Agricultural drainage water management in arid and semi ... - FAO.org

16Defining the problem and seeking solutionsThe terms simple and complicated in relation to engineering or functional and scientific modelsare rather subjective. The distinction between scientific and functional refers not only to thepurpose of modelling and the intended uses, but also implicitly to the approaches on which themodels are based.The three main groups of modelling approaches are mechanistic, empirical and conceptualapproaches. Mechanistic, or as Woolhiser and Brakensiek (1982) define them, physically-basedmodels are based on known fundamental physical processes and elementary laws. In groundwatermodelling, this approach is also known as the Darcian approach. As this approach is based onelementary laws it should be, theoretically, valid under any given condition and therefore itstransferability is extremely high. On the other hand, empirical approaches are based on relationsthat are established on an experimental basis and are normally only valid for the conditionsunder which they have been derived. Finally, conceptual calculation approaches are based onthe modeller’s understanding of fundamental physical processes and elementary laws, but theseare not used as such to solve a problem. Instead, a concept of the reality is used to tackle aproblem. The best-known example is the bucket-type approach to describe the flow of waterthrough unsaturated soil.Scientific models make use of mechanistic calculation approaches whenever possible andavoid the use of empirical and conceptual approaches. In contrast, functional models mightinclude any of the three calculation approaches. Here, mechanistic approaches might be includedas long as they do not conflict with other required model characteristics such as simplicity andshort calculation time. Empirical and conceptual approaches are used in functional models asthe only concern is that the model serves its intended purpose.Regional modelsThe employment of a range of drainage water management options results in certain benefits,interactions and trade-offs not only in the place where the measure is implemented but also inadjacent and downstream areas. To enable decision-makers, managers and engineers to choosefrom different options and to study the effects of various alternatives, regional simulation modelswill need to be employed. Regional models normally include three main calculation modules, i.e.water flow and salt transport in the unsaturated or vadose zone, through the groundwater zoneand through the irrigation and drainage conduits. Regional models normally require large amountsof data, and model calibration and validation is a time-consuming exercise. It is beyond thescope of this report to introduce the various models that have been developed and the readermay refer to Skaggs and Van Schilfgaarde (1999) and Ghadiri and Rose (1992) for more detail.The following sections introduce only some basic calculation considerations of water flow andsalt transport in the unsaturated or vadose zone as these form the basis of several of the calculationmethods presented in this publication. Where the water table in agricultural lands is controlledby subsurface drainage or where the water table is close to the rootzone, water and salt transportto and from the groundwater is considered as well. However, this publication introduces nospecific groundwater models or calculation procedures for water and salt transport in the saturatedzone.ROOTZONE HYDROSALINITY MODELSPrinciples of rootzone hydrosalinity modelsRootzone hydrosalinity models may range from simple conceptual to complex scientific models.In the more simple models, the spatial component in the control volume (i.e. the crop rootzone)