goals; and

2. To provide technical guidelines for the planning and preparation of preliminary designs of drainage water management options.

This publication consists of two parts. Part I provides the framework and technical guidelines on the drainage water management measures for decision-makers, planners and engineers. After the introduction (Chapter 1), Chapter 2 presents guidelines for defining the problem and alternative approaches in seeking solutions. Chapter 3 provides a framework for the selection and evaluation of drainage water management options. Chapter 4 deals with factors affecting drainage water quality. Chapters 5 to 8, respectively, present the guidelines and details on each of the four options related to water conservation, drainage water reuse, drainage water disposal and drainage water treatment. It is beyond the scope of this publication to provide technical details and guidelines to prepare detailed designs. Design engineers may refer to the numerous references provided later in the text.

Part II presents summaries of five case studies from India, Pakistan, Egypt, the United States of America, and the Aral Sea Basin. They illustrate how various countries or states deal with the issue of drainage water management in the context of water scarcity, both in quantitative and qualitative terms, under differing degrees of administrative and policy guidelines and regulations as well as differing degrees of technological advancements and possibilities. The attached CD-ROM contains the full text of these case studies.