Models in Irrigation andWater Management - Practices of Irrigation and On-farm Water Management - page 381

interest are related to one another. By definition, model is a simplified representation
of reality. It serves as a means of predicting and examining an existing or proposed
system's performance under a set of conditions specified by the users. A model can
come in many shapes, sizes, and styles. It is important to emphasize that a model
is not the real world but only an approximation of reality (human construct) to help
us better understand real world systems. In general all models have an information
input, an information processor, and an output of expected results (Fig. 10.1 ) .

Fig. 10.1 Schematic
representation of a model

Model
(Processor)

Input Output

Model is a tool to obtain promising field management strategies. With good mod-
els, realistic estimation of crop yield (or other expected outputs) can be simulated
for various environmental conditions. The models are valuable for outscaling the
experimental findings to new environments.
Models are not meant to exclusively represent all elements of a system. The trick
is to balance the complexity of reality with the principle of parsimony (the simpler
the better). The form of a model depends on the problem to be addressed, the client
sponsor of model development, the state of knowledge about the prototype, and the
use of the model by a potential decision maker.

10.2.2 Different Types of Model

Models can be broadly classified into two major groups:

• Physically based (or process based), and

• Empirical models

10.2.2.1 Physically Based or Process-Based Model

In this type of model, the processes are conceptually represented using mathe-
matical equations. A process-based model normally represents (conceptualizes) all
important physical processes occurring during an event as well as between events
(if applicable). Such a model is derived from theoretical approaches to causal
relationship.
For example, for modeling storm event pollution load from urban catchment,
such models represent a description of the hydrological rainfall-runoff transfor-
mation process with associated erosion, pollution buildup and washoff, and other
quality components.
Process-based models are accurate over a wide range of conditions.

10.2.2.2 Empirical Models (or Black Box Models)

This type of models is mathematical model, and inputs and outputs are related by
empirical equation. The internal mechanism of the process (or causal relationship)