A very famous quote about modeling “With poor assumptions, a man can make more mistakes with a computer in a milli-second, than he could in a lifetime of common sense”. An assembly of concepts in the form of a mathematical equation that portrays understanding of a natural phenomenon is called model. Hydrologic models are simplified, conceptual representations of the Hydrologic cycle.

Why Modeling is Required?

Modelling has become an integral part of the decision making process for water engineers and managers. Recent research in hydrologic modelling tries to have a more global approach to the understanding of the behaviour of hydrologic systems to make better predictions and to face the major challenges in water resources management [Source: Training workshop on Hydrological Modelling by Hollings Foundation, October 2019].

Recently, Hydrological models have taken over the most important tasks in problem solving in hydrology (UNESCO, 1985). It is valuable for studying potential impacts of changes in land use or climate. As one report of CSDMS (Community Surface Dynamics Modelling System)56 Hydrological models are Present.

Types of Modelling

  1. Black box Model

In science and engineering, a black box is a device, system or object which can be viewed in terms of its inputs and outputs (or transfer characteristics), without any knowledge of its internal workings. Its implementation is “opaque” (black). Almost anything might be referred to as a black box: a transistor, an algorithm, or the human brain.

  • Conceptual Model

A conceptual model is a representation of a system, made of the composition of concepts which are used to help people know, understand, or simulate a subject the model represents. People receive information, process this information, and respond accordingly many times each day. This sort of processing of information is essentially a conceptual model (or mental model) of how things in our surrounding environment work.

  • Deterministic model

A deterministic model is one that has an exact solution that relates the independent variables of the model to each other and to the dependent variable. For a given set of initial conditions, a deterministic model yields the same solution each and every time.

  • Lumped models

Do not account for the spatial distribution of input variables or parameters. Do not require a digital elevation model (DEM) as input. Also Known as “Low Rank” Models. Ex- SRM (Snowmelt-Runoff Model), WATBAL etc.

  • Semi-distributed models

Models Parameters are partially varying in space by dividing the basin into sub basins.

It has two types:

  1. Kinematic wave theory models (KW models, such as HEC-HMS),
  2. Probability distributed models (PD models, such as TOPMODEL).
  • SWAT Model

SWAT is a semi-distributed physically based parameter model which is used to simulate the flow of very small to very large watershed not only in the US but also in the whole world. SWAT can simulate the flow process in a broad range of watersheds. The SWAT hydrological model was established and has been used by Hydro-Quebec for 20years. It is presently used for forecasting of inflows on all ranges of the watershed. Studies showed that SWAT can work efficiently for the simulation of hydrological studies. The following conclusions have been drawn, which are also on the basis of previous studies:

(i) The efficiency of the SWAT model is very high for the hydrological studies for the large catchment. (ii) Satisfactory simulation is obtained for daily, monthly, seasonally and annual runoffs. (iii) The performance of the snow-melting process of SWAT is satisfactory. (iv) Projection of stream flows under climate change is possible. (v) SWAT is in the public domain.

Need of Hydro-climatic modeling

Water is an essential element for survival of living things. It is vital factor for economic development and augmenting growth of agriculture and industry especially in the perspective of rapidly increasing population and urbanization. Many zones face scarcity of freshwater or subject to pollution. Thus, the availability and the sustainable use of the water resources become the core of the local and national strategies and politics in these regions. To deal with water management issues, one must analyze and quantify the different elements of hydrologic processes taking place within the area of interest. Obviously, this analysis must be carried out on a watershed basis because all these processes are taking place within individual micro watersheds. Hydrological processes and their local scattering have always direct relation to weather, topography, geology, and land use of watershed in addition to the impact of human activities. A watershed is comprised of land areas and channels and may have lakes, ponds or other water bodies. The flow of water on land areas occurs not only over the surface but also below it in the unsaturated zone and further below in the saturated zone.

The use of a watershed model to simulate these processes plays a fundamental role in addressing a range of water resources and environmental and social problems. The development of remote sensing (RS) techniques and Geographic Information System (GIS) capabilities has encouraged and improved the expanded use of watershed models worldwide. GIS is a suitable tool for the efficient management of large and complex database and to provide a digital representation of watershed characteristics used in hydrologic modeling. It has added confidence in the accuracy of modeling by providing more practical approach toward the watershed conditions, defining watershed characteristics, improving the efficiency of the modeling process and ultimately increasing the estimation capabilities of hydrological modeling.

Salman Khan is lecturer at Civil Engineering Department, Sarhad University, Peshawar.

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