1) Environment and Environmental are used in their broadest possible sense to include not only physical and biological systems but also social and economic systems and relationships.
2) Environmental Impacts and Effects are taken to include both the influence of the wider environment on a project and the influence of the project on the environment. They are also taken to include both "positive" and "negative" changes.
3) A Project is defined as a specific set of human aedvities in a particular location intended to modify the environment for human benefit. Using the above definitions, a Project s prime aim is to maxi-mize environmental enhancement while minimizing environmentalchange. In terms of the assessment of envirunmentai change it includes not only the direct works of the project but also the indirect changes in infrastructure and population associated with it.
4) Irrigation/drainage Planner, Manager, Engineer and Profes- sional are terms used to describe those people who, by virtue of their specific technical training in the techniques of irrigation, drainage and flood control are given responsibility for planning, designing, imple- menting and managing irrigation, drainage and flood control project. Use of thase terms is not intended to be restrictive in relation to the disciplines of training which such people have received but relates to their assigned responsibilities and functions in a given situation.
8.2 Current Approaches and Priorities in Environmental
Assessment
The initial impetus to introduce methods of environmental as- sessment was provided by growing public concern about the large number of unforeseen, adverse environmental impacts which were re- ported during the 1960s due to new development projects particularly those constructed in areas where little previous infrastructure develop- ment had taken place. As a result, many countries have now adoptedformalized (and statutory) procedures for environmental impact as- sessment (ETA) to be applied to all new projects within specified ca- tegories. In almost every case, irrigation, drainage and flood control projects are included within the list of project types for which an EtA is normally required although in some countries projects smaller than a specified size may be exempt.
Whilst putting a useful check on projects which might otherwise have caused lasting environmental damage, the introduction of statu- tory EtA procedures is now acknowledged to have some significant shortcomings, the principal ones being:
1) that EtA concentrates on negative aspects of environmental change and results in a situation of conflict between those who have planned a project and those who undertake the EIA on it;
2) that due to the detailed information required and the number of specialists engaged in an EtA it is not usually undertaken until a late stage in project planning when changes to the project to mitigate ad- verse effects are difficult and costly;
3) that dialogue between environmental specialists and project planners to identify beneficial modifications to the project design or conceptisnotfacilitatedbyEIA；
4)that available EIA procedures are too gener~in scope and re。 quire the user to exercise considerable knowledge and skill each time they a”applied to projects of a particular type such as irrigation or drainage；and
5) that EIA procedures are intended for use only by people with particular expertise in social and environmental impact aSsessment and overlook the potential savings in time and cost Whieh could beachievedifnon-specialistsweretodo someofthepreliminarydatacol-lection as part ofnormM planning activities．
The procedure descdbed in this report attempts to overcome some ofthe atIOVe difficulties．
9 Interrelation of Irrigation and Drainage
Irrigation in the world today covers approximately 160 million ha,excluding areas under natural flooding. About half of this total is foundin the arid and semi-arid subtropical zones. It was particularly inthese zones that the special drainage measures demanded by irrigationcame to be recognized, as over the years those areas with insufficientdrainage began to show rising groundwater tables and increasing salinity.
About two thirds of the total irrigated area has been brought underirrigation since the beginning of this century, but only recently has itbecome generally accepted that the installation or improvement ofdrainage forms an indispensable part of any irrigation project.
In ancient times, as in the days of the various Babylonian king-doms, salinity and water logging gradually reduced the productivityof the land. Market records have shown that in such a situation thecultivation ofwheat--a crop sensitive to soil salinity- gave way tothe more tolerant barley, but that finally large areas had to be aban-doned and the farmers moved to new land. The rise and fall of thevarious kingdoms in Mesopotamia were evidently closely related tothis changing state of agriculture.
The Imperial Valley in California, comprising 200 000 ha, wasbrought under irrigation about 1910. Only fifteen years later the pro-duetivity ofthis area was severely threatened since no provisions had been made for the discharge of the superfluous i~igation water andthe salts that were brought to the area at a rate of 800 ks per ha witheach irrigation application．Large parts ofthe valley went out ofpro-duction and it was this catastrophe that provided the impulse for re-search into proper methods of re．．establishing and maintaining suffi-ciently low salt concentrations in the soil．Due to the work of the U-S．Salinity Laboratory at Riverside，California，and that of other in-stitutions，the remedy for drainage problems in irrigated lands is atpresent well．known，but only in minor part ofthe affected，subtropicalareas have the necessary works been carried out．It is estimated thatapproximately 50 million ha of irrigated lands still do not have the re-quired drainage facilities．
The particular effects of irrigation on the criteria and the designof a drainage system are more dominant when rainfall is of lesser im-portance for the growth of crops than irrigation is．
……